Influence of chronic and excessive nitrogen influx on forest ecosystems connected to the Tokyo metropolitan area

The impact of large cities on the environment is a concern not only on the environment of the urban area but also on the environment of the surrounding areas connected to the city and even the global environment. Therefore, it is necessary to determine the impact of large cities on the surrounding environment at various scales. Owing to human activities in megacities, various air pollutants are emitted into the atmosphere from fixed and mobile emission sources, causing severe air pollution problems. In particular, excessive anthropogenic nitrogen (N) emissions have become an urgent environmental issue that needs to be addressed globally. The impact of increased N emissions on a global scale also extends to forest ecosystems. The excessive N influx into forested areas alters the N dynamics of the ecosystem. To determine the impact of the Tokyo metropolitan area, one of the megacities in the world, on the surrounding environment, we investigated the influxes of N and sulfur (S) from bulk precipitation (BP) and throughfall (TF) at 12 forest sites surrounding the Tokyo metropolitan area. At the majority of the forest sites, atmospherically derived N and S influxes from TF were greater than those from BP. The N influx from TF in the forested area was equal to or greater than the amount of N deposited in the urban area. Air pollutants, mainly gases and aerosols, emitted from the Tokyo metropolitan area were transported to the inland forested areas by mesoscale meteorological and topographical factors. In addition, the forest canopy structure efficiently captured the dry deposition and accelerated the influx of N, particularly from the atmosphere into the forest ecosystems. The chronic and excess N influx due to human activities in the megacity has already resulted in high nitrate leaching at forested stream sites bordering the Tokyo metropolitan area.