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.