Divalent cations outweigh metal oxides in stabilizing organic carbon in humid subtropical grassland soils

• Permanganate-oxidizable carbon and microbial biomass carbon were robust indicators of soil organic carbon stabilization. • Exchangeable calcium and magnesium demonstrated the highest explanatory power for labile organic carbon proportions. • Fe-oxides were found to be more effective at stabilizing organic carbon than Al-oxides. Soil organic carbon (SOC) stability and protection are crucial for carbon sequestration and achieving climate mitigation goals. Physiochemical protection is a key mechanism of SOC stabilization, which is vital for global carbon cycling and has received considerable interests in recent years. However, the extent to which soil chemical properties determine SOC stabilization remains less well understood, especially in humid subtropical grasslands. This study investigates relationships between labile carbon fractions and various soil chemical properties to infer their protection effects on SOC. Our analyses of sandy and infertile soils in subtropical grasslands (south-central Florida, USA) reveal four key findings: (1) exchangeable calcium and magnesium ions are powerful stabilizing agents, demonstrating stronger influences on labile organic carbon proportions than any metal oxides; (2) among metal oxides, only iron (Fe)-oxides significantly affect labile carbon stabilization, while aluminum-oxides show no protective effects; (3) total Fe-oxides and poorly crystalline Fe-oxyhydroxides exhibit overall similar level of carbon protection; and (4) the proportions of permanganate-oxidizable carbon and microbial biomass carbon to total carbon are robust indicators of SOC stabilization, while water extractable organic carbon is less responsive to soil chemical protection. These findings suggest that management practices that enhance divalent cation availability and maintain soil conditions conducive to Fe-oxides formation could improve SOC stability in subtropical acidic and sandy grassland soils.