Soil retention, tree uptake, and tree resorption of15NH4NO3and NH415NO3applied to trembling and hybrid aspens at planting

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 of15N derived from either15NH4NO3or NH415NO3applied 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 of15N 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 of15NH4+than15NO3–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.