The effects of electrochemical reducing power on enrichment, growth, and ammonium production of freeliving diazotrophs from rhizosphere soil were evaluated. Soil bacteria were cultivated in a conventional bioreactor (CBR) and an electrochemical bioreactor (EBR), both containing a neutral red-modified graphite felt (NR-GF) cathode and a platinum anode, but with electricity charged to the EBR only. Temperature gradient gel electrophoresis identified 21 species from rhizosphere soil, and 17 and seven species from the CBR and EBR, respectively, after 40 days of incubation. Six species from the CBR and five species from the EBR were diazotrophs. The bacterial community biomass and the ammonium content in the bacterial culture were, respectively, 1.6 and 2 times higher in the EBR than in the CBR. These results indicate that the electrochemical reducing power generated from the NR-GF may be a driving force in the activation of enrichment, growth, and N2-fixing metabolism of diazotrophs.