Jökull - 01.12.2006, Blaðsíða 30

Y. Wang and M. J. Wooller
likely to be C3, given the relative biogeography of C3,
C4 and CAM plants (Sage et al., 1999). Therefore the
δ13C ranges for C4 and CAM plants are not consid-
ered further in our paper.
TOC in lake sediments can be composed of phyto-
plankton (with δ13C values between -42‰ to -26‰)
and/or biomass from higher plants (Dean and Stuiver,
1993) (Figure 1). The δ13C values of aquatic macro-
phytes also vary considerably, ranging from -50‰ to
-11‰ (Keeley and Sandquist, 1992), but are mostly
in the range of -30‰ to -12‰ (Boutton, 1991). Over-
lapping δ13C signatures for the different potential end
members (e.g. terrestrial plants vs. phytoplankton)
composing lake sediments (Figure 1) necessitates re-
lying on additional lines of evidence including δ15N
and C/N, to determine sources of organic matter.
The δ15N values of plants are determined not only
by the δ15N of source nitrogen such as the relative
abundance of different nitrogen compounds (NH+4 ,
NO−3 and amino acids), but also by plant physiolog-
ical factors such as different nitrogen (NH+4 , NO
−
3 )
uptake mechanisms, different pathways of assimila-
tion, different physiologies of host plants and myc-
orrhizal symbionts and recycling of nitrogen in the
plant (Evans, 2001; Robbins et al., 2002; Robinson,
2001). Interspecific differences in δ15N values are at-
tributable to a number of factors. For instance, plants
can exploit different soil horizons and increasing δ15N
can be correlated with soil depth (Nadelhoffer and
Fry, 1988). Relatively deep-rooted plants (such as
Carex) can have higher foliage δ15N values compared
to shallow-rooted species such as lichens and mosses
that use less processed nitrogen (Kendall, 1998). Dif-
ferent sources of nitrogen can also have markedly dif-
ferent δ15N values (Figure 1) so that the δ15N value
in plants reflect that of the nitrogen source (e.g. pre-
cipitation, soil, inorganic fertilizer, organic fertilizer,
and animal waste) when plant demand exceeds the ni-
trogen supply (Evans, 2001; Peterson and Fry, 1987).
The δ15N of sources available to plants can also be al-
tered by fractionations associated with a large number
of N cycling processes (Evans, 2001; Robbins et al.,
2002; Robinson, 2001). For example, there are large
isotopic equilibrium effects (19-30‰) associated with
the volatilization of ammonia, where the resulting am-
monia has a very negative δ15N value (Figure 1) (e.g.
Erskine et al., 1998; Tozer et al., 2005). It is also pos-
sible for nutrient limitation, particularly phosphorus
limitation, to influence the δ15N values of plants. For
example, phosphorus limitation has been related to
decreases in some plant’s demand for nitrogen, result-
ing in negative plant δ15N values (Jones et al., 2004;
McKee et al., 2002).
C/N values of sediments can also be used to
indicate the proportions of autochthonous and al-
lochthonous sources of organic matter in ecosystems
(Wetzel, 1983; Meyers et al., 1998) (Figure 1). The
C/N values of terrestrial organic matter tend to be
variable (10-44) although the majority of values are
greater than 20 (Herczeg et al., 2001). The C/N val-
ues of lacustrine algae and aquatic plants are typi-
cally lower than those of terrestrial plants and range
between 4 and 10 (Herczeg et al., 2001). Lacustrine
sediments with C/N values of between 10 and 20 rep-
resent a mixture of aquatic and higher plant material,
whereas C/N values > 20 indicate a greater proportion
of material from higher plants (Meyers, 1994) (Fig-
ure 1). Plankton has an average C/N values of ∼6,
with most diatoms varying between 5 and 8, and the
C/N value of fresh water macrophytes is between 12
and 30 (e.g. Lamb et al., 2004) (Figure 1). In addi-
tion to considering multiple lines of evidence to ef-
fectively understand the composition of sediments it
is also beneficial to have site specific data on mod-
ern potential end members that could compose those
sediments (e.g. Wooller et al., 2005). These data can
also reveal intriguing and sometimes unexpected pat-
terns in modern ecosystems (e.g. Wooller et al., 2005,
Tozer et al., 2005, Fogel et al., submitted). A part of
our field research in Iceland involved the collection of
lichens and senescent plants from four lakes with the
aim of assessing the modern variations in δ13C, δ15N
and C/N values to characterize the modern ecosystem.
To our knowledge there have been no stable isotope
analyses of plants and lichens from Iceland. Our data
could ultimately aid in interpretations of our future
measurements on lake cores from Iceland and provide
a novel ecophysiological perspective of the modern
Icelandic vegetation.
28 JÖKULL No. 56