Response of nitrogen oxide emissions to grazer species and plant species composition in temperate agricultural grassland

Agriculture is an important source of the greenhouse gas nitrous oxide (N2O) and the atmospherically important nitric oxide (NO). We evaluated the effects of different grazers and plant species composition on N2O and NO emissions in temperate grassland. Paddocks were grazed rotationally by either cattle or sheep. Mean N2O emissions were 38.7 μg N2O-N m−2 h−1, mean NO emissions 2.4 μg NO-N m−2 h−1. Cumulative NO-N emissions were larger for sheep- than for cattle-grazed paddocks. Plant species composition was insignificant compared to the effect of grazers on N oxide emissions. In a controlled application experiment, plots with cattle excreta showed larger N2O emissions than plots with sheep excreta, reaching peak emissions of 1921 μg N2O-N m−2 h−1 on cattle urine patches compared to 556 μg N2O-N m−2 h−1 on sheep urine patches, related to different N-inputs per excretion. Peak emissions of dung-treated plots were much smaller. The N2O emission factors were 0.4% for cattle urine, 0.5% for sheep urine, 0.05% for cattle dung and 0.09% for sheep dung. N oxide emissions on the paddock scale were larger for sheep- compared to cattle-grazing, despite larger emissions per cattle excretion. We attributed this to the more even spread of sheep excreta compared to cattle excreta.