Nitrogen sources and sinks in Iowa soils: biogeochemical links between carbon inputs, nitrate leaching, and nitrous oxide emissions
Nitrogen (N) lost from agricultural soils in the forms of nitrate (NO3) and nitrous oxide (N2O) have become major environmental concerns. Because N cycling is coupled with organic carbon (C) cycling, management practices that influence soil organic C inputs and cycling may affect reactive N losses. Management practices have been proposed to reduce N losses, including perennial vegetation buffers (PVB) and overwintering non-legume cover crops. However, the effects of these practices on N losses depend on the biogeochemical interactions between soil N and C cycling. This thesis presents investigations of the effects of these management practices on NO3 and N2O losses from row crop systems in Iowa, USA. In PVBs, soil organic matter and plant biomass acted as sinks for NO3 inputs. However, denitrification, stimulated by organic C inputs from perennial vegetation, appeared to be the most important NO3 sink. These results indicate that integration of perennial vegetation into agricultural landscapes can return substantial amounts of N to the atmosphere and decrease watershed NO3 losses in the long term. The effects of cover crops on N2O emissions were found to vary with N fertilizer rate, and cover crops increased N2O emissions at an economical N rate. These results indicate that overwintering cover crops should not be expected to consistently decrease N2O emissions from agricultural soils, even when they do decrease NO3 availability for denitrification. In row crop systems with PVBs and cover crops, mineralizable C inputs to soils are a key factor influencing the biogeochemical N transformations that lead to N retention or losses. Considering the interactions between C and N cycling in agricultural soils is necessary to understand and predict the effects of management practices on environmental N losses.