Geoscience Reference
In-Depth Information
Fig. 2. Changes in the NO
3
concentrations in the stream and in the groundwater at
UG512, DG75, and DG190 from 1989 to 1999. The well codes and locations are shown
in Fig. 1 (after Ohte
et al
.
4
).
The catchment was covered by semimature vegetation (a mixed stand
of
Pinus densiflora
and
Chamaecyparis obtusa
) before the PWD diebacks
of the early 1990s. Over 25% of the Matsuzawa catchment was suffered by
PWD (Fig. 1a). The total basal area of
Pinus
in the PWD portion decreased
from 18.6 to 1.9 m
2
ha
−
1
from 1989 to 1998.
6
Sampling design of waters were described in Ref. 4. Three observation
wells were installed in the catchment: two were located at the furthest down-
stream reach of the groundwater body (DG75, DG190), and one at the fur-
thest upstream reach of the groundwater body (UG512, see Fig. 1b). The
respective depths of the well bottom and the bedrock are 75 and 445 cm for
DG75, 190 and 450 cm for DG190 and 512 and more than 800 cm for UG512.
It should be noted that the water sampled from DG190 could include rela-
tively deeper groundwater than samples taken from DG75.
Visible dieback of pine stands has been observed since 1990.
6
Decreased
N uptake by roots and increased N supply from litter fall caused by the
1992-1994 pine dieback caused a threefold increase in NO
3
and cations
(Ca
2+
and Mg
2+
) concentrations of streamwater and subsurface ground-
water.
7
Unusual increase in NO
3
concentration of streamwater occurred
since 1994. From 1992 to 1996, remarkable seasonal peaks was observed
in the stream NO
3
concentrations during the rainy season from July to
August (Fig. 2)
4
.
3. Alterations in Nutrient Status by PWD
It was estimated that leaf litter during PWD supplied nitrogen and calcium of
0.17 kmol ha
−
1
and 0.08 keq ha
−
1
respectively, and tree branches and stems
