Drivers of nitrous oxide fluxes in winter wheat

Nitrous oxide (N₂O) is an important greenhouse gas (GHG) and ozone-depleting substance. The agricultural sector is the predominant anthropogenic source of N₂O, primarily due to the use of nitrogen (N) fertilizers. Thus, policies are being discussed to reduce N₂O emissions across Europe. However, the scarcity of high-resolution N₂O flux data hinders our understanding of the mechanisms driving N₂O emissions, and the development of effective mitigation strategies.

In this study, we measured high-resolution (10 Hz) N₂O concentration over the duration of a winter wheat cropping season and calculated half-hourly N₂O fluxes using eddy covariance. Our objective was to disentangle the roles of management practices, abiotic conditions, and biotic factors affecting N₂O fluxes and to track how their respective contributions change over time. Using a random forest model trained with management, environmental, and vegetation data, we applied SHAP (SHapley Additive exPlanations) analyses to investigate the drivers of N₂O fluxes.

As expected, N fertilization and soil moisture emerged as the main drivers with the largest contributions to the N₂O fluxes. Moreover, the net ecosystem exchange of CO₂ (NEE) was the third most important driver, highlighting the critical role of plant-microbe competition for soil N. N₂O fluxes indeed peaked during periods of low crop growth, when plant N uptake was limited, leaving available soil N accessible to N₂O-producing microorganisms. This study suggests that applying N fertilizers during periods of high crop N demand, rather than at the onset of the growing season, could significantly reduce N₂O emissions and the GHG footprint of crop production.