Different impacts of an electron shuttle on nitrate- and nitrite-dependent anaerobic oxidation of methane in paddy soil

Quinones, redox-active functional groups in soil organic matter, can act as electron shuttles for microbial anaerobic transformation. Here, we used (CH4)-C-13 to trace C-13 conversion (C-13-CO2 + C-13-SOC) to investigate the influence of an artificial electron shuttle (anthraquinone-2,6-disulfonate, AQDS) on denitrifying anaerobic methane oxidation (DAMO) in paddy soil. The results showed that AQDS could act as the terminal electron acceptor for the anaerobic oxidation of methane (AOM) in the paddy field. Moreover, AQDS significantly enhanced nitrate-dependent AOM rates and the amount of C-13-CH4 assimilation to soil organic carbon (SOC), whereas it was remarkably reduced nitrite-dependent AOM rates and C-13 assimilation. Ultimately, AQDS notably increased the total DAMO rates and C-13 assimilation to SOC. However, the electron shuttle did not change the percentage of C-13-SOC in total C-13-CH4 conversion. These results suggest that electron shuttles in the natural organic matter might be able to offset methane emission by facilitating AOM coupled with the denitrification process.