Maintenance of leaf N controls the photosynthetic CO₂ response of grassland species exposed to 9 years of free-air CO₂ enrichment

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 N_mass 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.