[Biochar](https://en.wikipedia.org/wiki/Biochar) is often heralded as a great combination of soil improvement and carbon sequestration. However, at least on the carbon sequestration front, it seems to be a mixed bag. [Biofuelwatch cites a paper from Nature (2011)](http://www.biofuelwatch.org.uk/wp-content/uploads/Biochar-Report3.pdf):

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> *“Persistence of soil organic carbon is primarily...an ecosystem property.”*' This is why some types of soil carbon which are highly 'unstable' in laboratory conditions have been found to remain in some soils for long periods of time whereas black carbon, which under laboratory conditions appears particularly stable, has been found under some circumstances to decompose quicker than other types of carbon. The authors
>  therefore warn: *“Sequestration strategies based on adding recalcitrant material to soils, whether through plant selection for recalcitrant tissues or through biochar amendments must be reevaluated.”*
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The linked text also claims that black carbon is not inert, but the exact mechanisms under which black carbon degrades are not well understood. 

They also mention several field trials where after 2 or 4 years, the carbon content of soil treated with biochar did not differ significantly from soil without biochar. So the claim that black carbon in the form of biochar has a very long lifetime in *all* soils seems to be false. As mentioned in the nature article, black carbon degradation seems to be very much a function of the soil ecosystem, which in turn depends on soil type; mineral, oxygen and moisture content; vegetation and farming practices etc.

Given the uncertainty of the lifetime of biocahr-C in the soil, is biochar actually a viable way of carbon sequestration? Are there specific application, e.g. biochar + specific soil type + specific farming style, that are shown to work as long term carbon sequestration?