Microbial mechanisms of biochar reducing methane emission in a rice-crayfish integrated system

Biochar application can mitigate methane (CH₄) emissions from paddy fields in rice monoculture cropping system. However, the response of CH₄ emissions to biochar application in rice-crayfish integrated system (RCS) remains unclear. A two-year field experiment (October 2022-October 2024) was conducted to explore the influences of biochar application on CH₄ emission, soil chemical properties and the community structures and abundances of methanogens and methanotrophs in RCS in Gaoyou City, Jiangsu Province, China. Biochar was applied annually at low rate (7.5 t ha^-1) and high rate (15 t ha^-1) in RCS, respectively. Biochar application significantly reduced annual CH₄ emissions by 33.6-43.5%, CH₄ intensities from rice source by 39.3-50.3%, and CH₄ intensities from crayfish source by 32.8-43.5%, averaged across all annual cycles. Reduction in annual CH₄ emissions resulted from biochar application was primarily attributed to decreased mcrA gene abundance, increased pmoA gene abundance, and a lower gene abundance ratio of mcrA to pmoA. Furthermore, changes in mcrA and pmoA gene abundances induced by biochar application significantly correlated with elevated soil redox potential, pH, ammonium and nitrate, as well as reduced soil dissolved organic carbon. Further analysis revealed that biochar application in RCS reduced methanogens abundance mainly by decreasing the relative abundances of Methanobacterium, Methanothermococcus and Methanocella, while enhanced methanotrophs abundance mainly by increasing the relative abundances of Methyloparacoccus, Methylocaldum and Methylogaea. Therefore, biochar application greatly mitigates CH₄ emission in RCS by altering community structures and abundances of methanogens and methanotrophs through modifications in soil chemical properties.