Nitrous oxide emissions from application of urea on New Zealand pasture

The use of nitrogen (N) fertiliser has been identified as a possible important source of nitrous oxide (N 2 O) from pastoral soils, and urea is the main form of N fertiliser used in New Zealand. The aim of this study was to examine the effects of urea application on N 2 O emissions from pastoral soil. A closed soil chamber technique was used to measure the N 2 O emissions from a pasture which received either 0 (control) or 50 kg N ha –1 (as urea) per application during different seasons between 2003 and 2005. Overall, urea fertiliser application generally increased N 2 O fluxes above control levels for up to 30 days, but the duration for which N 2 O levels were elevated depended on the season. These increases in the N 2 O fluxes were largely caused by a combination of changes in the soil mineral N level due to urea application, and moisture content of soil in different seasons. Nitrous oxide emissions were higher during the winter and spring measurement periods when the soil water‐filled pore space (WFPS) was mostly above field capacity, and the emissions were lower during the summer and autumn measurement periods when the soil WFPS was below field capacity. The estimated N 2 O emission factors for urea ranged between 0 and 1.56% of the urea‐N applied, with a calculated average emission factor of 0.56%. The findings of the seasonal measurements suggests that a reduction in the use of N fertilizers under wet winter or wet spring conditions in New Zealand could potentially reduce N 2 O emissions from pastoral soil.