Bacillus smithii XT-3 inoculation reduces ammonia emissions and promotes humification during co-composting of Siraitia grosvenorii residue and pig manure.

The co-composting of Siraitia grosvenorii residue and pig manure frequently faces challenges associated with substantial nitrogen loss and insufficient humification. To address these issues, the Bacillus smithii strain XT-3 was inoculated into the co-composting system to enhance nitrogen retention and promote humus formation during composting. The underlying mechanisms were further elucidated through metagenomic analysis. Inoculation with strain XT-3 reduced cumulative NH3 emissions by 27.8% and increased humic acid content by 17.02%. Temporal variations in humic substances presented a synchronous decline in fulvic acid coupled with humic acid accumulation, facilitating faster attainment of compost maturity thresholds. Furthermore, XT-3 reshaped the microbial community involved in nitrogen metabolism by enhancing the relative enrichment of Bacillota during the thermophilic stage and elevating the relative abundance of Pseudomonadota and Bacteroidota during the cooling stage, along with strengthened positive microbial interactions. Functional annotation of KEGG orthologs indicated that XT-3 improved the genetic potential of ammonia assimilation pathways throughout composting. Collectively, these findings demonstrate that XT-3 may facilitate synergistic nitrogen retention and enhanced humification by modulating microbial community structure, strengthening positive microbial interactions, thereby providing a theoretical basis for the precise regulation of functional microorganisms during composting.