Effects of time-controlled grazing on the magnitude and spatial distribution of N2O emissions from subtropical pastures in Australia

Time-controlled grazing, i.e. short intensive grazing periods followed by a long rest, is promoted as a management form to counteract grassland degradation, increasing soil health, drought resilience and the sustainability and profitability of pastoral farming. Emissions of the potent greenhouse gas nitrous oxide (N₂O) are known to respond to N substrate availability. Effects of grazing management on the distribution of N inputs and ensuing N₂O emissions remain however largely unknown. This study investigated effects of continuous vs. time-controlled grazing on the magnitude and the spatial distribution of N₂O emissions using a paired site approach. Emissions of N₂O were measured before and after a simulated rainfall event across two extensively managed pasture sites in subtropical Queensland. Both sites were subdivided into four strata with 31 N₂O sampling points per site, based on the distance to the water point. Mean N₂O emissions across strata ranged from 23.5 to 22.8 g N₂O-N m^-2 day^-1 and increased to 63.6 and 42.0 g N₂O-N m^-2 day^-1 after the simulated rainfall event, for the continuous and time controlled grazing site, respectively. Emissions differed between strata, with highest emissions exceeding 60 g N₂O-N m^-2 day^-1 within 100 m of the watering point and in shaded/forest areas, decreasing with distance to the water point. The spatial response of N₂O emissions was consistent with NO₃^- concentration in the soil, likely reflecting areas of herd concentration with increased urine and dung deposition providing N substrate for N₂O formation. Emissions of N₂O were lower in shaded and forested areas, as well as in strata with >500 m distance to the water point under time controlled grazing as compared to continuous grazing management. The lack of treatment effect on NO₃^- availability and overall N₂O emissions however shows no clear benefits of  time controlled grazing on the distribution of N substrate availability under the conditions of this study, demanding further research to evaluate its benefits in regards to N₂O mitigation from extensively managed pastures.