Progress 09/01/05 to 08/31/09
Outputs
OUTPUTS: The long-term goal of our research program is to make possible accurate and precise predictions of the effects of soil management on the amount of carbon that can be stored in soils of the upper Midwest. These predictions are needed by land managers and by those charged with monitoring the contribution that soils make to store carbon on a national scale. In this project, we focused specifically on the nature and quantity of recalcitrant soil organic matter in classic drainage sequences of soils. Our two primary objectives were to: (1) Characterize the spatial distribution of soil organic matter in cultivated Mollisols at the hectare scale, and (2) Characterize and quantify recalcitrant soil organic matter in cultivated Mollisols as a function of landscape position and depth. At two field locations in central Iowa (14 and 17 hectares, respectively), we sampled the soils in transects and determined the organic C, inorganic C, and bulk density at 0-5, 5-15, and 15-30 cm depths at each site. From this data set, we determined that the stock of soil organic carbon (SOC) in well-drained soils (0-30-cm depth) could be statistically distinguished from that of somewhat poor and poorly drained soils. We used both transect data and predicted SOC values from the soil-survey database to generate statistically equivalent distributions of SOC stocks at the site. Finally, we compared the generated maps with maps developed by geostatistical interpolation techniques to determine the degree to which intensive sampling decreased uncertainty in estimation of SOC stocks over the whole field. Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of soil organic matter (SOM) vary with landscape position To answer this question, we selected four soils at the two research sites mentioned above, two developed in till and two developed in loess. Two were well-drained and two were poorly drained soils. We used solid-state nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (IR) spectroscopy, and thermal analysis to characterize the soil organic matter and to determine if soil at different landscape positions had different levels of recalcitrant components in the organic matter. PARTICIPANTS: Michael L. Thompson, PI, Agronomy Department, Iowa State University; Klaus Schmidt-Rohr, PI, Chemistry Department, Iowa State University; Xiaowen Fang, graduate student, Chemistry Department, Iowa State University; Teresita Chua, assistant scientist, Agronomy Department, Iowa State University; Jessica Hutchison, research associate, Agronomy Department, Iowa State University; Nicholas Leete, undergraduate research assistant, Agronomy Department, Iowa State University. TARGET AUDIENCES: The target audience for this project was the scientific community, which was reached primarily through refereed journal articles. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
We characterized both aboveground and belowground crop residues as well as soil organic matter by using Fourier-transform infrared spectroscopy and advanced solid-state nuclear magnetic resonance spectroscopy. Using multiple techniques, we tested the hypothesis that the accumulation of recalcitrant components in soil organic matter is regulated by landscape position and drainage class. We have found that differential thermogravimetry is a promising approach to obtain a simple, semi-quantitative estimate of the recalcitrant fraction of soil organic matter. But thermal analyses were challenging to interpret. Thermal analysis of the particulate organic matter (POM) fractions reflected the nature of the crop residues that dominated POM at the time of sampling. Readily oxidizable compounds dominate, but it is likely that much hemicellulose (pectin) and cellulose had already been removed by decomposition, concentrating lignin in the POM. Our thermal characterization of model/reference compounds suggests that substantial portions of hemicellulose, protein, and lipids are lost over the temperature range from 110 to 350 deg C. Whole SOM is enriched in difficult to oxidize compounds - probably because most charcoal particles are larger than clay size. In contrast to the NMR data, our thermal studies suggest that the well drained soils accumulate difficult-to-oxidize material. Diffuse reflectance Fourier-transform infrared spectroscopy provides rapid insight concerning the functional groups present in samples of soil organic matter and crop residues. Interpretation of the infrared spectra led to these conclusions: In the particulate organic matter (POM) fraction, the aromatic C-H and aliphatic C-H were greater in the loess-derived than the till-derived soils. There was a considerable IR signal in the aromatic C-H region of clays, with the well-drained soils being less than the poorly drained soils. We found that it was imperative to treat soil with HF to concentrate OM and to improve the IR interpretations. We believe that there is potential for infrared spectroscopy to provide an economical quantification of soil organic matter components, as demonstrated by strong correlations of carbonyl-C and aromatic-C IR peak areas with NMR spectroscopy. Solid-state nuclear magnetic resonance spectroscopy provides the most definitive data, allowing us to conclude that OM in clay fraction of poorly drained soils had more aromatic C than that of well-drained soils. The quantitative NMR analyses allowed us to conclude: (1) that HF treatment (with or without heat) had little impact on the organic carbon functional groups in the samples, (2) that organic carbon was detectable even in untreated soil materials, (3) that the aromatic components of soil organic matter were enriched to ~52% in the poorly drained soils, compared with ~46% in the well-drained soils, and (4) that nonpolar, non-protonated aromatic carbon, interpreted as a proxy for charcoal carbon, dominated the aromatic carbon in all soil samples. It composed 69 - 78% of aromatic C and 27 - 36 % of total organic C in the unfractionated soil and clay-fraction samples.
