Scientists probe vegetables' carbon cost

30 March 2009 by Tom Marshall

Retailers and the public need to rethink concepts like 'carbon footprints' and 'food miles' according to new research.


These initiatives aim to give people a clearer idea of how much greenhouse gas is emitted to produce a piece of food and transport it to them, allowing them to make better-informed choices as consumers.

The measures take into account things like the amount of fuel used to bring them from producer to retailer, and have been the subject of growing interest from a public increasingly concerned about human greenhouse gas emissions.

But they don't consider other factors such as the carbon dioxide emitted into the atmosphere from the soil in which vegetables are grown. Plants absorb carbon from the atmosphere as they photosynthesise, but carbon dioxide is also emitted as both the plants and the microorganisms in the soil around them respire.

Once soil carbon budgets were taken into account, the researchers found that vegetables' 'carbon footprints' varied so much from place to place and between different crops that it was almost impossible to make generalisations.

Land management and its effect on carbon emissions is not a trivial issue; changes in land use are estimated to have released 156 gigatonnes of CO2 into the atmosphere between 1850 and 1990, around half as much as was released by burning fossil fuels, and soils accounted for about a quarter of this.

Current methods of labelling products with carbon footprints are fundamentally flawed, because they don't take account of soil carbon emissions.

- Professor Davey Jones

Soils in the UK are estimated to have emitted CO2 at around a third of the rate of industrial sources. Though the issue of food miles and the fuel used to transport fruit and vegetables between countries and continents has attracted much attention, they only account for around a thirteenth of the CO2 given off by soils. But it's estimated that improved land management could sequester up to 150 gigatonnes of carbon over the next 50-100 years.

The paper appears in the Journal of Applied Ecology; it's the first attempt to understand how carbon moves through vegetable fields throughout the growing cycle in a variety of climates.

Most of the carbon absorbed by a plant is rapidly released back into the atmosphere, but some is stored for a longer time in organic matter in the soil. The researchers measured all these variables to look at the total CO2 absorbed and emitted by a variety of crops grown in the UK, Spain and Uganda.

"Current methods of labelling products with carbon footprints are fundamentally flawed, because they don't take account of soil carbon emissions," says Professor Davey Jones, a specialist in soil and environmental science at Bangor University and one of the paper's authors. "Annual variability combined with geographical variability mean it's almost impossible to come up with a single number that you could put on a food packet - you need to take account of exactly where the crops were grown."

That's because the food inside often come from crops grown in many different nations, and each crop will release a different amount of CO2 into the atmosphere.

The research highlights other issues in this complex area. Some crops cause far more CO2 to be emitted than others. Brassicas such as broccoli are comparatively benign in the CO2 they emit over their lifetimes, largely because only some of the plant's biomass is harvested, and the rest is returned to the fields where it breaks down slowly and adds carbon to the soil. This means that over their whole lives, broccoli plants tend to put more carbon into the soil than they emit.

Other crops, such as lettuces, are far more harmful as almost the whole biomass of the crop is removed from the field for consumption, leaving little to rot back down into the earth and replenish the soil's stock of organic carbon. Lettuce plants are therefore a net source of carbon.

"Crops like lettuce pose a challenge. Should we stop growing lettuce? It has very little real nutritional value and often ends up in the household dustbin. Further, it's not good for the soil because you're constantly removing all the biomass from the land - you're taking away year after year without putting anything back. Innovative rotations may help with this in the future though", explains Jones.

One implication in the short term could be that it is a bad idea to grow plants like lettuce in hot climates like Uganda's, since heat increases the soil's rate of respiration and hence exacerbates the crop's already-hefty carbon emissions.

Irrigation can make this even worse - it means several growing seasons can be squeezed into a year, so soil bacteria are respiring and emitting CO2 for much more of the year.

On the other hand, another part of the research proposed an alternative perspective which suggests it's better to grow these plants in Africa. From an ethical standpoint, Jones notes, "Workers on export farms in countries like Kenya benefit from better work and pay conditions compared to their non-export counterparts. Whereas in Britain farms generally employ young migrant farm workers who are then vulnerable to all the insecurities and worries of working abroad" he says. "So ethically it is better to grow lettuces abroad - but there is a trade-off between ethics and carbon emissions."

In the longer term, a return to more traditional and sustainable methods of cultivation is desirable, Jones argues. "We're big on crop rotation and returning organic matter to the soil," he says. "In the long term we ought to be going back to the old systems where you put as much organic matter as possible back into the soil and you rotate crops and let the land rest rather than just growing year after year."

The research comes out of the £24m Rural Economy & Land Use (RELU) programme, which aims to draw together research across different disciplines to understand the social, economic, environmental and technological challenges that rural areas face.

RELU is an interdisciplinary collaboration between the Economic & Social Research Council (ESRC), the Biotechnology & Biological Sciences Research Council (BBSRC) and NERC. Additional funding comes from the Scottish Government and the Department for Environment, Food & Rural Affairs.