Synergistic effects of spent mushroom substrate and biochar on rape straw composting: Humification, metal passivation and microbial community dynamics

This study investigated the effects of spent mushroom substrate (SMS) and biochar on organic matter humification, heavy metal passivation, and microbial community dynamics during composting of heavy metal-contaminated rape straw. A 42-day aerobic composting experiment revealed that both additives accelerated organic matter degradation and promoted fulvic acid to humic acid transformation. The combination of 5% biochar and 5% SMS exhibited the best passivation effect on Cu, Fe, Mg, Mn, and Zn, with rates of 69.41%, 75.06%, 9.67%, 45.20%, and 72.81%, respectively, significantly higher than other treatments (P <0.05). Notably, exogenous additives significantly influenced microbial community succession. The variable community strongly correlated with CuF3, MgF4, ZnF3, and ZnF4 contents (P<0.01), playing a crucial role in accelerating heavy metal passivation. This study elucidates the synergistic mechanism of SMS and biochar in heavy metal passivation, providing new insights for sustainable utilization of contaminated rape straw.