Horrors! Swedish researchers have discovered that adding biochar to soil causes microorganisms to increase in number!

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.

I think that if they looked more closely they'd find that new organic matter is being produced at a higher rate too. More is produced and more is consumed since the soil is more alive. And I think they may be using the word humus in a confusingly loose way. Humus is quite stable in soil, the residue of raw organic matter that has already been processed by soil micro and macro organisms. It seems that they are referring to the raw, undigested organic matter as humus. I've heard this done before, and it isn't always an important distinction, but it is in this case. Both biochar and fully mature humus will remain in soil and accumulate over time, while the more ephemeral raw organic matter is produced and consumed cyclically in the production of ever more of the truly durable humus.

Other tests have established that biochar stimulates mychorrizal fungi and rhizobial bacteria, two of the most beneficial soil microorganisms. That it also stimulates other types of bacteria and fungi that decompose - i.e. rot - dead organic matter is no surprise.

It's important to parse the claims of these researchers. Organic matter will rot and release GHGs whether there is biochar present or not. It will not accumulate unless there are special conditions such as an acidic peat bog inimical to soil microorganisms. Biochar may accelerate decomposition by stimulating soil microorganisms, or by merely raising the PH, sweetening acidic soil. These are not bad things and will result in increased growth of new organic matter, sucking down CO2 released earlier.

A more important question is "just how durable is biochar?" We have some information about this but not full knowledge.