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

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

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.