Geobacterplays a key role in mercury (Hg) methylation and reduction in rice fields.Biochar boosts microbial electron transfer, but its impact on Hg reduction and methylation remains unclear. This study investigates how oxidative (OBC) and reductive (RBC) biochar influence Hg reduction and methylation by Geobacter sulfurreducens PCA. OBC increased electron donating capacity (EDC) but reduced electron accepting capacity (EAC), while RBC decreased electric resistance. Correlation analysis revealed biochar's electron exchange capacity (EEC) positively correlated with -OH (r = 0.73*), O-CO (r = 0.81*), COO/CO (r = 0.73*), -NH (r = 0.67*), and electron transfer number (n) (r = 0.99**), but negatively with -CH (r = -0.70*) and -NH