This study aimed to investigate the effect of swine manure composting with microbial inoculation (MI) and without MI (CK) on heavy metal (Cr, Cd, and Pb) fractions, humic substance (HS), and metabolism pathway. The results showed that MI could passivate the heavy metal Cr and reduce the proportion of exchangeable (EXC) fraction of Cd, but it does not affect the EXC fraction of Pb. Compared to CK, HS, humic acid (HA), and fulvic acid (FA) were significantly increased with MI at the maturity stage. The propagation of Proteobacteria (day 4) and Firmicutes (days 12 and 24) was strengthened with MI. Canonical correlation analysis found that HA and Firmicutes were positively correlated with heavy metal (Cr, Cd, and Pb) residual (RES) fraction, and FA was positively correlated with Proteobacteria. Moreover, MI can significantly increase amino acid metabolism and carbohydrate metabolism by day 4, enhance the metabolism of enzyme families and glycan biosynthesis by day 12, and improve membrane transport. Overall, MI could facilitates the increase in HA and FA content and transfer of heavy metal (Cr, Cd, and Pb) fractions, it particularly helps increase the RES fraction.

This study evaluated enzyme activity, available nitrogen, and bacterial succession during pig manure composting with and without microbial inoculation (ABB and CK, respectively). ABB reached the thermophilic stage 2 days than CK. Cellulose, urease, phosphatase, and sucrase activities were higher in ABB than in CK on days 12-24 of composting, but catalase activity was lesser in ABB than in CK throughout composting. NH4+-N and NO3--N were significantly increased in ABB at the maturity stage. 16S rRNA sequencing revealed Nocardiopsaceae, Bacillaceae, Streptosporangiaceaec, Flavobacteriaceae, and Caldicoprobacteraceae as the dominant bacteria at the family level. Metabolism function analysis revealed that human diseases were reduced and carbohydrate metabolism was increased in ABB. Correlation analysis revealed that urease, sucrose, and phosphatase were significantly correlated with bacteria at the species level, whereas NH4+-N and NO3--N were not significantly correlated. These results indicated that microbial inoculation accelerated the composting process and significantly regulated microbial functions.

This study determined the physicochemical changes and bacterial community succession in the pig manure composting process with microbial inoculant. Microbial inoculant could prolong the thermophilic stage by 2 days and increased the germination index (GI). Analysis with 16S rDNA showed that the Chao1 and Shannon indices increased at the thermophilic stage in the treatment (T), while those of the control (C) decreased. Microbial inoculant increased the relative abundance of Flavobacterium and Solibacillus in 4-12 and 12-24 days, respectively. Acinetobacter was reduced at 4-12 days. The key physicochemical factors affecting microbial successions were revealed by canonical correspondence analysis (CCA) and correlation analysis. Linear discriminant analysis (LDA) effect size (LEfse) analysis showed that there were 78 biomarkers, while in piles T and C, there were 35 and 43 biomarkers, respectively. These results indicated the addition of microbial inoculant improved the maturity and fertility, as well as significantly regulating the microbial community structure.