Do species and functional groups differ in acquisition and use of C, N and water under varying atmospheric CO₂ and N availability regimes? A field test with 16 grassland species

• To evaluate whether functional groups have a similar response to global change, the responses to CO₂ concentration and N availability of grassland species from several functional groups are reported here. • Sixteen perennial grassland species from four trait-based functional groups (C₃ grasses, C₄ grasses, non-leguminous forbs, legumes) were grown in field monocultures under ambient or elevated (560 μmol mol^-1) CO₂ using free-air CO₂ enrichment (FACE), in low N (unamended field soil) or high N (field soil +4 g N m^-2 years^-1) treatments. • There were no CO₂ × N interactions. Functional groups responded differently to CO₂ and N in terms of biomass, tissue N concentration and soil solution N. Under elevated CO₂, forbs, legumes and C₃ grasses increased total biomass by 31%, 18%, and 9%, respectively, whereas biomass was reduced in C₄-grass monocultures. Two of the four legume species increased biomass and total plant N pools under elevated CO₂, probably due to stimulated N-fixation. Only one species markedly shifted the proportional distribution of below- vs aboveground biomass in response to CO₂ or N. • Although functional groups varied in responses to CO₂ and N, there was also substantial variation in responses among species within groups. These results suggest that current trait-based functional classifications might be useful, but not sufficient, for understanding plant and ecosystem responses to elevated CO₂ and N availability.