Biochar influences soil fertility, N 2 O emissions, and atmospheric CO 2 budgets, and because of its quinone and aromatic structures, it is redox-active. Here we demonstrate that biochar concentrations of 5 and 10 g L -1 stimulate both the rate and the extent of microbial reduction of the Fe(III) oxyhydroxide mineral ferrihydrite (15 mM) by Shewanella oneidensis MR-1, while lower biochar concentrations (0.5 and 1 g L -1 ) have a negative effect on ferrihydrite reduction. Control experiments showed that biochar particles and not biochar-derived water-soluble organic compounds are responsible for the stimulating and inhibiting effect. We also found that biochar changed the mineral product of ferrihydrite reduction from magnetite (Fe 3 O 4 ) to siderite (FeCO 3 ). Our study suggests that biochar can influence soil biogeochemistry not only indirectly by changing the soil structure and chemistry but also by directly mediating electron transfer processes, i.e., by functioning as an electron shuttle.