Driving forces of the humification process in composting: from a microbial perspective

In composting, humification is critical for final compost quality and effectiveness and involves small molecules polymerizing into humic substances (HS). Microbial metabolism, as the core process of composting, drives this transformation. However, the full relationship between microbial metabolic processes and humification-related polymerization remains unclear in current research. This review first examines humification polymerization mechanisms from the perspective of precursors through functional group reactions and radical oxidative coupling. Then, it systematically elucidates the central role of microbial extracellular and intracellular metabolisms in driving the humification process. On the one hand, the effects of hydrolases and oxidoreductases of extracellular metabolism on the polymerization are elaborated in detail. On the other hand, it delves into the dual contributions of intracellular metabolism: catabolism supplies essential precursors and active energy for polymerization reactions through substance degradation and oxidative phosphorylation, while anabolism is directly involved in the biosynthesis of humus precursors. This work aims to provide a comprehensive theoretical framework for understanding the microbial driving mechanisms behind humification.