The discussion following the previous post brings up a complicated issue - accelerated decomposition of organic matter in fertile soil. For example, a study found that forest floor leaf litter decomposed more rapidly when biochar was present.

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 is no surprise, it is what we should expect from knowledge of how biochar functions. More importantly, it is what we desire rather than a disappointment.

The confusion comes from not considering the whole system. When organic matter is mineralized by microbes CO2 is released, but the nutrients released from the organic matter accelerate growth allowing trees to draw down more CO2. The net effect is reduced CO2 in the atmosphere.

Other studies show that increased productivity takes place below ground to a significant degree. Trees make more fine roots. They are sloughed off each year below ground and accumulate. In a way the leaf litter is converted from forest floor debris to root hairs which endure for longer.

Still other studies show other benefits from improved soil fertility in forests. The foliage of fertile forest trees reflect more solar radiation, increasing planetary albedo and reducing heating.

To evaluate a forest management system such as using biochar the narrow findings of various studies must be combined to yield a fuller picture over time.

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?"

I think that this is a short sighted and narrow view. I'm all in favor of using biochar in agriculture, and even in favor of transporting some forest litter to cultivated fields as biochar, but to understand the dynamics of such amended soil systems the focus has to be deeper than just the surface layer.