Changes of bacterial necromass and their roles in humus conversion during organic wastes composting from different sources

[Display omitted] •Dynamics of BNC and its contribution to composting humus conversion were revealed.•BNC varied from 0.90 to 13.05 mg/g, with the highest levels observed in KW.•BNC/MBC ratio ranged from 8% to 444% depending on feedstock type.•Firmicutes, Proteobacteria, and Gemmatimonadota contributed to BNC formation.•In KW, core bacteria drove BNC conversion into humic acid. This study compared the changes of bacterial necromass carbon (BNC) in composting of three distinct organic wastes [sewage sludge (SW), kitchen waste (KW), and pig manure (PM)] and their relationship with bacterial communities and humus formation. Results revealed that BNC content significantly differed across treatments, with KW exhibiting the highest level at 13 mg/g, followed by PM, where BNC changed between 8 % and 444 % of microbial biomass. Humification index and degree of polymerization indicated that PM had higher humification potential. Network analysis showed that key bacterial phyla contributing to BNC included Firmicutes in KW and Proteobacteria and Gemmatimonadota in SW and PM. Structural equation modeling demonstrated that BNC promoted the formation of humic acid in KW, while core bacteria facilitated the conversion of fulvic acid to humic acid in PM. These findings underscored the crucial role of bacterial necromass in enhancing humification and highlighted the distinct humification processes in composting.