Complete organic crop switch would raise UK emissions

Ploughing up pasture land for use to grow vegetables releases organic carbon as greenhouse gases. Credit: Andrew Stawarz/Flickr

Succeeding in the struggle to feed the increasing number of people in the world could hinder the fight against climate change if we relied on organic crop farming alone. That’s because it produces less food compared to the same area farmed using more usual methods, a team of British scientists said last week.

“Widescale adoption of organic crop production in the UK is always likely to require conversion of pasture to arable land,” agricultural consultant Rob Carlton told Simple Climate. Pasture land that livestock graze on contains more soil organic carbon than arable land, and changing over can release almost two tonnes of carbon per hectare each year. Thanks to those emissions, growing the same amount of crops in the UK with organic methods as existing farming does would produce the equivalent of three times as much CO2. But using aspects of the “environmentally-friendly” organic methods and more usual ones together might actually reduce such greenhouse gas emissions and help meet targets to slow global warming.

How we humans feed ourselves is an easy-to-overlook source of greenhouse gases that warm the planet. However in the UK it produces almost one-tenth of our emissions, Rob explained. “The UK is aiming for an 80 per cent reduction on 1990 greenhouse gas emissions by 2050 and a proportion of these cuts should come from the agricultural sector,” he said. “Globally agriculture accounts for over 14 per cent of greenhouse gas emissions, rising to 26 per cent when forestry and land use change are included. However, the need for emissions reductions should be viewed against demands on agriculture which are increasing as the population and consumption increases and farmers diversify into industrial and fuel crops.”

Need to feed

Leaving fields planted with clover or grass in between growing crops can trap carbon that might otherwise be emitted as greenhouse gases. Credit: .FuturePresent./Flickr

Rob had previously studied what it might mean for climate change if the amount of crops farmers in the UK could produce fell. Along with other scientists, including the University of Aberdeen’s Pete Smith, he worked out how much greenhouse gas would be produced converting other land to grow arable crops like vegetables and cereals. When discussing these ideas at a scientific meeting he found that Jon West from Rothamsted Research, which claims to be the world’s oldest agricultural research station, was doing similar work. Jon and his Rothamsted colleague Bruce Fitt therefore teamed up with Rob and Pete to look at yields and emissions from different farming methods. “This study is one of the first of its kind in that it looks at the complete arable production for the UK, an exceptionally complex task,” Rob said.

In a research paper published in the European Journal of Plant Pathology last Friday, the scientists counted up greenhouse gas emissions for all crops under four different farming systems. The two most familiar were conventional systems and organic ones, which strictly limit the use of synthetic chemicals like fertilisers and fungicides. Organic farming instead uses manure and rotation, where some fields are planted with grass or clover instead of crops, to boost fertility. These also trap extra carbon in the soil. However, lower soil nitrogen levels and higher disease rates mean organic yields are still lower than conventional farming.

The third system, reduced tillage, digs up slightly less leftover plant material than conventional farming. This saves energy and helps build up more soil organic carbon, but the waste can make weeds harder to remove and can also harbour crop diseases. The fourth, integrated, approach both exploits fertilisers and fungicides to boost yields and manure and crop rotation to trap carbon. Rotation still means that more space is needed than in conventional farming, and some pasture would need to be converted if it were adopted across the UK.

Assumptions left to question

The scientists gathered figures on greenhouse gas emissions produced by different farming steps and when making materials, like fertiliser and equipment, but didn’t consider emissions from livestock grazing. They combined this with crop data from the UK’s Department for the Environment, Food and Rural Affairs. This showed them that the emissions from land conversion needed for countrywide organic farming would far outweigh the benefits of the extra carbon it would trap. Reduced tillage cut emissions by a fifth compared to conventional farming, by contrast. Yet Rob and his colleagues still said the method may not be suitable for all soils and warn about the extra disease risk. “The control of crop disease will continue to have an essential role in both maintaining productivity and decreasing greenhouse gas emissions,” they wrote.

Source of emissions	Emissions for the whole of the UK (equivalent to millions of tonnes of CO2)
	Conventional	Organic	Reduced till	Integrated
Crop	15.7	17.3	15.1	14.4
Soil organic carbon trapped in long term arable land	0.0	-12.4	-1.9	-12.4
Soil organic carbon depleted from converted pasture	0.0	43.4	0.0	17.9
Total (crop plus soil organic carbon)	15.7	48.3	13.2	19.8

Greenhouse gas emissions if the whole of the UK used one farming system

Though this paper suggests the integrated approach would increase emissions if it were used in the whole country, its borrowings from organic farming have little downside in some areas, Rob said. “In regions where cropping intensity is already close to organic practice, such as North West England, land conversion may not be necessary,” he noted. He also now wants to investigate whether it’s possible to trap as much carbon when fields are left with grass or clover in instead of crops for shorter periods. “There is a possibility that the integrated system could reduce emissions if soil organic carbon sequestration could be achieved at a higher cropping intensity or if conversion of pasture to arable land gives rise to lower carbon depletion rates than those assumed in the study,” the consultant said.

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