Physicochemical Properties, Molecular Structures, and Stability of Organo-Calcium Carbonate Complexes: The Critical Role of Carbon Sources, Formation Processes, and OC/Ca Ratios

Calcium carbonates in calcareous soils play a crucial role in organic carbon (OC) sequestration. However, how calcium carbonates interact with OC and the stability of these interaction products remain unclear. In this study, organo-calcium carbonate complexes (OCC) were prepared through adsorption and coprecipitation using wheat and maize straw-derived dissolved organic matter (DOM) under alkaline conditions. The physicochemical properties and molecular structures of the OCC were characterized by XRD, ATR-FTIR, solid-state 13 C NMR, and STXM-XANES. The OC immobilization was higher in the OCC by adsorption but with lower stability compared to coprecipitation at equivalent 0–280 molar OC/Ca ratios. Increasing molar OC/Ca ratios led to a decline in O-alkyl C but an enrichment of carboxyl, aromatic, and alkyl C components in the OCC. The STXM-XANES analyses showed that Ca 2+ was primarily coordinated with carboxyl groups, with secondary interactions involving polysaccharides. Wheat and maize straw-derived OCC differed in the proportions of carboxyl C and O-alkyl C fractions according to the 13 C NMR analysis. Ternary Ca-carboxylate-polysaccharide colocalization and Ca-polysaccharide were more prominent for maize-derived OCC, whereas wheat-derived OCC was dominated by Ca-carboxylate binding. Adsorption facilitated multilayer OC coatings on calcite, while coprecipitation trapped OC within amorphous calcium carbonate during nucleation. The study provides new insights into carbon-calcium carbonate interactions at the molecular level, which should contribute to long-term OC sequestration in calcareous soils.