Soil organic carbon (SOC) contents can change in surface soils in response to land use changes. In agricultural fields converted to perennial grasslands, SOC can increase on decadal timescales. However, reported rates of SOC accumulation have been widely variable, leading to uncertainty in both predictions of carbon (C) sequestration with different management practices and the mechanisms responsible for soil C accumulation. Three state factors— vegetation, soil texture, and climate— are correlated with SOC contents on regional scales and are therefore likely to explain variation in SOC accumulation rates. We used the Century model to explore how variation in these state factors may affect the relative magnitude of changes in soil C on decadal timescales. We parameterized the model using data from a grassland chronosequence study of former agricultural fields in western Minnesota, USA. We found that modeled variation in grassland vegetation composition, soil texture, and climate directly influenced the size and turnover time of SOC pools and that interactions among the factors strongly affected SOC pools. These results contradict empirical results from these systems, where vegetation and soil texture were found to have negligible influence on SOC accumulation rates over 40 years. Differences between modeled and measured results may arise from variability in initial conditions among field sites due to differences in historical agricultural management practices.