Microbial community evolution, functional promotion and elemental cycling by magnetic biochar-loaded Achromobacter xylosoxidans BP1 in chromium-contaminated soils

Cr(VI) is a ubiquitous environmental contaminant that is highly poisonous and soluble. The combined use of biochar and microorganisms to immobilize Cr in soils is gaining popularity worldwide. This study involves the preparation of magnetic biochar-loaded Cr(VI)-reducing bacteria ( Achromobacter xylosoxidans BP1) composite (MRBC@BP1) and the exposure to remediation trials over 70 days to enhance Cr fixation capacity, elemental cycling, and soil community function. This study utilized 16S rRNA and metagenomic sequencing to investigate the relationship between magnetic biochar, introduced functional bacteria, and native microorganisms. The findings indicated that MRBC@BP1 enhanced the concentrations of SOM, Ah-N, and A-P by 3.608, 1.539, and 5.799 times, respectively, while reducing Cr(VI) soil and TCLP-Cr(VI) levels to 8.651 and 0.742 mg/kg, respectively. Additionally, MRBC@BP1 improved the diversity of the soil microbial community by increasing the abundance of dominant bacteria, particularly Achromobacter, which showed a significant increase (p < 0.05) at the genus level. The results related to elemental cycling suggested that adding MRBC@BP1 enhanced P cycling, N and S metabolism, and C sequestration. According to FAPROTAX analysis, MRBC@BP1 effectively removed Cr(VI) soil and facilitated nitrate metabolism. This study investigates the positive interactions between soil community structure and ecological functions concerning magnetic biochar and functional bacteria composites. Additionally, it seeks to develop and implement a low-carbon, environmentally friendly remediation technology system for Cr-contaminated soil by preparing highly effective materials to remove Cr(VI).