Controlled irrigation and straw-decomposing microbial inoculant mitigate CH4 emissions and increase rice yield in straw return mollisols of paddy fields

Straw-decomposing microbial inoculant (DMI) is widely adopted in rice production to promote the decomposition of straw returned into soil. However, the effects of DMI on CH 4 emissions and yield under straw return remain unclear, especially under different irrigation regimes. To address these knowledge gaps, a 2-year field experiment was conducted in Northeast China’s mollisols with four treatments: controlled irrigation (CI) + straw return (CS), CS + DMI (CSD), flooded irrigation (FI) + straw return (FS), and FS + DMI (FSD). Soil properties, such as oxidation–reduction potential (Eh), pH, dissolved organic carbon (DOC), microbial biomass carbon (MBC), and the abundance and community composition of methanotrophs and methanogens, were measured to elucidate mechanisms of CH 4 emission reduction. The results indicated that the application of DMI increased DOC and MBC, resulting in a slight increase in CH 4 emissions under both irrigation regimes. In contrast, CI effectively suppressed CH 4 emissions, regardless of whether DMI was applied or not. The cumulative CH 4 emissions of CS and CSD were significantly reduced by 34.21% and 33.40% compared to FS and FSD, respectively. It was attributed to the higher soil Eh and lower pH under CI relative to FI, which reduced the abundance of mcrA genes and increased the abundance of pmoA genes. In addition, CSD treatment resulted in the highest rice yield, with a significant increase of 8.64% compared to FSD. The structural equation model indicated that the irrigation regime had a stronger impact on CH 4 flux than DMI. Overall, CSD achieved the lowest yield-scaled CH 4 emissions (38.74% lower than FSD), suggesting CI + DMI is a sustainable strategy for reducing CH 4 emissions while maintaining high rice yields. • Straw-decomposing microbial inoculant (DMI) increased DOC and MBC, slightly elevating CH 4 emission. • Controlled irrigation (CI) + DMI treatment significantly increased rice yield. • CI reduced CH 4 emissions by enhancing soil oxidation. • CI + DMI treatment decoupled rice yield from CH 4 emissions. • The impact of irrigation regime on CH 4 emission flux was more significant than DMI.