Jökull - 01.12.2006, Blaðsíða 34

Y. Wang and M. J. Wooller
The %N and %C value of one of the herbs from
the site were 4.1% and 41.0%, respectively. The
δ15N and δ13C values (average of the duplicates)
of this plant were 3.1‰ and -26.4‰, respectively.
The second plant from the site had a %N of 1.2%
and %C of 15.9%, with δ15N and δ13C values of
5.4‰ and -25.4‰ (Figure 3 and Table 1). The δ15N
values of these two plants from Lake Stífluvatn, with
a mean of 4.3‰, were the most positive values among
the entire samples analyzed. The surface sediment
sample from the lake yielded a %N of 0.3%, a %C
of 3.1% (C/N=9.4), δ15N value of -0.1‰ and δ13C
value of -28.6‰.
The %N of specimens taken from Lake Arnarvatn
stóra ranged from 0.5% to 2.5%, with a mean of 1.2%.
The %C ranged from 40.8% to 55.8% with a mean of
47.6% (Table 1). Plants from Lake Arnarvatn stóra
had the lowest δ15N values compared to the samples
from other lakes with a mean of -6.9‰; and the lowest
value was -12.4‰. The δ13C values of plants ranged
from -30.9‰ to -24.6‰ with a mean of -28.0‰. A
surface sediment sample from the lake yielded a %N
of 1.1, %C of 6.3 (C/N of 5.6), δ15N of -1.5‰, and
δ13C of -26.3‰ (Table 1).
In general, the δ13C of the terrestrial plants and
lichens analyzed from all lake sites fell within a range
of -23.3‰ to -31.9‰, whereas the δ13C of the aquatic
plants analyzed ranged from between -11‰ to -15‰
(Figure 3). The δ13C of sediment samples from the
lake sites ranged from -20.0‰ to -28.4‰. Among
these four samples, δ13C values from Lakes Litla-
Viðarvatn (-20.0‰) and Torfadalsvatn (- 20.6‰) were
less negative than the values from Stífluvatn (-26.3‰
) and Arnarvatn stóra (-28.4‰ ). The δ15N values
of lake surface sediments were very similar, around
-1.4‰, with the exception of the δ15N from Lake
Stífluvatn, which was slightly higher (-0.1‰).
DISCUSSION
The range of δ13C values for the terrestrial plants in
Table 1 are typical for C3 plants; C3 plants are the
main constituent of mid to high latitude vegetation
(Sage et al., 1999). The δ15N values of the plants
and lichens (Figure 3) are surprisingly negative (down
to ∼ -12‰) for terrestrial plants (Figure 1), although
plants from temperate ecosystems are noted as having
δ15N values that are often more negative than tropical
plants (Figure 1). Negative δ15N values were even
seen in the Carex samples analyzed (Table 1), which
was also surprising given that deep rooted plants,
such as the Carex discussed by Kendell (1998), can
have higher δ15N values compared with lichens and
shallow rooted plants. Negative δ15N values in plants
can be related to a number of mechanisms (Figure 1).
For instance, some phosphorus limited ecosystems
have been noted as having plants present with negative
δ15N values (McKee et al., 2002). The δ15N values
of the plants analyzed from Iceland could be related
to phosphorus limitation. The majority of soils in
Iceland are classified as andisols and composed of
lava or ash, which is characterized by a frost-heaved
gravel layer at the surface (Arnalds, 2004). Soil
nutrients can leach out relatively easily, especially
from the ash particles, which have high surface area
to volume ratios. Some Icelandic soils have been
noted as having low organic contents (≤10 g kg−1),
and very low levels of nitrogen (Arnalds and Kimble,
2001) and total phosphorus (181 mg/100 g and
309 mg/100 g) (Simpson, 2002). A future survey of
δ15N values in Icelandic plants could also examine
the phosphorus content of the soils in which the
plants grow. Very negative δ15N values (< -20‰)
have also been noted in plants and lichens utilizing
ammonia in the atmosphere (Erskine et al., 1998;
Tozar et al., 2005; Fogel et al., submitted), which has
a very negative δ15N value (Tozar et al., 2005). Acid
traps and ammonia detectors could also be placed in
Icelandic vegetation (as in Fogel et al., submitted) to
establish the isotopic composition of any ammonia
present in the atmosphere in Iceland. Our δ15N
findings certainly warrant further investigation.
We found that the aquatic plants examined had
relatively less negative δ13C ranging from -11.5‰
to -14.2‰(mean=-12.9‰) compared to the terrestrial
plants (mean=-28.0‰) (Figure 3). These values are
typical for submerged aquatic plants in a freshwater
lake (Figure 1), although the exact mechanism
resulting in these values can vary (Keeley and
Sandquist, 1992). These mechanisms include the
utilization of different carbon sources by aquatic
32 JÖKULL No. 56