Pine wilt disease (PWD) is caused by a non-native pest that has spread extensively throughout Japan. Previous research has indicated that most infected trees have died and the litter deposited has resulted in changes to stream-water chemistry, particularly increased nitrate (NO (3) (-) ) concentrations. In this study, we divided stream nitrogen (N) export into N loss due to PWD and baseline N leakage without disturbance based on long-term monitoring. The annual N export was 110.0 mol N ha(-1) year(-1) in 1990 and 749.8 mol N ha(-1) year(-1) in 1997, and had decreased to 37.0 mol N ha(-1) year(-1) in 2005. N export under PWD influence was estimated to be 3697 mol N ha(-1), and N loss due to PWD was 2810 mol N ha(-1). N loss due to PWD was three times larger than baseline N leakage for the disturbed period. These changes in plant-herbivore relationships could affect N status in a forest ecosystem. So-called "semi-natural" disturbances related to non-native species invasion and increases of atmospheric N deposition caused by human activity will increase. Long-term monitoring studies of various aspects are necessary to offer insight into this ecosystem.