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

Abstract

The concentrations of NH 4 and NO 3 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 NH 4 and NO 3 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 NH 4 NO 3 ratio in rain water was about 1.0 in November to March, suggesting the equal amount of NH 4 and NO 3 is supplied to the forests of Yakushima Island as nutrients before the spring growth of plants. The concentrations of nutrients, especially NH 4 and NO 3, in river water on this island are extremely low (e.g. St. Y1, Apr 1994–Dec 1995: NH 4 1.1 μeq l −1 NO 30.20 μeq l −1, SO 4 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 NH 4 and NO 3 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 NH 4 and NO 3 by Cryptomeria japonica, as a background feature of the characteristically slow-growing but thick mature forest.