Refining the New Zealand nitrous oxide emission factor for urea fertiliser and farm dairy effluent

Abstract

Applications of urea fertiliser and farm dairy effluent (FDE) to New Zealand pastures are the second and third largest sources of nitrous oxide (N₂O) emissions, after emissions from excreta deposited during grazing (urine and dung). New Zealand currently employs emission factors (EF₁) (percentage of N applied which is emitted as N₂O) of 0.48% and 1% for urea fertiliser and FDE, respectively, for calculating its national N₂O inventory. The country specific emission factors for urine and dung are 1% and 0.25% respectively. Because FDE has a higher organic nitrogen (N) content than urea, and because it is a diluted mixture of urine and dung, the mean FDE EF1 is expected to be less than 1%. With a recent increase in research trials measuring EF₁ for FDE and urea, the objective of this study was to refine New Zealand-specific EF₁ values for these N sources. We analysed urea fertiliser and FDE N₂O emission data from 45 EF₁ field trials conducted in New Zealand. This meta-analysis yielded a combined (urea and FDE) EF₁ mean of 0.46% (95% confidence interval of 0.07% and 0.90%), with EF1 means for urea and FDE of 0.59% and 0.25%, respectively. There was no statistical difference between urea fertiliser and FDE EF₁ values. However, we recommend separate country-specific EF1 means of 0.6 and 0.3% for urea fertiliser and FDE, respectively, for New Zealand's agricultural soils N₂O emissions inventory due to the different origin and characteristics of these N sources.