Timing of psychrotrophic cellulolytic bacterial inoculation affects humification and heavy metal immobilization during low-temperature chicken manure composting

The relatively low ambient temperatures during spring in northeastern China can severely limit the self-heating capacity of aerobic composting, resulting in insufficient compost maturity before agricultural application. In this study, psychrotrophic cellulolytic bacteria were inoculated at different phases of the composting process. The results showed that, except for the control, all composting treatments reached temperatures that met the sanitation standards of the United States Department of Agriculture. The initial stage (D0) treatment outperformed the control, with pile temperatures rapidly reaching 65 °C and remaining above 45 °C for 10 d. Additionally, D0 treatment substantially enhanced humification, increasing the humification index by 125% and humic acid content by 226% relative to the control. This treatment enhanced heavy metal immobilization, with the most pronounced reduction observed for EXC-Pb (87.14%). Microbial analysis revealed the preferential enrichment of cellulose-degrading bacteria ( Pseudoxanthomonas and Cellvibrio ) and taxa potentially involved in heavy metal immobilization ( Devosia ) in the D0 treatment. Compared with the control, D0 increased microbial network complexity, with cooperative interactions rising from 76.08% to 86.71%. Structural equation modeling suggested that microbial communities were associated with HMs bioavailability through physicochemical property- and humification-related pathways (β = 0.342, P < 0.05). In saline-alkali soil pot trials, treatment with organic fertilizer modified by D0 produced the highest pakchoi ( Brassica rapa subsp. chinensis) biomass while simultaneously enhancing soil fertility. This study provides evidence that initial inoculation with a psychrotrophic cellulolytic consortium improves humification and heavy metal immobilization during low-temperature composting, with potential implications for cold-region composting.