Type-Specific Restructuring and Nitrogen–Sulfur Functional Degradation of Black-Soil Bacterial Communities Driven by Different Coal-Based Solid Waste Dumping in an Extremely Cold Open-Pit Coal Mine

Coal-based solid waste (CBSW; fly ash, weathered coal, coal gangue) from open-pit coal mining in extremely cold regions differentially releases salinity/alkalinity, heavy metals and acids, altering surrounding black-soil properties and reshaping its bacterial communities. We compared healthy farmland black soil (CK) with three dumping treatments—fly ash (FA), weathered coal (WC) and coal gangue (CG) (six replicates each; n = 24)—using full-length 16S rRNA sequencing (PacBio CCS), physicochemical, phospholipid fatty acid and enzyme-activity analyses. The wastes showed contrasting stress regimes: fly ash was slightly alkaline, saline and heavy-metal-enriched; weathered coal was strongly acidic and saline; and coal gangue was acidic–sulfidic. Dumping significantly altered soil pH, salinity and metal speciation, reducing organic matter by 36–61% and microbial biomass carbon by 45–74%. Alpha diversity responded in a type-specific manner: WC had the lowest Shannon index (4.72, −55.1%) and was dominated by Pseudomonadota (80.6%) with methylotrophy being enriched; FA retained the highest diversity (11.23); and CG was dominated by Actinobacteriota (45.1%). PERMANOVA and ANOSIM confirmed significant community separation. Nitrification was nearly lost under WC, while sulfate respiration was enriched under CG and sulfur respiration under FA, indicating type-specific restructuring of nitrogen and sulfur cycling. These results show that CBSW dumping drives restructuring and nitrogen–sulfur functional degradation of black-soil bacterial communities through type-specific environmental filtering, with weathered coal causing the most severe degradation, informing classified remediation.