Geochemical Journal, Vol. 42, pp. 481 to 492, 2008
Stable isotope studies of moss sulfur and sulfate from bog surface waters
GRZEGORZ SKRZYPEK,1,2* TASUKU AKAGI,3 WOJCIECH DRZEWICKI 2 and MARIUSZ-ORION J˛EDRYSEK 2
1
West Australian Biogeochemistry Centre, John de Laeter Centre of Mass Spectrometry, School of Plant Biology (MO90),
The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
2
The Laboratory of Isotope Geology and Geoecology, Institute of Geological Sciences, The University of Wroc l aw,
Cybulskiego 30, 50-205 Wroc l aw, Poland
3
Department of Earth and Planetary Sciences, Faculty of Sciences, 33 Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
(Received November 19, 2007; Accepted June 24, 2008)
Two moss genera (Sphagnum and Polytrichum) were collected seasonally in two close (~0.45 km distance) but environmentally different locations, an Open bog and a Spruce forest at Hala Izerska (the Izerskie Mts./SW Poland), for the
stable isotope analyses of plant in-body sulfur. Simultaneously, surface water was collected in places of moss growth and
along the creek discharging the bog, for stable isotope analysis of sulfate sulfur (5 locations/5 times in growing seasons).
The δ34S value of the analyzed mosses varies from 3.99 to 10.24‰ for Sphagnum and from 4.18 to 6.48‰ for Polytrichum.
The δ34S value of aqueous sulfate in creek waters, depending on location and season, ranges from 3.72 to 20.26‰. The
significant correlation between the plant in-body sulfur concentration and the isotopic composition was observed for
Sphagnum as well as loose correlation between δ34S of sulfates in surface water and moss in-body sulfur. The fractionation
factor, possibly caused by two processes of sulfate assimilation by Sphagnum and sulfate reduction by bacteria, calculated
based on Rayleigh’s distillation model equals about 4‰. The high correlation and simultaneous increase of δ34S(SO42–)
and δ13C(DIC) values downstream the creek discharging the bog suggest that the lighter isotopes of carbon and sulfur (12C
and 32S) are preferentially removed, probably due to assimilation by plants. The present results imply the original signature of the source of sulfur in the environment is greatly altered by the biological activities in bog water.
Keywords: sulfur, stable isotope, peat, bog, Sphagnum, Polytrichum
pogenic (δ34S from –3 to 10‰, SOx as acid rains), and
(3) geogenic (δ34S, mostly <+10‰, variable according to
the type of sediments). Sulfur is an intensively cycled
element in peatlands because of its high lability caused
by redox changes between valences of +6 (SO42–) to –2
(S2–) (e.g., Bartlett et al., 2005). The main source of atmospheric sulfur delivered to bogs in Central Europe is
anthropogenic in origin. Therefore, SOx can be considered as the major form delivered with precipitation. Since
the 19th century anthropogenic pollution of air has generally resulted in lower δ34S values of atmospheric sulfur,
but local isotope composition is controlled by conditions
at the place of precipitation such as oxidation/reduction
conditions, bacterial activity and water table variations
(J˛edrysek and Skrzypek, 2005).
Sulfur is mostly assimilated by plants in the form of
sulfate from the soil water through the root system or directly from the atmosphere in the form of SO2 through
stomata. Assimilation of sulfur by lower plants, as Sphagnum gametophytes, is less complicated and faster than in
the case of higher plants due to lack of any root system or
stomata. Sphagnum has single-cell thick leaves without
cuticle cover and is thus directly exposed to water in en-
INTRODUCTION
Raised ombrotrophic bogs are supplied with nutrients
and water exclusively through precipitation. Therefore,
the concentration, as well as the stable isotopic composition of sulfates from bog surface water is close to those
of the major form of precipitation, be that rain, mist or
snow (Blaś et al., 2002). Bog vegetation uses this sulfur
reservoir to maintain the life process. Therefore, δ34S values of peat-forming plants can potentially be used as tracers of different sources of sulfur (Thompson and Bottrell,
1998; Nriagu and Glooschenko, 1992). However, variations of temperature, the amount of precipitation, and the
retention time of water on the bog during the growing
season may result in isotope fractionations of sulfates
sulfur available for plants from water.
Three major potential sources of sulfur in terrestrial
ecosystems can be identified: (1) marine (δ 34S from 20
to 21‰, mainly sulfates and dimethylsulfide), (2) anthro*Corresponding author (e-mail: [email protected])
Copyright © 2008 by The Geochemical Society of Japan.
481
vironment and takes up sulfates directly to cells. The SO2
from the atmosphere or H 2S yielded during peat decay
could be considered as a secondary, minor source (Bartlett
et al., 2005). However, H2S must be oxidized to sulfates
prior to assimilation; it is toxic itself for nonvascular
plants (Coulson et al., 2005; Trust and Fry, 1992), but its
contribution is small in comparison to sulfates (Thompson
and Bottrell, 1998).
