Nitrate leaching from Japanese temperate forest ecosystems in response to elevated atmospheric N deposition

The effects of elevated atmospheric nitrogen (N) deposition on N cycling in Japanese forest ecosystems are reviewed here to assess the sensitivity of nitrate (NO ₃ ⁻ ) leaching from forests to streams in response to this deposition. Long-term monitoring of atmospheric N deposition throughout Japan suggests that long-range transport of air pollutants from East Asia accounts for recent increases in atmospheric N deposition in Japan. A new conceptual model of N saturation proposes that kinetic N saturation can be related to unavoidable hydrological nitrate (NO ₃ ⁻ ) loss from Japanese forests having an Asian monsoon climate with warm and wet summers. Soil microbes, including fungi and ammonia-oxidizing archaea, are important players in these forest ecosystems, affecting nutrient cycling in response to increased atmospheric N deposition. Similarly, a new framework based on a mycorrhizal-associated nutrient economy provides an understanding of NO ₃ ⁻ leaching in soils from temperate forests, suggesting that arbuscular mycorrhizal (AM)-dominated forests leach more NO ₃ ⁻ in response to N deposition than ectomycorrhizal fungi-dominated forests. Because Japanese cedar and cypress are AM-associated trees, they are likely susceptible to NO ₃ ⁻ leaching. Maturation and poor management of such plantations may accelerate NO ₃ ⁻ leaching. Grazing by increasing deer populations also enhances NO ₃ ⁻ leaching from the soil system. We argue that several factors, including long-range transport of air pollutants, an Asian monsoon climate setting, as well as maturation and poor management of AM-associated plantations, will make Japanese temperate forests more sensitive to increased atmospheric N deposition in East Asia over the next decades.