A high-density plantation of three genotypes of Populus was exposed to an elevated concentration of carbon dioxide ([CO(2)]; 550 micromol mol(-1)) from planting through canopy closure using a free-air CO(2) enrichment (FACE) technique. The FACE treatment stimulated gross primary productivity by 22 and 11% in the second and third years, respectively. Partitioning of extra carbon (C) among C pools of different turnover rates is of critical interest; thus, we calculated net ecosystem productivity (NEP) to determine whether elevated atmospheric [CO(2)] will enhance net plantation C storage capacity. Free-air CO(2) enrichment increased net primary productivity (NPP) of all genotypes by 21% in the second year and by 26% in the third year, mainly because of an increase in the size of C pools with relatively slow turnover rates (i.e., wood). In all genotypes in the FACE treatment, more new soil C was added to the total soil C pool compared with the control treatment. However, more old soil C loss was observed in the FACE treatment compared with the control treatment, possibly due to a priming effect from newly incorporated root litter. FACE did not significantly increase NEP, probably as a result of this priming effect.