NO emission factors for cattle urine: effect of patch characteristics and environmental drivers.

Urine patches from grazing cattle are hotspots of nitrous oxide (NO) emissions. The default IPCC emission factor for urine patches (EF) 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 NO emissions and corresponding EF 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 value (p > 0.05). EF 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. R = 0.60) or average WFPS 30 days after urine application using a linear model (Adj. R = 0.45). The derived precipitation model was used to simulate EF weekly over the last 20 years showing no significant differences between the seasons of a year. The resulting overall average EF was 0.67%. More field studies are needed across sites/regions differing in climate and soil properties to implement a country-specific EF for Switzerland and to improve the quantification of NO emissions at the national scales.