Zero-valent iron drives the passivation of Zn and Cu during composting: Fate of heavy metal resistant bacteria and genes

Livestock and poultry manures usually contain substantial heavy metals, especially Cu and Zn, which may induce the enrichment of heavy metal resistant genes (HMRGs) and heavy metal resistant bacteria (HMRB). This study aims to clarify the relationship among heavy metal bioavailability, HMRB and HMRGs during composting, so as to assess and alleviate potential ecological risks of manure application to land. In this study, zero-valent iron (ZVI) was used as passivator, and HMRGs and their host microorganisms were investigated by metagenomic analysis. The results showed that ZVI effectively promoted the passivation of Zn and Cu, and the bioavailability of Zn and Cu decreased by 8.25% and 19.84%, respectively. Proteobacteria was the main HMRB in composting, among which Escherichia, Pseudomonas and Salmonella contained abundant HMRGs, and the succession of these bacteria was closely related to the fate of HMRGs. Spearman correlation analysis showed that metal exchangeable and reducible fractions were positively correlated with the abundance of most HMRGs. Further-more, structural equation model showed that heavy metal passivation can also indirectly reduce the abundance of HMRGs by regulating the succession of HMRB. These results suggested that the fate of HMRGs is closely related to the chemical speciation of heavy metals in composting habitats.