Abstract: The available N and its dynamic changes in eight typical plots of Pinus sylvestris var.mongolica plantations and meadows in southeastern Keerqin sandy area were studied by ion exchange resin bag method.It was showed by the results that the contents of NH₄⁺-N, NO₃^--N, mineral N and relative nitrification ratio in the soil varied as 0.18~1.54 μg/d·g dry resin, 0.96~2.05 μg/d·g dry resin, 1.23~3.58 μg/d·g dry resin, and 0.76~.97 respectively in the growing season, and NH₄⁺-N was the most important form due to intense nitrification in these ecosystems.Furthermore, the differences of above indexes in 4 different aged Pinus sylvestris var.mongolica plantations with 2 different management regimes were analyzed.It was indicated by the results that the above four indexes increased along with the stand ages, while the corresponding values in the grazing prohibited plots were lower than those of in the grazing plots because the nitrification process in the former plots was limited.Meanwhile, it was found out that the N transformation procedure in the soil was altered by grazing prohibition, which increased NH₄⁺-N proportion and decreased the relative nitrification ratio.In conclusion, property amelioration of sandy soil by forestation was a very long process, and it was influenced by many factors, especially by human disturbances.