Vegetation Change and the Biogeochemistry of Grassland and Savanna Ecosystems

VEGETATION CHANGE AND THE BIOGEOCHEMISTRY OF GRASSLAND AND SAVANNA ECOSYSTEMS

Sponsoring Institution

National Institute of Food and Agriculture

Project Status

TERMINATED

Funding Source

HATCH

Reporting Frequency

Annual

Accession No.

0138237

Grant No.

(N/A)

Project No.

TEX06945

Proposal No.

(N/A)

Multistate No.

(N/A)

Program Code

(N/A)

Project Start Date

Jun 6, 2008

Project End Date

Jun 5, 2014

Grant Year

(N/A)

Project Director
Boutton, TH, W.

Recipient Organization
TEXAS A&M UNIVERSITY
750 AGRONOMY RD STE 2701
COLLEGE STATION,TX 77843-0001

Performing Department
Ecosystem Science & Management

Non Technical Summary
Soil organic matter (SOM) represents the largest pools of organic carbon (C) and nitrogen (N) in the terrestrial environment, but an incomplete understanding of multi-process soil/plant/microbe interactions limits our ability to quantitatively account for the storage and dynamics of these elements in global budgets. Species-dependent controls on plant chemical and physical composition, microbial community structure and activity, climate, and edaphic factors, all play a role in determining SOM stabilization and decomposition. Current concepts of the physical and biological controls over soil C and N storage emphasize that long-term stabilization of SOM results largely from the interplay among three factors: (i) physical protection within soil aggregates, (ii) inherent chemical recalcitrance of the organic matter, and (iii) association with mineral surfaces. This proposal will document and quantify how these protective mechanisms interact following vegetation change from C4 grassland to C3 woodland dominated by N-fixing tree legumes. The primary goal of this project is to determine the quantitative significance of microbe community structure and enzymatic activity, soil microfabric, and the specific chemical forms of organic C and N in the stabilization of SOM along a chronosequence from remnant grasslands to woodland. Specifically, a chronosequence (120 yrs) of C3 woody plant invasion into a subtropical C4 grassland will be utilized as a model system to investigate storage and dynamics of SOM in soil physical fractions. Natural C-13 and N-15 isotopic variations induced by vegetation change from C4 grassland to C3 woodland will be a key tool for tracing sources and fluxes of SOM in all phases of the project. The project will address the following questions: (1) How does soil physical structure determine C accrual and dynamics over time following woody plant invasion? (2) What is the chemical composition, source, and turnover rate of the plant and microbial carbon that is stabilized? (3) What is the role of shifting populations of soil microbes and enzyme activity in the respiration of litter and SOM fractions and how do they impact aggregation dynamics? (4) What is the relative accessibility of the new C3 SOM pools to microbial decay and can we relate physically identifiable SOM fractions with calculated mean residence time (MRT) to potential respiration in inoculation experiments? Innovative molecular, isotopic, and microbiological methods will be used to develop a more fundamental understanding of processes that control SOM storage and dynamics. This work has potential to benefit the SOM modeling community which is searching for biologically, chemically, and physically meaningful approaches to modeling of SOM dynamics. This work should yield an enhanced understanding of the role of soil processes in biogeochemical cycles and the earth system, which will be of immediate significance to both scientists and policy-makers as mankind considers the potential for manipulating the carbon cycle in order to mitigate the potential for global climate change.

Animal Health Component

(N/A)

Research Effort Categories

Basic

(N/A)

Applied

(N/A)

Developmental

(N/A)

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
102	0110	1070	20%
102	0630	1070	10%
102	0799	1020	10%
102	0799	1060	10%
102	0799	1070	20%
206	0110	1060	10%
206	0630	1070	10%
206	0799	1070	10%

Knowledge Area
206 - Basic Plant Biology; 102 - Soil, Plant, Water, Nutrient Relationships;

Subject Of Investigation
0110 - Soil; 0799 - Rangelands and grasslands, general; 0630 - Chaparral and shrub lands;

Field Of Science
1060 - Biology (whole systems); 1070 - Ecology; 1020 - Physiology;

Keywords

carbon cycle

nitrogen cycle

stable carbon isotopes

stable nitrogen isotopes

soil organic matter

soil carbon turnover

organic matter stabilization

soil physical structure

soil organic chemistry

soil microbial communities

savanna ecology

vegetation change

Goals / Objectives
Current concepts of the physical and biological controls over soil C and N storage emphasize that long-term stabilization of SOM results largely from the interplay among three factors: (i) physical protection within soil aggregates, (ii) inherent chemical recalcitrance of the organic matter, and (iii) association with mineral surfaces. This proposal will document and quantify how these protective mechanisms interact following vegetation change from C4 grassland to C3 woodland dominated by N-fixing tree legumes. The primary goal of this project is to determine the quantitative significance of microbe community structure and enzymatic activity, soil microfabric, and the specific chemical forms of organic C and N in the stabilization of SOM along a chronosequence from remnant grasslands to woodland. Specifically, a chronosequence (120 yrs) of C3 woody plant invasion into a subtropical C4 grassland will be utilized as a model system to investigate storage and dynamics of SOM in soil physical fractions. Natural C-13 and N-15 isotopic variations induced by vegetation change from C4 grassland to C3 woodland will be a key tool for tracing sources and fluxes of SOM in all phases of the project. The project will address the following questions: (1) How does soil physical structure determine C accrual and dynamics over time following woody plant invasion? (2) What is the chemical composition, source, and turnover rate of the plant and microbial carbon that is stabilized? (3) What is the role of shifting populations of soil microbes and enzyme activity in the respiration of litter and SOM fractions and how do they impact aggregation dynamics? (4) What is the relative accessibility of the new C3 SOM pools to microbial decay and can we relate physically identifiable SOM fractions with calculated mean residence time (MRT) to potential respiration in inoculation experiments? This work has potential to benefit the SOM modeling community which is searching for biologically, chemically, and physically meaningful approaches to modeling of SOM dynamics. This work should yield an enhanced understanding of the role of soil processes in biogeochemical cycles and the earth system, which will be of immediate significance to both scientists and policy-makers as mankind considers the potential for manipulating the carbon cycle in order to mitigate the potential for global climate change.

Project Methods
Research will be conducted at the Texas Agricultural Experiment Station La Copita Research Area located 65 km west of Corpus Christi, Texas in the eastern Rio Grande Plains of the Tamaulipan Biotic Province. This area has been classified as a Prosopis-Acacia-Andropogon-Setaria savanna (Kuchler, 1964), but contemporary vegetation is subtropical thorn woodland, having experienced significant woody plant expansion in recent history. A chronosequence approach will be used to quantify C and N pool sizes and their rates of change in litter, roots, and soils following woody plant encroachment. Remnant grasslands will be sampled to characterize C pool sizes at time zero (i.e. prior to woody encroachment), whereas clusters, groves, and drainage woodlands of known age will be sampled to evaluate changes in C pools at different points in time following woody invasion. Ages of woody communities have been determined and stands range in age from approximately 10-130 years. All soil and plant fractions will be quantified, dried, and ground for %C, N and δ13C, and δ15N analysis. Soil organic matter fractions will be isolated by wet sieving and density fractionation. Each of the soil fractions and litter and plant components will be subjected to detailed chemical and molecular characterization for plant components including cellulose, lignin, cutin and suberin acids, and amino acids in order to document changing chemistry due to microbial action and source. Additionally, difference in the δ13C value of the C3 and C4 sources will be expressed at the molecular level allowing for biopolymer-specific calculations of MRT upon compound-specific stable carbon isotope analysis. Aliquots of the same soils collected as part of the work described above will also be examined for enzyme activities. Mineralizable soil carbon and nitrogen pools will be determined by lab incubations. The potential recalcitrance of SOM derived from woody plants in whole soils and in physical fractions will be determined by analysis of the δ13C of CO2 released in long term respiration studies.

Progress 06/06/08 to 06/05/14

Outputs
Target Audience: The target audience for this research is primarily the scientific community working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meetings of the American Geophysical Union. In addition, knowledge acquired through my research program was transferred to graduate students in my ESSM 600 (Principles of Ecosystem Science and Management) and ESSM 622 (Biogeochemistry of Terrestrial Ecosystems) classes at Texas A&M. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? The following graduate students received training during the past year as part of research program: Julie Foote - Ph.D. Student, Texas A&M University Rachel Rosenstein - Ph.D. Student, Texas A&M University Yong Zhou - Ph.D. Student, Texas A&M University Ryan Mushinski - Ph.D. Student, Texas A&M University Fiona Soper - Ph.D. Student, Cornell University Anais Dion - M.S. Student, Texas A&M University How have the results been disseminated to communities of interest? Results were disseminated to scientists working in ecology and biogeochemistry by giving presentations at meetings of the American Geophysical Union. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? Soil organic matter in coarse-textured soils is more vulnerable to environmental disturbances due to reduced potential for soil organic carbon (SOC) stabilization in aggregates or organo-mineral complexes. In the sandy loam soils of the Rio Grande Plains region of southern Texas, woody encroachment has resulted in the rapid accrual of root and leaf tissues derived from trees and shrubs into poorly physically protected (macroaggregate >250 µm) and non mineral-associated (free light fraction <1.0 g/cm3) soil fractions. To determine the impact of changing plant input chemistry on the potential degradability of accumulating SOC fractions, we measured the quantity and isotopic composition of respired CO2 from year-long incubations of the macroaggregateand free light soil fractions along a grassland to woodland successional chronosequence. During incubation of both fractions, the proportion of SOC respired from older woody stand soils (~40-90 yrs) relative to recently established woody stands (<40 yrs) and remnant grassland soils decreased. We interpreted this decrease with woody stand age to result from a change in plant input chemistry with more lignin and aliphatic structures combined with a progressive shift to more non-hydrolysable, poorly accessible forms of soil organic nitrogen, resulting in a system with slower short-term decay dynamics. The δ13C values of respired CO2 from all landscape elements indicated a selective release of older grassland-derived SOC in the first month of the macroaggregate incubation, possibly due to the disruption and rapid microbial utilization of this SOC after the soil fractionation process. Due to the sensitivity of these rapidly-cycling soil fractions to environmental disturbance and their capacity to influence longer-term SOC dynamics, understanding their decay dynamics is essential for understanding the mechanisms of SOC stabilization. This is especially important in coarse-textured soils where large SOC stocks may be present in physical fractions that are relatively unprotected from decomposition.

Publications

Type: Journal Articles Status: Published Year Published: 2014 Citation: Scott DA, Eaton RJ, Foote JA, Vierra BJ, Boutton TW, Blank GB, Johnsen KH. 2014. Soil ecosystem services in loblolly pine plantations 15 years after harvest, compaction, and vegetation control. Soil Science Society of America Journal 78: 2032-2040.
Type: Journal Articles Status: Published Year Published: 2014 Citation: Dou F, Hons FM, Wright AL, Boutton TW, Yu X. 2014. Soil carbon sequestration in sorghum cropping systems: Evidence from stable isotopes and aggregate size fractionation. Soil Science 179: 68-74.
Type: Journal Articles Status: Published Year Published: 2014 Citation: Ansley RJ, Boutton TW, Jacoby PW. 2014. Root biomass and distribution patterns in a semi-arid mesquite savanna: Responses to long-term rainfall manipulation. Rangeland Ecology and Management 67: 206218.
Type: Book Chapters Status: Published Year Published: 2014 Citation: Ahmed I, Boutton TW, Karim A, Strom KB, Fox JF. 2014. Monitoring soil organic carbon loss from erosion using stable isotopes. IN: Soil Carbon Sequestration for Climate, Food Security, and Ecosystem Services, pp. 130-138, G Halldorsson, F Bampa, AB Thorsteinsdottir, BD Sigurdsson, L Montanarella, A Arnalds, Eds. JRC Science and Policy Reports, European Union. Luxembourg.
Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Pyle LA, Hockaday WC, Boutton TW, Zygourakis K, Kinney TJ, Masiello CA. 2014. Chemical and isotopic thresholds in charring: Implications for the interpretation of charcoal mass and isotopic data. Abstracts, Annual Meeting of the American Geophysical Union 2014: B41A-0008.
Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Ahmed I, Karim A, Boutton TW, Strom KB. 2014. Prediction of uncertainty in watershed scale sediment provenance model using tracers. Abstracts, Annual Meeting of the American Geophysical Union 2014: H23L-1048.
Type: Conference Papers and Presentations Status: Published Year Published: 2014 Citation: Kulawardhana RW, Feagin RA, Popescu SC, Boutton TW. 2014. Salt marsh carbon distributions as affected by relative sea level history and land cover conversions: Case study using remote sensing based estimates in Spartina alterniflora dominated salt marshes in Galveston, Texas. Abstracts, 11th International Symposium on Recent Advances in Environmental Health Research 2014: O-30.

Progress 01/01/13 to 09/30/13

Outputs
Target Audience: The target audience for this research is primarily the scientific community working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meetings of the American Geophysical Union, the Ecological Society of America, and the North American Carbon Program.. In addition, knowledge acquired through my research program was transferred to graduate students in my ESSM 600 (Principles of Ecosystem Science and Management) and ESSM 622 (Biogeochemistry of Terrestrial Ecosystems) classes at Texas A&M. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? The following graduate students received training during the past year as part of research program: Ilsa Kantola - Ph.D. Student, Texas A&M University Julie Foote - Ph.D. Student, Texas A&M University Rachel Rosenstein - Ph.D. Student, Texas A&M University Yong Zhou - Ph.D. Student, Texas A&M University Ryan Mushinski - Ph.D. Student, Texas A&M University Courtney Creamer - Ph.D. Student, Purdue University Fiona Soper - Ph.D. Student, Cornell University How have the results been disseminated to communities of interest? Results were disseminated to scientists working in ecology and biogeochemistry by giving presentations at meetings of the American Geophysical Union, Ecological Society of America, North American Carbon Program, and the Texas Chapter of the Wildlife Society. What do you plan to do during the next reporting period to accomplish the goals? Goals for the coming year include: (1) Examination of the potential for symbiotic nitrogen fixation by woody legumes that are invading areas that were once grassland in the southwestern USA. (2) Preliminary studies to evaluate the influence of invasive woody legumes on trace gas emissions from ecosystems. (3) Develop the instrumentation and methodology necessary to partition the contributions of nitrification vs. denitrification to N2O production in terrestrial ecosystems.

Impacts
What was accomplished under these goals? Spatial patterns of soil d15N reflect variation in rates of N-cycling processes across landscapes. However, the manner in which soil d15N is affected by vegetation and topoedaphic properties under nonsteady state conditions is understood poorly. Here we propose and evaluate a conceptual model that explains how soil d15N values will respond to changes in disturbance regimes (intensification of grazing and removal of fire) and the resultant invasion of a subtropical grassland by woody vegetation dominated by Prosopis glandulosa (honey mesquite), a N-fixing tree legume. Spatially-specific sampling along a catena (hill-slope) gradient where woody plants are known to have displaced grasses over the past 100 years revealed a positive relationship between soil d15N and d13C, and a negative relationship between NDVI and soil d15N on upland portions of the landscape, indicating that plant cover is a critical determinant of d15N spatial patterns. Because the dominant woody invader is a N-fixer, its invasion has increased N input and reduced soil d15N. However, while honey mesquite also invaded and came to dominate lowland portions of the landscape, soil d15N values in woodlands of intermittent drainages were significantly elevated relative to those in uplands. This is likely attributable to higher soil moisture, clay content, and total N in the lower portions of the catena gradient, which create conditions favoring more rapid N-transformation rates, higher preferential 14N losses (e.g., gaseous), and thus enrichment of 15N. Thus, while spatial and temporal variation of soil d15N has the potential to be an indicator of disturbance-induced changes in the net N balance, its sensitivity is compromised in topoedaphic settings with where rates of N-transformation are high. Continued improvements in our understanding of controls over the spatial variability of soil d15N at the landscape-scale will enhance our ability to use d15N as a diagnostic tool for inferring N dynamics under both steady-state and disturbed conditions.

