Effects of regenerative agricultural measures on soil nitrous oxide emissions in organic farming

Agricultural soils represent a significant source of greenhouse gas (GHG) emissions at the global scale, with nitrous oxide (N₂O) playing a major role. Promoting more sustainable agricultural practices is not only essential for climate change adaptation, but also for mitigating emissions from agriculturally used soils. Several management measures may contribute to both mitigation and adaptation, including long-term low-intensity tillage systems. When combined with other management measures associated with regenerative agriculture, such as intensive intercropping and undersowing, compost and mulch applications, and the use of biostimulants, their effects on N₂O emissions are poorly understood. The objective of this study was to assess the impact of these management practices on soil N₂O emissions.

Since 2010, the factors tillage, compost, and mulching, and since 2020 the factor vitalization (compost tea and ferments), have been implemented in a split-plot design with fourfold repetition in an organically managed long-term field experiment on a Luvisol in central Germany. Five treatments ranging from conventional ploughing to progressively intensified regenerative management (reduced tillage, reduced tillage with compost, reduced tillage with compost and mulch, and reduced tillage with compost, mulch, and vitalization) were selected for weekly N₂O flux measurements (closed static chamber method). Emissions were monitored from October 2021 to October 2023, covering winter intercrop vetch-triticale, potato cultivation, and a winter wheat-pea mixture. During potato cropping, reduced tillage plots were tilled to a depth of 0.12 m prior to planting, and mulching was applied as dead mulch (green rye, C:N ratio of 39:1). Living mulch was established in the winter wheat-pea mixture as an undersown crop (clover and ryegrass).

Cumulative emissions (728 days) were 9.28 (standard error: ±0.78) kg N₂O-N ha^-1 in the ploughed control treatment, whereas reduced tillage without additional factor combination resulted in slightly lower emissions of 8.06 (±0.53) kg N₂O-N ha^-1. Compost application with reduced tillage promoted slightly higher emissions compared to the ploughed control with 10.20 (±0.75) kg N₂O-N ha^-1. However, the combination of reduced tillage, compost, and mulching significantly reduced emissions compared to reduced tillage with compost alone, resulting in 6.90 (±0.35) kg N₂O-N ha^-1.  The factor vitalization on top of the reduced tillage, compost, and mulching treatment showed no further effect, with 7.17 (±0.79) kg N₂O-N ha^-1. Mulching contributed to the emission reduction through the combined effects of dead and living mulch: dead mulch effectively damped soil temperatures, reducing heat stress during the warm, dry summer of 2022, which likely enhanced nitrogen (N) uptake by the potatoes, in combination with nitrogen immobilization due to its high C:N ratio, thereby reducing a high post-harvest N₂O emission peak observed across all treatments. In the winter wheat-pea mixture, two consecutive emission peaks occurred post-harvest; these were mitigated by the enhanced growth of the undersown living mulch, likely resulting in N uptake during this critical period. Our findings indicate that mulching can overcompensate the tendency of long-term compost applications to increase N₂O emissions when both management measures are combined.