Improving Nitrous Oxide Reporting In Agricultural Inventories of Greenhouse Gas Emissions: A UK Case Study

The UK is committed to ambitious targets to reduce greenhouse gas emissions. Under the government's Climate Change act of 2008 there is a long-term legally binding requirement to reduce national emissions by at least 80% below the 1990 baseline by 2050, with interim reductions of at least 34% by 2020. Agricultural emissions made up 9% of the total greenhouse gas emissions in 2014, and the nitrous oxide emissions from agriculture are associated with some of the highest uncertainties in national inventory reports. Nitrous oxide emissions from agriculture have remained relatively constant over the past five years, despite significant reductions of emissions in CO2 and CH4. This places increased pressure agriculture to reduce emissions if overall emission reduction targets are to be met. For this reason a large programme of research has been undertaken in the UK in order to reduce uncertainties associated with agricultural greenhouse gas reporting and to support the development of mitigation actions. Nitrous oxide emissions were measured from the range of nitrogen sources that are added to soils under contrasting soil and climatic conditions across the UK. Experiments used standard protocols for experimental design, measurements, data handling and statistical analysis. A parallel process of verification and modelling of the data obtained from these research activities was undertaken. The Greenhouse Gas Platform research has provided improved understanding of emissions from agricultural sources. The observed N2O Emission Factor (EF1 is the proportion of N2O derived from a nitrogen source) for fertilisers applied to grasslands was 1.3 %, however, the Emission Factor for fertiliser applied to arable crops ranged between 0.5-0.8 % (depending on the nitrogen source) which is below the 1% Tier 1 default value. Nitrous oxide emissions from grazing returns by cattle were also significantly lower than default IPCC values with Emission Factors from dung of 0.2%, and urine of 0.7%, (as compared with the default EF from these sources of 2%). These new Tier 2 N2O Emission Factors are currently being applied to the UK's inventory reports decreasing the relative importance of N2O as a greenhouse gas.