δ18O-NO3- as an indicator of the nitrogen saturation stages in the stream water

We measured δ15N and δ18O of stream water nitrate to elucidate the nitrogen (N) cycle in the nitrogen saturated forest ecosystems. Stream water samples were taken at 24 points in the Chichibu mountainous region near Tokyo metropolitan area where watersheds were covered with deciduous and coniferous mixed-forest. Sampling was done in several days in August 2008 at base-flow during growing season. Although the sampling points were distributed within a small area (30 km x 20 km), nitrate concentrations varied greatly (25.6 to 237.1 μmol L-1), which covered all stages of nitrogen saturation as defined by previous studies (i.e. stage 2: 0.5-1 mg L-1 of N at base-flow throughout the growing season, stage 3: >1.0 mg N L-1 ( > 71.4μmol L-1)). Up to the concentration of 150 μmol L-1, nitrate showed positive correlation with δ18O-NO3- (-8 - +4‰). At the points which showed nitrate concentration larger than 150 μmol L-1, δ18O-NO3- showed constant value (6‰). We anticipated that the positive correlation between the δ18O-NO3- and nitrate concentration is reflecting the increasing contribution of the nitrate produced through nitrification to the stream water nitrate. We assumed thatδ18O-NO3- of nitrification origin in the studied area is near 6‰, thus the stream water nitrate whose concentration is larger than 150 μmol L-1 is mostly derived from nitrification.