Effects of microbial consortium on the bacterial co-occurrence network and metabolic functions in kitchen waste composting systems

Kitchen waste (KW) composting often suffers from prolonged processing time and strong odor emissions due to the high moisture content and complex organic composition of the substrate. This study aimed to elucidate how inoculation with an immobilized bacterial consortium (IBC) regulates the microbial community, co-occurrence network structure, and metabolic functions in a KW composting system, thereby improving composting efficiency and mitigating odor generation. A composting system inoculated with an IBC composed of six functional bacterial strains was established, with a non-inoculated treatment serving as control. The physicochemical parameters of the compost, including temperature, moisture content, pH, and germination index (GI), were continuously monitored throughout the 15-day process. Bacterial community composition and succession were analyzed via 16S rRNA gene sequencing. Co-occurrence networks were constructed for different composting phases to reveal changes in microbial interactions. Functional Annotation of Prokaryotic Taxa (FAPROTAX) was applied to predict metabolic pathways related to carbon, nitrogen, and sulfur cycling. Partial Least Squares Path Modeling (PLS-PM) was used to explore causal relationships among physicochemical conditions, microbial community structure, network complexity, metabolic functions, and composting efficiency. The IBC treatment sustained a longer and more stable thermophilic phase than the control, accelerating compost maturity, with the GI reaching 88.89% on day 15 compared to 58.89% in the control. Inoculation significantly reshaped the bacterial community structure and enhanced deterministic assembly processes, guiding microbial succession toward functional guilds specialized in organic degradation and nutrient transformation. The inoculated compost exhibited greater network complexity, characterized by increased node and edge numbers, higher average degree, and reduced path length and network diameter, indicating stronger microbial connectivity and synergistic metabolic cooperation. Functional prediction showed that carbon cycling was dominated by chemoheterotrophy and aerobic chemoheterotrophy, both increasing over time, while fermentation functions gradually declined. In the nitrogen cycle, nitrite respiration and dissimilatory ammonification were most active during the early phase, but nitrogen fixation became dominant in the later cooling and maturation stages. Sulfur respiration pathways were markedly suppressed in the inoculated group, implying the inhibition of reductive sulfur metabolism and reduced potential for odor emission. PLS-PM analysis further demonstrated that microbial inoculation reversed the relationship between physicochemical properties and bacterial community from negative to positive, promoting the enrichment of core functional taxa. The relationship between community structure and metabolic function shifted from diversity-driven to functional taxa-driven patterns. Although the direct effect of network complexity on composting efficiency declined, it indirectly enhanced system functionality through improved robustness and cooperative stability. The immobilized bacterial consortium effectively optimized the composting physicochemical environment, reconstructed microbial interaction networks, and reinforced functional coupling among key taxa. These integrated effects accelerated organic matter degradation, shortened the composting period, and reduced odor emissions. The study provides new ecological insights into the microbial regulatory mechanisms of KW composting and supports the development of efficient, low-emission, and sustainable biotechnological strategies for organic waste recycling.