Sustained increases in plant production in response to elevated atmospheric carbon dioxide (C02) concentration may be constrained by the availability of soil nitrogen (N). However, it is possible that plants will respond to N limitation at elevated CO2 concentration by increasing the specific N uptake capacity of their roots. To explore this possibility, we examined the kinetics of 15NH and 15N03 uptake by excised roots of Populus tremuloides Michx. grown in ambient and twice-ambient CO2 concentrations, and in soils of low- and high-N availability. Elevated CO2 concentration had no effect on either NH4+; or NO3- uptake, whereas high-N availability decreased the capacity of roots to take up both NH4+ and NO3-. The maximal rate of NH4+ uptake decreased from 12 to 8 umol/g/h, and Km increased from 49 to 162 umol/l, from low to high soil N availability. Because NO3 uptake exhibited mixed kinetics over the concentration range we used (10- 500 umol/l ), it was not possible to calculate Vmax and Km. Instead, we used an uptake rate of 100 umol/g/h as our metric of NO3 uptake capacity, which averaged 0.45 and 0.23 umol/g/h at low- and high-N availability, respectively. The proximal mechanisms for decreased N uptake capacity at high-N availability appeared to be an increase in fine-root carbohydrate status and a decrease in fine-root N concentration. Both NH4 and NO3 uptake were inversely related to fine-root N concentration, and positively related to fine-root total nonstructural carbohydrate concentration. We conclude that soil N availability, through its effects on fine-root N and carbohydrate status, has a much greater influence on the specific uptake capacity of P. tremuloides fine roots than elevated atmospheric CO2. In elevated atmospheric C02, changes in N acquisition by P. tremuloides appeared to be driven by changes in root architecture and biomass, rather than by changes in the amount or activity of N-uptake enzymes.