<p>This study aimed to investigate the effects of manganese dioxide (MnO₂) on promoting humification and stabilizing heavy metals (HMs) during sludge composting. The results showed that, compared with the control group (CK), the addition of MnO₂ increased the humus content by 13.72% and the humic acid/fulvic acid ratio (HA/FA) by 13.46%. Meanwhile, the bioavailability of As, Fe, Mn, and Zn in the treatment group (M) decreased by 16.70%, 8.34%, 19.07%, and 15.14%, respectively. The study also demonstrated that exogenous MnO₂ acted as a passivator without causing the negative effects typically associated with external additives, such as inhibited microbial activity or interference with the composting process. Metagenomic analysis revealed that MnO₂ regulated heavy metal resistance genes (HMRGs) and altered the community structure of heavy metal-resistant bacteria (HMRB). Correlation analysis indicated that the bioavailability of As, Fe, Mn, and Zn was negatively correlated with the abiotic factor humus (HS), but positively correlated with most HMRGs as biological factors. These findings suggest that using MnO₂ as an additive could serve as a potential strategy to simultaneously enhance the degree of humification and heavy metal stabilization in sludge composting, offering significant advantages.</p> Graphical Abstract <p></p>