Elevated [CO2] reduces CH4 emissions from rice paddies under in situ straw incorporation

Rice paddies contribute to ∼48% of greenhouse gas emissions from cropland, with ∼94% from methane (CH4). Elevated atmospheric CO2 concentrations (eCO2) due to human activities, generally stimulate the rice growth, and in turn affect CH4 emissions from rice paddies. However, the effects of eCO2 on CH4 emissions from rice paddies are still unclear under in situ straw incorporation, the popular agricultural practice. Therefore, we conducted a 3-yr field experiment to investigate the effects of eCO2 on CH4 emissions under in situ straw incorporation in the rice-wheat cropping system, using the open-top chamber technology. We found that eCO2 reduced the CH4 emissions from rice paddies by 10.9–23.8%, but increased rice plant biomass by 4.2–35.6%. The eCO2 reduced the soil NH4+ and NO3- concentrations, but did not affect the soil dissolved organic C. The eCO2 did not affect the abundance of methanogens and CH4 production potential, whereas it stimulated the abundance of methanotrophs and CH4 oxidation potential by 102.5% and 15.1%, respectively. The eCO2 also shifted the community composition of methanotrophs and reduced the relative abundance of type Ⅱ methanotrophs by 8.5%. The random forest analysis identified that soil CH4 oxidation potential is the most important factor affecting CH4 emissions. Our findings indicate that eCO2 can reduce the CH4 emissions from rice paddies under in situ straw incorporation mainly through increasing the soil CH4 oxidation potential. Our study suggests the effects of eCO2 on CH4 emissions from global paddies may be overestimated and underline the need for smart agricultural management to reduce CH4 emissions.