Synergistic Addition of Biochar and Gypsum Improves Compost Quality from Canteen Food Waste and Chinese Medicine Residues by Reshaping Microbial Networks and the Physicochemical Environment

Canteen food waste (FW) and Chinese medicine residues (CMR) are difficult to compost. Additive addition can enhance compost maturity, probably by reshaping microbial communities. Yet, the synergistic effect of biochar and gypsum on compost quality remains unknown. Thus, we conducted a four-treatment (control, CK; gypsum, T1; biochar, T2; and biochar + gypsum, T3) composting experiment, integrating 16S rRNA sequencing, co-occurrence network analysis, and partial least squares path modeling (PLS-PM). Compared to CK, T3 increased the comprehensive maturity (CM) index by 70.36%, extended thermophilic phase duration by 42.8%, and elevated the germination index (GI) to 108.11% at maturity. These enhancements were accompanied by the highest microbial biomass carbon and cellulase activity. The final compost with T3 treatment had pH = 8.61, EC = 3.14 ms cm−1 and contained 515.7 mg kg−1 MBC, while the activities of sucrase and cellulase reached 32.56 U and 4.39 U, respectively. The dominant bacterial community shifted from Bacillus during the thermophilic phase to Pusillibacter (Bacillota, Oscillospiraceae) at maturity. Core amplicon sequence variants (ASVs) exhibited positive correlations with the CM index and the humic acid to fulvic acid ratio. Furthermore, biological indicators were the principal drivers of the maturation process, whereas the additives primarily functioned through abiotic modulation. These findings provide a mechanistic basis for improving the co-composting of FW and CMR using composite additives.