Molecular-level insights into directional condensation mechanism of phenolic acids during humic substance formation

The regulation of humic substance formation during aerobic fermentation of organic solid waste has gradually become a research hotspot in related fields. The metabolic byproducts of lignocellulose have the potential to act as precursors for the synthesis of humic substances. This study, grounded in a robust framework of metabolic intermediate indicators, selected representative pure phenolic acid intermediates to conduct condensation experiments. The results showed that condensation reactions occurred rapidly under simulated aerobic fermentation conditions, and the degree of humification gradually deepened over time. Gallic acid and vanillic acid were considered the best condensation combinations. Condensation and rearrangement reactions produced 114 distinct compounds with a basic benzene ring structure and various functional side chains. These include lipids, lignin-like substances, unsaturated hydrocarbons, aromatic organic compounds, tannins, and a small carbohydrate component. At the same time, interconversion between different condensation substrates can occur to form gallic acid, yielding partially similar metabolites. Condensed tannins were identified as the dominant products, accounting for 65% of the relative abundance in the phenolic acid condensation system. This study could lay the foundation for the regulation technology of directed condensation humification of humic substance precursors during aerobic fermentation. These findings provide valuable insights for optimizing organic waste management and enhancing soil fertility through targeted humification strategies.