Effects of Land Use Change on the Organic Carbon Fractions in an Arid Soil.

Due to the scarcity of arable land in arid areas, native rangelands are often converted to irrigated agricultural lands, but it is unclear how this land use change affects soil organic carbon (C) dynamics. Cropping and irrigation affect vegetation C inputs and soil moisture, both of which may have large effects on soil organic C dynamics. The main objective of this research is to evaluate the effects of conversion of arid rangelands to irrigated agriculture on organic C sequestration in soils. We are comparing soils under native shrubs with soils in an adjacent alfalfa field that has been tilled since 1956 near Reno, NV. The close proximity of the native and tilled areas ensures that soils were likely similar prior to the land use conversion. We used density and particle size fractionation techniques to quantify changes in organic C in the surface horizon. The soils at the study site also provided us with a unique opportunity to study ‘deep’ organic matter as both soils had buried A horizons. This will allow us to assess how land use change affects buried C. Initial results show that shrubland surface soils contain more organic C than the alfalfa surface soils. Furthermore, a larger fraction of the organic C in the alfalfa soil is contained in the occluded rather than free-floating fractions, suggesting that overall C in the alfalfa soils is more recalcitrant. These findings are supported by a laboratory incubation showing that shrubland soils have higher microbial respiration rates than alfalfa soils. We will further characterize different fractions using DRIFT spectroscopy. This research will aid in achieving a more complete understanding of the processes that regulate C dynamics in managed and unmanaged systems in (semi)arid regions, which can help us understand how soils will respond to land use change and a changing climate.