Improvement and Stability of Soil Organic Carbon: The Effect of Earthworm Mucus Organo-Mineral Associations with Montmorillonite and Hematite

Improving soil carbon storage and stability plays an important role in the development of sustainable agricultural production and mitigating climate change. Earthworms are widely distributed in soil environments; earthworm mucus (EM) can interact with natural mineral materials to form EM–mineral association, enriching soil carbon storage. However, it is unclear how minerals affect the formation and oxidation degradation of EM–mineral associations. Herein, the interactions between EM and natural mineral materials (hematite ore (HO) and montmorillonite (MT)) were investigated. The carbon stability of EM–mineral associations was analyzed based on their chemical oxidative resistance. EM interacted with HO/MT through ligand exchange, hydrogen bonding interaction, and electrostatic attraction. Compared to EM that was extracted under pH 5.0 (EM5) or 9.0 (EM9), EM obtained at pH7 (EM7) contained more protein and polysaccharide components, and was greatly adsorbed by HO/MT. Moreover, EM showed a stronger sorption affinity to MT than HO. The stronger oxidation resistance of EM–MT than EM–HO was revealed by its higher carbon retention, suggesting the vital role of MT in protecting biogenically excreted organic carbon from degradation. Earthworms in neutral environments could substantially promote the establishment of organo-mineral associations. This study provides guidance for promoting soil carbon sequestration through agricultural management and is beneficial to the sustainability of the soil.