This study investigates the mineral control of soil organic carbon storage and turnover, focusing on how soil minerals influence the long-term storage and cycling of organic carbon in soils. The research uses radiocarbon analyses to explore the relationship between soil mineralogy and organic carbon along two natural gradients—soil age and climate—in volcanic soil environments. The study reveals that the accumulation and subsequent loss of organic carbon are largely driven by changes in the millennial-scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development. The findings suggest that non-crystalline minerals play a significant role in stabilizing organic carbon, with their abundance accounting for more than 40% of the variation in organic carbon content across different soil horizons and ages. The study also shows that the ability of soils to retain carbon is due to the capacity of non-crystalline minerals to stabilize large quantities of organic carbon for thousands of years. These results have implications for understanding the global carbon cycle, as they highlight the importance of soil mineralogy in controlling the storage and turnover of organic carbon. The study also suggests that the mechanisms identified may be generalizable to many soils of humid environments, where weathering initially produces metastable, reactive minerals that later transform to stable, less-reactive products. The findings contribute to a better understanding of the role of soils in the global carbon cycle and the factors that influence the long-term storage and turnover of organic carbon.This study investigates the mineral control of soil organic carbon storage and turnover, focusing on how soil minerals influence the long-term storage and cycling of organic carbon in soils. The research uses radiocarbon analyses to explore the relationship between soil mineralogy and organic carbon along two natural gradients—soil age and climate—in volcanic soil environments. The study reveals that the accumulation and subsequent loss of organic carbon are largely driven by changes in the millennial-scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere–ecosystem carbon fluxes during long-term soil development. The findings suggest that non-crystalline minerals play a significant role in stabilizing organic carbon, with their abundance accounting for more than 40% of the variation in organic carbon content across different soil horizons and ages. The study also shows that the ability of soils to retain carbon is due to the capacity of non-crystalline minerals to stabilize large quantities of organic carbon for thousands of years. These results have implications for understanding the global carbon cycle, as they highlight the importance of soil mineralogy in controlling the storage and turnover of organic carbon. The study also suggests that the mechanisms identified may be generalizable to many soils of humid environments, where weathering initially produces metastable, reactive minerals that later transform to stable, less-reactive products. The findings contribute to a better understanding of the role of soils in the global carbon cycle and the factors that influence the long-term storage and turnover of organic carbon.