Perennial pastures reduce nitrous oxide emissions in mixed farming systems in a semi-arid environment

Perennial pastures play a crucial role in mixed farming systems by supplying feed for livestock, restoring soil fertility, reducing deep drainage, providing an opportunity to manage herbicide-resistant weeds and breaking soil-borne disease cycles. However, to our knowledge there is no data on the role of perennial pastures in mitigating N2O emissions from the phased crop rotations in semi-arid environments. Two 4-year field experiments were conducted in a semi-arid environment in southern Australia to (a) evaluate the role of perennial pastures in mitigating N2O emissions in mixed farming systems, and (b) compare the cumulative N2O emissions from different pasture mixes. Results showed that the annual N2O emissions were 31% lower from chicory-based pastures and 12-17% lower from perennial grass-based pastures compared with lucerne-based pastures. During the pasture phase, actively growing pastures kept N2O emissions at a relatively low level (59 g N2O-N ha-1 year-1), but N2O emissions increased significantly upon termination of the pastures. Results showed that the N2O emitted during the summer (December to February) after the pastures were terminated accounted for 70% of the total N2O emissions in the final pasture year. Furthermore, perennial grass and chicory-based pastures were highly productive during favorable conditions, leading to a low N2O emission intensity. It is suggested that emphasis be placed on utilizing highly persistent species to foster a longer and more productive pasture phase, and to manage N-supply in the transition between pasture and crop phases as this is where the greatest risk of N2O emission exists.