Progress 07/01/14 to 06/30/17
Outputs
Target Audience:Our main target audience reached includes the scientific community and undergraduate students. The sceintific community was reached by presenting at professional meetings, and writing scientific papers. Undergraduate students were reached through formal teaching of courses and by having undergraduate students participate in ongoing research. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?The project provided funding for one graduate student and several undergraduate students. One undergraduate student conducted an independendent research project that resulted in a journal article that is currently in review. The graduate student continues to work in the soil science field. How have the results been disseminated to communities of interest?Our results have primarily been disseminated through presentations at professional meetings and peer-reviewed publications. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Impact statement Soils in semi-arid rangelands can contain substantial amounts of carbon (C) as soil organic matter. Many rangelands have been converted to irrigated agriculture to meet growing the demand for food production but it is unclear what the fate of organic C is upon conversion. This study compared soil organic carbon (SOC) dynamics in an alfalfa field that has been under irrigation for more than five decades with an adjacent unmanaged shrubland at the University of Nevada, Reno Main Station Field Laboratory on the eastern boundary of Reno, Nevada. Particle size and density fractionations of surface soils (0-10 cm) were performed to assess quantitative changes in quantity and quality of SOC following land-use change. Additionally, a buried A horizon (90-100 cm) was sampled in both sites to determine whether decomposition at depth is regulated by environmental or (bio)chemical factors. An eight-week laboratory incubation at constant temperature and moisture levels was performed to assess the decomposability of organic matter in each soil type. To assess changes in soil CO2 dynamics between the unmanaged shrubland and irrigated cropland, surface soil CO2 efflux and soil CO2 concentrations at 10, 30 and 75cm depths were measured every month over the 12-month duration of the study. The conversion to irrigated cropland reduced the amount of labile C in the soil and increased the amount of recalcitrant soil C compared to the shrubland soils most likely as a result of more favorable conditions for decomposition under irrigation. The lab incubation showed that, under optimal environmental conditions, microbial respiration was lower in the alfalfa field soils than in the shrubland soils reflecting the larger amount of recalcitrant C present in the alfalfa soils. Still, increased belowground biological activity in the cropland due to the elimination of water stress through irrigation resulted in larger soil CO2 concentrations and effluxes, especially during the growing season despite shrubland soils having larger amounts of labile C. Data from the buried horizons showed that these horizons contained more recalcitrant SOC than the surface soils and differences between the land use types were smaller than those for the surface soils. Still, the land use change had some effect on SOC dynamics even at this depth despite these buried horizons having formed when vegetation was most likely similar between alfalfa field and shrublands. Overall our data indicate that land-use conversion may result in significant soil C losses from semi-arid soils through decomposition of labile C sources that accumulate under shrub vegetation. Despite more stable C being present in the alfalfa field, the more favorable conditions for biological activity will increase CO2 emissions from these managed lands. Main Objective Throughout previous decades, land-use change has contributed to rising atmospheric carbon dioxide (CO2) concentrations by reducing carbon (C) storage and increasing C emissions from previously natural ecosystems. Due to the scarcity of arable land, semi-arid rangelands are often converted to irrigated croplands, which is likely to have a large effect on soil organic carbon (SOC) due to changes in C inputs to the soil as well as environmental factors regulating decomposition. The objective of this study was to quantify the long-term effects of converting an unmanaged semi-arid shrubland into irrigated agricultural land on SOC dynamics. We sampled surface soils (0-10 cm) in a native rangeland dominated by rubber rabbitbrush (Ericameria nauseosa) and yellow rabbitbrush (Chrysothamnus viscidiflorus) with an adjacent alfalfa (Medicago sativa) field that has been under irrigated agriculture for at least five decades near Reno, NV. Additionally, a buried organic matter-rich, A horizon (90-100 cm) was sampled in both sites to determine if decomposition at depth is regulated by environmental or (bio)chemical factors. We conducted