Geoscience Reference
In-Depth Information
supplied more than 7.3 kmol ha
−
1
and 3.7 keq ha
−
1
of nitrogen and calcium,
respectively.
7
In 1989, there was no apparent difference in soil nitrogen miner-
alization rates between the pre-PWD site and the SZ (nondefoliation site sur-
rounding DG75): 0.6-7.5 mgN kg
−
1
at the PWD site versus 0.6-9.1 mgN kg
−
1
at the SZ site in 28 days (Fig. 3a and d). There was little nitrification at either
site; mineralized nitrogen remained as NH
4
. In 1991, soil nitrogen mineral-
ization rates at both sites were accelerated (Fig. 3b and e
7
).
At the
Chamaecyparis
site (surrounding DG75), the high mineralization
rate was not constant, whereas at the PWD site it was still high in 1997.
The mineralization rate on the soil surface at the PWD site was three
times higher in 1997 than in 1989 (Fig. 3c). It was significantly higher
than at the
Chamaecyparis
site (
p
=0
.
04, Fig. 3f). In 1997, more than
90% of mineralized nitrogen was nitrified at the PWD site. The ratio of
nitrification to mineralization was significantly larger at the PWD site than
at the
Chamaecyparis
site (
p
=0
.
04, Fig. 3c and f
7
).
It was estimated that dry matter of approximately 82.8 Mg ha
−
1
and
nitrogen at 7.39 kmol ha
−
1
were added to the forest floor from tree die back
with PWD, which were three to five times larger than that due to litter
fall. Increase of net nitrogen loss via the stream was much smaller than
the nitrogen addition by PWD, although the stream NO
3
concentration
increased. Vegetation uptake (regeneration) at the PWD site was impor-
tant for considering the retention mechanism of nitrogen. Alternatively,
microbial nitrogen immobilization could have affected the nitrogen budget,
since the PWD litter, including stems and branches, had a relatively high
C/N ratio.
8
Fig. 3. Nitrogen transformation rates through the soil profile of SZ (a-c) and PW (d-f)
sites pre-PWD (1989; a and d), during-PWD (1991; b and e), and post-PWD (1997;
c and f). Black indicates NO
3
;whiteshowsNH
4
.
7