Publications
- Fang, X., T. Chua, K. Schmidt-Rohr, and M.L. Thompson. 2009. Quantitative 13C NMR of whole and fractionated Iowa Mollisols for assessment of organic matter composition. Geochim. Cosmochim. Acta. In press.
- Laird, D.A., M.A. Chappell, D.A. Martens, R.L. Wershaw, and M.L. Thompson. 2008. Distinguishing black carbon from biogenic humic substances in soil clay fractions. Geoderma. 143:115-122
- Mao, J.D., X. Fang, K. Schmidt-Rohr, A.M. Carmo, L.S. Hundal, and M.L. Thompson. 2007. Molecular-scale heterogeneity of humic acid in particle-size fractions of two Iowa soils. Geoderma 140:17-29.
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Progress 09/01/07 to 08/31/08
Outputs
OUTPUTS: Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of SOM vary with drainage class and therefore with landscape position We studied four soils in central Iowa, two developed in till and two developed in loess. Two were well-drained and two were poorly drained soils. We treated the samples with hydrofluoric (HF) acid dissolve minerals and to concentrate the organic matter. We used solid-state nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (IR) spectroscopy, and thermal analysis to characterize the soil organic matter and to determine if soil at different landscape positions had different levels of recalcitrant components in the organic matter. PARTICIPANTS: Michael L. Thompson, Agronomy Department, and Klaus Schmidt-Rohr, Chemistry Department, Iowa State University. TARGET AUDIENCES: The target audience for this project is the scientific community, which is reached primarily through refereed journal articles. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The quantitative NMR analyses allowed us to conclude: (1) that HF treatment (with or without heat) had little impact on the organic carbon functional groups in the samples, (2) that organic carbon was detectable even in untreated soil materials, (3) that the aromatic components of soil organic matter were enriched to ~52% in the poorly drained soils, compared with ~46% in the well-drained soils, and (4) that nonpolar, non-protonated aromatic carbon, interpreted as a proxy for charcoal carbon, dominated the aromatic carbon in all soil samples. It composed 71 - 82% of aromatic C and 27 - 36 % of total organic C in the unfractionated soil and clay-fraction samples. Interpretation of the infrared spectra led to these conclusions: In the particulate organic matter (POM) fraction, the aromatic C-H and aliphatic C-H were greater in the loess-derived than the till-derived soils. There was a considerable IR signal in the aromatic C-H region of clays, with the well-drained soils being less than the poorly drained soils. We found that it was imperative to treat soil with HF to concentrate OM and to improve the IR interpretations. There is potential for infrared spectroscopy to provide an economical quantification of soil organic matter components, as demonstrated by strong correlations of carbonyl-C and aromatic-C IR peak areas with NMR spectroscopy. Thermal analyses were more challenging to interpret. Thermal analysis of the POM fractions reflected the nature of the crop residues that dominated POM at the time of sampling. Readily oxidizable compounds dominate, but it is likely that much hemicellulose (pectin) and cellulose had already been removed by decomposition, concentrating lignin in the POM. Our thermal characterization of model/reference compounds suggests that substantial portions of hemicellulose, protein, and lipids are lost over the temperature range from 110 to 350 degrees C. Whole SOM is enriched in difficult to oxidize compounds - probably because most charcoal particles are larger than clay size. In contrast to the NMR data, our thermal studies suggest that the well drained soils accumulate difficult-to-oxidize material. We are currently working to determine the reasons for the apparent conflict in conclusions derived from different analytical techniques.
Publications
- Chua, T., Fang, X., Schmidt-Rohr, K., and Thompson, M.L. 2008. Spectroscopic and Thermal Assessment of Organic Matter in Iowa Mollisols. Soil Sci. Soc. Am. Annual Meetings, Houston, TX, Oct., 2008. Program and Abstracts.
- Fang, X., Chua, T., Schmidt-Rohr, K., and Thompson, M.L. 2008. NMR Spectroscopic Assessment of Recalcitrant Soil Organic Matter in Iowa Mollisols. Soil Sci. Soc. Am. Annual Meetings, Houston, TX, Oct., 2008. Program and Abstracts.
- Laird, D.A., Chappell, M.A., Martens, D.A., Wershaw, R.L., and Thompson, M. 2008. Distinguishing black carbon from biogenic humic substances in soil clay fractions. Geoderma 143:115-122.
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Progress 09/01/06 to 08/31/07
Outputs
OUTPUTS: We have completed three-times-per-year sampling of aboveground and belowground biomass in four soils at each of two research locations in Iowa. We have fractionated organic matter from soils collected at those locations, and we have begun analytical work to characterize the chemical composition of the organic matter in both the soils and crop residues. Prelimiary results of this characterization are given in the outcomes section. These results were presented at the Soil Science Society of America meeting in November 2007.
PARTICIPANTS: Michael L. Thompson and Teresita Chua, Agronomy Department, Iowa State University. Klaus Schmidt-Rohr and Xiaowen Fang, Chemistry Department, Iowa State University
TARGET AUDIENCES: The target audience for this project is the scientific community, which is reached primarily through refereed journal articles.
