Rye Cover Crop Effects on Direct and Indirect Nitrous Oxide Emissions

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Core Ideas A rye cover crop in a Midwest corn–soybean rotation did not reduce direct N2O emissions. NO3 leaching and indirect N2O emissions were reduced with a winter rye cover crop. Large year‐to‐year variations in precipitation were a major determinant of N2O losses. Winter cover crops can have a pronounced effect on N cycling in agricultural ecosystems. By reducing available soil mineral N during active growth and by providing a substrate for denitrifying bacteria after they are killed, cover crops can potentially influence soil N2O emissions. However, there have been conflicting reports regarding cover crop effects on increasing or decreasing N2O emissions. In this study, direct soil emissions of N2O were measured for a 10‐yr period in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation, with and without a winter rye (Secale cereale L.) cover crop. Additionally, NO3 leaching losses during the period were measured and used to estimate indirect N2O emissions by applying the Intergovernmental Panel on Climate Change (IPCC) EF5 emission factor of 0.0075 kg N2O‐N kg−1 NO3–N leached. Direct N2O emissions measurements were measured using vented static chambers from March 2004 through December 2012. Annual N2O emissions in corn years were significantly greater than emissions in soybean years (P = 0.013). Under corn annual direct N2O emissions ranged from 2.3 to 12.5 kg N2O‐N ha−1 yr−1, and under soybean annual emissions ranged from 1.02 to 6.94 kg N2O‐N ha−1 yr−1. Cumulative direct N2O emissions were calculated each year by numerical integration and summed to compute 10‐yr cumulative emissions for the rotation. Total direct emissions during the 10‐yr period in the rye and no‐rye treatments were not significantly different. The rye cover crop did effect estimated indirect emissions (P = 0.046), which were 1.25 kg N2O‐N ha−1 yr−1 in the rye treatment and 2.69 kg N2O‐N ha−1 yr−1 in the no‐rye treatment. There was no significant difference (P = 0.155) between 10‐yr cumulative total N2O emissions (direct + indirect) of the rye cover crop treatment (55.7 kg N2O‐N ha−1) and the no cover crop treatment (63.8 kg N2O‐N ha−1).