Greenhouse gas (CO2, N2O, CH4) fluxes from intensively managed grassland during a N2O mitigation experiment

Agriculture is the main contributor to anthropogenic emissions of nitrous oxide (N₂O) and methane (CH₄). Greenhouse gas (GHG) mitigation options in agricultural ecosystems are therefore urgently needed. In contrast to carbon dioxide (CO₂), continuous measurements of N₂O and CH₄ fluxes are still scarce. In this study, we investigated management and environmental drivers of N₂O and CH₄ fluxes, which were measured using eddy covariance in a predominantly mown temperate grassland. A N₂O mitigation experiment took place at the site where two parcels surrounding the eddy station were managed differently: at the experimental parcel, organic fertilizer was replaced by an increased legume (clover) proportion while the other parcel was still fertilized with slurry. Random forest gap-filling models were able to capture intermittent emission peaks well, with a better performance for half-hourly N₂O than for CH₄ fluxes. The unfertilized clover parcel showed reduced N₂O emissions (4.4 and 2.7 kg N₂O-N ha^-1 yr^-1) compared to the fertilized parcel (6.9 and 5.9 kg N₂O-N ha^-1yr^-1) over two years, which confirmed the results from the first years of the experiment. The net ecosystem CO₂ exchange (NEE) showed high interannual variability, which had a substantial influence on the grassland GHG budget. Overall, reducing fertilization and increasing the legume proportion were effective N₂O reduction measures. N₂O and CH₄ fluxes play an important role in the GHG budget of agricultural ecosystems as they can partly offset the ecosystem CO₂ uptake.