Irrigation scheduling with soil gas diffusivity as a decision tool to mitigate N₂O emissions from a urine-affected pasture

Abstract

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.