Differential contributions of microbial necromass to humification: Hyperthermophilic vs. conventional thermophilic composting.

Microbial necromass plays a crucial role in the microbial-driven conversion of organic matter within composting systems. However, its role in the humification process of high-temperature composting has not been thoroughly investigated. In this study, a 52-day full-scale trough hyperthermophilic composting (hTC) and conventional thermophilic composting (cTC) were conducted. The cTC exhibited significantly higher accumulation of bacterial necromass carbon (BNC) (3.67 to 6.35 mg/g) and fungal necromass carbon (FNC) (8.12 to 13.07 mg/g) compared to hTC, which had lower BNC (1.33 to 2.84 mg/g) and FNC (1.93 to 8.32 mg/g). Environmental factors including temperature and C/N had a negative influence on MNC accumulation in hTC. Additionally, the reduction in microbial activity and diversity during the heating phase of hTC further affected the MNC accumulation. The bacterial phyla Proteobacteria and Firmicutes, together with the fungal phyla Ascomycota and Basidiomycota, were identified as key microbial groups closely associated with MNC accumulation. Microbial-mediated biochemical processes governing MNC dynamics can influence the composting humification process. Differential contributions of microbial necromass to humification were observed: in cTC, MNC dynamics played a significant role in promoting humification, whereas the contribution of MNC accumulation to humification was notably limited in hTC. hTC likely enhances the humification rate more significantly through high temperatures (typically exceeding 80 °C) and the distinctive thermophilic microorganisms, facilitating polymerization and transformation of small molecule organic matter into stable humic substances. This study quantitatively evaluated the dynamic changes of necromass carbon in high-temperature composting and offers new insights into the roles of necromass in this process.