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The Effects of Land Management on Organic Matter Dynamics in a Semi-Arid Nevada Soil
AuthorTrimble, Brittany R.
AdvisorVerburg, Paul S.
Natural Resources and Environmental Science
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Land-use change has significantly contributed to rising global atmospheric carbon dioxide (CO2) concentrations by reducing carbon (C) storage and increasing C emissions from soils. Soils represent the second largest C pool on Earth, with drylands comprising approximately 21% of the globe’s soil organic carbon (SOC). While research regarding the effects of land-use change on SOC in more mesic regions has typically shown an overall reduction in SOC, it is relatively unclear how the land use change from native vegetation to irrigated cropland will affect SOC dynamics in semi-arid regions. Surface soils (0-10 cm) and subsoils (90-100 cm) of an alfalfa field that has been under irrigation for more than five decades, and of an adjacent unmanaged shrubland were collected at the University of Nevada, Reno Main Station Field Laboratory on the eastern boundary of Reno, Nevada. Soils were fractionated using particle size and density fractionation methods and each fraction was analyzed for C, nitrogen (N) content and C and N isotopic composition. Soil CO2 concentrations and effluxes were measured monthly in the same sites for the 12-month duration of the study.Carbon and N analysis of particle size and density fractions revealed that irrigation and management significantly reduced the amount of C and N in the soil. The amount of C in the labile fractions from both the particle size fractionation and density fractionation was significantly smaller and the relative amount of C in recalcitrant fractions was larger in the alfalfa field compared to the native vegetation. The differences in δ13C values of both stable and labile soil organic matter reflected differences between dominant vegetation types, but these differences were only significant for density fractions. Both fractionation methods revealed differences in δ15N values between soil types, again reflecting differences in vegetation. An eight-week laboratory incubation at constant temperature and water content revealed that the shrubland soil had a higher potential rate of decomposition than the alfalfa field soil, even though alfalfa SOM had a lower C/N ratio, likely as a result of water limitations at the field site allowing for greater accumulation of labile C in the shrubland soil. Additionally, decomposition of organic matter in the buried A horizon from each site was limited by substrate quality rather than environmental conditions. Land conversion to irrigated agriculture resulted in larger soil CO2 concentrations and effluxes, especially during the growing season. This was true despite shrubland soils having larger amounts of labile C available for decomposition. The source of respired CO2 for each soil type remains unclear, though CO2 δ13C values reflected differences in δ13C isotopic values for the SOM and vegetation between the two sites. The results from this study suggest that converting a semi-arid shrubland into irrigated cropland may cause an overall loss of SOC that can contribute to rising atmospheric CO2 levels, though the relative amounts of recalcitrant C may increase in semi-arid soils following management.