Mineral-specific effects on humification and bacterial community succession in industrial-scale spent mushroom substrate composting

Spent mushroom substrate (SMS) is a lignocellulose-rich agricultural by-product whose slow humification can limit the quality and agronomic value of the resulting compost. This study compared four low-cost mineral amendments: shale powder (S), mullite-based soil conditioner (SC), flue-gas desulfurization gypsum (DG), and rare-earth silico-titanate ore powder (TO). Each mineral was added at 10% of SMS dry mass during 40-day industrial-scale composting. Mineral structure, physicochemical properties, humic fractions, dissolved organic matter (DOM) fluorescence components, and bacterial succession were evaluated. Results showed that mineral amendments accelerated pile heating, prolonged the thermophilic phase, and promoted compost humification. Relative to CK, final humic acid (HA) content increased by 20.6, 28.1, 10.7, and 7.9% in S, SC, DG, and TO, respectively, while the humification index (HI) increased by 25.9, 41.5, 14.9, and 16.0%, respectively, highlighting SC as the strongest overall humification response. EEM-PARAFAC analysis revealed mineral-specific DOM transformation patterns: the relative abundance of the highly humified C3 component increased by 9.1 and 17.3% in S and SC, respectively, whereas DG preferentially increased C2 by 18.8%, and TO caused a slight 2.3% decrease in C3 relative to CK. SC increased the relative proportion of the highly humified C3 component by 17.3%, whereas DG increased that of C2 by 18.8% relative to CK. Mineral amendments also reshaped bacterial succession in a stage-dependent manner. SC favored heat-adapted core genera associated with organic matter transformation and humification during the thermophilic and cooling phases, whereas DG and TO exerted stronger selection for maturation-associated Actinomycetota, particularly Nonomuraea . Mantel analysis further revealed significant associations between mineral-induced shifts in the core bacterial community and changes in composting conditions and humification characteristics (Mantel r = 0.323–0.728, FDR-adjusted p < 0.05). These findings demonstrate that mullite-based conditioner provided the most effective enhancement of SMS compost maturity and humification, providing a scientific basis for the targeted selection of low-cost mineral amendments and the valorization of agricultural by-product.