Biochar-composting facilitates hexavalent chromium reduction and improves soil quality and microbial community...

The use of biochar-composting (BCing), where biochar is added at the beginning of the composting process, has greatly increased in agriculture over the last decade. However, few studies have investigated the role of BCing in the remediation of Cr-contaminated soil. In this study, three treatments: compost without biochar (C), biochar-composting with large particle size biochar (LBCing), and biochar-composting with small particle size biochar (SBCing), were applied at two different rates (1% and 5%) to assess their influence on Cr(Ⅵ) reduction, soil quality, and soil microbial community structure. The C, LBCing, and SBCing treatments significantly decreased the soil Cr(VI) content (P < 0.05), with reduction rates of 91.14% (C), 89.87% (LBCing), and 94.94% (SBCing) at the 5% application rate. The SBCing treatment had the highest Cr(VI) reduction efficiency overall. Furthermore, all the treatments significantly decreased the water-soluble Cr and leaching Cr contents in the soil and promoted the conversion of exchangeable Cr to oxidizable fractions, which increased environmental stability. The three treatments also increased electrical conductivity and organic matter levels, but decreased the redox potential. The high application rate (5%) increased bacterial diversity (Shannon index), reduced fungal diversity, and reshaped microbial communities toward ones that were dominated by Actinobacteriota, Firmicutes (bacteria), and Ascomycota (fungi, >99%). The Mantel test results indicated that Cr(VI) content and Cr speciation were significantly correlated with the soil microbial community. This suggests that the application of BCing to Cr-contaminated soil significantly enhances Cr(VI) reduction, improves soil quality and microbial community structure, and is a sustainable strategy for Cr remediation.