Switchgrass is a potential bioenergy crop that could promote soil C sequestration in some environments. We compared four switchgrass cultivars on a well-drained Alfisol to test for differences in biomass, C, and N dynamics during the fourth growing season. There was no difference ( P > 0.05) among cultivars and no significant cultivar × time interaction in analyses of dry mass, C stocks, or N stocks in aboveground biomass and surface litter. At the end of the growing season, mean (±SE) aboveground biomass was 2.1 ± 0.13 kg m −2, and surface litter dry mass was approximately 50% of aboveground biomass. Prior to harvest, the live root:shoot biomass ratio was 0.77. There was no difference ( P > 0.05) among cultivars for total biomass, C, and N stocks belowground. Total belowground biomass (90 cm soil depth) as well as coarse (≥1 mm diameter) and fine (<1 mm diameter) live root biomass increased from April to October. Dead roots were <10% of live root biomass to a depth of 90 cm. Net production of total belowground biomass (505 ± 132 g m −2) occurred in the last half of the growing season. The increase in total live belowground biomass (426 ± 139 g m −2) was more or less evenly divided among rhizomes, coarse, and fine roots. The N budget for annual switchgrass production was closely balanced with 6.3 g N m −2 removed by harvest of aboveground biomass and 6.7 g N m −2 supplied by fertilization. At the location of our study in west Tennessee, intra-annual changes in biomass, C, and N stocks belowground were potentially important to crop management for soil C sequestration.