Nitrate contamination of mountainous headwater streams from adjacent agricultural and pasture lands beyond the watershed boundary in eastern Hokkaido, Japan

放牧・耕作地由来の窒素が分水嶺を超えて森林渓流水質に与える影響

福島 慶太郎*; 岩崎 健太*; 小田 義也*; 境 優*; 堅田 元喜*; 山口 高志*; 久保田 智大* ; 永野 博彦   ; 渡辺 誠*; 小嵐 淳   

Fukushima, Keitaro*; Iwasaki, Kenta*; Oda, Yoshiya*; Sakai, Masaru*; Katata, Genki*; Yamaguchi, Takashi*; Kubota, Tomohiro*; Nagano, Hirohiko; Watanabe, Makoto*; Koarashi, Jun

森林における水・窒素の流出および収支を定量的に把握する上で、「集水域」が空間スケールの最小単位となる。大気から集水域内に流入した反応性窒素が、植物-土壌間の内部循環系に取り込まれ、その一部が渓流を通して集水域外へと流出する。この流入と流出のバランスをもって、生態系内の窒素保持機能が評価される。本発表では、集水域の水収支が閉じていない可能性のある森林における、渓流水の窒素濃度の形成メカニズムについて考察する。

Nitrogen retention in the forest ecosystem can be mainly evaluated as a balance between input of atmospheric reactive N and hydrological N loss at a watershed scale. As an assumption for the evaluation, it has been proposed that the watershed is completely "closed", namely, the source of water flowing out from the watershed is rainwater fell into the watershed. This means that exchange of N transported with water between the watershed and the adjacent watershed beyond the watershed boundary is ignored. However, is this assumption applicable for all forested watersheds? If not, nitrogen retention in the watershed may not be evaluated accurately. In our presentation, we aim to elucidate the source of streamwater nitrate-N in the forested watershed whose water balance are not possibly closed.