Differential regulation of humification by Maillard reaction precursors during the co-composting of pig manure and spent mushroom substrate

The rapid expansion of intensive agriculture has generated massive amounts of pig manure (PM) and spent mushroom substrate (SMS), posing severe environmental challenges. Although aerobic co-composting sustainably recycled these wastes, natural humification was typically slow, yielding immature humus (HS) with severely limited agronomic value. Therefore, this study comprehensively investigated the differential regulatory effects of distinct Maillard reaction precursors (MRPs)—glucose (Glu), catechol (Cat), and glycine (Gly)—on humification dynamics, humic acid (HA) structural evolution, and overall compost quality during a 120-day controlled mesophilic incubation experiment conducted under controlled 28 ± 0.5°C conditions, rather than traditional variable-temperature thermophilic composting systems. Chemical properties and HS fractionation were systematically analyzed alongside spectroscopic evaluations (3D-EEM, FTIR), principal component analysis (PCA), and structural equation modeling (SEM). Results demonstrated that all tested MRPs significantly mitigated initial phytotoxicity and enhanced ultimate compost maturity, achieving final T values (0.53–0.75) well below the established highly mature threshold (≤ 0.8). Specifically, Glu exhibited the strongest comprehensive regulatory effect (PCA score: 0.772, p < 0.05, significantly higher than Cat, Gly, and CK treatments); by stimulating microbial metabolism as inferred from DOM fluorescence properties and HS transformation dynamics, it maximized stable humin (Hu) accumulation and achieved the highest humic-extracted acid (HE) content (57.7 g/kg, significant difference relative to the other three groups at p < 0.05). The Cat treatment effectively alleviated salinity stress, significantly reducing electrical conductivity by a pronounced 63.5% ( p < 0.05, significantly lower final EC than CK , Glu, and Gly ) via phenolic oxidative coupling, while retaining the highest total intensity for aliphatic structures. Furthermore, Gly supplementation successfully facilitated the highest degree of HA aromatization, evidenced by a drastic 36.8% reduction in the H/C atomic ratio ( p < 0.05, the largest H/C decline among all treatments ). SEM supported our hypothesized causal framework and provided statistical evidence indicating that MRPs acted as pivotal drivers, exerting a significant direct positive effect on final compost quality by synergistically accelerating humification. These findings elucidate precursor-specific pathways of humification, providing a targeted and data-supported regulatory strategy to successfully enhance HS quality and effectively promote the high-value recycling of agricultural organic wastes.