The impact of elevated CO2 and N-fertilization on soil C-cycling in Lolium perenne and Trifolium repens pastures were investigated under Free Air Carbon dioxide Enrichment (FACE) conditions. For ten years, swards were exposed to ambient or elevated CO2 (35 and 60 Pa p CO2) and received a low and high rate of labeled (15N) N fertilizers. The CO2 added in the FACE plots was depleted in 13C compared to ambient (delta 13C -40 per mille) thus the C and N inputs could be quantified. The maximum amount of labile C in the T. repens sward was estimated at 10.4+/-1.3 Mg ha-1 and 9.7+/- Mg ha-1 in the L. perenne sward. Mean residence time (MRT) for newly sequestered soil C was estimated at 3.5 years for T. repens and 4.2 years for L. perenne. An average of 34 % and 7 % of total soil C in the 0-10 and the 10-25 cm depth respectively in the T. repens sward and 30 % and 8 % respectively in the L. perenne sward was derived from FACE after 10 years exposure. The majority of the change in soil delta 13 occurred in the first three years of the experiment. We found no treatment effect on total soil C and FACE derived C was not detected in the 50-75 cm depth layer. In the 0-10 cm depth, total N is similar under L. perenne and T. repens swards, regardless of CO2 treatment. Whereas total 15N under T. repens is smaller than under L. perenne, due to symbiotic atmospheric 14N2 fixation in the legume T. repens. Leached 15N was not observed in the 50-75 cm depth layer of all treatments. Although the rate of C and N cycling was affected by species, the soil in this intensively managed grassland ecosystem did not become a sink for additional C under elevated CO2 conditions.