Our ability to predict whether elevated atmospheric CO2 will alter the cycling of C and N in terrestrial ecosystems requires understanding a complex set of feedback mechanisms initiated by changes in C and N acquisition by plants and the degree to which changes in resource acquisition (C and N) alter plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumnal senescence (early vs. late) under experimental atmospheric CO2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO2 concentrations were manipulated with open-top chambers, and soil-N availability was modified in open-bottom root boxes by mixing different proportions of native A and C horizon soil. Net N mineralization rates averaged 61 ng N/g/d in low-N soils and 319 ng N/g/d in high-N soils. After 2.5 growing seasons, we harvested above- and belowground plant components in each chamber and determined total biomass, N concentration, N content, and the relative allocation of biomass and N to leaves, stems, and roots. Elevated CO2 increased total plant biomass 16% in low-N soil and 38% in high-N soil, indicating that the growth response of P. tremuloides to elevated CO2 was constrained by soil-N availability. Greater growth under elevated CO2 did not substantially alter the allocation of biomass to above- or belowground plant components. At both levels of soil-N availability, elevated CO2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO2 significantly increased whole-plant N content in high-N soil (ambient = 137 g N/chamber; elevated = 155 g N/chamber), but it did not influence whole-plant N content in low-N soil (36 g N/chamber). Our results indicate that plants in high-N soil obtained greater amounts of soil N under elevated CO2 by producing a proportionately larger fine-root system that more thoroughly exploited the soil. The significant positive relationship between fine-root biomass and total-plant N content we observed in high-N soil further supports this contention. In low-N soil, elevated CO2 did not increase fine-root biomass or production, and plants under ambient and elevated CO2 obtained equivalent amounts of N from soil. In high-N soil, it appears that greater acquisition of soil N under elevated CO2 fed forward within the plant to increase rates of C acquisition, which further enhanced plant growth response to elevated CO2.