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This paper investigates the possible contribution of Short Rotation Cultures (SRC) to carbon sequestration in both current and elevated atmospheric CO2 concentrations ([CO2]). A dense poplar plantation (1 x 1 m) was exposed to a [CO2] of 550 ppm in Central Italy using the free-air CO2 enrichment (FACE) technique. Three species of Populus were examined, namely P. alba L., P. nigra L. and P. x euramericana Dode (Guinier). Aboveground woody biomass of trees exposed to elevated [CO2] for three growing seasons increased by 15 to 27%, depending on species. As a result, light-use efficiency increased. Aboveground biomass allocation was unaffected, and belowground biomass also increased under elevated [CO2] conditions, by 22 to 38%. Populus nigra, with total biomass equal to 62.02 and 72.03 Mg ha-1 in ambient and elevated [CO2], respectively, was the most productive species, although its productivity was stimulated least by atmospheric CO2 enrichment. There was greater depletion of inorganic nitrogen from the soil after three growing seasons in elevated [CO2], but no effect of [CO2] on stem wood density, which differed significantly only among species.

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.

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