N₂O and CH₄ fluxes from intensively managed grassland: The importance of biological and environmental drivers vs. management

Agriculture is the main contributor to anthropogenic nitrous oxide (N₂O) and methane (CH₄) emissions. Therefore, mitigation options are urgently needed. In contrast to carbon dioxide, eddy covariance measurements of N₂O and CH₄ fluxes are still scarce, and thus little is known how environmental and biotic drivers as well as management affect the net N₂O and CH₄ exchange in grasslands. Thus, we investigated the most important drivers of net ecosystem N₂O and CH₄ fluxes in a temperate grassland, and continued a N₂O mitigation experiment (increased clover proportion vs. fertilization with slurry). Random forest gap-filling models were able to capture intermittent emission peaks, performing better for half-hourly N₂O than for CH₄ fluxes. The unfertilized clover parcel (parcel B) continued to show lower N₂O emissions (4.4 and 2.7 kg N₂O-N ha^-1 yr^-1) compared to the fertilized parcel (parcel A; 6.9 and 5.9 kg N₂O-N ha^-1 yr^-1) for 2019 and 2020, respectively. Tier 1 nitrogen (N) emission factors of 2.6 % and 1.9 % were observed at the fertilized parcel during the study period. Lower soil N concentrations indicated a lower N leaching risk at the clover than at the fertilized parcel. Annual CH₄ emissions (including periods with sheep grazing) were similar from both parcels, and ranged from 25 to 38.5 kg CH₄-C ha^-1. The most important drivers of both N₂O and CH₄ fluxes were lagged precipitation and water filled pore space, but also management (for N₂O from parcel B; CH₄ from parcel A). Biotic variables such as vegetation height and leaf area index were important predictors for the N₂O exchange, while grazing temporarily increased CH₄ emissions. Overall, reducing N fertilization and increasing the legume proportion were effective N₂O reduction measures. In particular, adjusting N fertilization to plant N demands can help to avoid high N₂O emissions from grasslands.