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

Abstract

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.

Keywords: Candidatus Methanoperedens nitroreducens; anaerobic oxidation of methane; free-air CO2 enrichment; metal reduction; methane cycle; paddy rice.