Wetlands can store large amounts of soil organic carbon (SOC). It is important to understand how conversion of wetlands to other land uses influences SOC storage. We examined SOC storage in subtropical hardwood swamps that had been converted to C-4 pastures in south central Florida. We used d¹³C signatures to assess the contribution of C-4 grasses to SOC pools in these converted swamplands. Using historical aerial photos, we selected three locations on cattle ranches where hardwood swamps had been converted to pastures, now dominated by Bahia grass (Paspalum notatum) or a mixture of native grasses (Andropogon virginicus, Panicum rigidulum). At each location we selected paired “treatments” that included an area of remnant hardwood swamp and an adjacent pasture that had been converted from the swamp in the past 20-40 years. All sites were dominated by muck soil underlain by poorly drained fine sand. We sampled the organic layer in three depth increments (0-5, 5-10, 10-xx cm) and mineral soil in four depth increments (0-10, 10-40, 40-70, 70-100 cm), and analyzed each sample for SOC and d¹³C. SOC differed among locations, treatments, and soil depth. Swamp soils had more total C in both organic (183 Mg/ha) and mineral soil (121 Mg/ha) layers than did the pastures soils (154 and 86 Mg/ha, respectively). The soil d¹³C signature was greater in the pastures than in the swamps throughout the soil profile, but the differences were greatest in the surface organic layer. Based on d¹³C signatures of vegetation and soil, we estimated that an average of 43% of SOC in the upper 5 cm of pasture organic soil, and 10% in the upper 10 cm of mineral soil, was derived from C-4 grasses. These wetland soils contained large stores of SOC which are still dominated by organic matter derived from the historical swamp vegetation.