Black Carbon in the Soil Carbon Cycle: Is it an Oxidation Resistant End-Product?

Black carbon (BC) is a very oxidation resistant product of incomplete combustion, and consists out of a range of combustion products such as char, charcoal, and soot. Due to its high recalcitrance, black carbon might act as a long-term carbon sink. Comparing the estimated production rate of BC with the effective pool, loss processes of BC must occur. However, the knowledge of black carbon degradation is still very low. Only three investigations have been conducted so far to compare fire protected sites with non protected sites. They found less charcoal in the protected site. Possible loss processes were explained by oxidation, what led to a better water solubility and therefrom a better mobility. Different studies measured directly microbial decomposition of ther-mal altered biomass. The decomposition decreased with increasing alter-nation temperature of the biomass. The structure of the biomass became more aromatic and therefore more stable. Glucose additions enhanced the mineralization of charred biomass. The degradation rate is influenced by molecular characteristics, soil properties and climate conditions. For instance, the stability of black car-bon increases with condensed aromatic structure and lower H/C and O/C ratio. Lignin-rich biomass is more stable to oxidation. The persistence of BC is affected by complexing with mineral particles or metals. The mineralization rate varies for different materials and combustion temperatures. It is less than 1 % for thermally altered biomass at combus-tion temperature higher than 200◦C. The half-life time of BC is probably less than a century. The oxidation of BC is enhanced in a first step and then leveled off. The remaining end-product is very recalcitrant. This paper reviews the current knowledge of degradation processes of BC in soils.