What question did this study set out to answer?

Investigate how molecular fingerprints of humic acids from different vegetation types affect their adsorption on clay minerals across varying pH levels.

April 25, 2026Open Access

Molecular Fingerprints of Vegetation-Derived Organic Matter Regulate pH-Dependent Adsorption on Clay Mineral: Implications for Soil Carbon Persistence

Key Points

Investigate how molecular fingerprints of humic acids from different vegetation types affect their adsorption on clay minerals across varying pH levels.
Conducted molecular dynamics simulations to model adsorption behaviors of humic acids.
Utilized spectroscopy techniques such as XPS and FTIR for molecular analysis of extracted humic acids.
Performed batch adsorption experiments to evaluate pH-dependent adsorption of humic acids from different vegetation sources.
Under acidic conditions (pH 5.3), carboxyl abundance dominated adsorption through ligand exchange and hydrogen bonding.
At alkaline pH (8.7), adsorption affinity reversed, with shrub-derived HA displaying the greatest stability due to compensatory interactions.
Shrub-derived HA showed a lower pH-induced loss (19.6%) compared to arboreal-derived HA (32.9%).

Abstract

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.

Molecular Fingerprints of Vegetation-Derived Organic Matter Regulate pH-Dependent Adsorption on Clay Mineral: Implications for Soil Carbon Persistence

Key Points

Abstract

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