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The purpose of this study is to investigate the effects of nano-sized or submicro Fe2O3/Fe3O4 on the bioreduction of hexavalent chromium (Cr(Ⅵ)) and to evaluate the effects of nano-sized Fe2O3/Fe3O4 on the microbial communities from the anaerobic flooding soil.The results indicated that the net decreases upon Cr(Ⅵ) concentration from biotic soil samples amended with nano-sized Fe2O3 (317.1 + 2.1 mg/L) and Fe3O4 (324.0 ± 22.2 mg/L) within 21 days,which were approximately 2-fold of Cr(Ⅵ) concentration released from blank control assays (117.1 ± 5.6 mg/L).Furthermore,the results of denaturing gradient gel electrophoresis (DGGE) and high-throughput sequencing indicated a greater variety of microbes within the microbial community in amendments with nano-sized Fe2O3/Fe3O4 than the control assays.Especially,Proteobacteria occupied a predominant status on the phylum level within the indigenous microbial communities from chromium-contaminated soils.Besides,some partial decrease of soluble Cr(Ⅵ) in abiotic nano-sized Fe2O3/Fe3O4 amendments was responsible for the adsorption of nano-sized Fe2O3/Fe3O4 to soluble Cr(Ⅵ).Hence,the presence of nano-sized Fe2O3/Fe3O4 could largely facilitate the mobilization and biotransformation of Cr(Ⅵ) from flooding soils by adsorption and bio-mediated processes.