Molecular fingerprints of vegetation-derived organic matter regulate pH-dependent adsorption on clay mineral: Implications for soil carbon persistence

Humic acids (HAs) extracted from soils under different vegetation restoration stages exhibit distinct molecular fingerprints, yet how these features interact with pH to regulate HA adsorption onto clay mineral such as montmorillonite, and the implications for soil carbon stability, remains poorly understood. Here, we combine molecular dynamics simulations, spectroscopy (XPS, FTIR), and batch adsorption experiments to unravel the mechanism by which humic acids (HA) extracted from soils under herbaceous, shrub, and arboreal vegetation exhibit distinct molecular fingerprints, thereby controlling pH-dependent adsorption behaviors. We show that arboreal-derived HA possesses the highest carboxyl content, whereas shrub-derived HA is enriched in aromatic and hydroxyl groups. Under acidic conditions (pH 5.3), carboxyl abundance governs adsorption via ligand exchange and hydrogen bonding. In contrast, at alkaline pH (8.7), electrostatic repulsion reduces overall adsorption but reverses the affinity order, with shrub-derived HA exhibiting the greatest stability due to compensatory hydroxyl H-bonding, Ca²⁺ bridging, and π–π interactions. Molecular dynamics simulations visually corroborate the dispersion of carboxyl-rich HA versus the sustained adsorption of aromatic/hydroxyl-rich HA under high pH. Our results demonstrate that the vegetation-driven carboxyl/hydroxyl ratio dynamically shifts the dominant adsorption mechanism from carboxyl coordination to hydroxyl–aromatic stabilization along a pH gradient. This work provides a molecular-scale framework for understanding and predicting SOM persistence in mineral–organic associations under environmental change. • Vegetation sources dictate HA’s pH-dependent fate via functional group heterogeneity. • Adsorption affinity reverses with pH: QLHA > BIHA > SDHA (5.3) → SDHA > BIHA > QLHA (8.7). • SDHA minimizes pH-induced loss (19.6% vs. QLHA’s 32.9%) via hydroxyl H-bonding/Ca²⁺ bridging. • Low pH: carboxyl dominates; high pH: hydroxyl/π-stacking compensates.