Amplitude and frequency of wetting and drying cycles drive N$_{2}$ and N$_{2}$O emissions from a subtropical pasture

Abstract

This study investigated the effects of irrigation frequency on N$_{2}$ and N$_{2}$O emissions from an intensively managed pasture in the subtropics. Irrigation volumes were estimated to replace evapotranspiration and were applied either once (low frequency) or split into four applications (high frequency). To test for legacy effects, a large rainfall event was simulated at the end of the experiment. Over 15 days, 7.9 ± 2.7 kg N$_{2}$ + N$_{2}$O-N ha$^{-1}$ was emitted on average regardless of irrigation frequency, with N$_{2}$O accounting for 25% of overall N$_{2}$ + N$_{2}$O. Repeated, small amounts of irrigation produced an equal amount of N$_{2}$ + N$_{2}$O losses as a single, large irrigation event. The increase in N$_{2}$O emissions after the large rainfall event was smaller in the high-frequency treatment, shifting the N$_{2}$O/(N$_{2}$O + N$_{2}$) ratio towards N$_{2}$, indicating a treatment legacy effect. Cumulative losses of N$_{2}$O and N$_{2}$ did not differ between treatments, but higher CO$_{2}$ emissions were observed in the high-frequency treatment. Our results suggest that the increase in microbial activity and related O$_{2}$ consumption in response to small and repeated wetting events can offset the effects of increased soil gas diffusivity on denitrification, explaining the lack of treatment effect on cumulative N$_{2}$O and N$_{2}$ emissions and the abundance of N cycling marker genes. The observed legacy effect may be linked to increased mineralisation and subsequent increased dissolved organic carbon availability, suggesting that increased irrigation frequency can reduce the environmental impact (N$_{2}$O), but not overall magnitude of N$_{2}$O and N$_{2}$ emissions from intensively managed pastures