[H-TT31] 環境トレーサビリティー手法の新展開

[HTT31-P05] 福岡市都市近郊の窒素飽和が下流域の河川水質に及ぼす影響評価

*篠塚 賢一¹、智和 正明¹、陀安 一郎²、由水 千景²、久米 篤¹ (1.九州大学大学院農学研究院、2.総合地球環境学研究所)

キーワード：窒素飽和, 河川水質, 窒素同位体, 土地利用

Nitrogen is often limiting nutrient for plant growth and is mainly supplied from atmosphere. Forests can act as a filter for atmospherically deposited nitrogen and maintain background concentration levels of nitrogen in streams. However, recent increases in atmospheric nitrogen deposition have resulted in a shift from net-nitrogen retention to high levels of net-nitrogen loss from forested, resulting in high nitrogen concentration stream water.
In the Tatara River Basin, Fukuoka City, nitrogen retention capacity of upland forests has decreased and nitrate concentrations in downstream water have been increasing (Chiwa et al., 2012). This study analyzed NO₃^--δ¹⁵N, NO₃^--δ18O in addition to NO₃^- concentrations in stream water in the Tatara River Basin to assess the impact of nitrogen saturation forest on downstream water quality.
In northern part of the basin, NO₃^- concentrations of upstream were lower than downstream. In contrast, in southern part of the basin, NO₃^- concentrations in upstream were lower than downstream. NO₃^--δ¹⁵N in upstream was significantly lower than downstream in both northern and southern parts of the basin. In contrast, little difference of NO₃^--δ¹⁸O was observed between upstream and downstream in both parts. It has been known that the value of δ¹⁵N and δ¹⁸O ratio due to human wastewater is 10 to 20‰ and -5 to 7‰, respectively (Kendall and others, 1995). Therefore, the different trends in NO₃^- concentrations from upstream to downstream between two parts could be caused by different amounts of human sewage to the downstream between two parts.