The information in this blog comes from the end of project report from the MIN-NO project, which is a Defra funded project looking at minimising nitrous oxide intensities of arable crop products. To read the full report and find out more click here.
The MIN-NO project (2009 - 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 emissions from UK agriculture (which excludes fertiliser manufacture). The greenhouse gas (GHG) intensity estimated with the MIN-NO smart EFs (which include reduced GHGs 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 arable food products are likely to have smaller GHG intensities than are being estimated at present. Also, bio-fuels 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.
Key messages for industry and policy
Most arable food products have significantly smaller GHG footprints than are being estimated by or on behalf of industry at present.
Biofuels made from N-fertiliser crops grown in the UK are more effective in reducing GHG than was previously thought. The impact of this finding will be enhanced further if the UK defines NUTS2 regional emissions estimates for biofuels in a similar way to that suggested by the MIN-NO model, e.g. depending on regional rainfall.
Mitigation of arable GHG emissions by reduced use of fertiliser N was estimated to be largely ineffective if indirect effects on land uses elsewhere were acknowledged.
As proposed in recent UK reviews, many potential GHG mitigation methods may be applicable to arable crops; these can be classed into four distinct themes.
i. Fertiliser systems (method of manufacture, formulation, application and timing) with low GHG emissions per kg nutrient
ii. Selection of species, varieties, and/or fertiliser systems that convert soil and fertiliser N more efficiently into harvestable biomass
iii. Sourcing of crop produce from regions with low rainfall and light soils hence low N2O emissions.
iv. Removal of crop residues if green, this applies to a minority of crops
Individually these approaches were estimated to have maximum mitigation potentials (on GHG intensities of crop produce) of -25%, -23%, -23% and approximately -16%/
The maximum GHG mitigation potential derived by aggregating all four mitigation approaches was around -30% for the harvested produce of most crops (grain, seed or root) hence from -5% to -35% for their food or fuel products, depending on the contribution of crop produce to total GHG footprint of the product.
This there are opportunities for industry to help further mitigate the GHG footprints of arable products through improved fertiliser systems (better regarded as crop nutrient supply systems), for example incorporating chemical inhibitors within fertiliser products, but their exploitation will depend on finding means of capturing some of the value e.g. through economic incentives offered by the supply chain.
Any improvements that the plant breeding industry can make int he N Use Efficiency of crop varieties will prove beneficial to GHG mitigation, but the scope will be modest, especially if further progress is made in fertiliser technology, because mitigation is multiplicative not additive.
The main opportunities for farmers to mitigate N2O emissions lie in selecting crop species and fertiliser systems. Unfortunately farmers using abated N fertilisers and following best practices have few other means of effective N2O mitigation at present (at least that could affect calculated GHG emissions). Even under- fertilising with N is counter - balanced by GHG effects through indirect land use change.
Thus the scope for the UK arable industry to further mitigate GHG intensities of its products is less than previously estimated, and GHG mitigation maxima could only be achieved if adequate and sustained incentives became available to support development and use of all the appropriate technologies.