Publications

Type: Journal Articles Status: Published Year Published: 2013 Citation: Creamer CA, Filley TR, Boutton TW. 2013. Long-term incubations of size-separated soil fractions to inform soil organic carbon decay dynamics. Soil Biology and Biochemistry 57: 496-503.
Type: Journal Articles Status: Published Year Published: 2013 Citation: Bai E, Boutton TW, Liu F, Wu XB, Archer SR. 2013. 15N isoscapes in a subtropical savanna parkland: Spatial-temporal perspectives. Ecosphere 4(1):4. http://dx.doi.org/10.1890 /ES12-00187.1
Type: Journal Articles Status: Published Year Published: 2013 Citation: Creamer CA, Filley TR, Olk DC, Stott DE, Dooling V, Boutton TW. 2013. Changes to soil organic N dynamics with leguminous woody plant encroachment into grasslands. Biogeochemistry 113: 307-321.
Type: Journal Articles Status: Published Year Published: 2013 Citation: Liu F, Archer SR, Bai E, Boutton TW, Wu XB, Gelwick F. 2013. Woody plant encroachment into grasslands: Spatial patterns of functional group distribution and community development. PLoS One 8(12): e84364. doi:10.1371/journal.pone.0084364.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Gann KR, Hewitt DG, Fulbright TE, Ortega-Santos A, Boutton TW, Ortega-Sanchez A. 2013. Foraging ecology and population parameters of unmanaged white-tailed deer in southern Texas. Abstracts, Texas Chapter of the Wildlife Society 2013.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Hines SL, Fulbright TE, Ortega-Santos A, Hewitt DG, Boutton TW, Ortega-Sanchez A. 2013. Investigating cattle grazing impacts on preferred deer forbs and ungulate grazing niches utilizing stable isotopes. Abstracts, Texas Chapter of the Wildlife Society 2013.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Ahmed I, Boutton TW, Strom KB, Karim A, Irvin-Smith N. 2013. Soil carbon distribution and loss monitoring in the urbanized Buffalo Bayou watershed, Houston, Texas. Abstracts, 4th North American Carbon Program Meeting, Poster 149.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Ahmed I, Karim A, Boutton TW, Strom KB. 2013. Spatial distribution of soil organic carbon in urbanized Texas Gulf Coast: An outlook on the future of carbon management. Abstracts, International Conference on Soil Carbon Sequestration for Climate, Food Security, and Ecosystem Services, Reykjavik, Iceland: p. 54.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Karim A, Ahmed I, Boutton TW, Strom KB. 2013. Bayesian simulation of soil organic carbon source monitoring: A case study in watershed hydrology and erosion. Abstracts, International Conference on Soil Carbon Sequestration for Climate, Food Security, and Ecosystem Services, Reykjavik, Iceland: p. 130.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Soper FM, Boutton TW, Sparks JP. 2013. Seasonal variation in nitrogen fixation in natural ecosystems a new approach to ?15N applications in a leguminous tree. Abstracts, Ecological Society of America Meeting 2013: COS 96-8. (Received Billings Award for Best Oral Presentation).
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Karim A, Ahmed I, Boutton TW, Strom KB, Fox J. 2013. Decision support system to guide land use fingerprinting using stable isotopes under hydrologic uncertainty. Abstracts, Annual Meeting of the American Geophysical Union 2012: H21B-1025.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Ahmed I, Karim A, Boutton TW, Strom KB. 2013. Watershed-scale stable isotope distribution and implications on soil organic carbon loss monitoring under hydrologic uncertainty. Abstracts, Annual Meeting of the American Geophysical Union 2012: H51C-1198.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Wellman RL, Boutton TW, Tjoelker MG, Volder A, Briske DD. 2013. Nitrogen transformations in response to temperature and rainfall manipulation in oak savanna: A global change experiment. Abstracts, Annual Meeting of the American Geophysical Union 2013: B43E-0543.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Foote JA, Boutton TW, Scott DA. 2013. Microbial responses to forest management in the western Gulf Coastal Plain, USA. Abstracts, Annual Meeting of the American Geophysical Union 2013: B51D-0298.
Type: Conference Papers and Presentations Status: Published Year Published: 2013 Citation: Boutton TW, Foote JA, Scott DA. 2013. Changes in soil nitrogen storage and dynamics in response to forest management in southeastern pine forest. Abstracts, Annual Meeting of the American Geophysical Union 2013: B21E-04.

Progress 01/01/12 to 12/31/12

Outputs
OUTPUTS: Woody plants have encroached into grasslands, savannas, and other grass-dominated ecosystems throughout the world during the last century. This dramatic vegetation change is likely driven by livestock grazing, altered fire frequencies, elevated atmospheric CO2 concentrations, and/or changes in atmospheric deposition patterns. Woody invasion often results in significant changes in ecosystem function, including alterations in above- and belowground primary productivity, soil C, N, and P storage and turnover, and the size and activity of the soil microbial biomass pool. The purpose of this study was to examine the relationships and interactions between plant communities and soil microbial communities in the Rio Grande Plains region of southern Texas where grasslands have been largely replaced by woodlands. Research was conducted along a successional chronosequence representing the stages of woody plant encroachment from open grassland to closed-canopy woodland. To characterize soil microbial community composition, soil samples (0-7.5 cm) were collected in remnant grasslands (representing time 0) and near the centers of woody plant clusters, groves, drainage woodlands, and playas ranging in age from 10 to 100 yrs. Ages of woody plant stands were determined by dendrochronology. Community DNA was extracted from each soil sample with a MoBio PowerMax Soil DNA isolation kit. The DNA concentrations were quantified on a NanoDrop ND-1000 spectrophotometer and diluted to a standard concentration. Pyrosequencing was performed by the Research and Testing Laboratory (Lubbock, TX) according to Roche 454 Titanium chemistry protocols. Samples were amplified with primers 27F and 519R for bacteria, and primers ITS1F and ITS4 for fungi. Sequences were aligned using BioEdit and the RDP Pipeline and analyzed in MOTHUR. PARTICIPANTS: Ilsa Kantola - Ph.D. Student, Texas A&M University; Julie Foote - Ph.D. Student, Texas A&M University; Rachel Rosenstein - Ph.D. Student, Texas A&M University; Courtney Creamer - Ph.D. Student, Purdue University; Fiona Soper - Ph.D. Student, Cornell University; Tim Filley - Professor, Purdue University; Diane Stott -Scientist, USDA National Soil Erosion Lab, Purdue University; Carrie Masiello - Professor, Rice University; Jason West - Professor, Texas A&M University; Tom Hallmark - Professor, Texas A&M University; Jed Sparks - Professor, Cornell University. TARGET AUDIENCES: The target audience for this research is primarily the scientific community working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meetings of the American Geophysical Union, the Soil Science Society of America, and the Ecological Society of America. To provide outreach to a broader and more general audience, I also gave two one-day workshops at the annual meeting of the Texas Master Naturalists Program. In addition, knowledge acquired through my research program was transferred to graduate students in my ESSM 600 (Principles of Ecosystem Science and Management) and ESSM 622 (Biogeochemistry of Terrestrial Ecosystems) classes at Texas A&M. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
Non-metric multidimensional scaling of the operational taxonomic units identified by pyrosequencing revealed that both bacterial and fungal community composition were significantly different between remnant grasslands and all woody plant community types. Phylum-level classification of the 16S bacterial sequences showed that five phyla (Acidobacteria, Actinobacteria, Proteobacteria, Bacteroidetes, and Gemmatiomonadetes) represented 85-91% of classifiable sequences in all landscape elements. The relative abundances of Acidobacteria were significantly higher (p<0.05) in grassland samples (29.5%) than in all wooded landscape elements (17.1-25.6%), while the relative abundances of Actinobacteria was lower in grasslands (8.8%) than wooded areas (16.1-19.7%). Phylum-level classification of fungal sequences showed that four phyla accounted for 61.8 to 86.3% of identified sequences. Ascomycota was the most common phylum in all samples (55.8-62.1%), with significant contributions from Basidiomycota, Chytridiomycota, and Blastocladiomycota. The largest change in fungal community composition at the phylum level was observed in the Chytridiomycota, which declined from 4.0% in the grasslands to 0.8-1.4% in the wooded landscape elements. These significant changes in microbial community composition that occur following grassland to woodland conversion may have important implications for key biogeochemical processes that influence ecosystem structure and function in this region.

Publications

Bai E, Boutton TW, Liu F, Wu XB, Archer SR. 2012. Spatial patterns of soil δ13C reveal grassland-to-woodland successional processes. Organic Geochemistry 42: 1512-1518.
Bai E, Boutton TW, Liu F, Wu XB, Archer SR, Hallmark CT. 2012. Spatial variation of soil δ13C and its relation to carbon input and soil texture in a subtropical lowland woodland. Soil Biology and Biochemistry 44: 102-112.
Hall SA, Boutton TW, Lintz CR, Baugh TG. 2012. New correlation of stable carbon isotopes with changing late-Holocene fluvial environments in the Trinity River basin of Texas, USA. The Holocene 22: 541-549.
Creamer CA, Filley TR, Olk DC, Plante AF, Peltre C, Top SM, Boutton TW. 2012. Degree of woody plant encroachment into grasslands controls soil carbohydrate and amino compound changes during long-term laboratory incubation. Organic Geochemistry 52: 23-31.
Soper FM, Sparks JP, Boutton TW. 2012. Temporal variation in nitrogen fixation during encroachment of Prosopis glandulosa into grasslands of the Rio Grande Plains. Abstracts, Ecological Society of America Meeting 2012: PS 49-101.
Boutton TW, Kantola IB, Filley TR. 2012. Changes in soil phosphorus fractions following woody plant invasion of grassland. Abstracts, Annual Meeting of the American Geophysical Union 2012: B33B-0524.
Filley TR, Creamer CA, Boutton TW. 2012. Changes in microbial community structure and carbon utilization patterns in response to woody encroachment into grasslands. Abstracts, Annual Meeting of the American Geophysical Union 2012: B33B-0526.
Jastrow JD, O Brien SL, Moran KK, Boutton TW. 2012. Soil carbon and nitrogen dynamics in deciduous forest exposed to twelve years of atmospheric CO2 enrichment. Abstracts, Annual Meeting of the American Geophysical Union 2012: B11F-05.
Kantola IB, Gentry T, Filley TR, Boutton TW. 2012. Biodiversity of soil microbial communties following woody plant invasion of grassland: An assessment using molecular methods. Abstracts, Annual Meeting of the American Geophysical Union 2012: B33B-0525.

Progress 01/01/11 to 12/31/11

Outputs
OUTPUTS: Many grass-dominated ecosystems around the world have experienced woody plant encroachment over the last century due to livestock grazing, fire suppression, and/or changes in climate and atmospheric chemistry. This land cover change is geographically widespread and has dramatic impacts on biogeochemistry. In southern Texas, encroachment by mesquite woodlands alters the ecosystem processes in a manner that causes C and N accrue belowground with increasing woody stand age. Soil C and N increases are predominantly in physically unprotected particulate soil organic matter fractions, where changes in the abundance and chemistry of lignin and aliphatic biopolymers suggest that selective accrual of purportedly more recalcitrant plant molecules may facilitate belowground C stabilization. To evaluate the importance of changing plant input chemistry and soil organic carbon (SOC) accessibility in belowground SOC stabilization, we measured the quantity and isotopic composition of respired CO2 during long term laboratory incubations of whole soil and size (>250 microns) and density (<1.0 g/cm3) separated soil fractions along a chronosequence of woody encroachment. PARTICIPANTS: PARTICIPANTS: Ilsa Kantola - Ph.D. Student, Texas A&M University; Julie Foote - Ph.D. Student, Texas A&M University; Rachel Rosenstein - Ph.D. Student, Texas A&M University; Courtney Creamer - Ph.D. Student, Purdue University; Fiona Soper - Ph.D. Student, Cornell University; Tim Filley - Professor, Purdue University; Diane Stott - Scientist, USDA National Soil Erosion Lab, Purdue University; Carrie Masiello - Professor, Rice University; Jason West - Professor, Texas A&M University; Tom Hallmark - Professor, Texas A&M University; Jed Sparks - Professor, Cornell University. TARGET AUDIENCES: TARGET AUDIENCES: The target audience for this research is primarily the scientific community working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meetings of the American Geophysical Union, the Soil Science Society of America, and the Ecological Society of America. To provide outreach to a broader and more general audience, I also gave two one-day workshops at the annual meeting of the Texas Master Naturalists Program. In addition, knowledge acquired through my research program was transferred to graduate students in my ESSM 600 (Principles of Ecosystem Science and Management) and ESSM 622 (Biogeochemistry of Terrestrial Ecosystems) classes at Texas A&M. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
During incubations of the two soil fractions, a greater proportion of total soil organic carbon (SOC) was respired from younger woody stands (14-23 yrs) and grasslands than from older woody stands (34-86 yrs), suggesting that increases in biochemically recalcitrant polymers with woody encroachment decreased microbial respiration from physically unprotected soil fractions. However, in the whole soil, proportional respiration was higher from older woody stands than from younger woody stands and grasslands. This was due to the greater overall allocation of SOC into physically unprotected soil fractions in older woody soils, revealing that microbial accessibility to SOC was more important than SOC chemistry during the whole soil incubation. The effects of changing plant chemistry and SOC accessibility with woody encroachment were also reflected in microbial community structure and function, as the ratio of fungal to bacterial phosphoplipid fatty acids (PLFA) and the activity of microbial N-acquiring enzymes (arylamidase and β-N-acetylglucosaminidase) were higher in woodland soil relative to grassland soils. Compound specific d13C analyses of extracted PLFA are being pursued to determine whether C utilization patterns differ among the landscape elements. This work provides important insights into the feedbacks between microbial community structure and function, plant input chemistry, and SOC accessibility and accrual in a globally relevant land-cover change.

