Abstract The uptake of atmospheric carbon by terrestrial ecosystems may play an important role in the global carbon cycle since every 6–7 years the whole atmospheric carbon content passes through the plant biomass. Major uncertainties in this area concern the persistency of growth stimulation by elevated CO2 and effects on carbon allocation to the soil compartment. In this study the effect of elevated CO2 on growth and carbon allocation of Lolium perenne was investigated. Plants were pretreated at 350 and 700 L L1 at two nitrogen levels (135 and 400 kg N ha1 yr1) for 14 months and subsequently crosswise transferred to ESPAS-phytotrons for a short-term treatment at 350 and 700 L L1 CO2 for three weeks. The pretreatment stimulated total shoot growth until the end of the experiment, although no CO2 pretreatment effects were observed in the yields of the last cut. The higher nitrogen level almost doubled shoot yield throughout the experiment. The fact that nitrogen stimulated shoot growth until the end of the experiment suggests that the disappearance of the growth stimulation of shoots by elevated CO2 was not primarily caused by exhaustion of nitrogen or other nutrients in soil. The nitrogen effect on root growth showed an interaction with the CO2 pretreatment. At the lower nitrogen level root dry weight was not increased at 700 L L1 CO2, whereas at the higher nitrogen level a strong increase was observed. This interaction indicates that nitrogen may have important implications for stimulating effects of elevated CO2 on root growth on the long-term and thus on carbon allocation to the soil.