1. We investigated the interactive effects of elevated CO2, supply of phosphorus (P) and legume presence in model communities of calcareous grassland. Half of the communities contained six graminoids and eight non-legume forb species. In the other half, four non-legume forbs were replaced by legumes.
2.Ecosystem responses. Above-ground phytomass (>5 cm) hardly responded to elevated CO2 alone. However, when P and legumes were combined, the CO2 effect on above-ground community phytomass in year two was a stimulation of 45% (P<0·001). Below-ground community dry matter was stimulated by elevated CO2 alone by + 36% (P<0·01), but was only + 20% (P<0·05) when legumes were present and P was added. At the final (late season) harvest the mean effect of elevated CO2 on total above- and below-ground phytomass was + 23% (P<0·001) and revealed no significant interactions among treatment combinations, because above- and below-ground effects of CO2 enrichment had opposite directions.
3.Functional group responses. When legumes were absent, graminoids increased their total above- and below-ground phytomass in elevated CO2 by 60% (P<0·001) but there was no increase when legumes were present. The response of forbs to CO2 was not significant, irrespective of co-treatment. Legumes, however, were significantly stimulated by P supply and their CO2 response was much larger when P was added (+ 55%, P<0·01 vs + 25%, NS).
4.Species responses. CO2 effects on species ranged from highly positive (+ 143%) to moderately negative (– 43%).
5. Our results demonstrate that the effect of CO2 enrichment in such natural grassland communities will be low on above-ground phytomass and largely below-ground if no additional nutrients are provided. N2-fixing legumes appear to be crucial for the community response to elevated CO2 but legume responsiveness is largely controlled by P availability.