Synergistic remediation of cadmium-contaminated soil by biochar-loaded nanoscale zerovalent iron and Eisenia fetida: Shifting bacterial communities from cadmium tolerance to metabolic resilience

Biochar-loaded nanoscale zerovalent iron (nZVI@BC) combined with earthworms is a promising strategy for Cd-contaminated soil remediation. However, the responses of soil bacterial communities, which are crucial indicators of soil health, remain poorly understood. This study systematically evaluated soil bacterial variations under BC/nZVI@BC- Eisenia fetida remediation through five perspectives: species composition, diversity analysis, differential species and biomarkers, co-occurrence networks, and metabolic pathways. The results revealed that Cd contamination enriched Cd-tolerant Actinobacteria and suppressed Proteobacteria , thereby destabilizing the bacterial community. BC- Eisenia fetida remediation partially restored sensitive bacteria but failed to fully reverse the community degradation. In contrast, nZVI@BC- Eisenia fetida remediation significantly increased the abundance of dominant bacteria, notably enriching Nocardioides and Blastococcus . This shift drove a community transition from Cd tolerance to functional recovery and enhanced ecological stability. It also significantly increased diversity and promoted the aggregation of dominant species. Twenty-two biomarkers were identified, with Actinobacteria indicating Cd tolerance stress, and Chloroflexi reflecting nZVI@BC-mediated redox. Co-occurrence networks clarified the interactions within bacterial community, revealing restructured ecological connectivity. Furthermore, functional prediction indicated that nZVI@BC- Eisenia fetida remediation activated basal energy metabolism and anaerobic pathways, inhibited non-essential secondary metabolism, and reshaped metabolic networks, thereby providing labile carbon sources and regulating redox conditions. This study provides a theoretical basis for understanding microbial responses to synergistic remediation of Cd-contaminated soil, and promotes soil sustainable utilization.