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Jökull - 01.12.2006, Qupperneq 34

Jökull - 01.12.2006, Qupperneq 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
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