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 greenhous e gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). A gricultural soils are generally considered to be the main source of N2O emi ssions in New Zealand, but production estimates to date are surrounded by g reat uncertainty. This paper reviews our current understanding of agricultu ral N2O emissions, and suggests directions for future research needs by eva luating the default emission factors of the 1996 Revised Guidelines for Nat ional Greenhouse Gas Inventories of the Intergovernmental Panel on Climate Change (IPCC) for the New Zealand situation. The emission factors calculate d 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. Applicati on of the IPCC methodology to New Zealand identifies herbivore excrement as the single largest potential source of anthropogenic NO emissions (about 5 0% of the total emission). In addition, research is also required for indir ect sources of N2O, because only limited overseas data, and none from New Z ealand, are available. The 1996 IPCC methodology does not account for varia tions in climatic and soil physical conditions, which are known to affect N 2O emissions. In the longer term, development of robust process-based model s, coupled with spatial and temporal data sets of the major drivers of N2O emissions, may therefore be a useful approach for obtaining national emissi on estimates for New Zealand. This will require long-term monitoring of N2O emissions, under various land uses and on a national network of sites.