wAbstract
@Nitrogen compounds generated by anthropogenic combustion deposits
in forest watersheds and induce nitrogen saturation of the area.
Because excess nitrogen is derived from atmospheric deposition,
this action is expected to uniformly affect a wide area of forest
soils. Geographically, heterogeneous nitrate concentration of
stream water within a small area has been attributed to the tree
type, geological setting and tree cut. In this article, we hypothesized
that the effect of the atmospheric nitrogen deposition in the
forest watershed may vary within a small area, and that such variation
is induced by the degree of air mass containing a high concentration
of nitrogen deposition of combustion origin. We measured major
ion concentrations, including nitrate, nitrite oxygen and nitrogen
stable isotope of nitrate sampled at 24 water streams in the Chichibu
region, which is 50-100 km from the Tokyo metropolitan area. The
nitrate concentration showed a wide range (35.6-237 Κmol L-1)
within 300 km2, which was explained sufficiently by
the air mass advection path and its contact with the mountain's
surface. The nitrate concentration showed a significant positive
correlation with chloride (r = 0.73; p0.001). As chloride originates
outside of the Chichibu region, the positive correlation between
two ions showed that the nitrate concentration of the stream water
was affected by the nitrogen compound from the Tokyo Metropolitan
area as a form of atmospheric deposition. Between the nitrate
concentration and the stable isotope ratio of oxygen of nitrate,
there was a positive correlation until nitrate concentration of
100 Κmol L-1. When the nitrate is over 100 Κmol L-1,
Β18O shows a stable value of ca. 5.7ρ. This indicates
that the nitrification proceeds when the nitrate concentration
was low to middle, but the reaction showed when the nitrate concentration
became high. Oxygen stable isotope of nitrate along with a set
of nitrate concentrations can be used as a good indicator of nitrogen
saturation.
Keywords: Nitrogen saturation; Nitrate; Oxygen stable isotope
of nitrate; Stream water; Atmospheric depositionx
1. Introduction
2. Methods
@2.1. Study area and sampling points
@2.2. Ion concentration and stable isotope ratio of the stream
water
3. Results
4. Discussion
@4.1. The heterogeneous effect of atmospheric nitrogen deposition
on the stream water
@4.2. Relationship between stages of nitrate saturation and Β18O
and Β15N of nitrate in the stream water
5. Conclusions
Acknowledgments
References