|| 要旨トップ | 目次 |||日本生態学会第59回全国大会 (2012年3月，大津) 講演要旨
一般講演（ポスター発表） P3-300J (Poster presentation)
The natural abundance of nitrogen and oxygen isotopes in NO3– is useful for identifying the source of nitrate in streamwater, because δ18O values in NO3– are typically higher with atmospheric deposition than in NO3– formed by microbial nitrification. To evaluate the source dynamics of NO3– during storms, we monitored the δ18O and δ15N of NO3– in streamwater and the spatial variation (rainfall, soil water, and groundwater) of isotopes in forested watersheds in Japan. The mean δ18O was higher for rain (59‰) than for soil water (–1.2‰) and groundwater (9.4‰). The mean δ15N of soil water (–2.3‰) was lower than for groundwater (1.4‰). The streamwater δ18O increased just after the onset of rainfall, and then both δ18O and δ15N decreased with discharge and were lowest at the peak flow. The changes in both the δ18O of streamwater NO3– during storms indicated that the atmospheric NO3– contribution to the stream increased (2~8%) at the onset of rainfall due to direct channel rainfall input, and then the contribution of nitrified NO3– increased at high flow. Moreover, the decreases in δ18O and δ15N at peak flow to below the baseline levels implied that groundwater denitrificated NO3– as an additional end-member for NO3– because the δ15N and δ18O values of soil water were clearly lower than those of groundwater.