Spatial and temporal nitrous oxide emissions from dairy cattle urine deposited onto grazed pastures across New Zealand based on soil water balance modelling

Nitrous oxide emissions from urine deposited onto soils during grazing are captured within the New Zealand national inventory by employing an annual average country-specific emission factor (EF3) of 1%. However, soil moisture is a key driver of N2O emissions, and we propose a soil water balance model can be used to determine spatially- and temporally-disaggregated emission factors to refine and improve the emissions estimate. We constructed a GIS-based water balance model that operates on regional and monthly scales and developed a predictive relationship between soil water content and EF3. Combined with estimated monthly cattle urine excretion, we calculated annual N2O emissions for four years, ranging between 6.6 and 7.5 Gg y−1. The associated, annual mean EF3 value was 0.9–1.0%. This is very similar to the currently employed country-specific EF3 value, which results in an annual N2O emission of 7.0–7.7 Gg y−1. Within-year variability in regional and monthly EF3 was much greater than the between-year variability in country-wide annual average EF3, reflecting a strong averaging affect across temporal and spatial scales.