What matters when? Temporal development of drivers and sources of nitrous oxide emissions in winter wheat

Abstract. Agriculture is the largest anthropogenic source of nitrous oxide (N2O), primarily due to nitrogen (N) fertilization. Understanding how the influence of key drivers and the relative contribution of source processes change throughout the cropping season is crucial for developing effective strategies to mitigate N2O emissions. In this study, we combined high-resolution eddy covariance flux measurements and stable isotope analyses over one winter wheat cropping season and the subsequent summer cover crop season. Two phases, crop establishment and early spring, were identified as critical periods for N2O emissions, characterized by a mismatch between N supply and plant demand, resulting in surplus soil mineral N and elevated N2O fluxes under favorable environmental conditions. Gross primary productivity (GPP), used as a proxy for crop N uptake, suppressed N2O emissions, especially under high soil moisture, highlighting the importance of active vegetation in mitigating emissions. Source partitioning, based on stable isotopes, revealed denitrification as the dominant process of N2O production, driven by poor soil drainage and high soil moisture. Over the nine-month winter wheat season, the Tier 1 N2O emission factor was 1.8 %, with cumulative emissions of 5.5 kg N2O-N ha−1, offsetting 70 % of the net CO2 uptake. Our findings emphasize the need to better synchronize N supply with crop demand and to adopt agronomic practices that promote rapid crop establishment to mitigate N2O emissions in cropping systems.