Cover Crop Effects on Nitrous Oxide Emissions: Role of Mineralizable Carbon

Nitrous oxide (N2O) emission from denitrification in agricultural soils often increases with N fertilizer and soil nitrate (NO3) concentrations. Overwintering cover crops in cereal rotations can decrease soil NO3 concentrations and may decrease N2O emissions. However, mineralizable C availability can be a more important control on N2O emission than NO3 concentration in fertilized soils, and cover crop residue provides mineralizable C input. We measured the effect of a winter rye (Secale cereale L.) cover crop on soil N2O emissions from a maize (Zea mays L.) cropping system treated with banded N fertilizer at three rates (0, 135, and 225 kg N ha(-1)) in Iowa. In addition, we conducted laboratory incubations to determine if potential N2O emissions were limited by mineralizable C or NO3 at these N rates. The rye cover crop decreased soil NO3 concentrations at all N rates. Although the cover crop decreased N2O emissions when no N fertilizer was applied, it increased N2O emissions at an N rate near the economic optimum. In laboratory incubations, N2O emissions from soils from fertilizer bands did not increase with added NO3, but did increase with added glucose. These results show that mineralizable C availability can control N2O emissions, indicating that C from cover crop residue increased N2O emissions from fertilizer band soils in the field. Mineralizable C availability should be considered in future evaluations of cover crop effects on N2O emissions, especially as cover crops are evaluated as a strategy to mitigate agricultural greenhouse gas emissions.