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Grasslands serve as crucial terrestrial carbon sinks, storing substantial amounts of organic carbon (OC) in aboveground vegetation, roots, and soils. However, effective carbon sequestration in grasslands depends on management practices. South African subtropical mesic grasslands are fire-adapted ecosystems, dependent on fire as a key ecological force shaping and sustaining them. Fire influences carbon sequestration by removing above-ground biomass, redistributing nutrients, and contributing inputs through thermal mineralisation. This study assessed the impact of varying fire frequencies and season of prescribed burns on soil organic carbon (SOC) stocks, total nitrogen (TotN), and carbon-to-nitrogen (C:N) ratios, as well as the stability of refractory organic matter (refractory OM) and black carbon (BC) fractions across soil depths up to 30 cm. Results showed that fire frequency and season of prescribed burns significantly influence SOC and TotN stocks, with annual winter and spring burns resulting in the highest SOC stocks and wide r C:N ratios in the top 5 cm of soil. Conversely, biennial and triennial burns showed reduced SOC stocks, with triennial burns displaying the lowest levels and even SOC loss below 5 cm. Over 20 years, SOC stocks increased under all fire regimes except for triennial burns, where stabilization or reduction was noted. Additionally, our findings indicate that increased fire frequency leads to the maintenance of BC within the soil, though BC remains stable across soil horizons. Refractory OM and BC levels were greater in areas that were unburnt or experienced longer burn intervals, likely driven by a decline in soil pH and increased acid saturation associated with less frequent fires. These patterns suggest that while regular fires can contribute to surface SOC stocks, the management of fire intervals is crucial for maintaining deeper, more recalcitrant carbon fractions. This emphasizes the beneficial role of regular grassland fires in carbon sequestration and suggests practical management strategies for fire-adapted mesic grasslands.