The boreal forests in the north of Canada, Russia and other countries that ring the Arctic burn every summer after lightning strikes their towering trees, releasing tons of carbon dioxide into the air as they turn to ash and charcoal in the flames. Some scientists have argued, however, that this climate-changing natural disaster might not be all bad from a global warming perspective: Charcoal is a stable way to store carbon in the ground, where the carbon-rich charcoal can safely stay for hundreds if not thousands of years. Or at least that's the theory of so-called biochar. A new study published today in Science shows that such charcoal may not keep as much carbon in the soil as previously believed.

The electrically charged surface of charcoal—the reason it is used in filters—absorbs simple organic compounds that, in turn, "serve as a good food source for microbes," says ecologist and study coauthor David Wardle of the Swedish University of Agricultural Sciences. "Because these compounds are carbon-rich, and are rapidly broken down by microbes on the surface of the charcoal, we get great accelerated [carbon] loss." In other words, instead of trapping carbon in the soil, charcoal, along with microbes, accelerates its release back into the atmosphere.

The ecologists discovered this by leaving hundreds of bags containing either pure charcoal, or the natural leaf litter of the forest floor, or a mix of charcoal and leaf litter at three sites in Sweden for a decade. They report that when they retrieved the bags in 2006, the contents of the bags with the mixture—the most like natural conditions after a forest fire—had shrunk by nearly 25 percent and lost a significant portion of their carbon within the first two years of the 10 year period.

The pure coal, however, remained nearly unchanged. "The amount of soil [carbon] lost as a result of adding charcoal to soil will likely partially counteract the amount of [carbon] sequestered by the charcoal itself," Wardle says.

Some experts have proposed using such charcoal, or biochar, as a way of offsetting the extra carbon dioxide emissions from a forest fire and dead trees. In fact, some have proposed that the vast swathes of forest killed in recent years by the pine beetle in Alaska and western Canada should be turned into such biochar.

But Wardle warns that rather than serve as a carbon sink, trees turned to charcoal could end up releasing even more carbon dioxide from the forest floor.

This does not mean that biochar might not find useful applications in agriculture, where it may enhance soil fertility as well as cut down on carbon emissions. "The most useful and easiest [place to apply biochar] would be to apply the biochar to agricultural soil that does not have a litter layer such as the one studied by the authors," says biogeochemist Johannes Lehmann of Cornell University, who studies the ancient biochar practices of the historical inhabitants of the Amazon.

But that means more research is needed to determine whether more carbon is captured or released using this process. Wardle says it's particularly important to see whether his findings apply to other ecosystems and types of charcoal. "In other words, is what we found a widespread phenomenon and is it ecologically important?"