Contrasting structural and functional adaptations in soil bacterial communities underlie multi-metal remediation by biochar versus nano zero-valent iron-loaded biochar

Abstract

Biochar (BC) and nano zero-valent iron-loaded biochar (nZVI@BC) have demonstrated great potential in remediation of heavy metal contaminated soil, yet their distinct impacts on soil bacteria remain unclear. This study comprehensively investigated soil bacterial responses to BC and nZVI@BC remediation of Pb, As, Cu, Cd and Zn co-contaminated soils through species composition, community diversity, species differences, biomarkers, association networks and pathways. The results revealed that at the phylum level, Actinobacteria increased while Proteobacteria decreased. nZVI@BC remediation uniquely increased Firmicutes, whereas BC decreased it. At the genus level, Nocardioides was enriched by nZVI@BC remediation, driven by elevated pH and nZVI-induced redox shifts. Although BC maintained bacterial diversity, nZVI@BC reduced richness and evenness. Beta diversity analysis indicated changes in community richness and composition, particularly highlighting the distinct separation of the nZVI@BC group from the control. Forty-six biomarkers were identified, with BC biomarkers linked to carbon metabolism, while nZVI@BC biomarkers correlated with heavy metal immobilization. Additionally, association networks revealed inverse correlations and positive interactions between taxa, explaining community dynamics. PICRUSt2 revealed BC activated pathways related to carbohydrate metabolism and heavy metal resistance, whereas nZVI@BC, through nano-enabled modulation of the soil microenvironment, conferred a distinct metabolic advantage over biochar. It enhanced reductive metabolism pathways, influencing microbial nitrogen utilization and cofactor synthesis, thereby optimizing nitrogen metabolism, reinforcing membrane structures, and enhancing pollutant metabolism. These findings provide new and valuable insights into the microbial mechanisms of BC and nZVI@BC remediation of soil co-contaminated with multiple heavy metals, offering a microbiome-driven remediation strategy.