Stage-specific bacterial-fungal synergy in industrial-scale hyperthermophilic composting: Community assembly dynamics...

This study aims to clarify how bacterial and fungal communities cooperate to drive humification during industrial-scale hyperthermophilic composting of municipal sludge, focusing on their functional division, community assembly, and cross-kingdom interaction networks. Results indicate that humic carbon increased by 9.40 mg g^-1, while humic-like fluorescence rose from 58.95 % to 85.79 % and protein-like signals declined, indicating enhanced humification. Procrustes and distance-based redundancy analyses revealed that bacteria degrade both readily degradable (e.g., proteins, polysaccharides) and recalcitrant (e.g., lignin, cellulose) substrates, thereby providing heat and humus precursors, whereas fungi primarily mediate recalcitrant degradation and the polymerization of humus precursors. β-nearest taxon index (βNTI)-based analysis revealed contrasting assembly regimes: deterministic processes accounted for 76.1 % of bacterial community assembly and shifted from deterministic (βNTI < -2) to stochastic (|βNTI| < 2) over time, whereas stochastic processes accounted for 73.9 % of fungal community assembly and shifted from stochastic to deterministic (βNTI >2), providing an ecological basis for stage-specific humification. Cross-kingdom co-occurrence networks resolved three modules: Module 1 (full stage), Module 2 (0-12 days) and Module 3 (25-30 days), comprising 48 %, 28 % and 23 % bacterial taxa and 43 %, 36 % and 21 % fungal taxa, respectively. Variance partitioning and Mantel tests, together with partial least squares path modeling, indicated that Module 1 primarily supplies humus precursors, whereas Modules 2 and 3 drive humic substance synthesis. Zi-Pi topological analysis identified keystone taxa in different modules as potential targets for steering humification through microbial regulation, providing a mechanistic basis for optimizing sludge composting. Overall, this study demonstrates that bacterial-fungal functional partitioning, community assembly dynamics, and cross-kingdom network organization jointly underpin humification efficiency in industrial sludge composting systems.