Compound Microbial Agent Improves Soil Redox Status to Reduce Methane Emissions from Paddy Fields

Paddy fields are considered a major source of methane (CH₄) emissions. Aerobic irrigation methods have proven to be efficacious in mitigating CH₄ emissions in paddy cultivation. The promising role of compound microbial agents in refining the rhizospheric ecosystem suggests their potential as novel agents in reducing CH₄ emissions from paddy fields. To explore a new method of using compound microbial agents to reduce CH₄ emissions, we conducted pot and field experiments over the period of 2022‒2023. We measured CH₄ flux, the redox potential (Eh) of the soil, the concentration of dissolved oxygen (DO) in the floodwater, and the gene abundance of both methanogens (mcrA) and methanotrophs (pmoA). The results showed that the application of the compound microbial agent led to a significant increase in the DO levels within the floodwater and an increase of 9.26% to 35.01% in the Eh of the tillage soil. Furthermore, the abundance of pmoA increased by 31.20%, while the mcrA/pmoA ratio decreased by 25.96% at the maximum tillering stage. Applying 45−75 kg/hm² of the compound microbial agent before transplanting resulted in a reduction of cumulative CH₄ emissions from paddy fields by 17.49% in single- cropped rice and 43.54% to 50.27% in double-cropped late rice during the tillering stage. Correlation analysis indicated that CH₄ flux was significantly negatively correlated with pmoA gene abundance and soil Eh, and positively related to the mcrA/pmoA ratio. Additionally, soil Eh was significantly positively correlated with pmoA gene abundance, suggesting that paddy soil Eh indirectly affected CH₄ flux by influencing the pmoA gene abundance. In conclusion, the pre-planting application of the compound microbial agent at a rate of 45‒75 kg/hm² can enhance the Eh in the rhizosphere and increase the abundance of the pmoA gene, thereby reducing CH₄ emissions from paddy fields during the tillering stage of rice growth.