Biochar derived from the pyrolysis at 500 degrees C with fresh biogas slurry and residue, was conducted to investigate its potential role in mediating the speciation and mobilization of As(V) and Fe(III) from arsenic contaminated tailing mine sediment, with consideration of the changes in microbial populations and dissolved organic matter (DOM). The reduction of As(V) (10-13%) and Fe(III) (12-17%) were partly in response to biochar abiotically causing desorption and reduction effect, but were predominantly (87-90% and 83-88% for As(V) and Fe(III)) attributed to biochar stimulating biological reduction. The level of As(III) released from sediment upon biochar amendment (656.35 +/- 89.25 mu g L-1) was significantly higher than the level released without biochar amendment (98.06 +/- 19.38 mu g L-1) after 49 days incubation. Although a low level of Fe(II) (0.81 +/- 0.07 mg L-1) was determined in the solution when amending with biochar, most of released Fe(II) (166.25 +/- 40.25 mg L-1) was formed as biochar-Fe(II)minerals composite. More importantly, biochar stimulated the DOM bioavailability in association with bacterial activities mediating As(V) and Fe(III) reduction. High-throughput sequencing results indicated biochar application shifted the soil microbial community and increased the relative abundance of As(V)-/Fe(III)-reducing bacteria, mostly Geobacter, Anaeromyxobacter, Desulfosporosinus and Pedobacter. The discovery of biochar-bacteria-DOM consortium may broaden new understanding into speciation and mobilization of metals, which arouses attention to exploit feasible bioremediation for metal-contaminated sediment. (C) 2016 Elsevier B.V. All rights reserved.