Pre-biodrying treatment enhances lignin-related pathways with phenolic hydroxyls as reactive cores to accelerate humification during composting

The innovative biodrying-enhanced composting (BEC) process produces highly matured fertilizer within 10 d. To clarify the biodrying-accelerated humification mechanism, structural and molecular variations in humic acid (HA) during BEC were compared to those during 16-d bioaugmented mechanical composting without biodrying. Results showed that BEC produced HA with significantly higher aromaticity and molecular size (p < 0.01). More aromatic skeletons, mainly from biodrying-enhanced lignin decomposition (p < 0.05), contributed to HA aromatization. Reactive phenolic hydroxyls on these skeletons facilitated the binding of other humic precursors, promoting HA elongation. Microbial analysis indicated that Bacillus, Sinibacillus, and Issatchenkia, enriched by drastic heating and dehydration during days 0–3, participated in lignin decomposition. Saccharomonospora, Georgenia, Oceanobacillus, Nigrospora, Kluyveromyces, and Aspergillus contributed to HA elongation during the maturation phase (days 3–9). This study’s findings that biodrying enhanced lignin-related humification pathways by enriching functional microorganisms provides a theoretical foundation for further improving compost humification efficiency.