Structural transformation strategy of lignin to humic acid during biological acclimated composting.

Humic acid (HA) generated during straw-based fertilization is a key component in restoring the quality of black soil. However, strategies for manipulating lignin structure to regulate HA formation during acclimated composting remain unclear. In this study, rice straw (RS), willow branches (TB), and pine needles (PN), each characterized by distinct lignin structures, were composted to investigate the microbial conversion pathways from lignin to HA. Four representative functional groups in lignin were identified: aryl-ether CO bonds, side-chain CH groups, dimeric OH groups, and benzene-ring skeletons. Structural differences among the three lignin types resulted in distinct exposure sequences of these functional groups, which in turn led to differing formation pathways for HA functional groups. Fourier transform infrared coupled with EEM-PARAFAC analysis further showed that the HA derived from RS possessed a more compact structure. This compactness was attributed to the rapid degradation of G-S-H type lignin, which releases phenolic hydroxyl groups that readily couple with aromatic carbon skeletons, thereby promoting the formation of complex HA (HAC3: 39 %). Microorganisms were confirmed as the primary drivers of lignin-to-HA transformation. Core microbial taxa were identified through Spearman correlation analysis. To further elucidate the coupling relationships between the degradation of specific lignin functional groups and the formation of HA, a functional mapping framework was constructed. Overall, this study provides a theoretical basis for enhancing lignin humification efficiency and offers valuable insights for the conservation and sustainable management of black soil.