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 1 −1 NO 3 0.20 μeq 1 −1 , SO 4 29.6 μeq 1 −1 , Cl 105 μeq 1 −1 , K 5.3 μeq 1 −1 , Mg 25.7 μeq 1 −1 , Ca 33.4 μeq 1 −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.