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.

Chlorination as a disinfectant of tertiary treatment is the most common form in municipal wastewater treatment plants in China. Excess residual chlorine in reclaimed wastewater is harmful to growth of lawn grass. However, upper limit of residual chlorine aimed at reclaimed wastewater reuse for urban green land irrigation has not been promulgated. Lab-scale potted experiments of seedling tall fescue (Festuca arundinacea Schreb.)(TF) and Kentucky bluegrass (Poa pratensis L.)(KBG) were performed to evaluate the effects of residual chlorine in reclaimed wastewater on these two kinds of lawn grass. The results showed that: relative aboveground biomass, photosynthetic rate and total chlorophll (Tchl) concentration reduced to great extent with increasing of residual chlorine. The activity of catalase (CAT) of TF went up under low concentrations and decreased under higher ones, which was different from the decline trend of the same index of KBG. Considered growth of the two kinds of turf grass and disinfectant of reclaimed water, the upper limits of residual chlorine in reclaimed water for landscape irrigation should be equal to 1.0 mg/L for TF and 0.8 mg/L for KBG.