particle size and density fractionations and each fraction was analyzed for δ13C, δ15N, percent C and percent nitrogen (N). Above- and belowground vegetation samples were also analyzed for percent C and N, δ13C and δ15N analysis to compare these with the various soil fractions. An eight-week laboratory incubation at constant temperature and moisture levels was performed to assess the decomposability of organic matter in each soil type. To assess changes in soil CO2 dynamics between the unmanaged shrubland and irrigated cropland, surface soil CO2 efflux and soil CO2 concentrations at 10, 30 and 75 cm depths were measured every month over the 12-month duration of the study. Soil CO2 collected from each soil depth was also analyzed for δ13C isotopic composition to determine the dominant biotic and abiotic sources of CO2 in each soil type. Carbon and nitrogen (N) analysis of particle size and density fractions revealed that irrigation and management significantly reduced the total amount of C and N in the soil. In addition, the total amount of C contained in the labile fractions derived from both the particle size and density fractionations was significantly smaller in the alfalfa field than in the shrubland. As a result, the relative amount of C contained in recalcitrant fractions was higher in the alfalfa field. The differences in δ13C values of the soil organic matter reflected differences between dominant vegetation types, but these differences were only significant for density fractions and not for the particle size fractions. The density fractionation data showed that alfalfa-derived C was present in the stable fractions despite these fractions having a supposedly long turnover time. Both fractionation methods revealed differences in δ15N values between soil types, reflecting differences in vegetation. The laboratory incubation showed that the shrubland soil had a higher potential decomposition rate than the alfalfa field soil, most likely due to the larger amount of labile vs. stable C in the shrubland soil. Water limitations likely allowed for greater accumulation of labile C in the shrubland soil. Decomposition in the subsoil (90-100 cm) of each site was limited by substrate quality rather than environmental conditions. This lower decomposability was supported by the fractionation data showing that the relative amount of stable C was much higher in the buried A horizons than in the surface horizons and differences between the land use types were smaller than those for the surface soils. Still, the land use change had some effect on SOC dynamics even at this depth despite these buried horizons having formed when vegetation was most likely similar between alfalfa field and shrublands. The conversion from a semi-arid shrubland to irrigated cropland also resulted in larger soil CO2 concentrations and effluxes, especially during the growing season. The higher CO2 production was most likely caused by more favorable moisture conditions in the irrigated cropland despite cropland soils having smaller amounts of labile C. It was unclear however how much of the respired CO2 originated from autotrophic (root) versus heterotrophic (microbial) respiration but δ13C of the soil CO2 values were consistent with differences in δ13C isotopic values for the soil organic matter (SOM) and vegetation at each study site. The isotope data further indicated that inorganic C did not contribute to belowground CO2 production. Overall our study showed that land-use conversion may result in significant soil C losses from semi-arid soils through decomposition of labile C sources that accumulate under shrub vegetation. Despite more stable C being present in the alfalfa field, the more favorable conditions for biological activity will increase CO2 emissions from these managed lands.
Publications
- Type:
Journal Articles
Status:
Under Review
Year Published:
2017
Citation:
Ketchian E., Trimble B., Poulson S., Verburg P.S.J. Determination of the concentration and isotopic composition of soil C in arid soils. Soil Science Society of America Journal
- Type:
Journal Articles
Status:
Under Review
Year Published:
2017
Citation:
Trimble B., Poulson S., Verburg P.S.J. Conversion of semi-arid rangelands to irrigated agriculture significantly alters quantity and quality of soil organic matter. Journal of Environmental Quality.
- Type:
Conference Papers and Presentations
Status:
Accepted
Year Published:
2016
Citation:
Verburg P.S.J., Ketchian E., Trimble B., Poulson S.R. 2016. Assessment of two methods for determination of quantity and isotopic composition of soil C in arid soils. Soil Science Society of America Annual Meeting. Phoenix, AZ.
- Type:
Conference Papers and Presentations
Status:
Accepted
Year Published:
2016
Citation:
Trimble B., Verburg P.S.J., Poulson S.R. 2016. Effects of Land Use Change on the Organic C Fractions in a Semi-Arid Soil. Soil Science Society of America Annual Meeting. Phoenix, AZ.
