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Charcoal is long-lived in soil but it is not permanent:


There is no doubt that in certain environments, charcoal is indeed recalcitrant. In a study of marine sediments in the North Pacific Basin, Herring (1985) found that “charcoal in the marine sediment is stable for several tens of millions of years” and that “charcoal forms a large percentage of the carbon content in the sediments”. Large accumulations of charred material with residence times in excess of 1000 years have also been found in soil profiles (Forbes et al 2006, Glaser et al 2001, Saldarriaga, et al 1986). Glaser et al (2003) attribute the presence of large stocks of pyrogenic black carbon in Amazonian dark earths, several hundred years after the cessation of activities that added it to the soil, to its chemical recalcitrance. Also, 14C ages of black carbon of 1000 to 1500 years from Amazonian Dark Earths suggest that it is highly stable (Glaser, 1999). Deposits of charcoal up to 9500 have been found in wet tropical forest soils in Guyana (Hammond et al, 2007), up to 6000 years old in Amazonia (Soubies 1979), and up to 23,000 years old in Costa Rica (Titiz & Sanford, 2007). (Source - PDF)


Soil biology plays a role in charcoal degradation.



Shnour (1966) reported a significant oxidation of graphite (the most stable form of black C) by microorganisms. Therefore, decomposition also of bio-chars can be expected. Experimental results are contradictory, and both rapid (Bird et al. 1999) and slow (Shindo 1991) decomposition of biomass-derived black C was reported. Notwithstanding the remaining uncertainty about its precise turnover, black C has been found to be the oldest fraction of C in soil, older than the most protected C in soil aggregates and organo-mineral complexes (Pessenda et al. 2001), which is commonly the most stable C in soil. This indicates that in quantitative terms biochar is stable, with decomposition leading to subtle, and possibly important changes in the bio-chemical form of the material rather than to significant mass loss. (Source - PDF)


In a study on the effect of glucose on microbial decomposition of black carbon in soils, Hamer et al (2004) found that “apparently, some microorganisms were able to live with BC as sole C source”. (Source - PDF)


It is worth noting that the longevity of BC in soils cannot be characterised by a single number. Pyrogenic BC is not a homogeneous substance (Hedges et al, 2000), and different fractions of it will decompose at different rates under different conditions. As Preston & Schmidt (2006) say, “Except for anoxic peats or permanently frozen soil, the high end for the half-life of PyC may be expected to be in the kY region (maybe 5–7 ky), for cold, wet environments, and for the Py fraction with more recalcitrant structure. At the other extreme, a half-life in the order of 100 y (Bird et al., 1999) may be not unrealistic for some fraction of PyC from boreal wildfire, with less thermal alteration and especially with surface exposure (unpublished field observations from Canadian and Siberian boreal forest sites)”.(Source - PDF)



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