Microbial necromass analogues reshape composting humification pathways

Microbial necromass is increasingly recognized as a key driver of soil carbon stabilization, yet its mechanistic role in compost humification remains elusive. Here, by integrating metagenomics, quantitative necromass tracing, and partial least squares structural equation modeling (PLS-SEM), the regulatory effects of two microbial necromass analogues-N-acetyl-d-glucosamine (GlcNAc) and chitin-on pig manure composting were systematically investigated. Both analogues significantly altered the physicochemical properties, microbial community composition, and necromass dynamics during composting. Chitin addition markedly enhanced early microbial biomass and bacterial diversity but inhibited humic acid (HA) formation, while promoting fulvic acid (FA) accumulation and resulting in humic substances (HS) with lower molecular complexity. In contrast, GlcNAc selectively stimulated bacterial proliferation during the maturation phase, reduced both bacterial and fungal diversity, and led to increased FA content, accompanied by reduced HS molecular complexity. Metagenomic and PLS-SEM analyses revealed that both analogues fundamentally reprogrammed humification metabolic pathways: chitin suppressed genes involved in complex precursor metabolism, whereas GlcNAc narrowed the functional repertoire and shifted humification toward simpler pathways. These analogue-driven microbial metabolic shifts decoupled necromass accumulation from stable HA formation and favored the accumulation of simpler humic fractions. This study provides the first mechanistic evidence that targeted regulation of necromass transformation can precisely optimize humification efficiency and molecular characteristics, laying a theoretical foundation for improved organic waste utilization and process control in composting.