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Progress 10/01/15 to 09/30/16
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?The project employed one graduate student and several undergraduate students throughout the year. One undergraduate student worked on the project as part of a senior thesis. Her work was presented at an international meeting in November 2016 and a manuscript is being developed for submission to an international peer-reviewed journal. How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?The field and laboratory work has been finalized and all data are being analyzed. We initiated a collaboration with the USDA-ARS to conduct advanced chemical analyses of all our samples to identify differences in chemistry between sites and organic matter fractions. The graduate student is finalizing her thesis and she is preparin two manuscripts for submission to international peer-reviewed journals.
Impacts
What was accomplished under these goals?
The main focus of the project is to assess the impacts of long-term management on soil properties and processes emphasizing C dynamics. To meet our goals we established 10 permanent plots in a location dominated by rabbitbrush (five plots under plant canopies and five plots in canopy interspace areas) while a second set of five plots was established in an adjacent alfalfa field. We continuously measured soil temperature, moisture and salinity at 10, 30 and 75 cm depth at each site. In addition, we measured soil CO2 efflux and soil CO2 concentrations at 10, 30 and 75 cm depths on a monthly basis in each plot. We conducted an intensive soil sampling of surface soils and measured soil texture, total C and N as well as soil chemical and physical characteristics. In addition, we conducted a particle size fractionation to measure organic C associated with clay, silt and sand fractions. We also conducted a density fractionation separating out organic C associated with heavy and light soil fractions. Most field and laboratory measurements have been completed and we are currently focusing on data analysis. A preliminary data analysis shows that soil CO2 efflux is higher in the alfalfa field compared to the native vegetation. Soil CO2 concentrations also tend to be higher in the alfalfa field supporting the CO2 efflux data. The soil CO2 samples are currently being analyzed for C-isotopic composition to determine the source (i.e. root vs. organic matter) of the CO2. In contrast, the fractionation data indicate that the total organic C content is higher in the rabbitbrush field and more organic C and N is contained in the labile fractions (sand and particulate organic matter fraction or light fraction) than in the alfalfa field. As a result, the higher soil CO2 efflux rates in the alfalfa field are most likely due to a larger contribution of root respiration to the total soil CO2 efflux of alfalfa compared to rabbitbrush rather than higher decomposition rates of soil organic matter. We incubated surface soil samples and measured microbial activity as a measure of decomposability of organic matter. Decomposability of the organic matter was higher in the rabbitbrush than in the alfalfa soils supporting the fractionation data that indicated that the rabbitbrush soils contain more labile organic matter. To further confirm this observation we conducted Fourier-transformed Infra-Red (FTIR) spectroscopy measurements of the different soil fractions and spectroscopy data are currently being analyzed. Two soil pits were dug in the native vegetation and alfalfa field revealing presence of a buried A horizon at approximately 1 m depth in the profile. We sampled these horizons to determine if organic matter quality differs from that in surface horizons. Microbial activity was much lower in the buried A horizons compared to the surface horizons indicating that the organic matter was more recalcitrant. Organic matter fractionation also showed that more C and N was contained in stable fractions compared to the surface horizons. Our results to date show that conversion of native vegetation to alfalfa will lower soil organic matter content and will decrease the decomposability of the organic matter. Overall belowground biological activity increased when native vegetation was converted to alfalfa but this increase was most likely primarily due to increased root respiration of the crops rather than microbial decomposition of organic matter. As part of this project we also tested different methods used for differentiating between soil organic and inorganic, carbonate-derived C. The results showed that most methods can produce erroneous results both in terms of amounts of C as well as the isotopic composition depending on the relative amounts of organic vs. inorganic C present in the soil.