Publications

Liu F, Wu XB, Bai E, Boutton TW, Archer SR. 2011. Quantifying soil organic carbon in complex landscapes. Global Change Biology 17: 1119-1129.
Creamer C, Filley T, Boutton TW, Kantola I, Oleynik S. 2011. Controls on soil carbon accumulation during woody plant encroachment into grasslands: Evidence from physical fractionation, soil respiration, and the isotopic composition of respired CO2. Soil Biology and Biochemistry 43: 1678-1687.
Rosenstein RW, Boutton TW, Tjoelker MG, Volder A, Briske DD. 2011. Root dynamics in response to elevated temperatures and altered rainfall regimes in oak savanna: A global change experiment. Abstracts, Ecological Society of America Meeting 2011: PS-1843.
Foote JA, Boutton TW, Scott DA. 2011. Soil carbon storage and dynamics in the western Gulf Coastal Plain as impacted by forest management. Abstracts, Ecological Society of America Meeting 2011: PS 66-126.
Kantola IB, Boutton TW, Filley TR, Hallmark CT. 2011. Changes in soil C, N, and P storage following woody plant invasion of grassland. Abstracts, Ecological Society of America Meeting 2011: PS 66-130.
Sekoni T, Boutton TW, Morgan CL, Knight RW. 2011. Growth and ecophysiological characteristics of an exotic woody invasive plant. Abstracts, Ecological Society of America Meeting 2011: PS 86-124.
Creamer CA, Filley TR, Boutton TW, Kantola IB. 2011. Using long-term soil incubations to determine drivers of soil organic carbon accumulation in response to woody encroachment. Soil Science Society of America Abstracts 2011: 354-2.
Creamer CA, Filley TR, Boutton TW, Stott DE, Olk DC. 2011. Changes in soil amino acid composition and microbial N acquisition strategies in response to woody invasion of grasslands. Soil Science Society of America Abstracts 2011: 123-33.
Rosenstein RW, Boutton TW, Tjoelker MG, Volder A, Briske DD. 2011. Soil C and N pools in oak savanna: Response to temperature and rainfall manipulation. Abstracts, Annual Meeting of the American Geophysical Union 2011: B31G-0390.
Rosenstein RW, Boutton TW, Tjoelker MG, Volder A, Briske DD. 2011. Root dynamics in post oak savanna: Response to global change forcing factors. Soil Science Society of America Abstracts 2011: 142-4.
Foote JA, Boutton TW, Scott DA. 2011. Soil carbon and nitrogen storage and dynamics in loblolly pine ecosystems as impacted by forest management. Soil Science Society of America Abstracts 2011: 142-3.
Sekoni T, Boutton TW, Morgan C, Knight R. 2011. Mycorrhizal status of Chinese tallow, an exotic invasive tree. Soil Science Society of America Abstracts 2011: 216-5.
Sparks JP, West JB, Boutton TW. 2011. Woody encroachment by nitrogen-fixing species: Impacts on nitrogen biogeochemistry expressed through nitrogen trace gases. Abstracts, Annual Meeting of the American Geophysical Union 2011: B31I-03.
Creamer CA, Filley TR, Boutton TW, Stott DE, Kantola IB. 2011. Drivers of belowground C accrual in response to woody plant encroachment of grasslands. Abstracts, Annual Meeting of the American Geophysical Union 2011: B13H-04.

Progress 01/01/10 to 12/31/10

Outputs
OUTPUTS: Many grass-dominated ecosystems around the world have experienced woody plant encroachment over the last century due to livestock grazing, fire suppression, and/or changes in climate and atmospheric chemistry. In the Rio Grande Plains of Texas, subtropical thorn woodlands dominated by N-fixing tree legumes have largely replaced grasslands and altered the biogeochemistry of this region. The purpose of this study was to assess the impact of this grassland-to-woodland transition on the size, distribution, and availability of soil P pools. A modified Hedley method was employed to fractionate soil P into pools based on organic and inorganic forms and relative plant-availability. Soil samples (0-10 cm) were collected in remnant grasslands and near the centers of woody plant clusters ranging in age from 14 to 86 yrs in a subtropical savanna parkland in southern Texas. Soil P was fractionated into resin-extractable inorganic P, bicarbonate-extractable organic and inorganic P, hydroxide-extractable organic and inorganic P, acid-extractable inorganic P, and residual inorganic P forms. P concentrations in these fractions were determined by colorimetry, and soil total P was determined by lithium fusion. Organic P was calculated from the difference between total and inorganic P. PARTICIPANTS: Ilsa Kantola - Ph.D. Student, Texas A&M University; Julie Foote - Ph.D. Student, Texas A&M University; Rachel Rosenstein - Ph.D. Student, Texas A&M University; Courtney Creamer - Ph.D. Student, Purdue University; Tim Filley - Professor, Purdue University; Diane Stott - Scientist, USDA National Soil Erosion Lab, Purdue University; Carrie Masiello - Professor, Rice University; Tom Hallmark - Professor, Texas A&M University. TARGET AUDIENCES: The target audience for this research is primarily the scientific community working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meetings of the American Geophysical Union, the Soil Science Society of America, the Geological Society of America, and the European Geophysical Union. To reach a broader and more general audience, I also gave two one-day workshops at the annual meeting of the Texas Master Naturalists Program. In addition, knowledge acquired through my research program was transferred to graduate students in my ESSM 600 (Principles of Ecosystem Science and Management) and ESSM 622 (Biogeochemistry of Terrestrial Ecosystems) classes at Texas A&M. PROJECT MODIFICATIONS: Not relevant to this project.

Impacts
Total P in whole soils increased dramatically from 102 mg P/kg soil in remnant grasslands to 166 mg P/kg soil in the oldest woody plant stands (70-85 yrs). P concentrations in all pools increased linearly with increasing woody plant stand age except acid-extractable phosphorus. The most dramatic increases were observed in the resin-extractable fraction (plant-available P), which increased from 3 to 13 mg P/kg soil, and in hydroxide-extractable P (the majority of the organic P in the system), which increased from 15 mg P/kg soil in grasslands to 26 mg/kg soil in the wooded clusters. Although the exact mechanisms by which soil P increase following woody invasion remain unknown, we suggest that the more deeply rooted woody plants are acquiring P from deep in the soil profile and tranferring it into the upper portion of the profile via litterfall and root turnover. Because P is generally the most limiting nutrient, increases in its availability could alter rates of biogeochemical processes, affect species interactions, and influence the future trajectory of woody invasion in this region.

Publications

Hollister EB, Schadt CW, Palumbo AV, Ansley RJ, Boutton TW. 2010. Structural and functional diversity of soil bacterial and fungal communities following woody plant encroachment in the southern Great Plains. Soil Biology and Biochemistry 42: 1816-1824.
Biederman L, Boutton TW. 2010. Spatial variation in biodiversity and trophic structure of soil nematode communities in a subtropical savanna parkland: responses to woody plant encroachment. Applied Soil Ecology 46: 168-176.
Kovda I, Morgun E, Boutton TW. 2010. Vertic processes and specificity of organic matter properties and distribution in vertisols. Eurasian Soil Science 43: 1467-1476.
Creamer CA, Boutton TW, Olk DC, Filley TR. 2010. Controls on microbial accessibility to soil organic carbon following woody plant encroachment into grasslands. Geophysical Research Abstracts 12: EGU2010-13990.
Boutton TW, Liao JD. 2010. Changes in soil nitrogen storage and d15N with woody plant encroachment in a subtropical savanna parkland landscape. Journal of Geophysical Research 115: G03019, doi:10.1029/2009JG001184, 2010.
Ansley RJ, Boutton TW, Mirik M, Kramp BA, Castellano MJ. 2010. Seasonal fire effects on herbaceous composition in a C3/C4 grassland invaded by Prosopis, a fire-resistant shrub. Applied Vegetation Science 13: 520-530.
Filley TR, Stott DE, Boutton TW, Creamer C, Olk D. 2010. Evidence for linkages between exoenzyme activity and soil organic matter chemistry following encroachment of leguminous woody plants into grasslands. Geophysical Research Abstracts 12: EGU2010-13033.
Stott DE, Filley TR, Creamer CA, Boutton TW. 2010. The influence of encroaching woodland on grassland soil enzymatic activities. Soil Science Society of America Abstracts 2010: 312-4.
Masiello C, Hockaday W, Zygourakis K, Dugan B, Rudgers J, Alvarez P, Pyle L, Kinney T, Sun H, Boutton T. 2010. Biochar research at Rice University: An overview. Abstracts, Geological Society of America Annual Meeting 2010: 27-12.
Pyle L, Hockaday W, Masiello C, Boutton T, LeCroy C. 2010. Production and isotopic composition of black nitrogen following experimental charring of plant materials. Eos, Transactions of the American Geophysical Union B41F-0387.
Creamer CA, Boutton TW, Kantola IB, Filley TR. 2010. Respiration dynamics of size-separated soil fractions. Eos, Transactions of the American Geophysical Union B41H-0415.
Kantola IB, Boutton TW, Filley TR, Hallmark CT. 2010. Changes in soil phosphorus fractions following woody plant invasion of grassland. Eos, Transactions of the American Geophysical Union B21C-0329.
Morgun EG, Boutton TW, Jessup KD. 2010. Assessment of the mobility and time of renewal of the densimetric fractions of organic matter in chestnut soils from the ratio of stable carbon isotopes. Eurasian Soil Science 43: 533-540.
Liu F, Wu XB, Bai E, Boutton TW, Archer SR. 2010. Spatial scaling of ecosystem C and N in a subtropical savanna landscape. Global Change Biology 16: 2213-2223.

Progress 01/01/09 to 12/31/09

Outputs
OUTPUTS: Grass-dominated ecosystems around the world are experiencing woody plant invasion due to human land uses. Vast regions in southern Texas have been transformed from open grasslands to subtropical thorn woodlands during the past 150 yrs. These woodlands are dominated by N-fixing tree legumes which are more productive above- and belowground, and store 2-3X more C and N than remnant grasslands. In tropical savannas and forests, it has been demonstrated that N-fixing plants are able to invest additional N in the acquisition of soil P. Accordingly, we hypothesized that soil acid phosphatase (AP) enzyme activity and concentrations of plant-available soil P (largely HPO4-2 and H2PO4-) would be greater in wooded areas dominated by N-fixing trees than in remnant grasslands where N-fixers are absent. We collected soils (0-7.5 cm) in remnant grasslands and in each of 4 different woodland types (clusters, groves, drainage woodlands, and playas) in a savanna parkland landscape in southern Texas. Plant-available soil P was determined by sorption onto anion exchange resin membranes placed in soil-water mixtures and shaken for 16 hr. P was desorbed from resin membranes using 0.5 N HCl and quantified colorimetrically using the Murphy-Riley technique. AP activity was determined using para-nitrophenyl phosphate as an analogue orthophosphate substrate, and then quantifying the p-nitrophenol (pNP) reaction product. PARTICIPANTS: Ilsa Kantola - PhD Student, Texas A&M University Julie Foote - PhD Student, Texas A&M University Rachel Rosenstein - PhD Student, Texas A&M University Courtney Creamer - PhD Student, Purdue University Tim Filley - Professor, Purdue University TARGET AUDIENCES: The target audience for this research is scientists working in the area of terrestrial biogeochemistry. Efforts to reach this group were achieved by attending the annual meeting of the American Geophysical Union. In addition, knowledge acquired through this research was transferred to graduate students in my ESSM 622 (Terrestrial Biogeochemistry) graduate class at Texas A&M. PROJECT MODIFICATIONS: Not relevant to this project.

Impacts
AP activity was 250 micrograms pNP/g soil/hr in grasslands, and increased linearly with time following woody plant invasion to 1400 micrograms pNP/g soil/hr in the oldest woody plant assemblages (90 yrs). Plant available P was 3 mg P/kg soil in grasslands, and ranged from 10 to 45 mg P/kg soil in wooded areas. Within each of the wooded landscape types, plant available P increased linearly with time following woody invasion and was correlated with soil AP activity. Results are consistent with prior studies showing that AP and plant available P are elevated under canopies of N-fixing plants, and suggest interaction between N and P cycles that may drive increased carbon sequestration following woody invasion in this landscape. Because P is often the most limiting nutrient in ecosystems, increased availability will likely alter rates of biogeochemical processes, influence species interactions, and determine the future trajectory of woody invasion in this region.

Publications

Dai X, Vietor DM, Boutton TW, Hons FM, Provin TL, White RH, Munster CL. 2009. Effect of composted biosolids on soil organic carbon storage during establishment of turfgrass sod. HortScience 44: 503-507.
Bai E, Boutton TW, Wu XB, Liu F, Archer SR. 2009. Landscape-scale vegetation dynamics inferred from spatial patterns of soil d13C in a subtropical savanna parkland. Journal of Geophysical Research 114: G01019, doi:10.1029/2008JG000839, 2009.
Biederman L, Boutton TW. 2009. Biodiversity and trophic structure of soil nematode communities change following woody plant invasion of grassland Soil Biology and Biochemistry 41: 1943-1950.
Jastrow JD, Moran KK, O'Brien SL, Boutton TW. 2009. Soil carbon and nitrogen dynamics in a deciduous forest exposed to ten years of atmospheric CO2 enrichment. Abstracts 2nd North American Carbon Program Meeting 2009: 48.
Filley T, Jastrow JD, O'Brien SL, Moran KK, Boutton TW. 2009. Selective stabilization of root and leaf-derived lignin and aliphatic biopolymers among soil organic matter fractions: Comparison of ambient and elevated CO2 rings in the sweetgum Free-Air CO2 Enrichment (FACE) experiment. Abstracts 2nd North American Carbon Program Meeting 2009: 113.
Balthrop L, Tribble J, Kantola I, Boutton TW. 2009. Plant-available phosphorus concentrations in soil are altered by woody plant invasion of grassland. Abstracts, Texas A&M Summer REU Poster Symposium 2009: 9.
Boutton TW, Kantola IB, Stott DE, Balthrop SL, Tribble JE, Filley TR. 2009. Soil phosphatase activity and plant available phosphorus increase following grassland invasion by N-fixing tree legumes. Eos, Transactions of the American Geophysical Union 90(52): B21B-0338.
Filley TR, Stott DE, Boutton TW, Creamer C, Olk D. 2009. Linkages between land cover, enzymes, and soil organic matter chemistry following encroachment of leguminous woody plants into grasslands. Eos, Transactions of the American Geophysical Union 90(52): B41B-0315.
Creamer CA, Boutton TW, Filley TR. 2009. Controls on microbial accessibility to soil organic matter following woody plant encroachment. Eos, Transactions of the American Geophysical Union 90(52): B53B-0392.
Kantola IB, Boutton TW, Filley TR, Hallmark CT. 2009. Carbon, nitrogen, and phosphorus increase in soil physical fractions following vegetation change from grassland to woodland. Eos, Transactions of the American Geophysical Union 90(52): B21B-0332.

