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 C-3 grassland species exposed to elevated atmospheric CO2 (560 mu mol 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 C-3 grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C-3 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 C-3 grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.