Nitrous oxide emissions from agricultural soils in New Zealand - a review of current knowledge and directions for future research

Should international protocols be ratified, New Zealand will become legally committed to limit its greenhouse gas emissions. The three major greenhouse gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Agricultural soils are generally considered to be the main source of N2O emissions in New Zealand, but production estimates to date are surrounded by great uncertainty. This paper reviews our current understanding of agricultural N2O emissions, and suggests directions for future research needs by evaluating the default emission factors of the 1996 Revised Guidelines for National Greenhouse Gas Inventories of the Intergovernmental Panel on Climate Change (IPCC) for the New Zealand situation. The emission factors calculated for New Zealand agricultural soils are generally within the range of the 1996 IPCC default values, but the limited amount of research data available hampers a full evaluation of the appropriateness of these factors for New Zealand. More long-term studies are needed to refine our emission factors, particularly those for animal urine and dung returned to pasture. Application of the IPCC methodology to New Zealand identifies herbivore excrement as the single largest potential source of anthropogenic NO emissions (about 50% of the total emission). In addition, research is also required for indirect sources of N2O, because only limited overseas data, and none from New Zealand, are available. The 1996 IPCC methodology does not account for variations in climatic and soil physical conditions, which are known to affect N2O emissions. In the longer term, development of robust process-based models, coupled with spatial and temporal data sets of the major drivers of N2O emissions, may therefore be a useful approach for obtaining national emission estimates for New Zealand. This will require long-term monitoring of N2O emissions, under various land uses and on a national network of sites.