Effect of Nitrogen Deposition in Forested Watersheds: Nitrogen Saturation and Nitrogen Runoff

Nitrogen occurs abundantly in the Earth's atmosphere in the form of nitrogen molecules. However, nitrogen molecules are stable and few organisms can utilize them directly. Nevertheless, through the artificial production of nitrogen compounds and exhaust gases from production facilities and motor vehicles, large amounts of nitrogen compounds are deposited in forest ecosystems, causing various problems. The responses of forests to deposited nitrogen differ depending on the environment. Those responses are summarized by region (Europe and the US, and Asia (Japan and China)) from previous studies. 1) While indicators of nitrogen saturation are valid in Europe and the United States, attention should be paid to hydrological conditions and other factors when applying them to Japan and other regions. 2) In recent years, high NO3- concentrations have been observed at forested watersheds in Japan due to air pollution near large cities and transboundary air pollution. 3) Significantly high NO3- concentrations have been observed at forested watersheds in China due to nitrogen deposition. 4) Nitrogen loading from rocks is not negligible in some areas. 5) New insights are being obtained into the functions of microbes involved in the nitrogen cycle in soil.