N Loss to Drainage and N2O Emissions from a Corn-Soybean Rotation with Winter Rye in Central Iowa As Simulated By RZWQM.

Cover crops are a viable management practice to reduce N loss to drain flow from agricultural land. Using the IPCC default emission factor has been reported to poorly estimate N2O flux, including in poorly drained U.S. Midwest soils, which suggests the need for process-based approaches under these conditions. Further, limited studies are available that simultaneously investigate NO₃ losses to subsurface drain flow and N₂O emissions. In this study, we used RZWQM to evaluate NO3 losses to drain flow and N₂O emissions in a corn-soybean system with a winter rye cover crop (CC) in central Iowa. Overall, the model results compared well to field observations for winter rye growth and N uptake, N loss to drain flow, and N2O flux. The model correctly simulated a decrease in average annual nitrate concentration in drain flow differences between NCC and CC differences over the study period (2002-2010) of approximately 1 mg N/L/yr, mostly because of increased precipitation. Some of the difference between simulated and observed N2O flux could be partly because of simulated peaks occurring between measurement periods. In contrast to previous research, monthly simulated N2O flux was generally greatest when N loss to leaching was greatest during corn years, most likely because relatively high rainfall occurred during the months' fertilizer was applied. The results suggest that RZWQM is a promising tool to estimate N2O emissions from subsurface drained corn-soybean rotations and to estimate the relative effects of a winter rye cover crop over different precipitation conditions on nitrate loss to drain flow.