The challenge to sustain a growing population without exacerbating climate change is clearly huge.
For instance, nitrogen fertilisers are highly beneficial, but somehow we must mitigate their side-effects. It may not be helpful to UK carbon accounting that a global average of nitrous oxide emissions from nitrogen fertiliser is used, says Roger Sylvester-Bradley of ADAS.
There's no disputing agriculture is responsible for producing about 70% of the UK's nitrous oxide, a very damaging greenhouse gas. But national accounting by the government, and product accounting by supermarkets and the biofuels industry, currently adopt the worldwide estimate that 1% of fertiliser nitrogen applied is emitted as nitrous oxide.
"We have little idea if 1% is about right or way off for the UK, so we need to measure emissions under our conditions. We must inform carbon accounting at the farm scale as well as having the necessary UK data to support global accounting," says Prof Sylvester-Bradley.
That's the objective of a LINK project involving organisations from all industry sectors. The UK agriculture industry is taking action to meet governments' targets of reducing agricultural greenhouse gases – for instance by 11% in England by 2020. Its plans will need reviewing and the project outcome will guide this.
At field level, Prof Sylvester-Bradley and his team are measuring the influence on emission levels of ammonium-nitrate applied two or three times in the spring at 40-160% of recommended rates. Plastic chambers placed around test crops are sampled about 50 times a season. Lids are put on the chambers for 40 minutes at a time, then the air is sampled and analysed for nitrous oxide.
The amount emitted will give a measure of the impact of a particular nitrogen management practice. The variables likely to affect emissions are crop species, rotation, soil type, local weather and method of crop residue disposal.
Prof Sylvester-Bradley expects some crops, for example nitrogen-fixing pulses, to have generally lower nitrous oxide emissions than winter wheat. Typically, crops in the drier east and south of the country – particularly on faster-draining light soils – are likely to have lower emissions than crops in the west or north.
Over the first two seasons of the project, nitrous oxide emissions were low because both springs were abnormally dry. For emissions to be notable, the topsoil must contain significant mineral N, and be sufficiently warm and wet.
"We are hoping for a wet spring this year, or we'll have to extrapolate to estimate the emissions for a typical season. By the end of the project, with measures of emissions from many crops and estimated carbon intensities of numerous products, we'll be much better placed to see how foods and biofuels can have smaller footprints," says Prof Sylvester-Bradley.
Crops perspective
Having UK-specific figures for nitrous oxide emissions has got to be better than using a global estimate for carbon accounting. This project will underpin plans to develop cropping systems in which growers can make the most of nitrogen fertilisers without worsening climate change.
Summary
Project no 3474: Minimising nitrous oxide intensities of arable products; ADAS, SAC, grower organisations, fertiliser manufacturers, food, drink and fuel producers, DEFRA and Scottish government under the Sustainable Arable LINK Programme; from July 2009 to July 2014.
HGCA PERSPECTIVE:
• carbon accounting, already important to regulators and the supply chain, will have implications at farm level
• need to understand greenhouse gas emissions and carbon footprints of arable products, to help develop mitigation strategies
• management of fertilisers and crop residues 'part of the solution', along with improving nutrient use efficiency and precision agriculture
• Interim report available on HGCA website: www.hgca.com/research
• Catch up with previous pieces from our Research in Focus series at www.fwi.co.uk/researchinfocus
Glossary
• Carbon dioxide – mildly damaging greenhouse gas, high levels in system
• Nitrous oxide – powerful global warming greenhouse gas, comparatively low levels
• Carbon footprint – all greenhouse gases due to a product, expressed as carbon dioxide equivalents. For arable products, mainly nitrous oxide, some carbon dioxide, a little methane
• Carbon accounting – estimating all greenhouse gases associated with a product