Irrigation Scheduling with Soil Gas Diffusivity as a Decision Tool to Mitigate N2O Emissions from a Urine-Affected Pasture

Pastures require year-round access to water and in some locations rely on irrigation during dry periods. Currently, there is a dearth of knowledge about the potential for using irrigation to mitigate N2O emissions. This study aimed to mitigate N2O losses from intensely managed pastures by adjusting irrigation frequency using soil gas diffusivity (Dp/Do) thresholds. Two irrigation regimes were compared; a standard irrigation treatment based on farmer practice (15 mm applied every 3 days) versus an optimised irrigation treatment where irrigation was applied when soil Dp/Do was ≈0.033 (equivalent to 50% of plant available water). Cow urine was applied at a rate of 700 kg N ha−1 to simulate a ruminant urine deposition event. In addition to N2O fluxes, soil moisture content was monitored hourly, Dp/Do was modelled, and pasture dry matter production was measured. Standard irrigation practices resulted in higher (p = 0.09) cumulative N2O emissions than the optimised irrigation treatment. Pasture growth rates under treatments did not differ. Denitrification during re-wetting events (irrigation and rain) contributed to soil N2O emissions. These results warrant further modelling of irrigation management as a mitigation option for N2O emissions from pasture soils, based on Dp/Do thresholds, rainfall, plant water demands and evapotranspiration.