Coupled anaerobic methane oxidation and metal reduction in soil under elevated CO2

Continued current emissions of carbon dioxide (CO2) and methane (CH4) by human activities will increase global atmospheric CO2 and CH4 concentrations and surface temperature significantly. Fields of paddy rice, the most important form of anthropogenic wetlands, account for about 9% of anthropogenic sources of CH4. Elevated atmospheric CO2 may enhance CH4 production in rice paddies, potentially reinforcing the increase in atmospheric CH4. However, what is not known is whether and how elevated CO2 influences CH4 consumption under anoxic soil conditions in rice paddies, as the net emission of CH4 is a balance of methanogenesis and methanotrophy. In this study, we used a long-term free-air CO2 enrichment experiment to examine the impact of elevated CO2 on the transformation of CH4 in a paddy rice agroecosystem. We demonstrate that elevated CO2 substantially increased anaerobic oxidation of methane (AOM) coupled to manganese and/or iron oxides reduction in the calcareous paddy soil. We further show that elevated CO2 may stimulate the growth and metabolism of Candidatus Methanoperedens nitroreducens, which is actively involved in catalyzing AOM when coupled to metal reduction, mainly through enhancing the availability of soil CH4. These findings suggest that a thorough evaluation of climate-carbon cycle feedbacks may need to consider the coupling of methane and metal cycles in natural and agricultural wetlands under future climate change scenarios.