N2O emission factors for cattle urine: effect of patch characteristics and environmental drivers.

Urine patches from grazing cattle are hotspots of nitrous oxide (N₂O) emissions. The default IPCC emission factor for urine patches (EF_urine) is 0.77% for wet climates and 0.32% for dry climates. However, literature reports a considerable range of cattle urine EF values and urine characteristics used in experimental studies, revealing contrary results on the effects of urine patch characteristics and seasonal pattern. Therefore, we examined N₂O emissions and corresponding EF_urine values in relation to urine patch characteristics (urine N concentration, urine volume, patch area, urine composition) and environmental drivers (precipitation, water filled pore space, soil temperature). Ten artificial urine application experiments were performed from July 2020 to June 2022 on a pasture located in Eastern Switzerland. Urine N concentration, patch area, volume and urine N composition showed no significant effects on the EF_urine value (p > 0.05). EF_urine varied, however, strongly over time (0.17-2.05%). A large part of the variation could be predicted either by cumulative precipitation 20 days after urine application using a second order polynomial model (Adj. R² = 0.60) or average WFPS 30 days after urine application using a linear model (Adj. R² = 0.45). The derived precipitation model was used to simulate EF_urine weekly over the last 20 years showing no significant differences between the seasons of a year. The resulting overall average EF_urine was 0.67%. More field studies are needed across sites/regions differing in climate and soil properties to implement a country-specific EF₃ for Switzerland and to improve the quantification of N₂O emissions at the national scales.