Planting deep-rooted perennial plants in former croplands has been proposed as a strategy for sequestering organic carbon in soil. Deep-rooted plants are thought to store more organic carbon in soil than shallow-rooted plants because root biomass stores carbon and helps to build soil organic matter over time. We tested this hypothesis by studying switchgrass, a deep-rooted perennial, at 12 sites across the USA where it had been planted and maintained for 10-30 years. We found that switchgrass consistently stores more carbon than annual crops plants in its root biomass, but that its effects on carbon storage in soil organic matter were not statistically detectable. Radiocarbon measurements suggested that switchgrass was adding newly fixed carbon deep in the soil without necessarily stimulating loss of older soil organic matter. Our study shows that switchgrass can increase belowground carbon storage, and that it does this primarily by growing deep roots.