Mechanisms of Enterobacter and Bacillus in promoting aerobic composting and immobilization of Cd in livestock and poultry manure

The "concentration effect" of heavy metals during aerobic composting of livestock and poultry manure and the associated pollution risks upon land application represent significant challenges in the agricultural waste resource utilization. Enhancing composting efficiency and passivating heavy metal Cd through microbial approaches are key to achieving safe disposal and resource recovery of manure. This study aimed to screen composite microbial strains capable of simultaneously promoting compost maturation and Cd passivation, investigating their mechanisms of action on the composting process, microbial community succession, and Cd speciation transformation. Cd-resistant strains were isolated and purified from chicken manure using in situ screening techniques, and a composite microbial inoculum was prepared using Enterobacter hormaechei (LB3), Enterobacter cloacae (LB4), and Bacillus velezensis (J-1-2). Composting experiments were conducted with a control group (CK) and two treatment groups: T1 (LB3+LB4) and T2 (LB3+LB4+J-1-2). Maturity parameters, Cd speciation distribution, and microbial community dynamics were monitored, with high-throughput sequencing and correlation analysis employed to elucidate the underlying mechanisms. The results demonstrated that the composite inoculum significantly optimized the composting process. The T1 group exhibited an extended thermophilic phase and more thorough organic matter degradation (lowest C/N ratio of 14.88), while the T2 group showed optimal nitrogen retention (highest NO3--N content of 1504 mg/kg and lowest NH4+-N content of 153 mg/kg). Microbial community analysis revealed that the Ace and Chao1 indices of T1 and T2 increased by 1.5－1.8 times compared to CK during the heating phase, while the Shannon index at maturity was 10.13% and 22.40% higher than CK, respectively. The Cd passivation efficiency was highest in T2 (66.7%), with the EX-Cd fraction decreasing from 27% to 9%. Notably, key taxa such as Thauera (Proteobacteria) showed a significant positive correlation with RES-Cd (p< 0.01). In conclusion, the composite inoculum accelerated organic matter decomposition and maturation by modulating microbial community structure, while synergistically passivating Cd through adsorption and complexation mechanisms involving key genera (e.g., Thauera). This study provides theoretical and technical support for the safe composting of livestock manure and heavy metal pollution control.