Progress 01/01/08 to 12/31/08

Outputs
OUTPUTS: Woody plant proliferation has been documented in many grass-dominated ecosystems throughout the world in recent history. This geographically extensive vegetation change is often attributed to land-use activities such as livestock grazing and fire suppression, but may also be related to changes in atmospheric composition, atmospheric deposition, and climate. Because plant species composition exerts strong control over soil organic matter storage and dynamics via the quantity and quality of litter inputs to soil, woody plant encroachment has strong potential to modify key C-cycle processes at ecosystem and global scales. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by N-fixing tree legumes have largely replaced grassland over the last century primarily as a result of livestock grazing and fire suppression. Previous studies in this region have documented significant increases in above- and belowground primary productivity and in soil C and N storage following woody plant proliferation in areas that were once grassland. These changes in rates of organic matter input to the soil which have resulted in increased soil C and N storage are likely to influence the pool size and activity of the soil microbial biomass (Cmic). Since soil microbes are often limited by C and/or N, increased C and N input could provide more resources to support a larger Cmic pool. On the other hand, the transition from grassland to woodland may result in lower quality organic matter inputs that are resistant to biological decay, thereby supporting less Cmic. Woody plants are generally considered to be more decay-resistant as a result of lignins, tannins, and other secondary compounds that are either absent or present in lower concentrations in herbaceous grassland plants. Furthermore, changes in the quantity and/or quality of organic matter inputs may shift the composition of the soil microbial community towards organisms that are either more or less efficient at converting organic substrates into Cmic, with potential consequences for the size of Cmic pool. As a first step towards evaluating these scenarios, we quantified soil microbial biomass carbon (Cmic), the ratio of Cmic to total Corg (Cmic/Corg), and the metabolic quotient or qCO2 (respiration rate per unit of Cmic) in soils from the Rio Grande Plains where grasslands have been invaded by woody plants. We utilized a chronosequence approach in which remnant grasslands represented time before woody invasion (Time 0), and woody plant stands ranging in age from 10-130 years represented time following woody encroachment. We hypothesized that: (1) Increased soil C and N content would result in increased Cmic with time following woody plant encroachment; (2) The Cmic/Corg ratio would decrease with increasing woody plant stand age due to the inherently more recalcitrant nature of woody plant litter compared to herbaceous litter; and (3) The qCO2 would increase with time after tree/shrub invasion reflecting lower microbial efficiency in processing low quality woody plant litter. PARTICIPANTS: Collaborators: Dr. Tim Filley, Purdue University; Dr. Julie Jastrow, Argonne National Laboratory; Dr. Terry Gentry, Texas A&M University; Dr. Steve Archer, University of Arizona; Dr. Jim Ansley, Texas AgriLife Research. Training Opportunities: Emily Hollister, PhD student, Texas A&M University; Ilsa Kantola, PhD student, Texas A&M University; Martha Ariza, PhD student, Texas A&M University; Rachel Wellman, PhD student, Texas A&M University; Lori Biederman, Postdoctoral Fellow, Texas A&M University. TARGET AUDIENCES: The primary target audience for this research is the research community in the field of ecosystem science. In addition, I have participated in a USDA grant (Hispanic Serving Institutions Program) aimed at encouraging Hispanic/Latino students at University of Texas San Antonio (UTSA) to pursue graduate studies and careers in environmental sciences. To better target underrepresented students, I am also involved in the Sloan Foundation Minority Fellowship Program and the Texas A&M Hispanic Leadership Program. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
The absolute size of the Cmic pool increased with time following woody plant invasion of grassland. The increases in Cmic are likely due to the greater above- and belowground productivity and larger pools of Corg and STN in wooded areas relative to remnant grassland. Cmic was generally correlated with C and N stores in litter, roots, and soils, indicating that microbes may have been C- and/or N-limited prior to woody plant proliferation in this dryland ecosystem. Although absolute values of Cmic increased following woody plant invasion of grassland, simultaneous linear decreases in the Cmic/Corg ratio with time indicate that the quality of the C available to microbes may have decreased over time as less microbial biomass is supported per unit of Corg. Increases in qCO2 with time following woody invasion also suggest that organic matter quality may be reduced since microbes in wooded areas appear to be less efficient at converting available C to biomass. Therefore, observed increases in Corg and STN following woody plant encroachment in this ecosystem may be a function of both greater inputs of poor quality C that is relatively resistant to decay, and the decreased ability of soil microbes to decompose this organic matter. EVENTS: Scientific presentations related to this research project were given at annual meetings of the American Geophysical Union, Soil Science Society of America, Society for Range Management, and Ecological Society of America. SERVICES: Advisory Panel Member: National Science Foundation, Washington, DC, 2008. Faculty Mentor: Sloan Foundation Minority Ph.D. Program, 2007-present. PRODUCTS: E.B. Hollister, Ph.D. Dissertation, Texas A&M University. [Present position: Postdoctoral Fellow, Texas A&M University]

Publications

Jastrow JD, Moran K, Boutton TW. 2008. Soil carbon responses to elevated atmospheric CO2 and tropospheric O3 in developing stands of trembling aspen, paper birch, and sugar maple. Abstracts, Facing the Future - A Joint Meeting of AspenFACE, SoyFACE, and SFB607.
Biederman LA, Boutton TW, Kantola IB. 2008. Nematode communities change following woody plant invasion of grassland. Abstracts, Ecological Society of America Meeting 2008: PS18-11.
Kantola IB, Boutton TW, Gentry TJ, Martin EC. 2008. Composition and diversity of soil microbial communities following vegetation change from grassland to woodland: An assessment using molecular methods. Abstracts, Ecological Society of America Meeting 2008: PS 7-100.
Hollister EB, Schadt CW, Palumbo AV, Boutton TW. 2008. Soil microbial diversity in a mesquite savanna: Response of bacterial and fungal communities to vegetation change. Abstracts, Ecological Society of America Meeting 2008: PS 38-29.
Ariza M, Applewhite J, Hallmark CT, Kantola IB, Moore DJ, Filley TR, Boutton TW. 2008. Glomalin and soil structure following woody plant invasion of grassland. Soil Science Society of America Abstracts 2008: 583-17.
Kantola IB, Boutton TW, Gentry TJ, Martin EC. 2008. Composition and diversity of soil microbial communities following vegetation change from grassland to woodland: An assessment using molecular methods. Soil Science Society of America Abstracts 2008: 583-3.
Boutton TW, Archer SR. 2008. Vegetation, land use history, and patterns of soil organic carbon in the southern Great Plains. Soil Science Society of America Abstracts 2008: 765-1.
Boutton TW, Rowe HI, Ariza MC, Miller RM, Filley TR. 2008. Mycorrhizal productivity following woody plant invasion of grassland. Eos, Transactions of the American Geophysical Union 89 (53): B23B-0415
Ariza MC, Boutton TW, Gonzalez-Chavez MC, Filley TR. 2008. Carbon and nitrogen storage in glomalin-related soil protein during grassland-to-woodland succession. Eos, Transactions of the American Geophysical Union 89 (53): B23B-0412
Filley TR, Stott DE, Dooling V, Sorg LP, Boutton TW. 2008. Activities of extracellular enzymes in soils following woody plant invasion of grassland. Eos, Transactions of the American Geophysical Union 89 (53): B23B-0414
Boutton TW, Liao JD, Filley TR, Archer SR. 2008. Belowground carbon storage and dynamics accompanying woody plant encroachment in a subtropical savanna. IN: Soil Carbon Sequestration and the Greenhouse Effect, 2nd ed., pp. 181-205, R. Lal and R. Follett, Eds. Soil Science Society of America, Madison, WI.
Liao JD, Boutton TW. 2008. Soil microbial biomass response to woody plant invasion of grassland. Soil Biology and Biochemistry 40: 1207-1216.
Bai E, Boutton TW, Liu F, Wu XB, Archer SR. 2008. Variation in woody plant δ13C along a topoedaphic gradient in a subtropical savanna parkland. Oecologia 156: 479-489.
Millard P, Midwood AJ, Hunt JE, Whitehead D, Boutton TW. 2008. Partitioning soil surface CO2 efflux into autotrophic and heterotrophic components using natural gradients in soil δ13C in an undisturbed savannah soil. Soil Biology and Biochemistry 40: 1575-1582.
Filley TR, Boutton TW, Liao JD, Jastrow JD. 2008. Chemical changes to non-aggregated particulate soil organic matter following grassland-to-woodland transition in a subtropical savanna. Journal of Geophysical Research 113: G03009, doi:10.1029/2007JG000564, 2008
Biederman L, Boutton TW, Whisenant SW. 2008. Nematode community development early during ecological restoration: The role of organic amendments. Soil Biology and Biochemistry 40: 2366-2374.
Harris WN, Boutton TW, Ansley RJ. 2008. Plant community and soil microbial responses to fire and clipping in a southern mixed grassland. Rangeland Ecology and Management 61: 580-587.
Bai E, Boutton TW, Liu F, Wu XB, Archer SR, Hallmark CT. 2008. Spatial variation of δ15N of woody plants along a topoedaphic gradient in a subtropical savanna. Oecologia (in press).
Bai E, Boutton TW, Wu XB, Liu F, Archer SR. 2008. Landscape-scale vegetation dynamics inferred from spatial patterns of soil δ13C in a subtropical savanna parkland. Journal of Geophysical Research (in press).
Archer SR, Boutton TW, McClaran MP, Throop HL, Wu XB. 2008. Grasses, shrubs, and the carbon cycle: Functional consequences of ecosystem state changes. Abstracts, Society for Range Management 2008.
Bartelink EJ, Yoder CJ, Grossman E, Boutton TW. 2008. Dietary variation in mainland and coastal California: Stable isotopic evidence from six late Holocene sites. Abstracts, Society for American Archeology 2008: Session 202.
Creamer CA, Boutton TW, Filley TR. 2008. Microbial accessibility of soil organic matter following woody plant encroachment into grasslands. Eos, Transactions of the American Geophysical Union 89 (53): B23B-0413
Wu XB, Liu F, Bai E, Boutton TW, Archer SR. 2008. Soil carbon in savanna landscapes: spatial pattern, uncertainty, and scaling. Eos, Transactions of the American Geophysical Union 89 (53): B11D-0397

Progress 01/01/07 to 12/31/07

Outputs
OUTPUTS: ACTIVITIES: Woody plant encroachment into grass-dominated ecosystems during the past century has been documented in North and South America, Australia, Africa, and Southeast Asia, and appears to be a consequence of human land-use activities, primarily livestock grazing and fire suppression. Major functional consequences of increased woody plant abundance in grasslands may include alterations of above- and belowground productivity, changes in the quality of litter inputs, modifications to rooting depth and distribution, altered hydrology, and changes in microclimate and energy balance. Additionally, many of the woody plant species that are encroaching into grasslands are capable of symbiotic N2-fixation, adding fixed N to N-limited ecosystems. These alterations in ecosystem characteristics following woody plant encroachment into grasslands have strong potential to modify C dynamics and storage, and may influence regional and global climate through feedback interactions. Despite the possibility that woody plant invasion could influence the C cycle at ecosystem and global scales, mechanisms and processes associated with C dynamics and storage in the affected systems are poorly understood. In this research, the impact of grassland-to-woodland conversion on the soil C cycle in the Rio Grande Plains of southern Texas is assessed by: (1) Quantifying rates of SOC sequestration following grassland-to-woodland conversion; (2) Evaluating the role of physical and biochemical mechanisms of SOC stabilization; (3) Assessing the relative importance of residual grassland-derived C4 carbon vs. C3 woodland-derived carbon inputs to total SOC content in bulk soil; and (4) Estimating turnover rates of SOC in bulk soil using the natural isotopic difference between C4 grasses and C3 woody plants. EVENTS: Scientific presentations related to this research project were given at the annual meetings of the American Geophysical Union, the American Chemical Society, the Soil Ecology Society, the North American Carbon Program, the Society for Range Management, the Polish Society for Soil Science, and the Ecological Society of America. SERVICES: Advisory Panel Member: National Science Foundation, Washington, DC, 2008. Faculty Mentor: Sloan Foundation Minority Ph.D. Program, 2007-present. PRODUCTS: Lui, Feng. 2007. Spatial pattern and uncertainty of soil carbon and nitrogen in a subtropical savanna landscape in southern Texas. Ph.D. Dissertation, Texas A&M University. [Present position: Postdoctoral Fellow, University of Wisconsin, Madison, WI] Bai, Edith. 2007. Spatial variation in soil and plant δ13C and δ15N values in a subtropical savanna: Implications for vegetation change and nutrient dynamics. Ph.D. Dissertation, Texas A&M University. [Present position: Postdoctoral Fellow, University of California, Davis] PARTICIPANTS: Collaborators: Dr. Tim Filley, Purdue University; Dr. Julie Jastrow, Argonne National Laboratory; Dr. Terry Gentry, Texas A&M University; Dr. Jim Ansley, Texas AgriLife Research (Vernon Center); Dr. Steve Archer, University of Arizona; Training Opportunities: Emily Hollister, PhD student, Texas A&M University; Ilsa Kantola, PhD student, Texas A&M University; Martha Ariza, PhD student, Texas A&M University; Rachel Wellman, PhD student, Texas A&M University; Lori Biederman, Postdoctoral Fellow, Texas A&M University TARGET AUDIENCES: The primary target audience for this research is the research community in the field of ecosystem science. In addition, I have participated in a USDA grant (Hispanic Serving Institutions Program) aimed at encouraging Hispanic/Latino students at University of Texas San Antonio (UTSA) to pursue graduate studies and careers in environmental sciences. During this past year I recruited a new Hispanic student from UTSA (Martha Ariza), and we are both involved in the Sloan Foundation Minority Fellowship Program and the Texas A&M Hispanic Leadership Program.

Impacts
Although woody plant invasion of grasslands has been geographically extensive over the past century, few robust generalizations regarding the biogeochemical consequences of this land cover change have emerged. The case study presented here indicates that woodlands have replaced grasslands over the past century in the Rio Grande Plains of southern Texas. Following this grassland-to-woodland transition, increased C storage in surface litter and roots was accompanied by significant increases in SOC (0-30 cm) from 2000 g C/m2 in grasslands to >5000 g C/m2 in older woodlands. Accumulation rates of SOC ranged from 12-43 g C/m2/yr. This is consistent with prior results obtained from field estimates and modeling at this same site, and is comparable to estimates reported for other ecosystems experiencing woody plant encroachment or from reforestation of agricultural lands. Evidence is presented for three important mechanisms driving carbon accumulation in the soil system following woody encroachment: (1) Higher rates of NPP in wooded areas relative to grasslands; (2) Shifts to biopolymer composition that should result in greater biochemical recalcitrance of litter inputs in wooded areas; and (3) Physical protection of organic matter inputs within the soil structure. Approximately 30-55% of the SOC that accumulates following woody encroachment is stabilized by physical protection in macro- and microaggregates, and by association with silt and clay. The remainder of the newly accrued SOC accumulates as free POM, which likely persists in the soil as a result of its biochemical resistance to decay, as indicated by the composition of its lignin fraction and its high concentrations of substituted fatty acids. A large portion of SOC in woodlands developing on grasslands (10-20% in surface soils and 60-80% at deeper depths) was comprised of older C derived from the previous grassland community. This older grassland C is relatively resistant to decay as little change was detected in the size of this pool in wooded areas older than 60 years. Hence, accumulations of SOC are due to both increased inputs of newer woodland-derived C and the retention of older, stabilized grassland-derived C. The significant increases in SOC documented in this study suggest that those portions of the Rio Grande Plains in southern Texas undergoing grassland-to-woodland conversion have been acting as a net sink for atmospheric CO2 over the past century. Grassland-to-woodland conversions have been geographically widespread in the world's drylands during this same time period, suggesting that changes in SOC storage and dynamics documented here could have significance for regional and global C cycles and potentially our climate system.

Publications

Jastrow JD, OBrien SL, Filley TR, Boutton TW. 2007. Nature and dynamics of carbon accrued in a forest soil during five years of atmospheric CO2 enrichment. Abstracts, US North American Carbon Program Meeting 2007: B.14.
Wu XB, Liu F, Bai E, Boutton TW, Archer SR. 2007. Spatial understanding and estimation of soil carbon storage in savanna landscapes. Abstracts, Society for Range Management 2007: 385CW.
Brewer EA, Boutton TW, Myrold DD. 2007. Woody plant encroachment into grassland alters the composition of soil microbial communities. Abstracts, Soil Ecology Society 11th Biennial International Meeting 2007: 62.
Hollister EB, Boutton TW, Schadt CW, Palumbo AV. 2007. Soil microbial diversity in a mesquite savanna. Abstracts, Soil Ecology Society 11th Biennial International Meeting 2007: 34.
Biederman L, Whisenant S, Boutton T. 2007. Organic amendments direct soil food web function early in restoration. Abstracts, Soil Ecology Society 11th Biennial International Meeting 2007: 26.
Jastrow JD, Six J, OBrien SL, Moran KK, Boutton TW. 2007. Comparison of density-based vs. particle size fractionations for investigating microaggregate stabilization of soil organic matter. Abstracts, Soil Ecology Society 11th Biennial International Meeting 2007: 73-74.
Moran KK, Jastrow JD, OBrien SL, Boutton TW. 2007. Changing soil sampling depth captures effects of tree species, elevated carbon dioxide, and ozone on soil carbon stocks in a forest ecosystem. Abstracts, Soil Ecology Society 11th Biennial International Meeting 2007: 82-83.
Liu F, Wu XB, Bai E, Archer S, Boutton TW. 2007. Spatial pattern of Prosopis glandulosa (honey mesquite) and its influence on soil C and N in a subtropical savanna landscape. Abstracts, Ecological Society of America Meeting 2007: PS40-2.
Bai E, Boutton TW, Liu F, Wu XB, Archer S. 2007. Spatial patterns of soil d13C in a subtropical woodland. Abstracts, Ecological Society of America Meeting 2007: PS40-6.
Morgun E, Boutton TW. 2007. Structural and isotopic characteristics of Chernozem organic matter transformations under vegetation change. Abstracts, 27th Congress of the Polish Society for Soil Science.
Filley TR, Jastrow JD, OBrien S, Boutton TW. 2007. Dynamics of lignin and plant aliphatic biopolymers in soil organic matter under elevated atmospheric CO2 in the sweetgum Free Air CO2 Enrichment (FACE) Experiment. Abstracts, American Chemical Society Meeting.
Boutton TW, Filley TR. 2007. Evolution of plant polymer chemistry of above- and belowground inputs in a thorn woodland chronosequence. EOS, Transactions of the American Geophysical Union 2007: B23D-1590.
Harris WN, Moretto AM, Distel RA, Boutton TW, Boo RM. 2007. Fire and grazing in the grasslands of the Argentine caldenal: Effects on plant and soil carbon and nitrogen. Acta Oecologica 32: 207-214.
Ansley RJ, Boutton TW, Jacoby PW. 2007. Mesquite root distribution and water use efficiency in reponse to long-term soil moisture manipulations. IN: Shrubland Dynamics: Fire and Water, pp. 96-103, RE Sosebee, DB Wester, CM Britton, ED McArthur, S Kitchens, Eds. Proceedings RMRS-P-47, USDA Forest Service, Rocky Mountain Research Station, Ft. Collins, CO.

