Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution.

[Display omitted] • Hydrochar-based composting enhanced reduction of heavy metals bioavailability. • Hydrochar addition increased temperature and extended the thermophilic phase. • Hydrochar addition promoted the humic acid (HA) formation during composting. • Bacterial community structure was significantly changed by hydrochar addition. • Bacterial bioremediation and HA complexation promoted heavy metals passivation. This study presented the effects of hydrochar on humification, heavy metals (HMs) bioavailability and bacterial community succession during composting. Results indicated that hydrochar addition led to elevated composting temperature, 7.3% increase in humic acid (HA), and 52.9% increase in ratio of humic acid to fulvic acid. The diethylene triamine pentacetic acid extractable Zn, Cu, Pb, and Ni were reduced by 19.2%, 36.3%, 37.8%, and 27.1%, respectively, in hydrochar-involved composting system. Furthermore, main mechanisms driving the reduced HMs bioavailability by hydrochar addition were revealed. The addition of hydrochar significantly modified the microbial community structure. Correlation analysis and microbial analysis demonstrated that relative abundance of bacterial groups connected with humification and HMs passivation were increased. Consequently, the HA formation was promoted and the HMs bioavailability were reduced through bacterial bioremediation and HA complexation. This study demonstrates the addition of hydrochar as a promising strategy to mitigate the HMs bioavailability during composting. [ABSTRACT FROM AUTHOR]