Abstract Rates of soil carbon (C) accumulation under 7 recently established tree plantations in Tennessee and South Carolina (USA) were estimated by comparing soil C stocks under the plantations to adjacent reference (nonplantation) sites. Estimated rates of C accumulation in surface (0– 40 cm ) mineral soil were 40– 170 g C m −2 yr −1 during the first decade following plantation establishment. Most soil C at each site was found in mineral-associated organic matter (i.e., soil C associated with the silt–clay fraction). Soils with high sand content and low initial C stocks exhibited the greatest gains in particulate organic matter C (POM-C). Labile soil C stocks (consisting of forest floor and mineral soil POM-C) became an increasingly important component of soil C storage as loblolly pine stands aged. Rates of mineral soil C accumulation were highly variable in the first decade of plantation growth, depending on location, but the findings support a hypothesis that farm to tree plantation conversions can result in high initial rates of soil C accumulation in the southeastern United States.