Influence of urine nitrogen concentration and environmental conditions on ammonia emissions and nitrogen uptake in...

Ammonia (NH₃-N) emissions from livestock urine patches are a key pathway of N loss in grazed pasture systems, contributing to air pollution, eutrophication, and indirect N₂O emissions. However, the interactive effects of urine N concentration (UNC) and prevailing seasonal conditions on NH₃-N losses and pasture performance remain poorly characterised. This study evaluated NH₃-N emissions, dry matter yield (DMY), N uptake, and nitrogen use efficiency (NUE) following urine application at five concentrations (0, 2.5, 5.0, 7.5, and 10.0 g N L^-1) during spring, summer, and autumn application periods in 2023. Emissions were measured over 15 days using a dynamic chamber system, and herbage was harvested at 4-week intervals for 12 weeks. A significant UNC × application period interaction (p < 0.0001) was observed for NH₃-N emissions, DMY, and N uptake. Peak NH₃-N losses occurred within 1-3 days across all treatments, but the magnitude varied markedly among seasonal application periods, with the highest cumulative losses in spring (up to 46.7 ± 3.4 kg N ha^-1 at 10.0 g N L^-1) and the lowest in summer (4.1 kg N ha^-1 at 2.5 g N L^-1). Emission factors ranged 0.5-3.4%, below both the IPCC (2006) Tier 1 default (20%) and the EMEP/EEA Tier 2 estimate (∼6%). While NH₃-N emissions increased with UNC, DMY gains and NUE plateaued beyond 5.0 g N L^-1, indicating that pasture N assimilation capacity was exceeded at higher urine N loadings. Short-term environmental conditions during each application period strongly influenced NH₃-N losses and pasture NUE following urine deposition. These findings highlight the importance of accounting for short-term environmental conditions when developing NH₃ mitigation strategies for pasture-based livestock systems.