Abstract: In order to improve the quality of the poplar plantations, the 6-year-old poplar plantations located in the Nanjing beach of Yangtze River were chosen to study thinning effects. Five thinning treatments(contrast, 30% intensity of the lower thinning, 40% intensity of the lower thinning, 50% intensity of the lower thinning and 50% intensity of the mechanical thinning)were designed in this experiment. The effects of soil available nitrogen supply, including inorganic nitrogen, DON(soil dissolved organic nitrogen)and SMBN(soil microbial biomass nitrogen)under different thinning treatments were investigated. The results showed that:(1)Soil available nitrogen was significantly affected by thinning treatments and the contents of available nitrogen, DON and SMBN were the highest in the treatments of 50% thinning intensities.(2)The contents of inorganic nitrogen, DON and SMBN showed a seasonal dynamic, and the highest contents of soil available nitrogen were in August.(3)The surface gathering of the available nitrogen was found for all the treatments, and the content at the soil depth of 0-5 cm was significantly higher than that of other two soil depths.(4)The correlation analysis showed a poor correlation among the contents of different types of soil available nitrogen within the experimental period. However, a significant and positive correlation between SMBN and DON as well as soil nitrate nitrogen was found during August. Results from the present study would provide a theoretical basis for the cultivating poplar large-diameter timber.