Progress 01/01/06 to 12/31/06

Outputs
Land uses and land cover changes frequently alter key ecosystem processes such as productivity and decomposition, with potential implications for the carbon cycle. In the southern Great Plains (USA), invasion of grassland systems by mesquite (Prosopis glandulosa), an N2-fixing woody species, is often managed through prescribed burning. As part of a broader effort to understand the effects of woody encroachment and prescribed fire on ecosystem carbon dynamics, we employed long-term laboratory incubations to quantify soil organic carbon (SOC) pools and their turnover rates in soils collected beneath grasses and woody vegetation in 3 long-term fire treatments (unburned controls, winter fire, and summer fire). Respiration kinetics were fit to parallel first-order decay models to estimate carbon pool (C_active and C_slow) sizes and fluxes. Both fire and vegetation exerted important controls over the magnitude and turnover rates of SOC. Regardless of fire treatment, total SOC, soil total N, and C_slow pools increased by 15- 20% under mesquite and were significantly larger than corresponding pools from soils collected beneath grasses (p< 0.05). Winter and summer fire treatments resulted in increased SOC beneath grasses but not beneath mesquite. The size of the C_active pool was consistent across vegetation types and fire treatments, accounting for 1.5-3% of total SOC. Winter fire treatment soils had significantly higher C_active turnover rates than those from summer fire treatments or controls (p < 0.05), paralleling trends of increased net primary productivity and soil microbial biomass that we have observed following winter fire. Both woody plant encroachment and attempts to manage encroachment with fire are likely to have significant impacts on the carbon balance of Great Plains grasslands.

Impacts
Many dryland ecosystems throughout the world have been invaded by woody plants that threaten biodiversity, modify ecosystem function and diminish the productivity of livestock production systems. Fire is a common land management tool that reduces the cover of woody species and favors conservation of native perennial grasses and other components of the biota. However, little is known regarding the impact of fire on C and N cycles in these systems. These results indicated that fire and the season during which it occurs can play a significant role in the biogeochemistry of mixed-grass/mesquite savannas. Our findings will have implications not only for land managers, but also for scientists and policy-makers who are now evaluating the potential for land management practices to alter ecosystem carbon storage and influence atmospheric CO2 concentrations and global climate.

Publications

Morgun E, Boutton TW. 2006. Vegetation change and organic matter in Chernozems: Peculiarities of state reflected by physical fractionation and stable isotope analyses. Abstracts, 18th World Congress of Soil Science 2006: 17013.
Bai E, Boutton TW, Liu F, Wu XB, Archer SR. 2006. Landscape-scale vegetation dynamics inferred from spatial patterns of soil δ13C in a subtropical savanna. American Geophysical Union Western Pacific Geophysics Meeting 2006: B31A-0035.
Hollister EB, Boutton TW, Ansley RJ. 2006. Influence of fire and woody plant invasion on the carbon dynamics of a southern Great Plains grassland system. Abstracts, Ecological Society of America 2006: 173.
Liu F, Wu XB, Bai E, Boutton TW, Archer SR. 2006. Spatial variability of soil organic carbon in a subtropical savanna landscape and its implications for sampling design. Abstracts, Ecological Society of America 2006: 233.
Hollister EB, Boutton TW, Ansley RJ. 2006. Land use and land cover changes in temperate savannas: Impact of woody encroachment and prescribed burning on soil carbon pools and flux rates. Soil Science Society of America Abstracts 2006: 92.113.
Brewer EA, Gentry TJ, Boutton TW, Myrold DD. 2006. Woody plant invasion of grassland alters the composition of soil microbial communities. Soil Science Society of America Abstracts 2006: 92.119.
McCulley RL, Boutton TW, Archer SR. 2006. Soil carbon cycling in subtropical savanna parkland: Response to water additions. Soil Science Society of America Journal (in press).
Filley T, Boutton TW (eds.). 2006. Ecosystems in Flux: Molecular and Stable Isotope Assessments of Soil Organic Matter Storage and Dynamics. Elsevier Science, Amsterdam (special issue of the journal Soil Biology and Biochemistry, Vol. 38, No. 11).
Filley T, Boutton TW. 2006. Ecosystems in flux: Molecular and stable isotope assessments of soil organic matter storage and dynamics. Soil Biology and Biochemistry 38: 3181-3183.
Liao JD, Boutton TW, Jastrow JD. 2006. Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural 13C and 15N. Soil Biology and Biochemistry 38: 3197-3210.
Liao JD, Boutton TW, Jastrow JD. 2006. Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland. Soil Biology and Biochemistry 38: 3184-3196.
Ansley RJ, Boutton TW, and Skjemstad JO. 2006. Soil organic carbon and black carbon storage and dynamics under different fire regimes in temperate mixed-grass savanna. Global Biogeochemical Cycles 20, GB3006, doi:10.1029/2005GB002670.
Dai X, Boutton TW, Ansley RJ, Hailemichael M, Jessup KE. 2006. Soil carbon and nitrogen storage in response to fire in a temperate mixed-grass savanna. Journal of Environmental Quality 35: 1620-1628.
Derner JD, Boutton TW, Briske DD. 2006. Grazing and ecosystem carbon storage in the North American Great Plains. Plant and Soil 280: 77-90.
Filley T, Jastrow JD, O'Brien SL, Boutton TW. 2006. Enhanced allocation of root-derived lignin and aliphatic biopolymers to soil organic matter under elevated atmospheric CO2 in the Sweetgum free air CO2 enrichment (FACE) experiment. Soil Science Society of America Abstracts 2006: 183.8.
Filley T, Boutton TW, Liao JD, Jastrow JD. 2006. Litter-driven changes to biopolymer characteristics of particulate soil organic matter fractions along a chronosequence of thorn woodland encroachment into grassland. Soil Science Society of America Abstracts 2006: 281.10.
Bai E, Boutton TW, Wu XB, Liu F, Archer SR. 2006. Spatial variation of soil δ13C values reflects pattern of woody plant encroachment in a subtropical savanna. Eos, Transactions of the American Geophysical Union 2006: B33C-1200.

Progress 01/01/05 to 12/31/05

Outputs
d13C and d15N values of plants are powerful tools in physiological ecology, ecosystem science, and global biogeochemistry, yet we know relatively little about their variation and controls at the landscape scale. In this study, we investigated landscape-scale spatial variations in the foliar isotopic composition of 3 woody plant species across a 308 m topoedaphic gradient, along which soil texture and plant resources (water and nitrogen availability) varied from upland (86 m) to lowland (84 m) portions of the landscape. The study was conducted in a subtropical savanna at the La Copita Research Area, approximately 60 km west of Corpus Christi, TX. Foliar d13C, d15N, leaf nitrogen concentration ([N]), and specific leaf area (SLA) were measured on all individuals of Prosopis glandulosa, Condalia hookeri, and Zanthoxylum fagara present within a belt transect 308 m long x 12 m wide. Soil texture, available soil moisture, and soil total N were measured at 1 m intervals along the center-line of the belt transect. Clay content, available soil moisture, and soil total N were all negatively correlated with elevation along the transect. Leaf d13C and d15N values for all 3 species increased by 1-4 o/oo with decreasing elevation along the transect. Contrary to theory and previous studies, d13C values were highest where soil water was most available, suggesting that some other variable could be overriding or interacting with water availability. Foliar [N] appeared to exert the strongest control over landscape-level variation, and was positively correlated with d13C of all species (R2= 0.58, p<0.0001). Since leaf [N] is positively related to photosynthetic capacity, plants with high [N] are likely to have low Ci/Ca ratios and therefore higher d13C values. d15N values of Zanthoxylum and Condalia were positively correlated with leaf [N] and soil water availability; however, these relationships were absent for Prosopis, an N-fixing tree legume. We speculate that the relationship between d15N and leaf [N] and soil water may reflect the fact that plants with high d15N values occurred on lower-lying portions of the landscape with relatively high N-transformation rates where soil mineral N was both relatively available and 15N-enriched. The lack of variation in d15N values of Prosopis suggests that it may be somewhat uncoupled from these processes because it is an N-fixer. Results of this study indicate that N-availability plays an important role in landscape scale variation in plant d13C and d15N.

Impacts
Results of this study contribute to our fundamental understanding of the controls over the carbon and nitrogen isotopic composition of plants and soils. More specifically, our results indicate an important role for the availability of soil nitrogen in determining both carbon and nitrogen isotope ratios of woody plants. These findings will improve our understanding of carbon and nitrogen isotope ratios of plants and soils as indicators of pattern and process at the ecosystem and landscape scales.

Publications

Dai X, Vietor D, Hanson N, Provin T, Munster C, Boutton T, White R. 2005. Cycling of composted municipal biosolids in turfgrass sod. Soil Science Society of America Abstracts 2005: 1343.
Wright A, Dou F, Hons F, Boutton TW. 2005. Soil organic matter fractions and changes in C-13 for a 20-year crop rotation and tillage study. Soil Science Society of America Abstracts 2005: 1427.
Jastrow JD, Miller RM, Matamala R, Norby RJ, Boutton TW, Rice CW, Owensby CE. 2005. Elevated atmospheric CO2 increases soil carbon. Global Change Biology 11: 2057-2064.
Egilla JN, Davies FT, Boutton TW. 2005. Drought stress influences leaf water content, photosynthesis, and water-use efficiency of Hibiscus rosa-sinensis at three potassium concentrations. Photosynthetica 43: 135-140.
Bai E, Boutton TW, Liu F, Wu XB, Archer SR, Jessup KE. 2005. Spatial patterns of soil C-13 and N-15 in a savanna parkland landscape. Abstracts, Society for Range Management 2005: 14.
Liao JD, Boutton TW. 2005. Soil microbial biomass mediates changes in soil carbon and nitrogen following grassland-to-woodland transitions. Abstracts, Society for Range Management 2005: 203.
Liu F, Wu XB, Bai E, Archer SR, Boutton TW, Jessup KE. 2005. Spatial pattern of vegetation in a subtropical savanna landscape. Abstracts, Society for Range Management 2005: 205.
Archer SR, Boutton TW, Wu XB, Liu F, Bai E. 2005. Spatial-temporal changes in soil properties following woody plant encroachment. Abstracts, Society for Range Management 2005: 8.
Hollister EB, Boutton TW, Ansley RJ. 2005. Carbon dynamics of temperate savannas: Effects of prescribed fire on microbial activity and potential carbon mineralization. Abstracts, Society for Range Management 2005: 171.
Dai X, Boutton TW, Hailemichael M, Ansley J, Jessup K. 2005. Soil carbon storage and dynamics in response to fire seasonality in a temperate mixed-grass savanna. USDA Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry 2005: 79.
Dou F, Wright AL, Hons FM, Boutton TW. 2005. Tillage, cropping sequence, and fertilization effects on 13C abundance in soil physical fractions. USDA Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry 2005: 252.
Filley T, Dria K, Gamblin D, Liao JD, Boutton TW, Jastrow JD. 2005. Dynamics of biopolymer turnover in soils following woody encroachment in a subtropical savanna. USDA Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry 2005: 87.
Jastrow JD, O'Brien SL, Dria KJ, Filley TR, Boutton TW. 2005. Nature and dynamics of carbon accrued in a forest soil during five years of atmospheric CO2 enrichment. Abstracts, Soil Ecology Society 10th Bienniel International Meeting 2005: 58-59.
Liu F, Wu XB, Archer SR, Boutton TW, Bai E, Jessup KE. 2005. Scaling of soil C and N and its relationship to vegetation and soil attributes in a subtropical savanna landscape. Abstracts, Ecological Society of America 2005.
Jahnsen HM, Boutton TW, Hallmark CT, Prochaska D, Wu XB, Archer SR. 2005. Spatial variation in soil texture in a savanna ecosystem. Soil Science Society of America Abstracts 2005: 7-8.
Filley T, Gamblin D, Boutton T, Liao J, Jastrow J. 2005. Changes to lignin phenol and hydroxy alkanoic acid stable carbon isotope composition and concentration in soil fractions from a grassland/woodland conversion in a subtropical savanna. Eos Transactions of the American Geophysical Union 86(52): B43A-0236.
Liu F, Bai E, Wu B, Archer S, Boutton T. 2005. Correlations of soil C and N with vegetation and soil attributes and their spatial scaling in a subtropical savanna landscape. Eos Transactions of the American Geophysical Union 86(52): B52B-08.
Bai E, Boutton TW, Liu F, Wu B, Archer SR. 2005. Spatial patterns of plant d13C and d15N along a topoedaphic gradient in a subtropical savanna landscape. Eos Transactions of the American Geophysical Union 86(52): B13B-08.

Progress 01/01/04 to 12/31/04

Outputs
Woody plant encroachment is prevalent in grassland regions worldwide. In the Rio Grande Plains of southern Texas, trees and shrubs have largely replaced grasslands over the past century. To evaluate the impacts of grassland-to-woodland conversion on ecosystem biogeochemistry, we measured soil organic C (SOC), soil total N (STN), litter and root C, and soil microbial biomass C (SMB-C) in remnant grasslands (Time 0) and in woody plant stands ranging in age from 10-130 years. Additionally, changes in Cmic/Corg (ratio of SMB-C to total SOC), and qCO2 (ratio of microbial respiration to SMB-C) were used to evaluate microbial efficiency in converting available C to SMB-C. SOC and STN increased up to 250% following woody plant invasion. SMB-C increased linearly from 300 mg C/kg soil in remnant grasslands to 600-900 mg C/kg soil in the oldest woodlands (>80 yrs). SMB-C in woodlands was positively correlated with total SOC, STN, litter C, and root C. The Cmic/Corg ratio decreased with increasing woody plant stand age suggesting that organic matter inputs from woodlands may be of poorer quality than grassland inputs. In addition, increasing qCO2 with increasing woodland age suggests that microbial populations in woodland soils may be less efficient at converting available SOC into biomass. Therefore, the size and metabolic characteristics of the SMB-C pool provide a mechanistic basis for understanding the increases in soil C and N storage following woody plant invasion of grassland.

Impacts
Given the geographically extensive nature of this vegetation shift, changes in SOC dynamics and storage documented here could have significance for regional and global carbon cycles. These results will have implications not only for land managers, but also for scientists and policy-makers who are now evaluating the potential for land management practices to alter ecosystem carbon storage and influence atmospheric CO2 concentrations and global climate.

