Abstract: We chose three different ages(5 years, 9 years and 15 years old)of poplar plantations in a coastal area in Dongtai city, Jiangsu province. The experiment used randomized block design with five nitrogen deposition concentration treatments, i.e. N0(CK), N1(5 g/(m²·a)), N2(10 g/(m²·a)), N3(15 g/(m²·a), N4(30 g/(m²·a))in the field. The results showed that:(1)The manimum content of soluble sugar and starch appeared in December, and soluble sugar in one year had the largest change range, the highest and the lowest differed about seven times. While the seasonal variation of structural carbohydrates(cellulose and lignin)was not obvious, and the cellulose and lignin content was relatively stable.(2)With the increase of exogenous nitrogen input, soluble sugar, starch and NSC content in the fine roots increased and then decreased, and the largest content at N2, while N4(the most nitrogen input)appeared below N0(control treatment). SC had no significant difference(p>0.05)among of levels of nitrogen increase treatments.(3)There were significantly positive correlation between fine root biomass and NSC, and no significant correlation between soil C, N and carbohydrates compositions, as well as temperature and humidity.