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

The Intergovernmental Panel on Climate Change methodology estimates that over 50% of total nitrous oxide (N(2)O) 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 N(2)O inventory. The objectives of this study were to refine the N(2)O emission factor for urine by simultaneously measuring N(2)O 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 N(2)O emissions were measured using closed flux chambers at regular intervals for 4-18 months following application. The N(2)O 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 N(2)O emissions had not reached background levels 4 months after urine application. To characterise urine-induced N(2)O emissions, we recommend measurements continue until N(2)O 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 N(2)O emission factor for urine.