Monitoring of nitrogen compounds on Yakushima Island, a world natural heritage site

The concentrations of NH4 and NO3 along with other cations and anions in atmospheric deposition (rain and snow) and river water were determined on Yakushima Island, a world natural heritage site. The results suggested that the major source of N compounds entering the ecosystem is atmospheric deposition, and that forest and river ecosystems in the high mountainous area are maintained on nutrient-poor granite bedrock. There was a seasonal change in the rainfall and in concentrations of NH4 and NO3 in rain. The amount of rainfall was relatively higher in April to October and the concentrations of N compounds were relatively higher in November to March. The NH4/NO3 ratio in rain water was about 1.0 in November to March, suggesting the equal amount of NH4 and NO3 is supplied to the forests of Yakushima Island as nutrients before the spring growth of plants. The concentrations of nutrients, especially NH4 and NO3, in river water on this island are extremely low (e.g. St. Y1, Apr 1994–Dec 1995: NH4 1.1 μeq l−1 NO30.20 μeq l−1, SO4 29.6 μeq l−1, Cl 105 μeq l−1, K 5.3 μeq l−1, Mg 25.7 μeq l−1, Ca 33.4 μeq l−1). The difference in the concentration of NH4 and NO3 in atmospheric deposition compared with river water would suggest a limited nutrient supply on the granitic precipitous island. It also implicated a chemical and biological uptake system in the ecosystem, such as uptake of NH4 and NO3 by Cryptomeria japonica, as a background feature of the characteristically slow-growing but thick mature forest.