Soil retention, tree uptake, and tree resorption of 15NH4NO3 and NH415NO3 applied 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 of 15N derived from either 15NH4NO3 or NH4^15NO3 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.