Grasslands store approximately one-third of terrestrial carbon (C) with most of it located belowground as soil organic carbon (SOC). Preserving and restoring SOC is essential for sustaining grassland ecosystem health and mitigating climate change. However, the effectiveness of soil management is constrained by limited understanding of where, how much, and how additional C can be stabilized in mineral-associated forms. Here, we combined a 2000-km field survey across temperate grasslands in China with machine learning to map the spatial distribution of mineralogical C deficit, defined as the unfilled capacity for long-term C stabilization through the formation of mineral-associated organic carbon (MAOC). We further conducted a ¹³C-labeled laboratory incubation experiment to identify key drivers of SOC formation efficiency. Random forest analysis revealed that mineralogical C deficit was primarily controlled by fine particle content, followed by mean annual temperature (MAT). Structural equation modeling showed that human disturbance indirectly reduced the deficit by reducing fine particles, while MAT increased it by altering soil chemistry and reducing plant cover. The largest deficits occurred in degraded and arid regions, totaling 0.78 ± 0.08 Pg C within the top 15 cm of soil. Isotope tracing further demonstrated that MAOC formation efficiency declined with increasing C deficit, as high-deficit soils were also biogeochemically degraded, characterized by lower SOC, nitrogen content, and microbial biomass, which limit C stabilization. Collectively, our study provides a spatially explicit assessment of soil C sequestration potential, highlights how degradation constrains SOC formation, and identifies priority areas for targeted restoration and climate mitigation.