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[Display omitted] • The roles of biochar on microbial growth and activity were systematically analyzed. • Biochar improved the microbial community diversity and population. • Biochar enhanced cellular enzyme activity and accelerated electron transfer. • Biochar promoted functional gene expression and regulated metabolic pathway. • Biochar targeted design to achieve high microbial activity in AD were suggested. The biochar is a well-developed porous material with various excellent properties, that has been proven with excellent ability in anaerobic digestion (AD) efficiency promotion. Current research is usually focused on the macro effects of biochar on AD, while the systematic review about the mechanisms of biochar on microbial behavior are still lacking. This review summarizes the effects and potential mechanisms of biochar on microorganisms in AD systems, and found that biochar addition can provide habitats for microbial colonization, alleviate toxins stress, supply essential nutrients, and accelerate interspecies electron transferring. Moreover, it also improves microbial community diversity, facilitates EPS secretion, enhances functional enzyme activity, promotes functional genes expression, and inhibits the antibiotic resistance genes transformation. Future research directions including biochar targeted design, in-depth microbial mechanisms revelation, and modified model development were suggested, which could promote the widely practical application of of biochar-amended AD technology. [ABSTRACT FROM AUTHOR]