Abstract: With the Pinus elliottii plantations along an urban-rural gradient in Nanchang City as test objectives, and by using ion-exchange resin (IER) bag, thi s paper studied the seasonal dynamics of soil available nitrogen in the plantati ons, and analyzed the effects of urbanization on soil nitrogen supply. The resul ts showed that the soil nitrogen supply in the plantations had an obvious season al fluctuation. Soil NH4+-N concentration was significantly higher in autum n and winter than in spring and summer, while soil NO3--N concentration, mi neral N concentration, and relative nitrification rate were in adverse (P＜0 05). The seasonal dynamics of soil available nitrogen was basically consistent with the growth rhythm of P. elliottii. The soil nitrogen availability and relative nitrification ratio along the gradient differed greatly, being signific antly higher in urban than in rural area (P＜005). Urbanization accelerate d soil N mineralization and nitrification, enhanced soil nitrogen supply capacit y, and increased soil NO3--N content. It was suggested that in the construc tion of urban forest, the plants with high N demand, especially with high NO3 -- N absorption capacity, should be introduced to mitigate the soil available N loss and its induced environmental pollution.

Key words: desert plant, root architecture, topological index,  fractal dimension, branching ratio, link length.