Elevated CO2 amplifies yield-scaled CH4 emissions in japonica rice paddies by enhancing soil C availability and methanogenesis

Although short-term elevated CO 2 (eCO 2 ) often stimulates CH 4 emissions from rice paddies, responses during the transitional stage of continuous CO 2 fumigation remain unclear, particularly for yield-scaled CH 4 emissions. This study aimed to elucidate how eCO 2 regulates yield-scaled CH 4 emissions in japonica rice paddies during the transitional stage through changes in soil C availability and soil methanogenesis. Over three consecutive rice seasons (2019–2021), using an open-top chamber platform established in 2016, CH 4 fluxes, rice biomass and yield, enzyme activities (invertase, urease, catalase), methane production potential (MPP), and the abundance and community structure of the methanogenic gene mcrA were measured under ambient CO 2 (CK) and eCO 2 (EC, CK + 200 μmol mol −1 ). The results showed that, across the three rice seasons, EC significantly increased the cumulative amount of CH 4 emissions and yield-scaled CH 4 emissions by 74.8% and 51.3%, respectively, while exerting no significant effect on average shoot biomass or grain yield. Mechanistically, eCO 2 significantly increased three-year seasonal averages of soil dissolved organic C (+24.0%) and enzyme activities (urease +14.7%, invertase +33.2%, catalase +8.4%), thereby boosting MPP (+63.5%) and mcrA gene abundance (+73.5%). This coincided with Methanoregula enrichment (+43.1–148.2%) and reduced relative abundances of several acetoclastic methanogen-related taxa. During the transitional stage of continuous CO 2 fumigation, eCO 2 enhanced CH 4 emissions by coupling increased soil C availability with stimulated MPP, and this increase exceeded the concurrent yield response, thereby amplifying yield-scaled CH 4 emissions in japonica rice paddies.