Effect of biochar addition on copper and zinc passivation pathways mediated by humification and microbial community evolution during pig manure composting

[Display omitted] •Optimal BC dose were 15% and 10% of Cu and Zn by identifying passivation pathways.•Adding 10% BC was optimal for enhancing microbial interactions.•Adding 20% BC inhibited humification and reduced passivation of Cu and Zn.•Phenolic hydroxyl and carboxyl groups are the main binding sites for Cu.•HA content and microbial community dominated morphological evolution of Cu and Zn. The mobility and bioavailability of Cu and Zn are the main threats associated with the land application of pig manure (PM) compost products. This study investigated the impacts of biochar (BC) concentration on passivation of Cu and Zn associated with the compost maturity. The results indicated that 15% and 10% BC favoured the passivation of Cu and Zn, respectively. BC promoted passivation of Cu by accelerating HA production and optimized the abundance of Firmicutes. BC promoted the passivation of Zn by increasing the high temperature peak and the corresponding pH (8–8.5). A higher level (15% and 20%) of BC altered the dominant bacterial phylum from Firmicutes to Proteobacteria. 20% BC inhibited the passivation of Cu and Zn by reducing the highest temperature and lowering the alkalinity of substrate. These results offer new insights into understanding how the addition of BC could reduce the risk of hazardous products during PM composting.