Biochar-Mediated Anaerobic Oxidation of Methane

Biochar was recently identified as an effective soil amendment for CH₄ capture. Corresponding mechanisms are currently recognized to be from physical properties of biochar, providing a favorable growth environment for aerobic methanotrophs which perform aerobic methane (CH₄) oxidation. However, our study shows that the chemical reactivity of biochar can also stimulate anaerobic oxidation of CH₄ (AOM) by anaerobic methanotrophic archaea (ANME) of ANME-2d, which proposes another plausible mechanism for CH₄ mitigation by biochar amendment in anaerobic environments. It was found that, by adding biochar as the sole electron acceptor in an anaerobic environment, CH₄ was biologically oxidized, with CO₂ production of 106.3 ± 5.1 μmol g^-1 biochar. In contrast, limited CO₂ production was observed with chemically reduced biochar amendment. This biological nature of the process was confirmed by mcr gene transcript abundance as well as sustained dominance of ANME-2d in the microbial community during microbial incubations with active biochar amendment. Combined FTIR and XPS analyses demonstrated that the redox activity of biochar is related to its oxygen-based functional groups. On the basis of microbial community evolution as well as intermediate production during incubation, different pathways in terms of direct or indirect interactions between ANME-2d and biochar were proposed for biochar-mediated AOM.