Temporal dynamics of N₂O emissions from pasture urine patches: drivers and source processes

Nitrous oxide fluxes from urine patches (F(N₂O)_urine) of grazing livestock are variable over time due to fluctuations in driving parameters, such as soil temperature, water-filled pore space (WFPS), and availability of the source substrates ammonium and nitrate. Therefore, the frequency and timing of flux measurements after urine application are important when determining cumulative F(N₂O)_urine. In this study, F(N₂O)_urine was measured in eight experiments at high temporal frequency using an automatic chamber system in a pasture located in Switzerland. A driver analysis using random forest identified the time since urine application as most important predictor for F(N₂O)_urine. The exponential decay in F(N₂O)_urine after urine addition was in parallel to decreasing soil ammonium but anticorrelated to nitrate concentration, suggesting that nitrification and nitrifier denitrification are major source processes. Since nitrate showed elevated concentrations up to 122 days after application, bacterial denitrification is most likely responsible for late F(N₂O)_urine peaks following an increase of WFPS. The isotopic composition of the emitted N₂O indicates that nitrification dominates the N₂O production immediately after urine application, while bacterial denitrification and nitrifier denitrification become more important with increasing time since urine application. The observed high-frequency emission time series were also used to simulate typical low-frequent manual chamber sampling schedules. They led to considerable deviations (up to ± 30%) of the time-integrated N₂O emission. However, the average bias was considerably smaller (- 5%) for the prescribed sampling schedule, indicating that manual chamber measurements can reasonably quantify average total N₂O emissions from urine patches under the present pedoclimatic conditions.