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 at mospheric deposition train and snow) and river water were determined on Yak ushima Island, a world natural heritage site. The results suggested that th e major source of N compounds entering the ecosystem is atmospheric deposit ion, 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 r ainfall 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 amo unt of NH4 and NO3 is supplied to the forests of Yakushima Island as nutrie nts before the spring growth of plants. The concentrations of nutrients, es pecially NH4 and NO3, in river water on this island are extremely low (e.g. St.Y1, Apr 1994-Dec 1995: NH4 1.1 mu eg l(-1) NO3 0.20 mu eq l(-1), SO4 29 .6 mu eg l(-1), Cl 105 mu eq l(-1), K 5.3 mu eq l(-1), Mg 25.7 mu eq l(-1), Ca 33.4 mu eq l(-1)). The difference in the concentration of NH4 and NO3 i n atmospheric deposition compared with river water would suggest a limited nutrient supply on the granitic precipitous island. It also implicated a ch emical and biological uptake system in the ecosystem, such as uptake of NH4 and NO3 by Cryptomeria japonica, as a background feature of the characteri stically slow-growing but thick mature forest.