In order to establish suitability of forest ecosystems for long-term storage of C, it is necessary to characterize the effects of predicted increased atmospheric CO2 levels on the pools and fluxes of C within these systems. Since most C held in terrestrial ecosystems is in the soil, we assessed the influence of Free Air Carbon Enrichment (FACE) treatment on the total soil C content (Ctotal) and incorporation of litter derived C (Cnew) into soil organic matter (SOM) in a fast growing poplar plantation. Cnew was estimated by the C3/C4 stable isotope method. Ctotal contents increased under control and FACE respectively by 12 and 3%, i.e., 484 and 107 gC/m2, while 704 and 926 gC/m2 of new carbon was sequestered under control and FACE during the experiment. We conclude that FACE suppressed the increase of Ctotal and simultaneously increased Cnew. We hypothesize that these opposite effects may be caused by a priming effect of the newly incorporated litter, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates.