Biochar promotes methane production at high acetate concentrations in anaerobic soils

Methane (CH4) is a major greenhouse gas emitted by the industry, agriculture and natural processes such as degradation of organic matter in anaerobic waters and soils. Addition of biochars to soils has been investigated to control greenhouse gas emission, but actually there are still current conflicting views: do biochars promote or suppress greenhouse gas emissions? In particular, the role of acetate, a product of organic matter degradation and substrate of methanogenesis, is poorly known. Here, we studied the effect of acetate concentrations on CH4 production by soils amended with biochar. Results show that biochar decreased CH4 release at low acetate concentration, below 0.2 mM. Gompertz modeling reveals that CH4 production rate can be reduced by up to 26.2% by biochar application. In apparent conflict, results show also that biochar stimulated methanogenesis at higher acetate concentrations, above 2 mM. Moreover, the application of CH3F, an inhibitor of acetoclastic methanogenesis, demonstrated that biochar mildly strengthened acetoclastic methanogenesis, thus confirming that biochar favors methane production. C-13 isotope analysis demonstrated that CH4 is derived from direct acetate cleavage when the methanogenic substrate, e.g., acetate, was relatively abundant. Data from 16S rRNA sequencing suggest that CH4 is produced by the metabolism of Methanosarcinaceae. This work provides extra perspective that promotion or suppression of CH4 release by biochar may depend on acetate concentration in terrestrial methanogenic environments.