Knowledge of the chromium (Cr) redox process in soil is important in addressing Cr bioavailability and risk assessment of contaminated soils. In this study, seven representative agricultural soils with different physicochemical properties were used to investigate the importance of microbially mediated Cr(VI) reduction and the response of soil bacterial community to Cr contamination. Chromium application increased soil bacterial diversity in Periudic Argosols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols, with an exception of Udic Ferrisols. The soil bacterial community responded to Cr contamination through changes in bacterial community structure, with Cr-resistant bacteria becoming the dominant species, and the percentage of Cr-resistant bacteria of total cultivable bacteria was 89.9, 75.2, 92.8, 65.3, 72.8, 77.3, and 65.4%, respectively, for Periudic Argosols, Udic Ferrisols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols. Bacillus, Escherichia, Deinococcus, Micromonospora, Methylobacterium, Massilia, Acidobacterium, Comamonas, Bradyrhizobium, and Arthrobacter were identified as the Cr-resistant bacteria. Moreover, our results demonstrated that microbial reduction was an important Cr(VI) reduction pathway, and the relative contribution of microorganisms to Cr(VI) reduction was 14.4, 44.0, 20.6, 34.9, 21.9, 21.7, and 22.0%, respectively for Periudic Argosols, Udic Ferrisols, Calcaric Regosols, Stagnic Anthrosols, Mollisols, Typic Haplustalfs, and Ustic Cambosols. Soil properties, especially Fe(II) and soil particle distribution, affected the microbially mediated Cr(VI) reduction. These results provide useful information for the bioremediation of Cr-contaminated soils under a wide range of environmental conditions.
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