Publications

Archer SR, Boutton TW, McMurtry C. 2005. Carbon and nitrogen storage in a savanna landscape: Field and modeling perspectives. Global Environmental Change in the Ocean and on Land, Shiomi and H Kawahata, Eds. Kluwer Academic, Dordrecht, Netherlands (in press).
Northup B, Zitzer S, Archer S, McMurtry C, Boutton TW. 2005. Aboveground biomass and C and N content of woody species in a subtropical thornscrub parkland. Journal of Arid Environments (in press).
Mora MA, Boutton TW, Musquiz D. 2005. Regional variation and relationships between DDE and selenium and stable isotopes in swallows nesting along the Rio Grande. Isotopes in Environmental and Health Studies (in press).
Dai X, Boutton TW, Glaser B, Ansley RJ, Zech W. 2005. Black carbon in a temperate mixed-grass savanna. Soil Biology and Biochemistry (in press).
McCulley RL, Archer SR, Boutton TW, Hons FM, Zuberer DA. 2004. Soil respiration and nutrient cycling in wooded communities developing in grassland. Ecology 85: 2804-2817.
Boutton TW, Archer SR, Liao JD. 2004. Grassland-woodland transitions in subtropical North America and their implications for the carbon cycle. Abstracts, American Quaternary Association 2004: 2-8.
Archer SR, Boutton TW, Wu XB, Liu F, Bai E. 2004. Spatial accounting of soil C and N in grasslands invaded by woody plants. Abstracts, Ecological Society of America 2004.
Harris WN, Boutton TW, Ansley RJ. 2004. Plant community and soil microbial dynamics in subhumid temperate grasslands: Effects of seasonal fire and clipping. Abstracts, Ecological Society of America 2004.
Liao JD, Boutton TW, Jastrow JD. 2004. Soil organic matter dynamics following land-cover change in a subtropical savanna: Insights from soil physical fractionation and stable isotopes. Eos Transactions of the American Geophysical Union 85(47): B53D-01.
Liao JD, Boutton TW, Jastrow JD. 2004. Carbon dynamics in soil physical fractions following grassland-to-woodland conversion. Soil Science Society of America Abstracts 2004: 4705.
Hollister EB, Boutton TW, Ansley RJ. 2004. Primary production and soil microbial biomass in a temperate mixed grass savanna. Soil Science Society of America Abstracts 2004: 4612.
Filley T, Boutton TW, Liao JD, Jastrow JD, Gamblin D. 2004. Dynamics of biopolymer turnover in soil physical fractions following land-cover change in a subtropical savanna. Eos Transactions of the American Geophysical Union 85(47): B53D-02.
Jastrow JD, O Brien SL, Dria K, Moran KK, Filley T, Boutton TW. 2004. The nature and dynamics of C accrued in a sweetgum forest soil during five years of atmospheric CO2 enrichment. Eos Transactions of the American Geophysical Union 85(47): B51E-03.
Archer SR, Boutton TW, Wu XB, Liu F, Bai E. 2004. Spatial and temporal patterns of soil carbon and nitrogen storage following woody plant encroachment into grassland. Eos Transactions of the American Geophysical Union 85(47): B11A-0136.

Progress 01/01/03 to 12/31/03

Outputs
In the Rio Grande Plains of southern Texas, grasslands dominated by C4 grasses (d13C = -13 o/oo) have undergone succession over the past 150 yrs to subtropical thorn woodlands dominated by C3 trees/shrubs (d13C = -27 o/oo). To evaluate soil organic carbon (SOC) storage and dynamics in these woodlands, we measured mass and isotopic composition (d13C) of SOC in remnant grasslands and woody plant stands (age 10-130 yrs). Increased carbon storage in surface litter and roots following woody invasion of grassland was associated with significant increases in SOC (0-30 cm) from 2000 g C/m2 in grasslands to >5000 g C/m2 in older woodlands. SOC accumulation rates (0-30 cm) ranged from 12.5 g C/m2/yr in upland woodlands to 41.5 g C/m2/yr in lowlands. d13C of SOC at 0-15 cm decreased exponentially from -18 o/oo in grasslands to -23 o/oo in woodlands >60 yrs old. At 15-30cm, d13C of SOC decreased from -15 o/oo in grasslands to -19 o/oo in woodlands >60 yrs old. These C-13 dynamics yielded SOC mean residence times of 40-80 yrs at 0-15 cm and 150-500 yrs at 15-30 cm, consistent with previous results based on C-14 dating. Therefore, soils in this region have been a net sink for atmospheric carbon over the past century. Most of this newly sequestered carbon is stored in the upper 15 cm of the profile and has mean residence times <100 yrs.

Impacts
Given the geographically extensive nature of this vegetation shift, changes in SOC dynamics and storage documented here could have significance for regional and global carbon cycles. These results will have implications not only for land managers, but also for scientists and policy-makers who are now evaluating the potential for land management practices to alter ecosystem carbon storage and influence atmospheric CO2 concentrations and global climate.

Publications

Archer SR, Boutton TW, McMurtry C. 2004. Carbon and nitrogen storage in a savanna landscape: Field and modeling perspectives. Global Environmental Change on Land and in the Oceans, H Kawahata and H Koizumi, Eds. Kluwer Academic, Dordrecht, Netherlands (in press).
Jessup KE, Barnes PW, Boutton TW. 2003. Vegetation dynamics in a Quercus virginiana - Juniperus ashei savanna: An isotopic assessment. Journal of Vegetation Science 14: 841-852.
Derner JD, Boutton TW, Briske DD. 2003. Grazing impacts on ecosystem carbon storage along a precipitation gradient in Great Plains grasslands. Abstracts, Society for Range Management 56: 61.
Boutton TW, Archer SR. 2003. Ecosystem carbon gain with woody plant invasion of grasslands. Abstracts, Society for Range Management 56: 25.
Archer SR, Boutton TW. 2003. Wooded grasslands: Functional consequences of an ecological oxymoron. Abstracts, Ecological Society of America 2003.
Harris WD, Boutton TW, Ansley RJ. 2003. Soil carbon and nitrogen cycling in subhumid temperate grasslands: Effects of seasonal fire and simulated grazing. Abstracts, Ecological Society of America 2003.
Boutton TW, Liao JD. 2003. Woody plant invasion of grassland: Changes in whole-soil carbon storage and dynamics. Eos Transactions of the American Geophysical Union 84(47): B31D-0330.
Liao JD, Boutton TW, Jastrow JD. 2003. Woody plant invasion of grassland: Storage and turnover of carbon in soil physical fractions. Eos Transactions of the American Geophysical Union 84(47): B31D-0331.
Gamblin D, Filley TR, Liao JD, Boutton TW, Jastrow JD. 2003. Woody plant invasion of grassland: Lignin and aliphatic biopolymer chemistry and carbon isotope composition in physical fractions. Eos Transactions of the American Geophysical Union 84(47): B31D-0332.
Bai E, Boutton TW, Archer SR, Wu XB, Liu F. 2004. Spatial patterns of soil properties in a savanna parkland: Soil organic carbon and total nitrogen. Abstracts, Society for Range Management 2004: 15.
Boutton AT, Rosol T, Boutton TW, Wu XB, Archer SR, Hallmark CT, Liu F, Bai E, Prochaska D. 2004. Spatial variation in soil texture in a subtropical savanna parkland landscape. Abstracts, Society for Range Management 2004: 40.
Boutton TW, Liao JD. 2004. Soil carbon storage and dynamics following woody plant invasion of grassland. Abstracts, Society for Range Management 2004: 41.
Liao JD, Boutton TW, Jastrow JD. 2004. Storage and turnover of carbon in soil physical fractions following woody plant invasion of grassland. Abstracts, Society for Range Management 2004: 217.
Liu F, Wu XB, Archer SR, Boutton TW, Bai E. 2004. Spatial patterns of soil properties in a savanna parkland: Bulk density and soil water content. Abstracts, Society for Range Management 2004: 221.

Progress 01/01/02 to 12/31/02

Outputs
Vegetation changes that alter productivity and/or decomposition in dryland ecosystems have potential to impact the global carbon cycle and climate. One of the most prevalent vegetation changes in grasslands and savannas worldwide is increased woody plant abundance. Despite the geographic dimensions of this vegetation change, we know little regarding its impact on the carbon cycle. In the Rio Grande Plains of southern Texas, grasslands have undergone succession toward subtropical woodlands over the past 100 yrs. We used a chronosequence approach to quantify changes in carbon storage in soils (to 1.5 m) and above- and belowground biomass in remnant grasslands, and wooded landscape elements ranging in age from 10-120 yrs. Whole-ecosystem carbon storage was 12.2 kg C m!2 in remnant grasslands, and 19.9-35.3 kg C m!2 in wooded landscape elements. Thus, C-storage increased 64-191% where woody plants replaced grasslands. Rates of whole-ecosystem C-sequestration ranged from 80-230 g C m!2 yr!1 over the past 100 years, similar to results from other temperate and tropical savannas. The geographic extent of woody encroachment into dryland ecosystems and its impact on carbon storage in those ecosystems are both documented poorly, so the impact of this change on the global carbon cycle is unknown at present. However, recent estimates suggest this vegetation change may sequester >0.1 Pg C yr!1 in the USA, and >0.8 Pg C yr!1 globally. If these approximations are reasonable, woody encroachment in drylands is a significant flux in the C-cycle that could affect the climate system.

Impacts
The U.S. government plans to reduce the greenhouse gas intensity of the U.S. economy by 18% in the next ten years, and most other nations of the world are planning similar reductions through participation in the Kyoto Protocol. These reductions can be met through reduced fossil fuel consumption, and/or by increased carbon storage in ecosystems. Results of this study provide key information regarding the potential for grasslands and savannas in the southwestern USA to store additional carbon in vegetation and soils, and therefore contribute to reduced net carbon emissions from the USA.

Publications

Davies FT, Olalde-Portugal V, Aguilera-Gomez L, Alvarado MJ, Ferrera-Cerrato RC, Boutton TW. 2002. Alleviation of drought stress of Chile ancho pepper (Capsicum annuum L. cv. San Luis) with arbuscular mycorrhiza indigenous to Mexico. Scientia Horticulturae 92: 347-359.
Nordt LC, Boutton TW, Jacob JJ, Mandel R. 2002. C4 plant productivity and climate-CO2 variations in south-central Texas during the late Quaternary. Quaternary Research 58: 182-188.
Derner JD, Johnson HB, Kimball BA, Pinter PJ, Polley HW, Tischler CR, Boutton TW, LaMorte RL, Wall GW, Adam NR, Leavitt SW, Ottman MJ, Matthias AD, Brooks TJ. 2003. Above- and belowground responses of C3-C4 species mixtures to elevated CO2 and soil water availability. Global Change Biology 9: 1-9.
Beazley MJ, Rickman RD, Ingram DK, Boutton TW, Russ J. 2003. Natural abundance of carbon isotopes (14C, 13C) in lichens and calcium oxalate pruina: Implications for archeological and paleoenvironmental studies. Radiocarbon (in press).
Harris W, Boutton TW, Ansley RJ. 2002. Management effects on carbon and nitrogen cycles of Rolling Plains grasslands. Abstracts Soil Survey and Land Resource Workshop 2002.
Liao JD, Boutton TW. 2002. Land cover changes and soil carbon dynamics in a subtropical savanna ecosystem. Abstracts Soil Survey and Land Resource Workshop 2002.
Wu XB, Archer SR, Boutton TW, Liu F, Jessup K. 2002. Spatial patterns of bulk density and soil water content in a savanna parkland landscape. Abstracts, Ecological Society of America 2002: 158.
Harris WN, Boutton TW, Ansley RJ. 2002. Soil respiration and microbial biomass in a subhumid temperate grassland: Role of fire and simulated grazing. Abstracts, Ecological Society of America 2002: 8.
Russ J, Rickman R, Beazley M, Boutton T. 2002. Predicting paleomoisture regimes based on stable carbon isotope ratios of oxalate residues from lichens. Geological Society of America Abstracts 34(6): 140-9.
Liao JD, Boutton TW. 2002. Soil carbon dynamics of a subtropical savanna ecosystem. Soil Science Society of America Abstracts 2002: S03-Liao163705-P.
Jessup KE, Boutton TW, Barnes PW. 2002. Vegetation dynamics and soil C and N storage in an oak-juniper savanna: An isotopic assessment. Soil Science Society of America Abstracts 2002: S08-Jessup140225-P.
Harris WN, Boutton TW, Ansley RJ. 2002. Soil respiration and microbial biomass in a temperate grassland: Effects of fire and simulated grazing. Soil Science Society of America Abstracts 2002: S07-Harris161156-P.
Liu F, Boutton TW, Wu XB, Archer SR, Jessup KE. 2002. Spatial patterns of soil properties in a savanna parkland landscape: Soil organic carbon and total nitrogen. Soil Science Society of America Abstracts 2002: S07-Boutton154224-P.
Wu XB, Archer SR, Boutton TW, Liu F, Jessup KE. 2002. Spatial patterns of soil properties in a savanna parkland landscape: Bulk density and soil water content. Soil Science Society of America Abstracts 2002: S07-Boutton152842-P.
Ansley RJ, Boutton TW, Kramp BA. 2002. Biogeochemical responses to fire seasonality and frequency in a temperate mixed-grass savanna: Storage and dynamics of soil carbon and nitrogen. Soil Science Society of America Abstracts 2002: S07-Boutton120448-P.
Boutton TW, Ansley RJ, Skjemstad JO. 2002. Biogeochemical responses to fire seasonality and frequency in a temperate mixed-grass savanna: Charcoal carbon. Soil Science Society of America Abstracts 2002: S07-Boutton121534-P.
Russ J, Beazely M, Rickman, Boutton TW. 2002. Carbon isotope abundances in lichen deposits might reflect past moisture trends. Eos Transactions of the American Geophysical Union 83(47): PP71B

Progress 01/01/01 to 12/31/01

Outputs
Grassland-to-woodland conversion has been geographically extensive in grassland ecosystems worldwide in recent history. In the Rio Grande Plains, TX grasslands have been largely replaced by subtropical woodlands over the past 100 yrs due to grazing and fire suppression. To evaluate the impact of this vegetation change on C storage and dynamics, long-term soil incubations and kinetic analyses were used to quantify SOC pool sizes and fluxes. Active C pools in remnant grasslands were twice the size of those in woodlands. Conversely, slow C pools in woodlands were 20% larger than those in grasslands. Mean residence times of SOC pools were longer in woodlands (active = 10-26 days; slow = 3-14 years) than in grasslands (active = 15-18 days; slow = 2-3 years). Pool sizes of total SOC were 150 to 600% larger in woodlands than in grasslands. Despite higher absolute soil respiration rates, it appears that slower C turnover in woodlands has caused C accumulation in more resistant fractions, and net C storage has occurred. Results are consistent with our prior studies indicating that woodland development reduces C turnover and increases SOC storage in this savanna parkland. Drylands where trees and shrubs have replaced grasses may be significant sinks for atmospheric CO2 and may help offset releases of C associated with other industrial and land-use activities.

Impacts
Grassland-to-woodland conversion during the past 100 years has been geographically extensive in grasslands and savannas worldwide, suggesting that changes in soil organic carbon storage and dynamics documented here could have significance for the global carbon cycle and climate.

