Abstract. Predicting forest responses to rising atmospheric [CO.sub.2] will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric [CO.sub.2] and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric [CO.sub.2] and soil-N availability on the flow of C and N in the plant-soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development ...