Assessing nitrous oxide emissions from grass-clover ley within a crop rotation using measurements and modeling

Nitrous oxide (N 2 O) is a significant greenhouse gas that contributes to climate change, with one of the major anthropogenic sources being agricultural fertilizer application. In Europe, a substantial fraction of nitrogen (N) fertilizers is applied in grass-based systems, including leys, which generate N 2 O emissions. The contribution of grass-clover leys to N 2 O emissions and the specific emission factors (EFs) are not well-documented. Therefore, we monitored N 2 O emissions in grass-clover ley over three full years (2021-2023) and assessed the feasibility of using the process-based model DayCent with a previously published multi-site calibration for Western European cropland to simulate N 2 O emissions. The monitoring was undertaken using an automatic time integrating chamber system in three treatments of a long-term fertilization experiment on a ley-arable rotation in Switzerland, including organic fertilization (slurry), mineral fertilization (ammonium nitrate and ammonium sulphate) and a control (unfertilized). The results showed mean annual N 2 O emissions of 0.51 ± 0.26 kg N ha − 1 yr − 1 , 0.53 ± 0.08 kg N ha − 1 yr − 1 and 0.02 ± 0.03 kg N ha − 1 yr − 1 from organic, mineral and control treatments, respectively, over the three study years. We found that the EF values, which were determined from measured emissions of the fertilized treatments after subtracting the control treatment, were 0.23 % for organic and 0.41 % for mineral fertilizer, which are much lower than the IPCC wet climate default EF values of 0.6 % and 1.6 %, respectively. For DayCent simulations, we used aboveground N yield to adjust for model parameters associated to plant C/N ratio and the biological N 2 fixation of the specific grass-clover mixture ley. The model yielded mean EFs of 0.26 % and 0.29 % for organic and mineral fertilizer, respectively, showing no significant difference (p > 0.05) to the corresponding measured EF values. The modeling results suggest that grass-clover ley under these conditions tends to have lower N 2 O emissions than the arable crops in the rotation. Our results indicate that using IPCC default EFs may overestimate the N 2 O emission from grass-clover ley under the studied soil conditions, and that DayCent with an average multi-site calibration with adjustment of few crop N parameters is able to reproduce the comparatively low N 2 O emissions and EFs for ley.