Effects of Mineral Adsorption on the Molecular Composition of Soil Dissolved Organic Matter: Evidence from Spectral Analyses

The adsorption of soil dissolved organic matter (DOM) by minerals is crucial for long-term carbon storage and has been extensively studied. However, the variations in soil DOM composition resulting from adsorption by different minerals have rarely been investigated from a spectral perspective. This study examined the impact of non‑iron (kaolinite) and iron-bearing (ferrihydrite) minerals on the concentration and characteristics of DOM from dark brown soil during a 60-h-long incubation experiment. Soil dissolved organic carbon was removed more efficiently by ferrihydrite (66.2 %) than by kaolinite (14.8 %). UV–Vis spectroscopic analysis indicated that DOM with high aromaticity and molecular weight exhibited a strong affinity for both ferrihydrite and kaolinite. The fluorescence of soil DOM decreased in the ferrihydrite treatment, with a notable reduction in humic-like fluorescent components identified through fluorescent excitation-emission matrix coupled parallel factor analysis. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that carboxyl groups were critical in DOM adsorption on ferrihydrite, participating in ligand exchange, hydrogen bonding, and electrostatic attraction. This study highlights the important role of iron-bearing minerals in soil carbon sequestration and demonstrates how different mineral alter soil DOM composition, potentially influencing the cycling of organic carbon in terrestrial systems.