Abstract/Summary

The majority of intensive grassland production systems consist of perennial ryegrass (Lolium perenne) monocultures which are heavily reliant on inorganic nitrogen (N) fertiliser inputs. Inorganic N fertiliser drives up the carbon footprint of such systems as it is a direct source of nitrous oxide (N2O), a potent greenhouse gas with 265 times the global warming potential of carbon dioxide. In recent years, there has been increased use of binary grass-legume and multispecies swards for livestock production. This trend is owing to the associated benefits of more diverse swards (increased yield production, drought resistance and animal performance) and the need to diverge from conventional systems in order to reduce artificial N inputs. This PhD thesis addresses the gaps in knowledge regarding the N2O emissions and emissions intensity of multi-species swards. Annual N2O emissions and emissions intensity of systematically varied monocultures and mixtures were assessed using a simplex experimental design and static gas chamber methodology. Furthermore, given that drought events are predicted to increase due to climate change, the impact of drought and re-wetting on N2O emissions were also assessed according to species identity and diversity. Moving from plot to systems scale, a comparison of two production systems was also made between a) a monoculture receiving moderate N inputs and b) a grass-legume mixture receiving low N inputs and their subsequent impacts on N2O emissions, livestock production and emissions intensity. Key findings of this thesis include highly significant reductions in N2O emissions intensity from a 6-species mixture compared to L. perenne monocultures at both high and moderate N applications; no long lasting legacy effects of drought on N2O emissions following N fertiliser application post-rewetting; and increased liveweight gain (2kg) in lambs reared on the low N grass-legume system compared with a standard production system. Overall, this thesis solidifies the role of multispecies swards in climate-smart livestock production systems.