Charcoal's green image blackened

The idea of using charcoal as a carbon sink may be over-optimistic.

Charcoal seems to promote the release of carbon from some types of soil. Credit: Getty

Unpalatable to bacteria and largely stable, charcoal has been touted as a powerful way to trap carbon. But it might actually prompt soil to release carbon at higher rates, new research indicates.

Charcoal (or biochar) has been heralded as a way to enhance agricultural yield while immobilizing large amounts of carbon, thereby mitigating climate change. But a ten-year trial in Swedish forests now shows that charcoal may promote the breakdown of humus, the thick layer of decomposing plant matter on the forest floor.

“Our results suggest that charcoal is not inert in soil,” says study author David Wardle, an ecologist with the Swedish University of Agricultural Sciences in Umeå.

The charred material still traps more carbon than it helps to release, he says. But its interaction with soil could at least “partially counteract” its ability to trap carbon. The results are published today in Science1.

Quick release

Wardle and his colleagues started their charcoal experiment to examine the effect of forest fires on soil ecology. To do so, they buried nearly 400 mesh bags containing either humus, charcoal, or a 50–50 mixture of the two in several sites in the Swedish boreal forest.

They periodically weighed the bags and measured the concentration of carbon and nitrogen in them: a loss of weight indicated that something was being degraded and either leached away or released as gas. After just one year, they began to see an unexpectedly large amount of mass was being lost from the bags containing the humus–charcoal mixture. Over ten years, these bags surprisingly released just as much carbon as did those containing just humus. Adding together the amount of carbon expected to be released by humus and charcoal independently clearly did not account for it all.

Based on previous experiments showing that charcoal has a lifetime of hundreds of years, Wardle and his colleagues reasoned the charcoal was stable, but was somehow enhancing the breakdown of humus.

This is not completely unexpected. Charcoal is known to provide a surface and favourable chemistry for microbes to live on while they break down humus, says Wardle. But the magnitude of carbon released in the experiment was surprisingly large.

Beyond boreal

Charcoal’s effect in more natural situations is not yet clear. Because the bags were buried at the start of the experiment, the test doesn’t mimic the natural weathering processes of the compound, says James Amonette, a geochemist at the Pacific Northwest National Laboratory in Richland, Washington. Chemical differences on charcoal surfaces, created by weathering and natural burial, could lessen the material’s impact on humus degradation.

Still, Amonette says, the study is “something we need to think about. We need lots more work to find out how charcoal interacts with soil constituents.”

The effect might also be different with different soil types. Humus is very rich in organic matter: as much as half of its weight is carbon. Other soils, such as the mineral soils used in agriculture, carry less carbon. And minerals interacting with organic matter may inhibit charcoal from boosting emissions of carbon dioxide, some suggest.

“This is a very good starting point for further studies, but I would say we need to confirm it for mineral soil,” says Johannes Lehmann, a soil biogeochemist at Cornell University in Ithaca, New York, and advocate of biochar. He admits that the study seems to put “less value on the strategy of net carbon sequestration with biochar”.


  1. 1

    Wardle, D. A., Nilsson, M.-C. & Zackrisson, O. Science 320, 629 (2008).

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Courtland, R. Charcoal's green image blackened. Nature (2008).

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