Field-scale management and environmental drivers of N2O emissions from pasture-based dairy systems

Emissions of N2O from two dairy farms with different grazing intensity responded non-linearly to increasing soil nitrate availability and total rainfall. Higher N2O emissions were observed immediately after grazing, due to the possible increased incidence of N deposition from animal excreta, increased soil compaction and low plant N uptake. The spatial distribution of N2O fluxes and soil nitrate contents reflected the effect of animal treading and excreta N deposition with N2O fluxes in the proximity of field gateways 11 times higher than the field average. Three years average annual N2O emissions were 6 times higher (9.3 ± 2.6 kg N2O-N ha−1 y−1) in the high grazing intensity farm than in the low grazing intensity farm (1.6 ± 0.2 kg N2O-N ha−1 y−1). This corresponded to 2% and 0.9% of the fertiliser N inputs lost as N2O for the high and low intensity farm, respectively. The GHG intensity (N2O emitted per kg of fat and protein corrected milk FPCM) for the intensive system was only almost two times higher than the non-intensive system (0.12 vs 0.08 kg CO2-eq per kg FPCM y−1 for the high and low intensity system, respectively). The high occurrence of N load hotspots near animal gateways in this study underlines the necessity to account for the spatial N variation in dairy grazing systems. Integrating the spatial management of N loads into improved farming practice has therefore significant scope to reduce N2O emissions and N losses from dairy grazing systems.