Urine patches from grazing cattle are hotspots of nitrous oxide (N2O) emissions. The default IPCC emission factor for urine patches (EFurine) 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 EFurine 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 EFurine value (p > 0.05). EFurine 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 EFurine weekly over the last 20 years showing no significant differences between the seasons of a year. The resulting overall average EFurine 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.

<p>Nitrous oxide (N<sub>2</sub>O) is a powerful greenhouse gas (GHG) with a global warming potential about 300 times that of carbon dioxide (CO<sub>2</sub>). In Switzerland, N<sub>2</sub>O emissions contribute to about 6% of the total GHG emissions, agriculture being responsible for more than 60% of the former. Understanding the processes driving N<sub>2</sub>O emissions from agricultural land is therefore of paramount importance for developing national GHG emissions inventories. Of relevance in this respect is the fact that about two-thirds of the agricultural lands are grasslands, part of which are managed as pastures.</p><p>Urine deposited by grazing animals has high N loads and induce increased nitrification and denitrification. Urine patches are hence hotspots for N<sub>2</sub>O emissions. In the IPCC Tier 1 method still in use in Switzerland for quantifying N<sub>2</sub>O emissions, a default EF<sub>3</sub> value of 2% is assumed for excreta (dung and urine). This does not properly account for the spatial heterogeneity of N returns from grazing animals. Recent studies have indeed shown that country-specific EF<sub>3</sub> are typically much lower than the default IPCC value. These results suggest that the use of IPCC Tier 2 and Tier 3 methods, that rely on the application of process-based models, is to be preferred for estimating countrywide N<sub>2</sub>O emissions.</p><p>In this work, we will apply the comprehensive process-based model ecosys to simulate N<sub>2</sub>O emissions from urine patches in a Swiss grazing system. We report on preliminary results from experiments aiming at modelling artificially applied urine patches. After showing that the model is able to reproduce the emission rates measured in a companion field trial, we use ecosys to examine N fractions lost to direct (N<sub>2</sub>O emissions) and indirect (ammonia volatilization, nitrate leaching and runoff) pathways for urine-N input rates varying from 500-2000 kg N ha<sup>-1</sup>. We also apply the model to understand the effects of seasonal variations in the environmental drivers on N<sub>2</sub>O EF. This work is part of a PhD conducted by the first author that aims at developing the scientific basis for establishing country-specific EFs for grazing-related N<sub>2</sub>O emissions in Switzerland.</p>

<p>In grassland ecosystems nitrogen (N) inputs are mainly attributed to fertilizer applications for increasing  herbage productivity and to excreta of grazing animals. Cattle, for instance, excrete 75-95 % of the N intake. Accordingly, dung and urine patches of grazing animals form hotspots of nitrate leaching and gaseous N emissions as ammonia (NH<sub>3</sub>) or the important greenhouse gas nitrous oxide (N<sub>2</sub>O). Global default emission factor (EF) values for N<sub>2</sub>O, 2.0 % for grazing based nitrogen inputs (EF3) and 1.0 % for nitrogen inputs via fertilizer applications (EF1) have been suggested by IPCC. However, some countries like New Zealand, Canada or the Netherlands have established country-specific EFs showing considerable regional differences.</p><p>In the present research study, we examine N<sub>2</sub>O emissions of a pasture field in Switzerland in relation to possible drivers. Field scale emissions by eddy covariance are measured in parallel to patch-scale N<sub>2</sub>O fluxes from controlled applications of urine, dung and fertilizer. The patch-scale fluxes are measured by a manually operated chamber ('fast-box') connected to an online gas analyzer. Besides estimating EF values on annual and seasonal basis, relevant factors that might control N<sub>2</sub>O fluxes like environmental conditions (weather parameters, soil moisture, soil temperature), vegetation characteristics (height, composition, nitrogen and carbon content) and pasture management (patch age, grazing, fertilization, cut events, interactive effects) are analyzed. </p><p>We present and discuss results of the first measurement year 2020. Three artificial urine applications during summer and autumn were performed. They show peak N<sub>2</sub>O fluxes of 279-1718 μg m<sup>-2</sup> h<sup>-1</sup> directly after application that decrease to near-background fluxes within 19-43 days. Using a simple linear interpolation of measured N<sub>2</sub>O fluxes, EF values of artificial urine patches vary between 0.57 and 2.44 % indicating a seasonal variability of N<sub>2</sub>O fluxes.</p>