The effects of elevated CO 2 concentration upon rhizodeposition of nitrogen were investigated on field-grown Lolium perenne planted in soil cores set into the resident soil of a intensively managed ryegrass sward treated with elevated CO 2 for nine consecutive years, under two contrasted N fertilisation regimes (Swiss FACE Experiment). The planted cores were excavated from the ambiant (35Pa pCO 2 ) and enriched (60Pa pCO 2 ) rings at two dates during the growing season (spring and early autumn). The cores were brought back to the laboratory for a pulse-labelling of ryegrass shoots with 15 NH 3 , in order to quantify 15 N-rhizodeposition.A recovery of 10–16% of the total 15 N administred to the plant was recovered in the plant–soil system 48h after the pulse-labelling; significant amounts of 15 N were released into the soil adhering (adhering soil: AS) to the roots (0.44μg 15 NgAS −1 and 0.60μggAS −1 in the spring and the autumn samplings, respectively).In the spring sampling, there was no effect of atmospheric CO 2 concentration on N rhizodeposition. In the autumn sampling, elevated CO 2 stimulated N rhizodeposition that amounted to 7.2 and 5.2mg 15 Nm −2 , under elevated and ambient CO 2 , respectively. Nitrogen rhizodeposition was higher at high N (56gNm −2 ) than at low N fertilisation (14gNm −2 ), whatever the sampling date investigated.The mechanisms by which elevated atmospheric CO 2 leads to a stimulation of the net root-released N flux remains to be investigated: was it caused by a higher nitrogen immobilisation by the microbial biomass and a reduced re-assimilation of mineralized N and/or by a stimulation of N efflux from roots? Concomitant to the observed reduction of C rhizodeposition, the stimulation of net N efflux suggests that the quality of root released compounds was modified under elevated CO 2 concentration.