Evaluating nitrous oxide emissions in low input systems using different cover crop strategies over the winter period

Summary

The integration of cover crops (CCs) in low-input systems is a widely adopted practice to re-capture surplus nitrogen (N) and avoid excessive losses to the environment by leaching or N₂O emissions. Closing the N cycle within an agricultural system is therefore economically beneficial and lowers the negative impact of inorganic N on soil and water bodies. However, it is debated if pollution swapping occurs to some extent and if N₂O emissions increases as a result of decreased N leaching. An experiment was conducted to systematically evaluate grass vs. non-grass CCs, frost tolerant vs. non-frost tolerant CCs as well as high residual N vs. low residual N from the pre-crop, in a low input system which receives no additional fertilizer. Furthermore, the extent to which N₂O losses occur from different seeded CC species and mixtures (Sinapis alba/Vicia sativa, Brassica rapa/Vicia villosa and Lolium perenne/Trifolium repens) was investigated over two experimental years on a sandy soil located in northern Germany. The annual N₂O emissions were investigated on a weekly basis using the static closed chamber method. The non-grass CCs had the highest annual N₂O emissions (2.5 kg N₂O-N ha⁻¹) compared to grass (1.9 kg N₂O-N ha⁻¹). The frost-killed CC led to higher N₂O emissions (3.3 kg N₂O-N ha⁻¹), especially after the first year when high residual N was still present due to the pre-crop. This indicates that the type of CC used, frost tolerance as well as residual N from the pre-crop will affect N₂O emissions. The low N₂O emissions observed in the current study indicate that pollution swapping as a result of avoided N leached due to the use of CCs, as shown in a previous paper from the same experiment, is not occurring under these low-input systems. Furthermore, grass used as CC has low N₂O emissions and a high N uptake and therefore the potential to close the N cycle and improve on-farm N budgets.