Carbon sequestration in soils might offset part of the increase of CO₂ in the atmosphere. Two contrasting subtropical grassland species, bahiagrass (BG), Paspalum notatum Flügge, and rhizoma perennial peanut (PP), Arachis glabrata Benth., a legume, were grown at Gainesville, Florida, USA, in field soil plots in four temperature zones (baseline-ambient, +1.5, +3.0, and +4.5 °C) in four temperature-gradient greenhouses, two each at 360 and 700 ppm CO₂. The soil had been in continuous cultivation for more than 20 years before plant establishment. Samples from the top 20 cm of each plot were collected before plant establishment and six years later, after the study ended. Soil organic carbon (SOC) increases across the six years were greater for BG than PP, 1.396 and 0.746 g/kg, respectively. Belowground biomass was also greater for BG than PP. Mean SOC gains in BG plots at 700 and 360 ppm CO₂ were 1.450 and 1.343 g/kg, respectively (no CO₂ effect). Mean SOC increases in PP plots at 700 and 360 ppm CO₂ were 0.949 g/kg and 0.544 g/kg, respectively (significant CO₂ effect). Overall, SOC increased only for the first temperature increment, and thereafter declined. Soil organic nitrogen (SON) accumulation patterns were similar to SOC increases. Mean annual SOC accumulation was 475 kg/ha per year, comparable with other studies. We conclude that carbon can be accumulated in soils converted to grassland species in humid, subtropical environments. The SOC accumulation will be greatest for species that have greater belowground biomass accumulation.