Biochar conductivity enhances methane generation in paddy soil by facilitating electron transfer mediated by dissolved organic matter

Abstract Biochar can regulate methane (CH 4 ) emissions from paddy soils. However, the mechanism through which biochar conductivity influences methanogenesis in paddy soils remains unclear. In this study, biochar samples with varying conductivity levels were prepared by incorporating different amounts of graphene. The dissolved organic matter (DOM) derived from biochar was completely eliminated before its application. The addition of conductive biochar in the paddy soil system increased CH 4 production by enhancing the electron transfer rate (ETR), as demonstrated by a significant positive correlation between CH 4 production and ETR. Electrochemical experiments conducted after the removal of DOM from paddy soil demonstrated that biochar enhanced the ETR in paddy soils by facilitating the electron transfer of dissolved organic matter. Anthraquinone-2,6-disulfonate (AQDS), a common analogue for quinone- and hydroquinone-containing molecules in DOM, facilitates electron transfer and serves as a model for electrochemically active DOM. An experiment with biochar and AQDS confirmed that biochar enhanced the ETR of AQDS, supporting previous findings. Following incubation, methanogenic archaea showed no significant change in relative abundance across systems, demonstrating that that biochar enhanced methanogenesis solely via accelerated ETRs, without altering the microbial community composition. This study deepens our understanding of how biochar conductivity affects methanogenesis and offers scientific guidance for optimising the use of biochar in paddy soils. Graphical Abstract