Oxidation of black carbon by biotic and abiotic processes

The objectives of this study were to assess the relative importance of either biotic or abiotic oxidation of biomass-derived black carbon (BC) and to characterize the surface properties and charge characteristics of oxidized BC. We incubated BC and BC-soil mixtures at two temperatures (30 °C and 70 °C), with and without microbial inoculation, nutrient addition, or manure amendment for four months. Abiotic processes were more important for oxidation of BC than biotic processes during this short-term incubation, as inoculation with microorganisms at 30 °C did not change any of the measured indicators of surface oxidation. Black C incubated at both 30 °C and 70 °C without microbial activity showed a decrease in pH (in water) from 5.4 to 5.2 and 3.4, as well as an increase in cation exchange capacity (CEC at pH 7) by 53% and 538% and in oxygen (O) content by 4% and 38%, respectively. Boehm titration and Fourier-transform infrared (FT-IR) spectroscopy suggested that formation of carboxylic functional groups was the reason for the enhanced CEC during oxidation. Analysis of surface properties of BC using X-ray photoelectron spectroscopy (XPS) indicated that the oxidation of BC particles was initiated on the surface. Incubation at 30 °C only enhanced oxidation on particle surfaces, while oxidation during incubation at 70 °C penetrated into the interior of particles. Such short-term oxidation of BC has significance for the stability of BC in soils as well as for its effects on soil fertility and biogeochemistry.