Minimising nitrous oxide intensities of arable crop products (MIN-NO)

Abstract

The MIN-NO project (2009 to 2014) used multi-site industry data, field experiments and modelling to improve estimates of nitrous oxide (N₂O) emissions associated with major UK arable crops and their products. Of 24 field experiments conducted in widely contrasting rainfall, soil and crop conditions, 21 showed direct N₂O emissions due to fertiliser nitrogen (N) to be less than the 1% default emission factor (EF) assumed by the Intergovernmental Panel on Climate Change. A simple model summarising these emissions predicted a 30-year average EF for arable land across the UK of only 0.46% of N applied.

A set of ‘smart’ EFs was devised for consideration by UK stakeholders, based on the MIN-NO model, other MIN-NO results and associated evidence. The smart EF for fertiliser N predicted a decrease in emissions of almost 10% of the previously estimated total N₂O-N emission from UK agriculture (which excludes fertiliser manufacture). The greenhouse gas (GHG) intensity estimated with the MIN-NO smart EFs (which include reduced GHG from fertiliser manufacture) expressed as emissions per tonne of UK feed wheat was 20% less than the ‘benchmark’ GHG intensity using a current default methodology. Smart EFs also gave reduced GHG intensities for harvested rapeseed, similar intensities for sugar beet and increased intensities for vining peas. Thus most UK arable food products are likely to have smaller GHG intensities than are being estimated at present. Also, biofuels made from N-fertilised crops could be considered more effective in reducing GHG emissions than is currently assumed.

However, prospects for mitigation of N₂O emissions associated with UK arable cropping are less than was thought previously. Farmers already using abated N fertilisers and following good practice lack any easy means of further mitigation. Feasible approaches tend to have economic costs, so further mitigation depends on the arable industry finding ways of capturing financially some of the value. Four feasible options were identified and, if all of these were aggregated, a combined GHG emissions mitigation potential of around -30% was estimated for the harvested produce of most crops, and from -5% to -25% for their food or fuel products. The best mitigation options appeared to lie in employing more sophisticated crop nutrient supply systems, and/or growing more N-efficient crops through better-informed selection of species and varieties. Other options, such as cultivation strategies to improve soil conditions, cannot be advocated without further research.