Measured and modelled estimates of nitrous oxide emission and methane consumption from a sheep-grazed pasture

Sheep grazing, one of the major land-uses in New Zealand occupies 7.1 million hectares and provides forage for about 40 million sheep. Spatially and temporally explicit data on nitrous oxide (N2O) and methane (CH4) fluxes from soils under this land use are needed at farm-scale to better estimate regional and national greenhouse gas inventories. In this study, we measured N2O and CH4 fluxes at the soil surface of a sheep-grazed pasture over a 20-month period, to assess the influence of soil moisture, temperature and seasonal variations in the emissions of N2O and CH4, and to calculate an N2O emission factor. The farm-scale N2O emissions were also simulated using a process-based NZ-DNDC model, and the model was tested against measurements of CH4. Measured annual N2O emissions from this pasture site are estimated to be 3.7 ± 2.2 kg N2O-N ha−1 year−1. The mean of daily N2O emission measurements (7.4 g N2O-N ha−1 day−1) for this sheep-grazed pasture was higher than the mean of daily emission measurements from ungrazed control site (3.4 g N2O-N ha−1 day−1) but significantly lower than reported value of 32.0 g N2O-N ha−1 day−1for pastures grazed by dairy cows on the same soil type at an adjacent farm. The emission factor (EF), i.e. amount of N2O-N emitted expressed as a percentage of excretal and fertiliser N input for this site, is 0.99, which is essentially the same as the New Zealand specific N2O emission factor of 1.0 for excretal N. The sheep-grazed pasture acted as a CH4 source and sink but was an overall annual CH4 sink (0.64 ± 0.19 kg CH4-C ha−1). Methane consumption was highest in the summer and lowest during the wetter, colder winter months. Thus mitigation strategies for reducing anaerobic N2O emissions will also favour greater CH4 oxidation at the soil surface. NZ-DNDC model simulated effectively general pulses and trends in both N2O and CH4 fluxes.