Many experiments conducted under controlled environmental conditions suggest that deciduous tree species are adapted to NO₃^– rather than NH₄⁺ uptake. To test this under field conditions, we studied soil retention, tree uptake, and tree resorption of ¹⁵N derived from either ¹⁵NH₄NO₃ or NH₄¹⁵NO₃ applied to trembling aspen (Populus tremuloides Michx.) and hybrid aspen (Populus tremula L. × P. tremuloides) at planting. Overall, the hybrid had greater dry-matter yield and took up more total N than the trembling aspen over two growing seasons after fertilization. The recovery of ¹⁵N per tree was also higher for the hybrid aspen than for the trembling aspen. Trembling aspen showed higher resorption efficiency of foliage N before abscission than hybrid aspen; however, the total amount of N retranslocated before leaf senescence was the reverse because of higher foliar dry-matter yield of hybrid aspen. The higher recovery of ¹⁵NH₄⁺ than ¹⁵NO₃^– by trees seemed to be governed by higher N retention potential of NH₄⁺ than NO₃^– in the soil. These results suggest that fertilization strategies need to be made based on both N acquisition capability of species and site-related factors, such as soil pH and immobilization–mineralization potential.