In recent years, due to global climate change and unpredictable human activities, the problem of water resource shortage and environmental water pollution has become widespread. Particularly, nitrogen pollution of water in China is a problem, owing to atmospheric nitrogen deposition and excess of nitrate, which results in a significant increase in water soluble nitrogen and eutrophication. To address the issue, an important multipurpose tree species, willow (Salix sp.), which has large biomass production and high resistance to environmental stress, has been studied, and as proved effective in the uptake and accumulation of pollutants from contaminated waters and soils. Hence, it was employed in the present experiment. In the current study, the seeding of Salix matsudana was selected to probe the characteristics of growth, nitrogen absorption/distribution, and physiological responses under different nitrogen treatments. These included poor nitrogen (1.4 mg/L), medium nitrogen (2.8 mg/L), high nitrogen (7.0 mg/L), and excessive nitrogen (28.0 mg/L), which are levels related to the water eutrophication classification standard. Nitrogen was added as ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) through water culture experiments. The results showed that the biomass of Salix matsudana increased with increasing nitrogen to some extent, but the growth was inhibited under excessive nitrogen concentrations, and NH₄⁺-N showed a higher inhibition than NO₃^--N. Meanwhile, Salix matsudana had a preference for NH₄⁺-N absorption over NO₃^--N when provided with the same concentration of nitrogen. The order of AT% contents, ¹⁵N absorption and Ndff% in different organs of Salix matsudana was, root > stem > leaf, and the content of NH₄⁺-N was higher than that of NO₃^--N. Furthermore, the difference showed an increasing trend with rising nitrogen concentration. Moreover, with increasing nitrogen concentration, the rate of ¹⁵N allocation was significantly different in different organs for the two kinds of nitrogen. Specifically, for NH₄⁺-N treatment, the ¹⁵N allocation rate showed a trend of increasing after first decreasing, and nitrogen levels peaked in the root of Salix matsudana. The maximum value appeared under medium nitrogen concentrations; while the ¹⁵N allocation rate in leaves showed a trend of gradually falling, and the maximum value appeared in the case of poor nitrogen concentration. Both inadequate and excessive nitrogen in solution showed significant effects on the physiological traits of willow seedlings. The chemical form of nitrogen also showed significantly different effects on physiological characterizations of willow. The root activity decreased by 50.61% and increased by 19.53% under excessive nitrogen when compared with the control. Furthermore, indicators such as root length, root surface area, root volume, root diameter and root tips of Salix matsudana, decreased by 30.92%, 29.48%, 19.44%, 27.01%, and 36.41% respectively, under NH₄⁺-N treatment. The root length, root surface area, root volume, root diameter, and root tips of Salix matsudana were decreased by 1.66%, 5.56%, 1.49%, 5.06%, and 25.72% respectively, under NO₃^--N treatment. Thus, it could be concluded that the influence of NH₄⁺-N stress on Salix matsudana was higher than that of NO₃^--N when seedlings were exposed to high concentration of nitrogen. Therefore NO₃^--N certain concentrations could be added to alleviate the damage caused by NH₄⁺-N on Salix matsudana.