Species specific effects of cover crops on nitrous oxide emissions in hemiboreal cereal production: The importance of freeze-thaw driven emissions

Including cover crops in cereal production systems has the potential to reduce greenhouse gas (GHG) emissions from agriculture by sequestering carbon (C) in soils. However, growing crops during the off-season interferes with cold season soil nitrogen (N) cycling, with uncertain effects on the emission of nitrous oxide (N2O), the dominant anthropogenic GHG in crop production. In this study, we investigated N2O emissions in barley production systems with seven different cover crops over two years and modelled soil organic C (SOC) accrual based on biomass production to calculate species-specific GHG balances. We measured N2O emission rates weekly using manual chambers during the cropping season and more frequently during the off-season with a field flux robot. Off-season emissions exceeded those from the growing season, accounting for 67 % (+/- 14 % SD) of annual emissions. Among the cover crops, oilseed radish had the largest N2O emissions and the largest off-season contribution, whereas a herb mixture and ryegrasses (Lolium perenne and Lolium multiflorum) emitted less N2O than barley grown without cover crops. The largest emissions were observed during diurnal freeze-thaw cycles and spring thaw. Based on the estimated GHG balances (measured N2O emissions vs. modelled SOC accrual), we conclude that winter-hardy cover crops have the potential to reduce overall GHG emissions. In contrast, frostsensitive cover crops support large N2O emissions during winter, undermining the mitigation effect of SOC accrual. Our findings emphasise the importance of including off-season N2O emissions in C farming schemes that use cover crops in temperate and hemiboreal cereal production systems.