Meta-analysis of New Zealand's nitrous oxide emission factors for ruminant excreta supports disaggregation based on excreta form, livestock type and slope class.

Globally, animal excreta (dung and urine) deposition onto grazed pastures represents more than half of anthropogenic nitrous oxide (NO) emissions. To account for these emissions, New Zealand currently employs urine and dung emission factor (EF) values of 1.0% and 0.25%, respectively, for all livestock. These values are primarily based on field studies conducted on fertile, flatland pastures predominantly used for dairy cattle production but do not consider emissions from hill land pastures primarily used for sheep, deer and non-dairy cattle. The objective of this study was to determine the most suitable urine and dung EF values for dairy cattle, non-dairy cattle, and sheep grazing pastures on different slopes based on a meta-analysis of New Zealand EF studies. As none of the studies included deer excreta, deer EF values were estimated from cattle and sheep values. The analysis revealed that a single dung EF value should be maintained, although the value should be reduced from 0.25% to 0.12%. Furthermore, urine EF should be disaggregated by livestock type (cattle > sheep) and topography (flatland and low sloping hill country > medium and steep sloping hill country), with EF values ranging from 0.08% (sheep urine on medium and steep slopes) to 0.98% (dairy cattle on flatland and low slopes). While the mechanism(s) causing differences in urine EF values for sheep and cattle are unknown, the 'slope effect' on urine EF is partly due to differences in soil chemical and physical characteristics, which influence soil microbial processes on the different slope classes. The revised EF values were used in an updated New Zealand inventory approach, resulting in 30% lower national NO emissions for 2017 compared to using the current EF values. We recommend using the revised EF values in New Zealand's national greenhouse gas inventory to more accurately capture NO emissions from livestock grazing.