Interactive priming of black carbon and glucose mineralisation

Black carbon (BC) in soil is important in the global C cycle, but only little is known about the mechanisms and rates of its degradation. We have investigated the influence of 14C-glucose addition on the mineralisation of charred maize and rye residues (thermally altered at 350 °C) and oak wood (thermally altered at 800 °C). The different BC materials were mixed with sand and incubated for 60 days at 20 °C. The samples received 20 μg glucose-C per mg black carbon at the beginning and at day 26 of the experiment. At the beginning, a nutrient solution [(NH4)2SO4 + KH2PO4] and an inoculum extracted from an arable soil were added. In the controls without glucose addition, between 0.3% (wood) and 0.8% (maize) of the initial charred materials were mineralised and 0.6–1.2% when glucose was added. The two glucose additions accelerated BC mineralisation, with the second addition inducing a stronger enhancement of mineralisation than the first one. A close correlation (r=0.94, p<0.001) between glucose mineralisation and additional BC mineralisation suggests that BC degradation may be due to co-metabolism, i.e. to the enhanced growth of microbial biomass and the accompanying increased enzyme production. On the other hand, glucose mineralisation was enhanced over the control by the presence of charred material, i.e., there was an interactive priming of BC and glucose mineralisation. We conclude that BC in soils may promote growth of microorganisms and the decomposition of labile C compounds rather than stabilise them against degradation.