Effect of irrigation scheduling on nitrous oxide emissions in intensively managed pastures

Denitrification is a key pathway of N loss in pastures and contributes to the low nitrogen use efficiency of these agroecosystems. Previous studies have shown irrigation to significantly increase nitrous oxide losses from fertilised agroecosystems. This study aimed to determine the effect of three irrigation frequencies on N2O emissions while maintaining pasture yield. High-resolution N2O data and biomass yields from each grazing were used to evaluate the mitigation potential of irrigation scheduling. Irrigation volumes were based on average monthly evapotranspiration and were applied at 4, 10 and 15 day intervals (High-Frequency, Medium-Frequency and Low-Frequency), with the aim of equal cumulative irrigation accounting for rainfall. Highest cumulative losses of 5855 ± 537 g N ha-1 were measured from the Low-Frequency treatment, which was significantly higher than the Medium and High-Frequency treatments with 3282 ± 190 g ha-1 and 3790 ± 448 g ha-1,respectively. Emission factors ranged between 0.49% and 1.17% for Medium-Frequency and Low-Frequency treatments respectively. Despite significant differences in losses, much of this variance was not directly related to individual irrigation events but to a treatment legacy seen during succeeding high rainfall events. These high losses from the Low-Frequency treatment have been attributed to the Birch Effect due to the severe drying of the soil profile between irrigation events; suggesting a future focus of research on wetting and drying cycles rather than absolute soil moisture contents.