Calcium Carbonate Constrains the Capacity and Saturation of Mineral-Associated Organic Carbon in Global Soils

Mineral-related organic carbon (MAOC) is the most stable and persistent component of soil organic carbon and the core of long-term carbon storage. However, the quantification of its capacity and saturation is insufficient on a global scale, especially in calcium-containing soils that cover large areas of the Earth's land. This study integrates regional soil sampling with a global dataset to systematically evaluate the limiting effect of calcium carbonate on the storage and saturation of MAOC. We utilized random forest machine learning models for both calcium-bearing and non-calcium-bearing soils to generate a global MAOC distribution map (0-30 cm), and revealed that although calcium-bearing soils cover nearly one-third of the global land area, they only account for 23% of the global MAOC reserves. Our findings highlight the role of calcium carbonate in carbon stabilization by restricting the mineral surface sites available for organic carbon retention. This makes a significant revision to the current global carbon model and provides a new understanding of the mechanism of soil carbon heterogeneity.