Determining underlying physiological patterns governing plant productivity and diversity in grasslands are critical to evaluate species responses to future environmental conditions of elevated CO2 and nitrogen (N) deposition. In a 9-year experiment, N was added to monocultures of seven C3 grassland species exposed to elevated atmospheric CO2 (560 μmol CO2 mol−1) to evaluate how N addition affects CO2 responsiveness in species of contrasting functional groups. Functional groups differed in their responses to elevated CO2 and N treatments. Forb species exhibited strong down-regulation of leaf Nmass concentrations (−26%) and photosynthetic capacity (−28%) in response to elevated CO2, especially at high N supply, whereas C3 grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C3 grasses (+68%) in elevated CO2, but not significantly for forbs. Differences in access to soil resources between forbs and grasses may distinguish their responses to elevated CO2 and N addition. Forbs had lesser root biomass, a lower distribution of biomass to roots, and lower specific root length than grasses. Maintenance of leaf N, possibly through increased root foraging in this nutrient-poor grassland, was necessary to sustain stimulation of photosynthesis under long-term elevated CO2. Dilution of leaf N and associated photosynthetic down-regulation in forbs under elevated [CO2], relative to the C3 grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.