Nitrous oxide emissions from grazed pastures: measurements at different scales

Estimates made by the Intergovernmental Panel on Climate Change (IPCC) and the Australian National Greenhouse Gas Inventory Committee (NGGIC) suggest that grazed pastures are substantial anthropogenic sources of nitrous oxide (N₂O), contributing 28% of all anthropogenic N₂O emissions globally and >43% for Australia. These estimates are based almost wholly on extrapolations of enclosure experiments to the field scale and uncertainty levels are high. Verification with direct field measurements is needed. This paper reports micrometeorological studies of N₂O emissions from Australian grazed pastures made at the same location on a variety of space scales. They included a mass-balance study employing a small test plot approximately 0.05 ha in area in which 14 sheep were grazed, tower-based flux measurements representing areas between 25 ha and 5 km² and convective boundary-layer budgets representing regions of order 100 km². The mass-balance study, which was considered to be the most reliable micrometeorological approach, gave an average emission over 8 days of 1.87 g N₂O–N head⁻¹ d⁻¹ corresponding to 11.5% of the nitrogen (N) voided by the animals in urine and dung. However, the data set included two days after rain on which emissions were an order of magnitude larger than on the other days in the study. For the latter, the emission of N₂O accounted for 3.9% of the N excreted. Although uncertainty levels remain high due to large temporal and spatial variability, the micrometeorological measurements suggested that N₂O emissions might be considerably larger than those predicted by NGGIC algorithms which use emission factors of 0.4% for urine and 1.25% for dung, but appear to be predicted more closely by IPCC algorithms which use 2% for both. The study has indicated ways to improve the precision of relevant micrometeorological approaches.