Recently, the Cultivated Land Quality Conservation Innovation Team from the Institute of Agricultural Resources and Regional Planning (IARRP), part of the Chinese Academy of Agricultural Sciences, has unveiled the molecular structure and mechanisms underlying the formation of straw-derived organo-calcium carbonate complexes (OCC) in calcareous soils, using Scanning transmission X-ray microscopy (STXM) and other advanced techniques. The study, led by the team, was published in Environmental Science & Technology (ES&T), a top-tier journal in the Chinese Academy of Sciences (CAS) Zone 1, under the title: "Physicochemical Properties, Molecular Structures, and Stability of Organo-Calcium Carbonate Complexes: The Critical Role of Carbon Sources, Formation Processes, and OC/Ca Ratios."

Calcareous soils, accounting for 30% of Earth's terrestrial surface, contain calcium carbonate (C_aCO₃) concentrations ranging from tens of percent to 95%, which play a vital role in the OC sequestration in the calcareous soils. The high C_aCO₃ content in calcareous soils facilitates two distinct OM stabilization processes: Organic matter (OM) can adsorb directly onto carbonate minerals or coprecipitate during C_aCO₃ crystallization to form organo-calcium carbonate complexes (OCC), which strongly influence carbon stability in these soils. However, the molecular mechanisms governing these processes have remained insufficiently understood.

Under alkaline conditions, OCC was synthesized through both adsorption and coprecipitation pathways using dissolved organic matter derived from wheat and maize straw. The physicochemical properties and molecular structures of the OCC were characterized by XRD, ATR-FTIR, solid-state ¹³C NMR, and STXM-XANES. At the molecular level, the study clarified how the critical role of carbon sources, formation processes, and OC/Ca ratios control the stabilization of OC in calcium carbonate systems. Key findings include: 

First, under the same OC/Ca molar ratio, adsorbed OCC retained more OC than coprecipitated OCC; however, the OC in adsorbed OCC was less stable and more prone to desorption.

Second, 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. At the molecular scale, Ca^2⁺ primarily coordinates with carboxyl C and can also interact with polysaccharides. Maize straw-derived OCC tended to form "Ca-polysaccharide" or "Ca-carboxylate-polysaccharide" ternary structures, whereas wheat straw-derived OCC was dominated by "Ca-carboxylate" binding.

This work provides a molecular-level explanation of how calcium carbonate drives differential stabilization of straw-derived OC through adsorption and coprecipitation processes in calcareous soils in northern China, offering important theoretical insights into long-term OC sequestration in calcareous soils.

The study was conducted by PhD candidate Liu Yichen of CAAS IARRP as the first author, with Professor Yang Jianjun as the corresponding author. It was supported by multiple funding sources, including the National Natural Science Foundation of China (42277024; 42277327), the Top-Notch Young Talents Program of China, the earmarked fund for China Agriculture Research System-03 (CARS-03), the Agricultural Science and Technology Innovation Program of the Chinese Academy of Agricultural Science (2026–2030), and the Central Public-interest Scientific Institution Basal Research Fund (No. Y2025YC83). 

Citation:

Liu, Y.; Darma, A. I.; Wang, J.; Xia, X.; Sun, M.; Zhao, Y.; Pang, L.; Liu, J.; Yang, J. Physicochemical Properties, Molecular Structures, and Stability of Organo-Calcium Carbonate Complexes: The Critical Role of Carbon Sources, Formation Processes, and OC/Ca Ratios. Environ. Sci. Technol. 2026.

Original paper link: https://pubs.acs.org/doi/full/10.1021/acs.est.5c12270