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

Abstract

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.