Nitrous oxide inventory
Estimating a nitrous oxide emission factor for animal urine
Nitrous oxide (N2O) is a greenhouse gas that contributes to global warming and to the destruction of the ozone layer. N2O is mainly produced in soils by microorganisms that reduce nitrogen oxides such as nitrates to nitrogen gas (denitrifying microbes), or convert soil ammonium to nitrate (nitrifying microbes). The extent to which denitrification or nitrification occurs depends on soil conditions.
Ratification of the Kyoto Protocol will legally bind signatory countries to limit emissions of greenhouse gases including N2O to agreed levels. Furthermore, signatories will be required to report their greenhouse gas emissions (inventory) each year.
New Zealand's N2O inventory is calculated using guidelines and methodology provided by the Intergovernmental Panel on Climate Change (IPCC). IPCC methodology estimates N2O emissions by applying an emission factor (i.e. the proportion of N emitted as N2O) to all anthropogenic N inputs into the system. For New Zealand's pastoral agriculture which, unlike other countries, is characterised by year-round grazing of clover-based pastures and relatively low N fertiliser use, IPCC methodology estimates that over 50% of N2O emissions in New Zealand are derived directly from animal excreta-N deposited during grazing, and a further 30% of total emissions are a result of indirect emissions from leached and volatilised excreta-N. So the emission factor for excreta-N has an important impact on New Zealand's total N2O inventory.
The IPCC methodology provides default values for emission factors. However, these default values do not account for the variations in climatic and physical conditions that can affect soil N2O emissions and, consequently, emission factors. The IPCC encourages the use of country-specific factors, especially for N inputs that are a key source of N2O. Use of country-specific factors in inventory is, however, only permitted if they can be defended and can withstand international peer-review of the methodologies and approaches used to calculate them.
Landcare Research, in collaboration with AgResearch and Lincoln University, has undertaken research to refine the N2O emission factor for animal urine. We simultaneously measured N2O emissions from 5 pastoral soils of different drainage classes, in 3 different regions in New Zealand following a single application of urine. We also tested various aspects of the soil cover method for determining emission factors.
Results showed that the N2O emission factors for cow urine estimated for the first 4 months after urine application varied greatly depending on rainfall and soil drainage. Emission factors ranged from 0.3 to 2.5% of the nitrogen applied in urine. The largest emission factor was found in a poorly drained soil; the lowest in a well-drained stony soil. These results suggest that using a single emission factor to estimate emissions from the whole of New Zealand may not be appropriate.
This research was funded through NZONet, by the Ministry of Agriculture and Forestry.