A mathematical model of total nitrogen balance in a watershed ecosystem was developed by deriving a time evolution equation of total nitrogen stored in the soil layer. The distribution of forest tree species was characterized by the growth rate and the stem volume of trees in this model. Applicability of this model was examined in a forested mountain area (Aoya River basin; 4, 500 ha) located in the central part of Japan. Total nitrogen loading in the stream water is generally small and the nitrogen fixation in trees becomes active in the forested watershed in which the growth rate and the stem volume of trees are large.
