Quantitative estimates of soil C input under ambient (35 Pa) and elevated (60 Pa) CO2-partial pressure (pCO2) were determined in a Free-Air Carbon dioxide Enrichment (FACE) experiment. To facilitate 13C-tracing, Trifolium repens L. was grown in a soil with an initial δ13C distinct by at least 5‰ from the δ13C of T. repens grown under ambient or elevated pCO2. A shift in δ13C of the soil organic C was detected after one growing season. Calculated new soil C inputs in soil under ambient and elevated pCO2 were 2 and 3 t ha–1, respectively. Our findings suggest that under elevated CO2 conditions, soil C sequestration may be altered by changes in plant biomass production and quality.