Impacts of Agricultural Management and Landscape Factors on Soil Carbon and Nitrogen Cycling

Agricultural management has altered soil carbon (C) and nitrogen (N) inputs, losses, and turnover rates. Understanding how management interacts with landscape factors to regulate soil C and N losses is essential to addressing climate change. Through research conducted in agricultural systems in Minnesota I investigated: (1) how the loss of corn root-derived C as carbon dioxide (CO2), and N as nitrous oxide (N2O) differed among five management systems, and (2) how hillslope position and soil moisture affected the size and turnover of soil C pools.;In a field study using stable isotope techniques, I found that the fraction of root-derived C and N emitted as CO2 and N2O, the C and N emission factors, were 35% and less than 1% respectively. Individually, each emission factor was lower in systems with increased rotation diversity. Conversely, the relationship between C and N emission factors differed with tillage and fertilization intensity, not with rotation diversity. The magnitude of root-derived C and N emission factors has agricultural policy implications. Currently an emission factor of 1% is used for all N inputs to agricultural systems. My research suggests that a lower emission factor would better reflect N2O emissions from belowground N sources.;In a laboratory study, both position and soil moisture significantly impacted the size and mean residence time of soil C pools along a low slope hillslope. Intact core sections of the upper four horizons from three hillslope positions were incubated at 50, 75, 90 and 100% water-filled pore space (WFPS) for 355 days. Total soil C (TC), N, and the resistant fraction of TC (64%) increased downslope. Under saturated conditions, 100% WFPS treatment, the size and mean residence time of the labile C fraction (<1% of TC) increased. Increased moisture, between 50% and 90% WFPS, also lengthened the mean residence time of slow C. In this low slope landscape I found effects of both position and moisture on C pool dynamics; soil moisture had the most significant impacts on labile C pool size and the slow C pool mean residence time.