Estimated nitrous oxide emissions from nitrogen fertilizer use on multispecies grassland compared to monocultures

Grassland agriculture faces increasing demands in terms of sustainability; economic, social, and environmental. Soils are critical to sustainable agriculture, in terms of maintaining soil fertility and quality, protecting water quality and mitigating greenhouse gas (GHG) emissions. There is evidence to suggest that greater sward diversity may have benefits in this regard. We report results from SmartGrass; a three year field study at two sites in Ireland investigating grass sward diversity along a gradient from perennial ryegrass (Lolium perenne L.) monoculture to grass-legume mixes to more complex grass-legume-herb mixes of up to nine species. Results reported include estimates of nitrous oxide (N2O) emissions from fertiliser nitrogen (N), soil temperature and moisture conditions, plant-available soil N, changes in soil organic carbon (C) and plant-available phosphorus (P). Estimated direct N2O emissions from N fertiliser (g N2O-N t DM-1 ha-1 yr-1) decreased from 146 for the monoculture at 250 kg fertiliser N ha-1 yr-1 to 35 for the monoculture at 90 kg fertiliser N ha-1 yr-1, to approximately 16 for the grass-legume and grass-legume-herb mixes, also at 90 kg fertiliser N ha-1 yr-1. This was due to a combination of the grass-clover and mixed swards maintaining high DM yields at low fertiliser N input, and the fact that the fertiliser N for these treatments was applied entirely as urea. These results indicate significant potential for more diverse swards to mitigate GHG emissions from fertiliser N use in grassland agriculture.