EFFECTS OF RIPARIAN DENTTRIFICATION ON STREAM NITRATE -EVIDENCE FROM ISOTOPE ANALYSIS AND EXTREME NITRATE LEACHING DURING RAENFALL-

摘要

The effects of riparian denitrification on stream nitrate were investigated by detailed soil water observations and isotope analysis at a small headwater catchment in an urban area near Tokyo, central Japan. In the base flow period, stream nitrate concentration (<100 μM) was comparable with that of riparian ground water which had less nitrate than unsaturated soil water. Nitrogen isotope analysis showed that the consumption of nitrate by denitrification took place in riparian ground water, suggesting that denitrification is an important process to control nitrate leaching to streams. During rainfall, the concentration of stream nitrate increased up to 400μM, which was comparable with that of pre-event soil water. The fact that soil water nitrate directly leached to streams indicated that the riparian denitrification process did not work during rainfall because of the rapid discharge of water. A decrease of denitrification effects is a possible reason for high stream nitrate concentration during rainfall.