Effect on water quality in irrigation reservoir due to application reduction of nitrogen fertilizer

Until 1997, tea farmers in the Makinohara district of Shizuoka Prefecture, Japan, applied around 1.2 ton/ha of nitrogen fertilizer per year to their tea fields. In general, uptake amount of nitrogen by tea plants is around 300–350 kg/ha. Then some part of the remainder of nitrogen fertilizer leach into ground water and flow out into the river. The other part of remainder of nitrogen fertilizer is accumulated in the soil layer. Following a recommendation by the local government, this amount was then gradually decreased to 660 kg/ha in 1999 and 540 kg/ha in 2000. Although nitrate nitrogen concentrations in local ground and river water have decreased, they remain high today. The river water runs off from a watershed in the Makinohara area and enters a small irrigation reservoir called Tanno Reservoir, where it has caused deterioration of the water quality, that is, acidification of the reservoir. In Japan, environmental standard for nitrate nitrogen is 10 mg/L in public water body and ground water. Here, the author developed the Water Quality Tank Model, and applied this model to investigate the nitrate nitrogen concentrations in the rivers and an irrigation reservoir called Tanno. The author applies these findings to demonstrate that nitrogen concentrations continue to remain high due to nitrogen accumulation in the soil layer, and that the amount accumulated would be reduced by a reduction in application. The simulation results demonstrate a small decrease in accumulation in the soil layer, and thus that the present high nitrate nitrogen concentrations will continue in ground, river and reservoir water will decrease only gradually.