Publications
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Progress 10/01/14 to 09/30/15
Outputs
Target Audience:
Nothing Reported
Changes/Problems:We had to find a new field site after the projected site turned out to be unsuitable for our study. For our managed site we were relying on having active crop production. The drought in Nevada caused irrigation to be absent and as a result no crops could be grown. We found a new site that was actually closer to the university where active cropping with alfalfa was ongoing. What opportunities for training and professional development has the project provided?A graduate student is working on this project as part of her thesis. Similar to last year, my lab and the lab of co-PI Leger had weekly joined lab meeting to discuss scientific papers about plant-soil interactions, experimental design as well as professional ethics including paper authorship and reviewing papers and proposals. How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?We will continue to analyze samples taken from the field site and conduct density fractionation of soil organic matter. In addition, we will assess how these fractions correlate with fractionation based on particle size. Individual soil organic matter fractions based on density and size fractionation will be chemically characterized using Fourier-Transformed Infra Red (FTIR) spectroscopy to determine how management regimes affect chemical composition of different soil organic matter fractions. In addition, we will measure stable carbon isotopic composition of each individual fraction. We will also sample excavate a soil pit in the alfalfa field and conduct a sampling with depth of this profile. Finally, we will measure isotopic composition of soil CO2 and soil-respired CO2 to assess the potential contribution of inorganic C to soil CO2 production.
Impacts
What was accomplished under these goals?
The main focus of the project is to assess the impacts of long-term management on soil properties and processes emphasizing C dynamics. To accomplish our goals we were looking for sites where native vegetation was present on similar landforms as fields that had been under long-term irrigation. The first year we had challenges finding a suitable field site for our project. We had identified a site at the Rafter 7 Ranch in the Walker basin and conducted an intensive sampling during the spring of 2015. However, for our project we had to rely on the presence of irrigated crops and the lack of water available for irrigation prevented us from working at the site. We ended up moving our project to Main Station Farm. During the spring we conducted a soil survey to find appropriate locations at Main Station Farm and we finalized field selection and instrumentation installation during the summer. We established 10 permanent plots in a location dominated by rabbitbrush (five plots under plant canopies and five plots in canopy interspace areas) while a second set of five plots were established in an adjacent alfalfa field. We installed sensors for continuous measurement of soil temperature, moisture and salinity at 10, 30 and 75 cm depth. In addition, we installed PVC rings for soil CO2 efflux measurements and gas wells at 10, 30 and 75 cm depths for soil CO2 measurements. Soil CO2 efflux and soil CO2 concentrations are measured manually on a monthly basis. We conducted an intensive soil sampling of surface soils and measured soil texture, total C and N and other soil chemical and physical characteristics. In addition, we conducted a density fractionation separating out organic C associated with heavy and light soil fractions. Preliminary data suggest textural differences between rabbitbrush and alfalfa field as well as differences in organic matter fractions. In addition, soil CO2 efflux is higher in the alfalfa field compared to the native vegetation. Two soil pits were dug in the native vegetation showing presence of a buried A horizon in the profile. We sampled these horizons to determine if organic matter quality differs from that in surface horizons.
Publications
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Progress 07/01/14 to 09/30/14
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? We have recruited a graduate student who started at the end of August. She has taken courses that help her prepare for coonducting the research as proposed in this project. In addition, my lab and the lab of co-PI Leger had weekly joined lab meeting to discuss scientific papers about plant-soil interactions, experimental design as well as professional ethics including paper authorship and reviewing papers and proposals. How have the results been disseminated to communities of interest? We presented our proposed research to representatives of the National Fish and Wildlife Foundation who own the property where the research is conducted. What do you plan to do during the next reporting period to accomplish the goals? We will finalize site selection and start installing equipment as well as take samples for lab analyses during the spring and summer of 2015. We will work closely with the site managers to keep them up to date on our progress and participate in any tours of the site. The site is used as a demonstration site for vegetation restoration work conducted in western Neavda and as a result receives many visits from the general public, farmers and ranchers, and other land managers. We will use these visits as an opportunity to demonstrate our research and disseminate our results to wider audiences. We will continue to hold joint lab meetings to engage the graduate students and help them develop their research skills.
Impacts
What was accomplished under these goals?
We have identified potential research sites to conduct our measurements using existing soil maps. We have interacted with site managers to help select research plots for destructive sampling and installation of soil moisture sensors and gas wells.
Publications
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