The objective of this experiment was to understand how atmospheric carbon dioxide (CO2) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO2 and interact with atmospheric CO2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO2 X N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/root box experimental units at UMBS in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted for by roots <0.4 mm in diameter, and specific root length (100-250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Elevated atmospheric CO2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO2 and soil N. Rates of root production and mortality were significantly greater at elevated CO2 when trees grew in high-N soil, but there were no CO2 effects at low soil N. Fine-root biomass increased 137-194% in high-N compared to low-N soil, and elevated atmospheric CO2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r=0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO2, when compared to ambient atmospheric CO2, increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r2=0.96). Our results indicate that atmospheric CO2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. Aspen-dominated ecosystems of the future are likely to have greater productivity fueled by greater nutrient uptake due to greater root length production. Further, it appears that elevated atmospheric CO2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.