Nitrogen (N) is essential for fast growth in Populus species. For successful forestation, it is necessary to estimate growth performance and N utilization efficiency under different N regimes. Saplings of P. cathayana were subjected to 0.25 (N deficiency), 2.5 (N normal), or 7.5 (N fertilization) mM NH4NO3 for six weeks, respectively. The morphological and physiological parameters relative to N responses were measured. In comparison with normal N, N deficiency stimulated root biomass, a reduced biomass of leaves and stem, and an elevated root to shoot biomass ratio; in contrast, the biomass of all tissues was unaltered under N fertilization. Concentrations of NH4+ in roots and leaves, and of NO3- in roots were decreased in response to N deficiency, corresponding to lower activities of glutamate dehydrogenase (GDH) and nitrate reductase (NR) in both roots and leaves, and of glutamate synthase (GOGAT) in the roots of P. cathayana exposed to N deficiency, whereas the opposite effect was observed in most cases under N fertilization. The imbalance of nutrients, accumulation of large amounts of stress hormones (ABA, JA, and SA), and reduced photosynthesis were observed in the plants treated with N fertilization. P. cathayana displays morphological and physiological plasticity to changes in N levels, and the growth driven by N appears closely related to the internal resource allocation and carbon/nitrogen partition. Our data indicate that moderately reducing the dose of N fertilizer may have both economic and environmental benefits in commercial poplar plantations.