Publications

Steelman KL, Rowe MW, Boutton TW, Southon JR, Merrell CL, Hill RD. 2001. Stable isotope and radiocarbon analyses of a black deposit associated with pictographs at Little Lost River Cave, Idaho. Journal of Archeological Science (in press).
Ansley J, Boutton TW, Jariel D, Kramp B, Skjemstad JO. 2001. Biogeochemical consequences of fire in temperate mixed-grass savanna: Soil C and N storage and the role of black carbon. Plant and Soil (accepted).
Nordt LC, Boutton TW, Jacob JJ, Mandel R. 2001. C4 plant productivity in relation to climate-CO2 variations in the southern Great Plains during the late Quaternary. Quaternary Research (accepted).
Steelman KL, Rickman R, Rowe MW, Boutton TW, Russ J, Guidon N. 2002. AMS radiocarbon ages of an oxalate accretion and rock paintings at Toca do Serrote da Bastiana, Brazil. IN: Archeological Chemistry, K Jakes, Ed. American Chemical Society, Washington, DC (in press).
Russ J, Beazley M, Rickman R, Ingram D, Boutton TW. 2001. Evaluating oxalate rock coating as a Holocene paleoclimate proxy. Geological Society of America Abstracts 33(6): 26144.
Liao JD, Boutton TW. 2001. Land cover changes and soil carbon processes in a savanna ecosystem. Soil Science Society of America Abstracts 2001: S3-Liao122449-P.
Wessman C, Archer S, Asner GP, Bateson A, Boutton TW. 2001. Regional NPP and carbon stocks in Southwestern USA rangelands: land-use impacts on the grassland-woodland balance. NASA Land Cover Land Use Change Meetings, Baltimore, MD (in press).
Boutton TW, Archer SR. 2001. Woody plant encroachment in grasslands and savannas: Significance for ecosystem and global carbon storage. Proceedings 9th US-Japan Workshop on Global Climate Change. Tokyo, October 9-11, 2001 (in press).
Boutton TW, Archer SR, Liao JD. 2001. Land cover changes and soil carbon dynamics: Insights from natural 13C and long-term incubations. Proceedings 9th US-Japan Workshop on Global Climate Change. Tokyo, October 9-11, 2001 (in press).

Progress 01/01/00 to 12/31/00

Outputs
Livestock grazing has the potential to change ecosystem structure and function and alter carbon (C) storage in plants and soils. We quantified C density (g C/m2) in soil (0-30 cm) and above- and belowground plant biomass in long-term grazed and ungrazed areas at 3 locations along a rainfall gradient in the Great Plains. d13C was used to estimate relative proportions of soil organic C and plant biomass C derived from C3 vs. C4 plant sources. Ecosystem C storage (vegetation + soil) in ungrazed areas increased along the precipitation gradient from 3236 g C/m2 in shortgrass prairie to 8997 g C/m2 in tallgrass prairie. Less than 3% of ecosystem C was located in aboveground biomass at all sites and both grazing treatments. Moderate grazing strongly increased the relative proportion of C derived from C4 grasses in above- and belowground biomass and soil in shortgrass prairie, and increased C storage in that ecosystem by 24%. In contrast, moderate grazing had little effect on the relative importance of C3-C4 species in midgrass and tallgrass prairies, and reduced ecosystem C storage by 8% in those grasslands. Grazing increased the relative importance of fine root C at all 3 locations, and shifted the distribution of root C upwards in the profile at midgrass and tallgrass prairies. Hence, long-term moderate grazing significantly increased ecosystem C storage in semiarid shortgrass prairie, but had less effect on C storage in the more mesic midgrass and tallgrass ecosystems.

Impacts
Livestock grazing is the primary land use activity on 50% of the terrestrial surface, yet we have little knowledge of how this activity may influence the carbon cycle at the ecosystem or global levels of organization. This study shows that grazing alters the storage of carbon in ecosystems by changing the structure and function of plant communities, and by changing rates of soil processes. Results of this study will enhance our understanding of grazing effects on the carbon cycle, and provide some indication of the potential for rangelands to store carbon and mitigate the anthropogenic greenhouse effect.

Publications

Archer SR, Boutton TW, Hibbard K. 2001. Trees in grasslands: Biogeochemical consequences of woody plant expansion. IN: Global Biogeochemical Cycles in the Climate System, pp. 115-137, E-D Schulze, SP Harrison, M Heimann, EA Holland, J Lloyd, IC Prentice, D Schimel, Eds. Academic Press, San Diego.
Boutton TW, Archer SR. 2000. Carbon sequestration: Understanding the basics. Abstracts 12th Annual Texas Plant Protection Conference 2000: 23.
Archer SR, Boutton TW. 2000. Carbon sequestration: A viable commodity? Abstracts 12th Annual Texas Plant Protection Conference 2000: 23.
Jessup KE, Barker KD, Barnes PW, Boutton TW. 2000. Vegetation dynamics of juniper-oak savannas on the Edwards Plateau of Texas: An isotopic assessment. Abstracts, Ecological Society of America 2000: 294.
Derner JD, Boutton TW, Briske DD. 2000. Vegetation dynamics and ecosystem carbon storage in the Great Plains: Role of grazing and climate. Soil Science Society of America Abstracts 2000: 347-348.
Boutton TW, Derner JD, Liao JD. 2000. Natural abundance of N-15 in grassland ecosystems: Role of grazing and climate. Soil Science Society of America Abstracts 2000: 347.
Archer SR, Boutton TW, Hibbard KA. 2001. Ecosystem simplification and ecological processes. Society for Range Management Abstracts (in press).

Progress 01/01/99 to 12/31/99

Outputs
The Rio Grande Plains were once relatively open grasslands and savannas dominated by C4 grasses (d13C = -13 o/oo), but have been largely replaced by woodlands dominated by C3 trees/shrubs (d13C = -27 o/oo) over the past 100 yr. To evaluate soil organic carbon (SOC) dynamics in these woodlands, we measured d13C of SOC in remnant grasslands and woody plant stands aged by dendrochronology. d13C of SOC decreased with increasing stand age from -19 o/oo in remnant grasslands to approximately -23 o/oo in woodlands older than 60 yr. Exponential models indicated that SOC in upland woodlands (sandy loam soils, 12 % clay) had mean residence times (MRT = 1/k) of 69 yr at 0-10 cm, and 119 yr at 10-20 cm. In contrast, SOC in lower-lying woodlands (sandy clay loams, 22 % clay) had MRTs of 130 yr at 0-10 cm, and 250 yr at 10-20 cm. Mass balance revealed that 40-60 % of SOC derived from the original grasslands remained after 80 yr of woodland development. However, over the same time period, accumulation of C from woody plants increased SOC by 33-60 % in upland woodlands and 250 % in lowland woodlands. Results demonstrate a high potential for soil carbon storage following succession from grassland to woodland, confirm the existence of a large mass of relatively inert organic carbon derived from the original grassland, and highlight the importance of soil texture and landscape position as determinants of SOC storage and dynamics.

Impacts
The sequestration of soil organic carbon in ecosystems where trees and shrubs have displaced grasses may represent a significant sink for atmospheric CO2 that could help offset releases of carbon associated with other industrial and agricultural activities. The identification of ecosystems capable of sequestering atmospheric CO2 in soils will help achieve the goals of the Kyoto Protocol aimed at mitigating greenhouse gas concentrations. Agricultural producers owning land capable of sequestering atmospheric carbon are likely to profit economically in the future.

Publications

Russ J, Loyd DH, Boutton TW. 2000. A paleoclimate reconstruction for southwestern Texas using oxalate residue from lichen as a paleoclimate proxy. Quaternary International 67 (in press).
Archer SR, Boutton TW, Hibbard KA. 2000. Woody encroachment in grasslands and savannas: Ecosystem consequences. Abstracts, Ecological Society of America 2000 (in press).
Jessup KE, Barker KD, Barnes PW, Boutton TW. 2000. Vegetation dynamics of juniper-oak savannas on the Edwards Plateau of Texas: An isotopic assessment. Abstracts, Ecological Society of America 2000 (in press).
Boutton TW, Archer SR, Midwood AJ. 1999. Stable isotopes in ecosystem science: Structure, function, and dynamics of a subtropical savanna. Rapid Communications in Mass Spectrometry 13: 1263-1277.
Moreno-Valdez A, Boutton TW. 1999. Dietary variation in the Mexican long-nosed bat (Leptonycteris nivalis) revealed by stable isotope analysis. Abstracts 79th Annual Meeting American Society of Mammalogists 1999: 200.
Boutton TW, Archer SR. 1999. Soil carbon dynamics in a subtropical savanna ecosystem. Soil Science Society of America Abstracts 1999: 227.
Archer SR, Boutton TW. 1999. Historic changes in tree/grass abundance: Implications for C and N storage. Soil Science Society of America Abstracts 1999: 364.
Liao JD, Boutton TW. 1999. Natural abundance of 15N in plants and soils of a subtropical savanna: Relationships with N-cycling processes and woodland development. Soil Science Society of America Abstracts 1999: 228.
Liao JD, Shaw NL, Anderson VJ, Monsen SB, Boutton TW. 1999. Reproductive biology of rush skeletonweed: Seed germination characteristics. Soil Science Society of America Abstracts 1999: 107.
Rickman R, Boutton TW, Russ J. 1999. The validity of radiocarbon ages of biogenic oxalate. Abstracts, American Chemical Society Annual Meeting.

Progress 01/01/98 to 12/31/98

Outputs
Although the influence of temperature on soil respiration is well-described, little is known regarding the effect of soil moisture on this key component of the carbon cycle. We hypothesized that increased soil moisture would enhance soil respiration and soil microbial biomass and activity in a semiarid, subtropical savanna parkland ecosystem. Soil respiration, microbial biomass carbon (MBC), and potential carbon mineralization rates (CMIN) were measured monthly in control and irrigated patches dominated by either grasses or woody plants. Soil respiration was greater in irrigated (4752 g CO2/m2/month) than control (2532 g CO2/m2/month) plots when averaged across all landscape elements. However, MBC and CMIN were significantly lower in irrigated (MBC = 310 mg C/kg soil; CMIN = 7.6 mg C/kg soil/day) than control plots (MBC = 411 mg C/kg soil; CMIN = 11.6 mg C/kg soil/day) for all months and across all landscape elements (p< 0.001). Therefore, the increase in soil respiration in response to irrigation does not appear to have been a function of enhanced microbial activity. We suggest that increased soil respiration in irrigated treatments was a consequence of increased root respiration resulting from improved plant carbon/water relations. Irrigation-induced decreases in microbial biomass and activity may have resulted from increased competition between microbes and roots for nutrients, increased leaching of nutrients from the surface soil, and/or adverse physical conditions for microbes at high soil water content.

Impacts
(N/A)

Publications

Nordt LC, Kelly EF, Boutton TW, Chadwick OA. 1998. Biogeochemistry of isotopes in soil environments: Theory and application. Geoderma 82: 1-3.
Boutton TW, Archer SR, Midwood AJ, Zitzer SF, Bol R. 1998. d13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma 82: 5-41.
Nordt LC, Hallmark CT, Wilding LP, Boutton TW. 1998. Quantifying pedogenic carbonate accumulations using stable carbon isotopes. Geoderma 82: 115-136.
Midwood AJ, Boutton TW. 1998. Soil carbonate decomposition by acid has little effect on d13C of soil organic matter. Soil Biology and Biochemistry 30: 1301-1307.
Boutton TW, Nordt LC, Kuehn DD. 1998. Late Quaternary vegetation and climate change in the North American Great Plains: Evidence from d13C of paleosol organic carbon. IN: Isotope Techniques in the Study of Environmental Change, pp. 653-662. International Atomic Energy Agency, Vienna, Austria.
Hays KA, Smeins FE, Boutton TW. 1999. Vegetation dynamics in a coastal oak savanna: Evidence from soil isotopic composition. Abstracts 52nd Annual Meeting, Society for Range Management (in press).
Boutton TW, Archer SR. 1998. Rates of soil carbon and nitrogen accumulation during succession from grassland to woodland in a subtropical savanna ecosystem. Agronomy Abstracts 1998: 218.
Jastrow JD, Miller RM, Boutton TW, Short SA, Cates EE, Schultz PA, Owensby CE. 1998. Changes in prairie soil organic matter fractions after exposure to elevated CO2. Agronomy Abstracts 1998: 304.
Jastrow JD, Miller RM, Boutton TW. 1998. Accrual of organic carbon in previously cultivated mollisols of the Prairie Peninsula. Agronomy Abstracts 1998: 224.
Boutton TW. 1999. Stable isotopes in ecosystem science. Abstracts Stable Isotope Mass Spectrometry Users Group Meeting 1999: 2-4.
McCulley RL, Boutton TW, Archer SR. 1999. Soil respiration and microbial biomass in a savanna parkland: The role of soil moisture. Bulletin of the Ecological Society of America (in press).
Liao JD, Boutton TW, Hoskisson AJ, McCulley RL, Archer SR. 1999. Woodland development and the N-cycle of a subtropical savanna parkland: Insights from d15N of plants and soils. Bulletin of the Ecological Society of America (in press).
Hubbard JA, Archer SR, Boutton TW, Ansley RJ. 1999. Interactive effects of fire and grazing on plant productivity and soil respiration. Bulletin of the Ecological Society of America (in press).
Hays KA, Boutton TW, Smeins FE. 1999. Vegetation dynamics in a Texas coastal oak savanna: Evidence from soil isotopic composition. Bulletin of the Ecological Society of America (in press).

Progress 01/01/97 to 12/31/97

Outputs
In southern Texas, woodlands dominated by highly productive, nitrogen-fixing trees/shrubs have increased recently in areas that were once grassland or savanna. Our previous work showed that both pool sizes and rates of accumulation of soil organic carbon (SOC) were significantly greater in these woodlands relative to remnants of the grassland they have largely replaced. To further assess the impact of grassland-to-woodland succession on the carbon cycle in this ecosystem, SOC turnover rates were determined by measuring the natural abundance of radiocarbon (C-14) in soil organic matter collected from remnant grasslands and woodlands. Mean residence times (MRT) of SOC were then calculated using a proportional replacement model (Harkness et al 1991) which utilizes the decay kinetics of the 1950's C-14 bomb spike to quantify the turnover rate of the SOC compartment. MRT's of soil organic carbon in the upper soil profile did not differ significantly between remnant grasslands and woodlands, and ranged from 40-100 yrs at 0-15 cm, and 300-500 yrs at 15-30 cm. Below 30 cm in the profile, MRT's in upland grasslands and woodlands (400-1800 yrs) were lower than those in lowland woodlands (900-2800 yrs), perhaps reflecting the greater clay content of the soil in the lower landscape positions. Thus, the natural abundance of radiocarbon suggests that rates of SOC turnover have not been influenced significantly by succession from grassland to woodland. SOC dynamics are now being evaluated further using both soil respiration measurements and natural C-13/C-12 ratios.

Impacts
(N/A)

Publications

Derner JD, Briske DD, Boutton TW. 1997. Does grazing mediate soil carbon and nitrogen accumulation beneath C4 perennial grasses along an environmental gradient. Plant and Soil 191: 147-156.
Derner JD, Briske DD, Boutton TW. 1997. What mechanisms confer ecological success to bunchgrasses. Abstracts, 50th Annual Meeting of the Society for Range Management: 43.
Boutton TW, Nordt LC, Kuehn DD. 1997. Late Quaternary vegetation and climate change in the North American Great Plains: Evidence from d13C of paleosol organic carbon. Abstracts International Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and Atmosphere, IAEA (Vienna), pp. 112-113.
Hubbard JA, Archer SR, Boutton TW. 1997. Rates of root biomass accumulation during succession from savanna to woodland. Bulletin of the Ecological Society of America 78(4): 260.
McCulley RL, Boutton TW, Archer SR, Zuberer DA, Hons FA, Hubbard AJ. 1997. Spatial and temporal variation in soil respiration and microbial biomass in a subtropical savanna parkland. Bulletin of the Ecological Society of America 78(4): 283.
Nordt LC, Hallmark CT, Wilding LP, Boutton TW. 1997. Quantifying pedogenic carbonate accumulations using stable carbon isotopes. Geoderma (in press).
Boutton TW, Archer SR, Midwood AJ, Zitzer SF, Bol R. 1997. d13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma (in press).
Midwood AJ, Boutton TW. 1997. Soil carbonate decomposition by acid has little effect on d13C of soil organic matter. Soil Biology and Biochemistry (in press).
Nordt LC, Kelly EF, Boutton TW, Chadwick OA (eds.). 1997. Biogeochemistry of Isotopes in Soil Environments: Theory and Application. Elsevier Science, Amsterdam (in press).
Boutton TW, Nordt LC, Kuehn DD. 1997. Late Quaternary vegetation and climate change in the North American Great Plains: Evidence from d13C of paleosol organic carbon. IN: Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere. International Atomic Energy Agency, Vienna, Austria (in press).

