In the aftermath

After a fire rages through a forest, what’s left is charcoal, which can remain in the soil for centuries, even millennia. Scientists hoping to capitalize on this persistence and sequester carbon by burying charred wood may be disappointed: Apparently, charcoal in forest soil stimulates microbial activity that accelerates carbon loss from organic material covering the forest floor.

Although scientists have long known that charcoal isn’t biologically inert, its effect on organic matter in soil is poorly understood, says David A. Wardle, a soil ecologist at the SwedishUniversity of Agricultural Sciences in Umeå. To address the dearth of information, Wardle and his colleagues conducted field experiments in the forests of northern Sweden.

Wardle’s team buried small mesh bags that had one of three fillings: charcoal, carbon-rich humus or a half-and-half mix of charcoal and humus. The charcoal was burnt wood from crowberry, the most common shrub found at the Swedish forest sites, and the humus was collected from those sites as well. The researchers left the bags at three types of sites: one in a mature, 450-year-old forest, one in a forest that had recently experienced a fire and one in a forest halfway to maturity. They recovered the bags one, two, four and 10 years later.

After 10 years, each bag of charcoal, on average, still weighed the same, but more than 26 percent of the mass in the bags containing only humus was missing, presumably organic matter that had been lost via decomposition. Using those data, the team expected the charcoal-humus mix to have lost about 13 percent of its mass in 10 years, says Wardle. Instead, about 23 percent of the mass in those bags disappeared, he and his colleagues report in the May 2 Science.

Analyses of the samples revealed a higher rate of microbial activity in the charcoal-humus mixtures than in humus alone, says Wardle. It wasn’t clear whether that activity accelerated the rate of decomposition or generated water-soluble substances that later leached from the bags. In particular, says Wardle, tiny fungi like to feed on the organic compounds that charcoal absorbs and concentrates.

The new report “provides really important findings,” says Tom DeLuca, a forest ecologist with The Wilderness Society in Bozeman, Mont. Most previous studies of charcoal’s effects on ecosystem processes have lasted only weeks or months, he notes. Insights about how charcoal alters decomposition rates in soil are particularly timely, he adds, because some scientists have suggested that burying large amounts of charcoal could be an effective way to sequester carbon.

The new results are particularly significant because boreal forests — those found at high latitudes of the Northern Hemisphere, like those in Sweden — hold about 703 billion metric tons of carbon, says David U. Hooper, an ecosystem ecologist at Western Washington University in Bellingham. Moreover, about 89 percent of that carbon sits in the soil, not the plants themselves.

Thus, he notes, a boreal forest fire would deliver a “triple whammy” of carbon dioxide to the atmosphere. Besides the effect described by Wardle and his colleagues, combustion of the trees and material lining the forest floor releases a large pulse of the greenhouse gas. Finally, he notes, charcoal left in the wake of a fire tends to darken the ground. That causes the ground to absorb more sunlight and warm considerably, thereby boosting long-term rates of decomposition and carbon dioxide production.