Abstract: Soil NH+ 4 -N and NO- 3 -N were the research objects, and tube closed-top incubation method was conducted to investigate the content and composition of soil mineral N in the original broadleaved Pinus koraiensis forest and the secondary forest. The results showed that soil NH+ 4 -N content of the original broadleaved P. koraiensis forest was always more than the secondary forest from April to October, and soil NO- 3 -N content of original broadleaved P. koraiensis forest was less than the secondary forest from May to July. In the original broadleaved P. koraiensis forest, soil NH+ 4 -N was always the dominating N form. In the secondary forest, soil NO- 3 -N was the dominating N form from May to July, and soil NH+ 4 -N was dominated in the other months. P. koraiensis and Betula platyphylla represented respectively the dominant tree species in the climax community and the pioneer tree species in the degenerated community,whose N absorption pattern was studied continuously by investigating their organ N content and growth. The results showed that N absorption trends of the two tree species were similar to soil mineral N content trends in the original broadleaved P. koraiensis forest and the secondary forest. From April to October, soil mineral N was absorbed gently by P. koraiensis, and it was absorbed rapidly by B. platyphylla from May to August, and its peak period was late June.