Rye Cover Crop Effects on Direct and Indirect Nitrous Oxide Emissions

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).