Progress 01/01/96 to 12/30/96

Outputs
In southern Texas, woodlands dominated by highly productive N2-fixing trees/shrubs have recently increased in areas that were once grassland or savanna. We hypothesized that greater N2-fixation and above- and belowground productivity in wooded landscape elements would increase soil respiration and microbial biomass relative to remnant grasslands. Soil respiration (using infrared gas analysis) and microbial biomass C (using chloroform fumigation-incubation technique) were measured monthly in upland grasslands and wooded patches, and lowland woodlands. Microbial biomass C was greater in lowland (690 +/- 45 mg C/kg soil) and upland woodlands (500 +/- 50 mg C/kg soil) than in grasslands (363 +/- 40 mg C/kg soil) throughout the year, perhaps as a result of differences in the quality and quantity of organic matter inputs. Despite these differences, soil respiration was comparable among landscape elements (annual mean +/- SE: grasslands = 1.8 +/- 0.2, upland woodlands = 2.0 +/- 0.2, and lowland woodlands = 2.1 +/- 0.2 micromoles CO2/m2/ sec), and showed little diurnal and seasonal variation. Respiration rates peaked in September (4-7 micromoles CO2/m2/sec) when soil moisture was greatest. The spatial and temporal distribution of net primary productivity and soil moisture both appear significant in controlling soil respiration and microbial biomass in this system.

Impacts
(N/A)

Publications

BOUTTON, T.W., ARCHER, S.R., MIDWOOD, A. J., and ZITZER, S.F. 1996. k13C values uof soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma (In press).
NORDT, L.C., HALLMARK, C.T., WILDING, L. P., and BOUTTON, T.W. 1996. Quantifying pedogenic carbonate accumulations using stable carbon isotopes. Geoderma (In press).
BOUTTON, T. W., and YAMASAKI, S.I. (Editors). 1996. Mass Spectrometry of Soils. Marcel Dekker, Inc., New York, NY.
ZITZER, S.F., ARCHER, S. R., and BOUTTON, T.W. 1996. Spatial variability in the potential for symbiotic N2 fixation by woody plants in a subtropical savanna ecosystem. J. Appl. Ecol. 33: 1125-1136.
RUSS, J., PALMA, R., LOYD, D., BOUTTON, T., and COY, M. 1996. Origin of the whellite-rich rock crust in the Lower Pecos region of southwest Texas & its significance to paleoclimate reconstructions. Quat. Res. 46: 27-36.
JASTROW, J.D., BOUTTON, T. W., and MILLER, R.M. 1996. Carbon dynamics of aggregate-associated organic matter estimated by 13C natural abundance. Soil Sci. Soc. Am. J. 60: 801-807.
BOUTTON, T.W. 1996. Stable carbon isotope ratios of soil organic matter ... of vegetation & Climate change. IN: Boutton, T.
W., Yamasaki, S.I. (Eds.), Mass Spectrometry of Soils, pp. 47-82. Marcel Dekker, Inc., New York, NY.

Progress 01/01/95 to 12/30/95

Outputs
13C of organic carbon (OC) in soils and paleosols integrates the relative contribution of C3 and C4 plants to the soil OC pool. Since the geographic distribution and relative productivity of C4 species are both correlated with temperature, 13C of paleosol OC should reflect long term vegetation dynamics and climatic variation. We determined 13C of OC (n=51) in 8 paleosols in central Texas. 13C of paleosol OC varied from -25.1 to -16.7, indicating that the proportion of C from C4 species ranged from 10 to 75% during the past 15,000 years before present (YBP). Three distinct periods of low C4 productivity occurred between 15,000 - 8,000 YBP. These events coincided precisely with 3 well documented episodes of glacial meltwater flux into the Gulf of Mexico (220 km from study area) via the Mississippi River, indicating strong coupling between marine and adjacent continental ecosystems. The effects of glacial meltwater in the Gulf ceased around 8,000 YBP; after this time, relative C4 productivity was strongly influenced by orbital forcing mechanisms. Holocene altithermal periods were characterized by high relative C4 productivity is highly correlated with previous climatic reconstructons for this region, indicating that 13C of paleosol OC may be a useful proxy for paleotemperature.

Impacts
(N/A)

Publications

Progress 01/01/94 to 12/30/94

Outputs
Grazing history and vegetation type are known to influence soil nutrient storageand turnover. We investigated the effects of grazing intensity (heavy, moderate, none) on soil nitrogen (N) and carbon (C) pool sizes and dynamics under different plant communities (shortgrass, midgrass, juniper, oak) in a juniper-oak savanna in west-central Texas. Grazing treatments were in place for 40 yr. Organic C, total N, microbial biomass C (MBC) and N (MBN), and potential C and N mineralization were measured on five samples (0-10 cm) in each treatment combination. Grazing had no significant effect on any of the variables, and there were no significant grazing x plant community interactions; however, plant community type was significant (p < 0.001) for all variables. Soil C and N concentrations (g kg(superscript -1)) were higher under oak (C = 96.7 /+-/ 9.3; N = 6.1 /+-/ 0.6) than all other communities (C = 54.4 /+-/ 3.3; N = 3.5 /+-/ 0.1). Microbial biomass (mg kg(superscript -1)) was greater under oak (MBC = 2358 /+-/ 495; MBN = 360 /+-/ 127) than all other communities (MBC = 1799 /+-/ 211; MBN 259 /+-/ 39). Similarly, potential mineralization (mg kg(superscript -1) d(superscript -1)) was greater under oak (C(subscript min) = 49.9 /+-/ 13.3; N(subscript min) = 7.8 /+-/ 3.5) than other communities (C(subscript min) = 32.2 /+-/ 6.8; N(subscript min) = 3.4 /+-/ 0.7).

Impacts
(N/A)

Publications

Progress 01/01/93 to 12/30/93

Outputs
In the Rio Grande Plains of southern Texas, subtropical thorn woodland has replaced relatively open grassland/savanna during the past 200 years. To investigate potential changes in the hydrologic cycle of this system, we determined the natural abundance of (superscript 2)H and (superscript 18)O in rainfall, soil water, and plant water in three habitats representing a successional chronosequence in the transition from open grassland to closed-canopy woodland. Precipitation was isotopically identical to groundwater and fell on the meteoric water line with a weighted mean (delta)(superscript 2)H of -22% and weighted mean (delta)(superscript 18)O of -4.3 %. (delta)(superscript 2)H and (delta)(superscript 18)O of precipitation did not vary seasonally, but showed a significant "amount effect". Correlations between (delta)(superscript 2)H and (delta)(superscript 18)O of soil water indicated higher evaporation rates in wooded areas than in grassland. We suggest that this result does not reflect higher evaporation rates, but rapid drying of the soil via transpiration followed by kinetic fractionation associated with evaporation in dry soils.

Impacts
(N/A)

Publications

Progress 01/01/92 to 12/30/92

Outputs
Carbon isotope ratios of soil organic matter (SOM) were used to test the hypothesis that subtropical thorn woodland replaced open grassland in southern Texas. (delta)(superscript 13)C values and (superscript 14)C ages of SOM from cores taken along transects closed-canopy woodland into remnant grasslands confirmed that C(subscript 3) woody plants displaced C(subscript 4) grassland. (delta)(superscript 13)C values of SOM inputs (litter, roots) in areas dominated by woody plants were near -25%o, as expected for a C(subscript 3) community. In contrast, (delta)(superscript 13)C of SOM was -19%o at 0-15 cm ((superscript 14)C age = post-modern), indicating that > 50% of soil carbon at this depth was derived from C(subscript 4) grasses. Furthermore, (delta)(superscript 13)C values of SOM at 30-60 cm ((superscript 14)C age = 700 /+-/ 65 yr) were near -14%o, indicating that nearly 100% of this organic carbon had a C(subscript 4) grass origin. These data provide direct, unambiguous evidence that this area was once open C(subscript 4) grassland, supporting conclusions based on other methods. Half of the woody plant species in this ecosystem belong to the Fabaceae and Rhamnaceae, families known to be capable of symbiotic N(subscript 2)-fixation. Greenhouse studies showed that the legumes Prosopis glandulosa (PG) and Acacia farnesiana (AF) formed effective root nodules when grown in native soils. In the field, (delta)(superscript 15)N values of leaves from PG (+2.1 /+-/ 1.1, n = 120) and AF (+ 0.5 /+-/ 1.

Impacts
(N/A)

Publications

Progress 01/01/91 to 12/30/91

Outputs
(delta)(superscript 13)C(subscript PDB) of soil organic matter (SOM) should record the long-term contribution of C(subscript 3) and C(subscript 4) plants to community productivity. To test the ability of this method to detect vegetation change, plant community characteristics and (delta)(superscript 13)C(subscript PDB) of SOM were compared on True Prairie sites with known vegetation history Species composition, shoot and root biomass, and (delta)(superscript 13)C(subscript PDB) of SOM were measured on an ungrazed area and an adjacent area grazed since 1969. In the ungrazed site, (delta)(superscript 13)C(subscript PDB) of SOM (-14(degree)/oo) was similar to that of above- and belowground biomass (-16(degree)/oo), suggesting that this site has been stable at about 70% C(subscript 4) biomass for a relatively long period of time. In the grazed site, (delta)(superscript 13)C(subscript PDB) of above- and belowground biomass (-22(degree)/oo) indicated a community comprised of of only 30% C(subscript 4) biomass, but (delta)(superscript 13)C(subscript PDB) of SOM (-16(degree)/oo) reflected the C(subscript 4)-dominated community that persisted on this site prior to grazi with known vegetation history, (delta)(superscript 13)C(subscript PDB) of SOM has accurately and directly documented vegetation change. This technique should be useful in reconstructing vegetation change arising from management practices and/or natural ecosystem processes in systems where past history is unknown.

Impacts
(N/A)

Publications

SWERHONE, G.D.W., HOBSON, K.A., VAN KESSEL, C., and BOUTTON, T.W. 1991. An economical method for preparation of plant and animal tissue for (delta)(superscript 13)C(subscript PDB) analysis. Commun. Soil. Sci. Plant Anal. 22: 177-190.
BOUTTON, T.W. 1991. Stable carbon isotope ratios of natural materials. I. Sample preparation and mass spectrometric analysis. In: Coleman DC, Fry B (eds.), Carbon Isotope Techniques, pp. 155-171. Academic Press, New York.
BOUTTON, T.W. 1991. Stable carbon isotope ratios of natural materials. II. Atmospheric, terrestrial, marine, and freshwater environments. In: Coleman DC, Fry B (eds.), Carbon Isotope Techniques, pp. 173-185. Academic Press, New York.
BOUTTON, T.W. 1991. Tracer studies with (superscript 13)C-enriched substrates: Humans and large animals. In: Coleman DC, Fry B (eds.), Carbon Isotope Techniques, pp. 219-242. Academic Press, New York.
BOUTTON, T.W. and FLAGLER, R.B. 1991. (superscript 13)C/(superscript 12)C ratios as indicators of plant physiological response to ozone and simulated acid rain. In: Stable Isotopes in Plant Nutrition, Soil Fertility, and Environmental Stud.
BOUTTON, T.W., ARCHER, S.R., ZITZER, S.F. and MIDWOOD, A.J. 1991. (delta)(superscript 13)C(subscript PDB) values of soil organic matter indicate vegetation change in a True Prairie ecosystem. Bull. Ecol. Soc. Am. 72(2):299.
JACOB, J.S., and BOUTTON, T.W. 1991. Isotopic characterization of the Asa paleo.

Progress 01/01/90 to 12/30/90

Outputs
The stable isotope laboratory has participated in interlaboratory comparisons ofC/C measurements of biological samples with twelve other stable isotope labs throughout the world. In all cases, delta C(PDB) values from our lab were within 0.1 /(00) of the results from all other labs. These results verify that both sample preparation and mass spectrometric procedures are performing optimally in our laboratory. Similar interlaboratory comparisons are underway for N/N and D/H. Work has begun to assess the potential of 16 woody plant species belonging to the Leguminosae and Rhamnaceae to fix nitrogen in woodland/savanna ecosystems in southern Texas. Sixteen woody plant species belonging to these two plant families were grown in the growth chamber and inoculated with soils collected at the La Copita Research Area in south Texas. Of these 16 species, only Prosopis glandulosa and Acacia farnesiana formed root nodules under these conditions. Acetylene reduction experiments indicated that these nodules were actively fixing atmospheric N(2.) Leaf tissue samples have been collected from La Copita and will be anlyzed for N/N ratios to determine the significance of N-fixation under field conditions. The effects of ozone and acid rain on growth and physiological response of shortleaf pine have been investigated in east Texas.

Impacts
(N/A)

Publications

Boutton, T.W., FLAGLER, R.B. 1990. _1_3C/_1_2C ratios as indicators of plant physiological response to ozone and simulated acid rain. Abstracts of the IAEA/FAO International Symposium .
BOUTTON, T.W., FLAGLER, R.B. 1990. Growth and water-use efficiency of shortleaf pine as affected by ozone and acid rain. Abstracts 83rd Annual Meeting of the Air and Waste Management Association 1990:142.
BUMSTED, M.P., BOOKER, J.E., BARNES, R.M., BOUTTON, T.W., et al. 1990. Recognizing women in the archeological record. In: Powers of observation: Alternative views in archeology, SM Nelson and AB Kehoe (eds), pp. 89-101.
HUEBNER, J.A. and BOUTTON, T.W. 1990. The isotopic ecology of bison in Texas. Abstracts 48th Plains Anthropological Conference, 1990:41-42.
JACOB, J.S., BOUTTON, T.W. 1990. Stable carbon isotope ratios of soil organic matter and carbonates from a paleosol in east-central Texas. Agron. Abstr. 1990-294.
SVEJCAR, T.J., BOUTTON, T.W. and TRENT, J.D. 1990. Assessment of carbon allocation with stable carbon isotope labeling. Agron. J. 82:18-21.

Progress 01/01/89 to 12/30/89

Outputs
Additional effort has been made towards the completion of a laboratory dedicatedto studies of rangeland biogeochemistry. This laboratory now consists of: 1) Two Nier-type, dual inlet, triple collector gas isotope ratio mass spectrometers capable of determining isotope ratios for carbon, nitrogen, oxygen, and hydrogen, 2) Four vacuum systems which can be used to prepare animal, plant, soil, and water samples for isotopic analysis by mass spectrometry, and 3) A Carlo Erba elemental analyzer capable of determining percent carbon, percent nitrogen, and percent sulfur on animal, plant, and soil samples. All of these systems are undergoing testing and evaluation against reference materials from the National Institute of Standards and Technology. In addition to efforts towards the development of a biogeochemical research laboratory, several research projects have been initiated: 1) Reconstruction of past vegetation history of a subtropical savanna ecosystem, 2) Assessment of stable isotope techniques for measuring plant water-use efficiency, 3) Impact of ozone and acid rain on plant physiological processes and nutrient allocation patterns, and 4) Studies of water uptake by plants using deuterated soil water. All projects have been initiated recently, and there are no data to report at this time.

Impacts
(N/A)

Publications