Biochar addition is a promising strategy for enhancing anaerobic digestion (AD), however, the mechanisms underlying its effectiveness, particularly the impact of biochar origin, remain unclear. This study directly compares the impact of corn stover-derived biochar (CSBC), mixed straw-derived biochar (MSBC), and digested sludge-derived biochar (SSBC) on methane production from AD. Results revealed that CSBC addition (10 g/L) yielded the most significant enhancement, increasing cumulative methane yield by 122.1% to 153.7 mL/g VS compared to the control. This superior performance correlated strongly with CSBC's abundant oxygen-containing functional groups (OFGs), which facilitated higher redox activity (kapp 12-15% greater than SSBC/MSBC) and electron transfer system (ETS) activity (81.1% increase over control). Microbial analysis indicated CSBC enrichment of key electron-transferring bacteria (e.g., Bacteroidota) and fostered syntrophy with methanogens. Conversely, MSBC primarily enhanced hydrolysis, while SSBC showed benefits in promoting hydrogenotrophic methanogens. These findings elucidate the critical role of biochar's surface chemistry and redox properties in mediating microbial interactions and electron transfer, providing crucial insights for designing tailored biochars to optimize waste valorization and energy recovery in AD. This study highlights the potential of SSBC in a circular economy context, and underscores the importance of linking biochar properties to functional microbial activity.