Emissions of N2O following field incorporation of leguminous and non-leguminous cover crops

Cover crop (CC) cultivation can reduce N leaching from agricultural fields. It has also recently been proposed as a measure to mitigate climate change due to its potential for increasing soil carbon (C) stocks and reducing soil nitrous oxide (N₂O) emissions during CC growth. However, the input of immature, low C/N ratio plant material to the soil can increase the risk of N₂O emission upon CC termination. In a field trial under humid temperate conditions, N₂O emissions from two leguminous CC species – hairy vetch (Vicia villosa) and crimson clover (Trifolium incarnatum) – and from two non-leguminous CC species – oilseed radish (Raphanus sativus) and winter rye (Secale cereale) – were measured in the spring from 24 days before to 54 days after incorporation. The vetch had a significantly higher shoot N concentration and consequently lower C/N ratio than the other three species, but this did not increase N₂O emissions relative to the fallow control. Indeed, oilseed radish was the only cover crop to significantly increase N₂O emissions by 290 g N₂O-N ha⁻¹ during the measurement period relative to the fallow control. These results suggest that factors other than the shoot C/N ratio, such as total biomass, N content or other biochemical parameters may be of greater importance for predicting N₂O emissions from immature cover crop residues.