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Biochar: part of the solution?

A short quote from Edinburgh University's website on biochar suggests quite a long period before buried char is oxidised and the CO2 released into the athmosphere:

Biochar from forest fires has existed in some soils for 10,000 years, whilst radiocarbon dating of the terra preta soils of Amazonia show that the carbon can persist in the soil for between 500 and 7000 years before present (Lehmann et al., 2009). One conservative estimate is that the Mean Residence
Time (MRT – the average time that biochar remains in the soil) is between 1000 and 2000 years for dryland conditions of northern Australia. A recent study confirmed that the MRT for black carbon in two Australian savannah regions was between 1,300 and 2,600 years (Lehmann et al., 2008). The half-life of biochar found in coastal temperate forests in western Vancouver
has been calculated as 6623 years (Lehmann et al., 2009).

There are, however, some other studies which show a much faster turn-over time for biochar. A study of fires in the Russian steppe concluded that the turn-over time of biochar was only 293 years. Meanwhile, biochar stocks formed after
savannah burning in Zimbabwe had a MRT of only a few decades. Lehmann et al. (2009) suggest that these much lower residence times might be explained by processes other than mineralization alone (such as leaching or erosion).

The longevity of biochar in soils should not be overestimated: an unknown but large-scale mechanism for removing black carbon appears to exist. We know that more black carbon is produced than is found in possible long-term sinks (e.g. ocean sediments and the soil organic carbon pool) (Woolf 2008).
Over-accumulation of black carbon in soils is also inconsistent with empirically-validated models of the response of carbon in soils. One of the critical research needs is better understanding of such processes on different timescales. In summary, whilst there are major scientific
questions to be addressed regarding longevity, there is good evidence that biochar, if managed correctly, will remain in soils for at least 1000 years and possibly much longer. These timescales look sufficient for biochar to qualify as a viable option for atmospheric CO2 reduction (since, to be
effective in tackling anthropogenic climate change, the carbon must be removed from the atmosphere for hundreds to a thousand years) (Shackley and Gough 2006).

I got interested in the burial of charcoal, several years back, when I remembered that charcoal is found in strata deposited around the time of the Cretaceous–Tertiary extinction event; that is 65M years ago.

At the moment, a couple of hundred years would be useful and I imagine that burial could take place where contact with oxygen is limited. There are many other considerations, of course, but biochar is not going to be lost in months!