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

Aerobic composting of livestock manure concentrates heavy metals like cadmium (Cd), elevating environmental risks after land application. This study screened composite microbial strains for simultaneous promotion of compost maturation and Cd passivation, exploring their mechanisms on the composting process, microbial community succession, and Cd speciation transformation. Three Cd-resistant strains-Enterobacter hormaechei (LB3), Enterobacter cloacae (LB4), and Bacillus velezensis (J-1-2)-were isolated from chicken manure, formulated into two composite inoculants (T1: LB3 + LB4; T2: LB3 + LB4 + J-1-2), and compared with an uninoculated control (CK) during composting. Maturity parameters, Cd species distribution, and microbial community dynamics were monitored. Results showed composite inoculants significantly improved composting: T1 extended the thermophilic phase and enhanced organic matter degradation; T2 achieved optimal nitrogen retention, with the highest NO₃^--N (1504 mg/kg, representing a 13.51% increase compared to CK) and lowest NH₄⁺-N (153 mg/kg, a 23.12% reduction compared to CK). Microbial community analysis revealed Ace/Chao1 indices in T1/T2 were 1.5-1.8 times higher than CK in the heating phase, while the Shannon index at maturity was 10.13% and 22.40% higher, respectively. T2 had the highest Cd passivation efficiency (66.7%), with exchangeable Cd decreasing from 27 to 9%. Inoculants promoted composting and Cd immobilization via microbial community modulation and adsorption-complexation mediated by key genera (e.g., Thauera), providing an effective strategy for safe reuse of livestock manure and heavy metal pollution mitigation.