The effect of post-harvest cover crop management on N2O emissions

Agricultural soils are the primary source of nitrous oxide (N₂O) emissions into the atmosphere, contributing 78% of total N₂O emissions. These emissions are influenced by different tillage practices and types of plant cover that are left after harvest. Cover crops (CC) are widely used in agriculture to take up excess nitrogen from the fields, thereby reducing nitrate leaching and increasing soil carbon accrual. However, despite these benefits, agricultural soils containing cover crops have often resulted in high N₂O emissions.

A two-year field study with measurement of N₂O emissions was conducted in Denmark using a long-term conservation agriculture experiment, including cover cropping, no tillage and crop rotation. The study focused on the influence of soil tillage and cover cropping on soil physical properties and N₂O emissions. The tillage systems included no tillage (direct seeding) and conventional ploughing; CC management included paired subplots with oil-seed radish (Raphanus sativus L.), where (i) the cover crop residues were terminated and removed in autumn (CC-rem), and (ii) the cover crop residues were killed by the frost and left in the field (CC-left). Bare soil treatment (i.e., without CC) is included as a reference. Spring oats (Avena sativa L.) grew as the main crop during the first year followed by spring barley (Hordeum vulgare L.) in the second year.

The first-year results for N₂O fluxes showed that there were no significant differences in N₂O emissions between the tillage practices. However, emissions varied significantly among CC treatments. Compared to the reference without CC, peak emissions (up to 74 µg N₂O-N m^-2 h^-1) were observed for both cover crop treatments. During the cropping season, most of the emissions occurred after fertilization. Especially, +CC-left emitted more N₂O than CC-rem during the main cropping season. Before establishing the main crop, CC-rem emitted more N₂O than CC-left. Volume-effective porosity, air permeability, bulk density and gas diffusivity are critical soil physical properties that influenced N₂O emissions among the cover crop treatments.