Pre-biodrying enhanced lignin degradation to promote aromatic macromolecular humic acid formation in double-phase composting

A 24-h pre-biodrying phase was introduced in 10-d double-phase biodrying-enhanced composting (BEC) to accelerate the humification of kitchen waste by promoting the generation of phenolic-rich aromatic skeletons, key humic precursors for the lignin-protein and polyphenol pathways. To explore how biodrying promotes phenolic hydroxyl accumulation, macromolecule decomposition, particularly lignin, was investigated. Compared to day 9 of 15-d conventional bioaugmented mechanical composting (BMC) without pre-biodrying, BEC achieved 78.32 ± 15.96 % higher lignin degradation by day 9 (p < 0.001), with stronger correlations to humification indicators and greater explanatory power (91 %) for humic acid UV-vis indices than other macromolecules. Structural analysis showed enhanced lignin demethoxylation (minimum 3.92/Ar) and β-O-4 cleavage (minimum 51.69 %), increasing phenolic hydroxyl accumulation (maximum 4.34 mmol·g^-1). Microbial analysis further revealed that oxygen availability, improved by frequent stirring and aeration during biodrying, enriched Bacillaceae and Issatchenkia, which accelerated bioheat generation and created thermophilic conditions favorable for Bacillus and Saccharomonospora capable of lignin demethoxylation (via vanAB, ligM) and β-O-4 cleavage (via ligD, ligL, ligE, ligF, ligG). High temperatures also enhanced microbial cooperation, enabling the lignin degraders to obtain nutrients, cofactors, and detoxification agents for basal metabolism and lignin degradation from Oceanobacillus, Gracilibacillus, Sinibacillus, Georgenia, Nigrospora, and Aspergillus, thereby reducing their metabolic burden and allowing them to focus on lignin degradation. This study is the first to unveil the humification mechanism during composting from the perspective of macromolecule decomposition, especially lignin, for the generation of phenolic-rich aromatic skeletons as key humic precursors, providing theoretical support for developing simple, effective strategies to accelerate composting.