17:15 〜 18:30
[AHW22-P02] Effect of stream water from nitrogen-saturated forests in the Koise River Basin on nitrogen concentrations in the main river
キーワード：森林、窒素飽和、水田
Around metropolitan Tokyo, high nitrate concentrations can occur in water from forest streams, which are nitrogen-saturated due to chronic excessive nitrogen deposition. Stream water from forests in hilly and mountainous areas should reduce nitrogen input to agricultural areas. However, this dilution effect is thought to be smaller in nitrogen-saturated forests. To test this in the Koise River Basin, where the forests are nitrogen-saturated, we periodically sampled water from the main river and a forest stream and compared the nitrate concentrations.
The average nitrate concentration in forest stream water for 2018–2019 was 8.5 mg/L. There was no significant seasonality, although temporal increases were observed during rainfall events. The average nitrate concentration in the main river water during the same period was 9.0 mg/L, and showed marked seasonal fluctuation; moreover, this was substantially lower than that in stream water from the forest in summer.
Denitrification in paddy fields in the basin is thought to be responsible for the summer decrease in nitrate concentrations in the main river. In the Koise River Basin, where forests are commonly nitrogen-saturated, the high nitrogen concentrations in forest stream water are thought to be reduced in paddy fields in the summer. It has been suggested that the stream water in this area is not reducing the nitrogen load to Lake Kasumigaura during the summer, and the forest may be a non-point source.
The Koise River Basin, located just outside metropolitan Tokyo, has been subjected to a large influx of anthropogenic nitrogen emitted by human activities over the years, causing the forest to become nitrogen-saturated. During this period, few coniferous plantations in the area have been thinned, which may have amplified the negative effects of nitrogen saturation in the region’s forests by maintaining the high dry deposition capture effect of a crowded canopy.
The paddy fields in the area are thought to purify the nitrogen from the forests during the summer via denitrification. However, as the number of abandoned paddy fields in the area increases, there are concerns about the reduced ability of paddy fields to control eutrophication in Lake Kasumigaura.
The average nitrate concentration in forest stream water for 2018–2019 was 8.5 mg/L. There was no significant seasonality, although temporal increases were observed during rainfall events. The average nitrate concentration in the main river water during the same period was 9.0 mg/L, and showed marked seasonal fluctuation; moreover, this was substantially lower than that in stream water from the forest in summer.
Denitrification in paddy fields in the basin is thought to be responsible for the summer decrease in nitrate concentrations in the main river. In the Koise River Basin, where forests are commonly nitrogen-saturated, the high nitrogen concentrations in forest stream water are thought to be reduced in paddy fields in the summer. It has been suggested that the stream water in this area is not reducing the nitrogen load to Lake Kasumigaura during the summer, and the forest may be a non-point source.
The Koise River Basin, located just outside metropolitan Tokyo, has been subjected to a large influx of anthropogenic nitrogen emitted by human activities over the years, causing the forest to become nitrogen-saturated. During this period, few coniferous plantations in the area have been thinned, which may have amplified the negative effects of nitrogen saturation in the region’s forests by maintaining the high dry deposition capture effect of a crowded canopy.
The paddy fields in the area are thought to purify the nitrogen from the forests during the summer via denitrification. However, as the number of abandoned paddy fields in the area increases, there are concerns about the reduced ability of paddy fields to control eutrophication in Lake Kasumigaura.