Geoscience Reference
In-Depth Information
4. Discharge Mechanisms of NO
3
NO
3
leaching from the near-surface horizon was prominent from the second
half of October to the mid-November in 1997. In 1998, NO
3
levels in soil
water began to increase in mid-August.
4
In association with the vertical
infiltration of soil water, NO
3
was transported from near the surface to
the lower layers with dispersion. The seasonal pattern, however, became
unclear compared with concentrations in the shallower layer. It is unlikely
that the seasonal signal in soil water directly affected the seasonal variation
in the stream NO
3
concentrations.
4
The fluctuation in streamwater NO
3
concentrations corresponded
strongly with groundwater levels at DG75, suggesting that volumetric
changes in the contribution of shallow groundwater from the DG75 zone
affected the seasonal variation in the stream NO
3
.
4
5. Conclusions
Although very high potential of N retention has been found in soil bio-
geochemical processes, the increase in stream NO
3
concentration was
remarkable. The mechanisms for the seasonal patterns in stream NO
3
con-
centrations in this case emphasize the importance of changes in hydro-
logical conditions in summer in Japan, where precipitation, groundwater
levels, and runoff rates are high. Previously, in Europe and the eastern
United States, the simultaneous effects of smaller pools of inorganic nitro-
gen due to high plant uptake and lower hydrologic transporting forces due
to low precipitation and high transpiration during the summer
9
may let
ones underrate the effect of hydrological conditions on seasonality in stream
NO
3
concentrations.
10
In order to precisely understand the mechanism behind the biogeo-
chemical responses to the catchment scale disturbances, including NO
3
leaching due to main canopy defoliation, hydrological information are cor-
respondingly necessary.
Acknowledgments
I would like to thank Masanori Katsuyama, Masatoshi Kawasaki, and
Keisuke Koba for their critical efforts on the field and laboratory works
for this project.