Impacts
To better predict the long-term fate of soil organic carbon, we seek to understand the transformation of crop residue components into recalcitrant components of soil organic matter. We characterize both aboveground and belowground crop residues as well as soil organic matter by using Fourier-transform infrared spectroscopy and advanced solid-state nuclear magnetic resonance spectroscopy. These analytical approaches are supported by elemental analysis and thermal analysis. Using multiple techniques, we are testing the hypothesis that the accumulation of recalcitrant components in soil organic matter is regulated by landscape position and drainage class. We have found that differential thermogravimetry is a promising approach to obtain a simple, semi-quantitative estimate of the recalcitrant fraction of soil organic matter. Diffuse reflectance Fourier-transform infrared spectroscopy provides rapid insight concerning the functional groups present in samples of soil
organic matter and crop residues, but the spectra are challenging to interpret. We are now testing quantitative goodness-of-fit comparisons of simulated composites with real spectra to permit better interpretations. Solid-state nuclear magnetic resonance spectroscopy provides the most definitive data, allowing us to conclude that OM in clay fraction of poorly drained soils had more aromatic C than that of well-drained soils. The non-protonated aromatic C was 70-80% of the total aromatic C, suggesting an important char-like component in the organic matter of these soils.
Publications
- Thompson, M.L., Schmidt-Rohr, K., Chua, T. and Fang, X. 2007. Spectroscopic assessment of recalcitrant soil organic matter in Iowa Mollisols. Soil Science Society of America Annual Meeting. New Orleans, LA.
- Mao, J.D., Fang, X., Schmidt-Rohr, K., Carmo, A.M., Hundal, L.S., and Thompson, M.L. 2007. Molecular-scale heterogeneity of humic acid in particle-size fractions of two Iowa soils. Geoderma 140:17-29.
- Laird, D.A., Chappell, M.A., Martens, D.A., Wershaw, R.L., and Thompson, M.L. 2008. Distinguishing black carbon from biogenic humic substances in soil clay fractions. Geoderma (In press).
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Progress 09/01/05 to 09/01/06
Outputs
This project is directed toward improving knowledge of carbon dynamics in soils. The research (1) deals with biogeochemical cycling of carbon, (2) focuses on field and laboratory studies that can support modeling approaches, (3) employs innovative molecular-scale techniques, and (4) examines both the spatial and temporal variability of belowground processes related to carbon. We are focused on understanding the nature of recalcitrant (decomposition-resistant) soil organic matter (RSOM) in cultivated Mollisols. RSOM includes charcoal, lipids, lignin, and humic substances and is a critical component in biogeochemical models of carbon cycling. Yet little is known about its spatial distribution at the landscape scale. We are characterizing and quantifying RSOM as a function of landscape position in two classic suites of cultivated Mollisols that are developed in till and loess. At each landscape position, we measure the seasonal and annual rates at which recalcitrant
components in crop biomass are added to soil, and we characterize the spatial distribution of RSOM at the study sites at the hectare scale. The soils investigated are models of row-crop soils in the upper Midwest. Here, we report progress in the third objective of the project: documenting the landscape-scale distribution of organic C. When the analyses of RSOM are completed, we will be able to prorate the values and report a landscape-scale distribution of RSOM. To identify the optimal spatial scales for assessing carbon storage in soils, the uncertainty in estimating stocks of soil organic carbon at field management scales must be assessed. The objective of this study was to compare alternative approaches for estimating the stocks of soil organic carbon of agriculturally managed fields. We studied two fields (15 and 17 ha) in Iowa where soils were developed in Wisconsinan till and loess, respectively. Soils occurred on characteristic landscape positions in quintessential drainage
sequences, ranging from well to poorly drained. The fields were sampled in grid patterns at 125 or 143 sampling locations to a depth of 30 cm. Organic carbon content and bulk density were determined for each of three depth increments. The stocks of organic carbon were estimated in three ways: (1) by using a county-specific, expert database to predict soil characteristics, (2) by using classical statistical analyses, in which (a) the data from all sampling points were lumped or (b) in which the data from each mapping unit were stratified and then compared with those from other mapping units, and (3) by using geostatistical analysis, in which data for the entire field were analyzed by semivariograms and kriged maps of the organic carbon distribution. Each approach resulted in a different estimate of the total organic carbon stock of the field. Each approach also yielded a different estimate of the uncertainty in the carbon stock value. For both fields, the expert database approach gave
the largest estimate of carbon stocks, while block kriging gave the lowest estimate. The uncertainty associated with each estimate ranged from 1 to 20% of the estimated stock.
Impacts
To compare the total amount of carbon stored in agriculturally managed fields with one another, differen sampling schemes will give somewhat different "answers" to the question. Intensive sampling and analyses are required to make an estimate with a high degree of confidence.
Publications
- Thompson, M.L., Chua-Ona, T., Hutchison, J. and Wu, Y.-F. 2006. Estimating Carbon Stocks at the Field-Management Scale. World Congress of Soil Science, Philadelphia, PA, July 2006. Abstract 115-51. (http://crops.confex.com/crops/wc2006/techprogram/P17515.HTM)
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