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article imagePaper Says Charcoal Limited as Carbon Sink

By Bob Ewing     May 2, 2008 in Environment
Fire-derived charcoal is thought to be an important carbon sink. However, a SLU paper in Science shows that charcoal promotes soil microbes and causes a large loss of soil carbon.
Biochar or charcoal made from biological tissues (e.g., wood) has been getting increased attention as a possible long term sink for carbon in the soil. Charcoal is carbon-rich and breaks down extremely slowly. These properties have lead to discussions by government officials around the globe about producing biochar in large quantities and then stored in soils.
It was thought that this in turn increase ecosystem carbon sequestration, and thereby counteract human induced increases in carbon-based greenhouse gases and help combat global warming.
The University press release says that a new study by Professors David Wardle, Marie-Charlotte Nilsson and Olle Zackrisson at SLU, the Swedish University of Agricultural Sciences, in Umeå, scheduled to appear in this Friday’s issue of the prestigious journal Science, suggests that these supposed benefits of biochar may be somewhat overstated.
The charcoal was prepared and mixed with forest soil, and left in the soil in each of three contrasting forest stands in northern Sweden for ten years.
When charcoal was mixed into humus, there was a substantial increase in soil microorganisms (bacteria and fungi). These microbes carry out decomposition of organic matter (carbon) in the soil, and consistent with this and the researchers found that charcoal caused greatly increased losses of native soil organic matter, and soil carbon, for each of the three forest stands.
Much of this lost soil carbon would be released as carbon dioxide, a greenhouse gas. Therefore, while it is true that charcoal represents a long term sink of carbon because of its persistence, this effect is at least partially offset by the capacity of charcoal to greatly promote the loss of that carbon already present in the soil.
The anticipated benefits of biochar in increasing ecosystem carbon storage may be overstated, at least for boreal forest soils. The effect of biochar on the loss of carbon already in the soil needs to be better understood before it can be effectively applied as a tool to mitigate human-induced increases in carbon-based greenhouse gases.
More about Carbon sink, Charcoal, Wood
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