Limitations of charcoal as an effective carbon sink

May 2, 2008

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. 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 study finds that the supposed 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.

Source: Swedish University of Agricultural Sciences

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2 / 5 (4) May 08, 2008
Some comments from Terra Preta Soil researchers, for more info or questions please visit; http://terrapreta.../?q=node

It has been immensely gratifying to see all the major players join the mail list , Cornell folks, T. Beer of Kings Ford Charcoal (Clorox), Novozyne the M-Roots guys(fungus), chemical engineers, Dr. Danny Day of EPRIDA , Dr. Antal of U. of H., Virginia Tech folks and many others .


From Christoph Steiner to the terrapreta List:

" The study of Wardle et al was simple and provides valuable data. 10 year
studies are a rare opportunity but other conventional means of C
sequestration (conservation tillage etc) were studied much longer. That
charcoal increases the decomposition of labile soil organic matter (SOM)
is a logical consequence of increased microbial biomass and activity.
This is the example of humus rich Swedish forest soils. The Terra Preta
example is different (low respiration rates in absence of an easily
degradable organic substance). Chernozems are a other example. Charcoal
can led to the formation of very persistent SOM and this in environments
and soils with low carbon sequestration capacity. Nobody proposed to
apply charcoal as a C sink in humus rich soils. It can be a mean of
carbon sequestration in depleted soils (e.g. southeastern US). Due to
agriculture most soils have lost 50% of there original carbon content.
The recalcitrance of charcoal allows SOM build up beyond the carrying
capacity of a soil.
In some cases increased decomposition might be even desired. Composting
of manures and other green biomass would be increased and emissions of
CH4 and N2O reduced. Organically applied nutrients might be faster
available for plants if applied with charcoal and leaching of nitrogen
This study proves once again the recalcitrant nature of charcoal and
shows that we have to do much more research in the field to determine
appropriate applications for charcoal as a C sink. I am confident that
there are many management options."

Here is another TP researcher's view of the article.

Edward Someus to Terrapreta List

show details 11:47 PM (6 hours ago) [Charcoal LOSS.pdf]


Dear Folke,

Pls find encl the publication text PDF. Need to be studied more in-depthly and discussed later on. The department of forest ecology and management, SLU (Swedish University of Agricultural Sciences) Umea Professor David Wardle, Professor Marie-Charlotte Nilsson and Professor Olle Zackrisson are high science Colleagues, and such rarely 10 long years executed study is of value and should be taken very seriously.

The 10 year study - in relation to the evaluation of forest wild fires - is executed in the Boreal forest sites in northern Sweden humus rich forest soils with high "dosis" 50/50 mixtures and 100% blank. The conditions and the scenario for wild fire boreal forest is very much different than for AGRO TP, which agricultural adaptation may also widely variate at different places and cultivation practices world wide.

The industrialized production soil-char composite products I develop is a microbiologically pre-mobilized organic/inorganic complex fertilizers with combined effects, with successful dosis 400 kg/ha up to 1000 kg/ha in granulated form with consideration 20 cm top soil. At this moment we have wide soil and climatic tests ongoing in Italy, The Netherlands, Germany, Israel, UK and Hungary.

I have distributed the publication to my Colleagues at different soil science groups at UK, NL, DE, IT Universities and for experience / knowledge exchange I will make follow up and direct discussion with the SLU Professors as well.

However, KEVIN may have right: "Reading between the lines of the article, one could guess at the general content of the Paper, but the article was presented in a shallow, sensationalist manner that would likely leave with the uninitiated reader with the impression that charcoal in soil was "a bad thing." "


Sincerely yours: Edward Someus (environmental engineer)
EMAIL 1: edward at
EMAIL 2: edward.someus at

Erich J. Knight
540 289 9750

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