Hongtao Liu, Shiyun Liu, Liting Huang, Jun Zhang, Lixia Wang, Caihong Huang, Dunqiu Wang
Composting effectively reduces the bioavailability of heavy metals (HMs) in livestock manure. However, the mechanisms by which physical, chemical, and biological passivators modulate microbial communities and HM-related functional genes to enhance HM immobilization remain unclear. This study investigated the effects of individual and combined additions of medical stone (physical agent), hydroxyapatite (chemical agent), and crayfish shell (biological agent) on modulating microbial community structure and functional gene abundance while deciphering their coupled associations with the bioavailability of HMs (Cu, Zn, Cr, As, Ni). The results demonstrated that medical stone exhibited the highest immobilization rate for As (84.80%), followed by Cu, which was mainly attributed to enhanced lacz1 abundance, increased microbial diversity, adsorption, and humification. Hydroxyapatite was most effective in immobilizing Cu and Zn, with immobilization rates of 82.02% and 81.05%, respectively, driven by enrichment of Actinobacteriota, particularly Saccharomonospora and Flavobacterium, as well as increased mt and lacz1 abundances, humification, and phosphate precipitation. Crayfish shell primarily immobilized Cu and Zn, with immobilization rates of 80.90% and 40.11%, respectively, by promoting microbial growth and diversity. Combined passivators primarily immobilized Cu and Zn, with hydroxyapatite and crayfish shell combination being most effective (immobilization rates of 80.03% and 64.98%, respectively); this was achieved by increasing the abundance of Cellvibrio (14-fold increase), Saccharomonospora, and the mt gene. These findings elucidate the mechanisms through which exogenous passivators mitigate the potential environmental risks of Cu, Zn, and As in livestock manure by modulating specific microbial communities and HM-related functional genes.