Estimating a nitrous oxide emission factor for animal urine from some New Zealand pastoral soils

Abstract

The Intergovernmental Panel on Climate Change methodology estimates that over 50% of total nitrous oxide (N₂O) emissions in New Zealand derive from animal excreta-N deposited during grazing. The emission factor for excreta-N as used by this methodology has an important impact on New Zealand's total N₂O inventory. The objectives of this study were to refine the N₂O emission factor for urine by simultaneously measuring N₂O emissions from 5 pastoral soils of different drainage class, in 3 different regions in New Zealand following a single application of urine; plus test various aspects of the soil cover method for determining emission factors. Cow urine and synthetic urine was applied to pastoral soils in autumn 2000 and N₂O emissions were measured using closed flux chambers at regular intervals for 4–18 months following application. The N₂O emission factors for cow urine estimated for the first 4 months after urine application varied greatly depending on rainfall and soil drainage class, and ranged from 0.3 to 2.5% of the urine-N applied, suggesting that adopting a single emission factor for New Zealand may be inappropriate. The largest emission factor was found in a poorly drained soil, and the lowest emission factor was found in a well-drained stony soil. Ongoing measurements on one of the soils resulted in an increase in emission factors as the N₂O emissions had not reached background levels 4 months after urine application. To characterise urine-induced N₂O emissions, we recommend measurements continue until N₂O emissions from urine-amended soil return to background levels. Furthermore, we recommend using real animal urine rather than synthetic urine in studies when determining the N₂O emission factor for urine.