Effects of a Composite Additive on Yield, Microbial Community, and Carbon–Nitrogen Transformation During Volvariella volvacea Cultivation on Spent Pleurotus eryngii Substrate

The addition of nutritional additives to the cultivation substrate is an effective way to increase edible fungus yields; however, the mechanisms by which these additives regulate the structure and function of the substrate microbial community to promote nutrient transformation remain unclear. In this study, a patented composite additive composed of soybean meal, corn protein, and wheat bran was added to a Volvariella volvacea cultivation system based on spent Pleurotus eryngii substrate after secondary fermentation, and the additive-treated (T) and control (ck) groups were compared across different cultivation stages. The additive increased V. volvacea yield by 26.4% (4.02 vs. 3.18 kg/m2) and increased fruiting body amino acid content by 16.43%, in addition to shifting carbon and nitrogen metabolism toward amino acid accumulation and biomass. Throughout cultivation, the additive sustained higher cellulase (CL) and alkaline protease (AKP) activities, increased cellulose degradation and organic nitrogen mineralization, and enlarged the pool of available carbon and nitrogen in the substrate. Co-abundance group (CAG) network analysis of 626 ASVs (577 bacterial and 49 fungal ASVs) revealed that the additive was associated with three potential bacterial–fungal functional modules in a stage-specific manner: a cellulose-degrading and nitrogen-accumulating module, a carbon-assimilating module, and a nitrogen-mineralizing module. The core taxa potentially associated with carbon and nitrogen transformation included the bacteria Pseudoxanthomonas and Thermopolyspora and the fungus Mycothermus, together with the additive-enriched genera Schlegelella, Niabella, and Chelatococcus. ASV-level resolution further revealed an intragenus functional division of Mycothermus, in which two ASV clusters were associated with cellulose degradation and nitrogen mineralization. These findings elucidate the mechanism whereby additives enrich functional microbial guilds, accelerate coupled carbon–nitrogen turnover, and ultimately increase the yield and quality of V. volvacea, providing a theoretical basis for the precise management of spent-substrate-based edible fungus cultivation.