Diversified crop rotation may improve production efficiency, reduce fertilizer nitrogen (N) requirements for corn (Zea mays L.), and increase soil carbon (C) storage. Objectives were to determine effect of rotation and fertilizer N on soil C sequestration and N use. An experiment was started in 1990 on a Barnes clay loam (U.S. soil taxonomy: fine-loamy, mixed, superactive, frigid Calcic Hapludoll) near Brookings, SD. Tillage systems for corn–soybean (Glycine max [L.] Merr.) rotations were conventional tillage (CS) and ridge tillage (CSr). Rotations under conventional tillage were continuous corn (CC), and a 4-year rotation of corn–soybean–wheat (Triticum aestivum L.) companion-seeded with alfalfa (Medicago sativa L.)–alfalfa hay (CSWA). Additional treatments included plots of perennial warm season, cool season, and mixtures of warm and cool season grasses. N treatments for corn were corn fertilized for a grain yield of 8.5 Mg ha–1 (highN), of 5.3 Mg ha–1 (midN), and with no N fertilizer (noN). Total (1990–2000) corn grain yield was not different among rotations at 80.8 Mg ha–1 under highN. Corn yield differences among rotations increased with decreased fertilizer N. Total (1990–2000) corn yields with noN fertilizer were 69 Mg ha–1 under CSWA, 53 Mg ha–1 under CS, and 35 Mg ha–1 under CC. Total N attributed to rotations (noN treatments) was 0.68 Mg ha–1 under CSWA, 0.61 Mg ha–1 under CS, and 0.28 Mg ha–1 under CC. Plant carbon return depended on rotation and N. In the past 10 years, total C returned from above- ground biomass was 29.8 Mg ha–1 under CC with highN, and 12.8 Mg ha–1 under CSWA with noN. Soil C in the top 15 cm significantly increased (0.7 g kg–1) with perennial grass cover, remained unchanged under CSr, and decreased (1.7 g kg–1) under CC, CS, and CSWA. C to N ratio significantly narrowed (–0.75) with CSWA and widened (0.72) under grass. Diversified rotations have potential to increase N use efficiency and reduce fertilizer N input for corn. However, within a corn production system using conventional tillage and producing (averaged across rotation and N treatment) about 6.2-Mg ha–1 corn grain per year, we found no gain in soil C after 10 years regardless of rotation.