Coupled anaerobic methane oxidation and metal reduction in soil under elevated CO.

Continued current emissions of carbon dioxide (CO ) and methane (CH ) by human activities will increase global atmospheric CO and CH concentrations and surface temperature significantly. Fields of paddy rice, the most important form of anthropogenic wetlands, account for about 9% of anthropogenic sources of CH . Elevated atmospheric CO may enhance CH production in rice paddies, potentially reinforcing the increase in atmospheric CH . However, what is not known is whether and how elevated CO influences CH consumption under anoxic soil conditions in rice paddies, as the net emission of CH is a balance of methanogenesis and methanotrophy. In this study, we used a long-term free-air CO enrichment experiment to examine the impact of elevated CO on the transformation of CH in a paddy rice agroecosystem. We demonstrate that elevated CO 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 CO 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 CH . 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.