Molecular Recalcitrance Govern Natural Organic Matter Stabilization on Clay Minerals Versus Interfacial Reactions on Iron Oxides

The interaction of soil organic matter (SOM) with minerals is crucial for carbon sequestration, yet the extent and mechanisms of clay minerals in stabilizing SOM and how they differ from iron (hydr)oxides remain unclear. Using humic acid (HA), kaolinite, and goethite to represent organic matter, clay minerals and iron (hydr)oxides, respectively, we addressed these issues through adsorption experiments, molecular characterization (HPSEC, NMR and FT-ICR-MS), chemical oxidation analysis, and computational chemistry (MD and DFT). The result showed that the interaction energy, expressed per mole of the modeled HA molecule, was stronger between HA and goethite (< −5000 kJ/mol) than with kaolinite (> −2400 kJ/mol). Elevated ionic strength (IS), acidity, and Ca2+ addition further enhanced this interaction, resulting in a higher chemical stability of HA adsorbed on goethite. For kaolinite, IS increment, acidity, and phosphate addition promoted adsorption of larger (10–50 kDa) aliphatic HA fractions, which are intrinsically more stable. Due to weaker interaction with kaolinite, adsorbed HA's molecular properties played a more important role in determining its chemical stability. Our results demonstrated that goethite stabilized HA through strong chemical bonding, whereas kaolinite acted more as a selective filter for inherently stable HA fractions. These findings provide new insights into mineral-specific mechanisms of carbon stabilization, elucidating the mechanisms that govern SOM persistence and advancing our understanding and prediction of organic carbon cycling in soil ecosystems.