Improving estimates of nitrate leaching for quantifying New Zealand's indirect nitrous oxide emissions

New Zealand's indirect nitrous oxide (N2O) emissions have been estimated to contribute approximately 23% of the national nitrous oxide inventory. However, there is great uncertainty about this figure. Currently the intergovernmental panel for climate change (IPCC) calculation is used, where a proportion (Frac(LEACH)) of nitrogen (N) applied to land as animal excreta or fertiliser is leached or runs off, and a further proportion is later emitted as N2O. New Zealand has been using a value of 0.15 (Frac(LEACH[NZ])) and this paper examined whether (Frac(LEACH[NZ])) is appropriate for New Zealand conditions. For a range of typical farming systems, we compared N leaching estimates calculated using the IPCC formula with estimates obtained using the OVERSEER (R) nutrient budget model calibrated for New Zealand conditions. The comparison suggested that Frac(LEACH[NZ]) is too high for dairy and sheep and beef farming systems. In contrast, the Frac(LEACH) values estimated for arable and intensive vegetable systems using the OVERSEER (R) model were much closer to 0.3 (the IPCC default value). However, in New Zealand, arable cropping and intensive vegetable farms occupy only a small proportion of land compared to pastoral farming. Based on this study, we suggest that a Frac(LEACH) value of 0.07 is appropriate for New Zealand conditions; this more accurately reflects the proportion of N applied to pastoral land that may leach or runoff. Using this value in the IPCC calculation for the New Zealand inventory would approximately halve the national estimate of indirect N2O emissions due to leaching or runoff of N, reducing the national estimate of N2O emissions by 12%.