Limitations of charcoal as a carbon sink

I've been hopping on the biochar bandwagon myself, so I solemnly take note of the following bulletin from Science Daily: 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.

There has been greatly increasing attention given to the potential of ‘biochar’, or charcoal made from biological tissues (e.g., wood) to serve as a long term sink of carbon in the soil. This is because charcoal is carbon-rich and breaks down extremely slowly, persisting in soil for thousands of years. This has led to the suggestion being seriously considered by policy makers worldwide that biochar could be produced in large quantities and stored in soils. This would in turn increase ecosystem carbon sequestration, and thereby counteract human induced increases in carbon-based greenhouse gases and help combat global warming.

However, 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. In their study, 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.

They found that 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, they 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....

Charcoal pile. Picture scanned in : Manuel de la métallurgie du Fer, Tome 1, by Adolf Ledebur, French version translated by Barbary de Langlade annoted par F.Valton, published Librairie polytechnique Baudry et Cie, 1895. page 69... where else but Wikimedia Commons