Microbial mineralization and assimilation of black carbon: Dependency on degree of thermal alteration

Abstract

Black carbon is degraded slowly in the environment and its formation can therefore be an effective sink for atmospheric CO 2. This study examined whether charcoal is assimilated by microorganisms or not and estimated the rate of mineralization depending on the degree of thermal alteration of the black carbon. Charcoals were produced at three different temperatures from homogeneously 14C labelled plant material and incubated in soil, and 14C in the evolved CO 2 and the microbial biomass was measured. Unlike parallel plant samples, CO 2 evolution from the charcoals showed no lag phase, but a period of faster CO 2 evolution for the first 5–8 days followed by a period of slow evolution. The mineralization of charcoal appeared to decrease with increasing temperature at which it was produced. This was also the case after the initial period of fast CO 2 evolution. With the techniques used, it was not possible to observe any microbial assimilation of charcoal, either because it did not occur, or because the methods used were not sufficiently sensitive. However, the lack of a lag phase in the CO 2 evolution from the charcoals is in line with earlier evidence that charcoal is initially oxidized at the surfaces by abiotic processes.