The uptake of sulfur during moss growth has been recorded as occurring with a low fractionation (Krouse,
1977; Krouse et al., 1991; Nrigau and Glooschenko, 1992;
Novak et al., 1994). The stable isotopic fractionation of
sulfur in plants (plant-source) ranges from –8 to 1‰, with
an average of –1.5‰ (Trust and Fry, 1992). In recent studies, Novak et al. (2001) reported δ34S values for Sphagnum in unforested locations that were lower, on average,
than the bulk precipitation by –0.7‰. However, the δ34S
value of bulk precipitation is not reflected exactly by the
stable isotopic composition of the sulfate taken up by
Sphagnum, which comes mostly from surface bog water
which is usually enriched in 34S due to bacterial sulfate
reduction. Bacterial activity differentially removes lighter
isotope (32S) of sulfur from water relative to the heavier
34
S, therefore a decrease of sulfur concentration is associated with the simultaneous increase of δ 34S of the remaining, unused sulfur in solution (SO42–) (Bartlett et al.,
2005).
The specific case of large δ34S variability (–4 to –12‰
difference between surface water and moss) has been
observed by Novak et al. (2001) and Bottrell and Novak
(1997) for Sphagnum from the British Islands. According to Bartlett et al. (2005), this large variability can be
attributed to two possible processes: a genuine significant isotopic fractionation, with plants preferentially assimilating 32S; or uptake of sulfur species other than by
Sphagnum. For instance, H2S depleted in 34S (relative to
surface waters) can be produced during peat decay in
anaerobic conditions and may diffuse up the peat profile,
where after oxidation it can be available to growing
mosses (Thompson and Bottrell, 1998; Bartlett et al.,
2005; Krouse et al., 1991).
Sphagnum species dominate the flora of bogs. Due to
the high preservation of organic matter in acid bogs, peat
may form depth deposits. Peat could serve as an archive
of past vegetation (Akagi et al., 2004; Skrzypek and
J˛edrysek, 2005), particularly their sulfur isotopic composition. However, it is not clear how far the δ34S of peat
reflects original stable isotopic composition of peatforming plants. The most common recognized mechanism,
which causes the variation of the stable isotopic composition of sulfur in the peat profile, is the partial decomposition of organic matter during peat formation. Sulfur may
be retained in peat as both organic forms and as a reduced inorganic form, depending on anaerobic conditions
482 G. Skrzypek et al.
(Bayley et al., 1986; Proctor, 1994; Chapman, 2002).
Usually, the diagenetic effects of biological sulfur reduction are negligible in the upper layers of the peat profile
(acrotelm) but can be significant in the deeper, much decomposed part of the catotelm. According to Coulson et
al. (2005), the isotopic composition of recent plant sulfur
is preserved with only minor changes in the catotelm, but
the type of peat is critical in determining the degree of
preservation. A peat needs to be anaerobic and acidic to
prevent decomposition, and the water table needs to be
stable. High variation in the water table may cause higher
decomposition and isotopic fractionation of sulfur due to
the oxidation of plant sulfur and the production of sulfates
as a result of biological activity. In the case of a relative
stable water level, when no significant changes in the type
of plants occur, peat horizons may potentially preserve
records of changes in isotopic compositions of plant sulfur
(Coulson et al., 2005).
In general, the sulfur concentration in moss tissues
reflects the concentration of sulfate in surface water because, in contrast to most vascular plants, mosses cannot
control sulfur uptake. According to previous studies,
plants take up sulfur from the environment with relatively
low isotope fractionation (Krouse, 1977; Thompson and
Bottrell, 1998; Novak et al., 1994). Therefore, their isotopic composition reflects the isotopic composition of the
sulfur source. The main source of water for peat-forming
mosses is standing water on the surface of a bog. In such
cases, the isotopic composition of sulfates depends mainly
on the original isotopic composition of SOx in precipitation and partially on bacterial activity (Bartlett et al.,
2005).
GOAL
The primary aim of the present investigation was to
test how far the stable isotope composition of sulfur in
peat forming vegetation reflected the sulfur isotope in
water and/or to obtain a general rule to explain the variation in isotope ratio of the vegetation in terms of sulfur
assimilation and reduction. The seasonal and spatial variation of sulfate sulfur isotopic composition and concentration in water around the sites and along the creek, discharging the studied bog would provide basic information and thus were studied as well.
MATERIALS AND METHODS
Location of sampling places
Sampling points for mosses and water collection were
selected on the raised peat bog “Nad Jagniecym
Potokiem” at Hala Izerska (Fig. 1). Hala Izerska is located at ~840 m altitude in the Izerskie Mts. (SW Poland). It is an elevated, flat-bottomed basin surrounded
Fig. 1. Sampling locations at Hala Izerska, SW Poland. Plant sampling stations: OB, Open bog; SF, Spruce forest. Sampling
places of surface waters: 1, Izera River; 2, 3, and 4, Jagniecy Potok Creek; 5, pool on the bog surface.
by mountain ridges up to 1100 m in altitude. This location causes specific cold and wet weather conditions with
low mean summer (13°C) and winter (–5°C) temperatures.
The annual mean temperature does not exceed 3.5°C. The
growing season starts late in April and last only ~175 days.
The total annual precipitation in the Izerskie Mts. is very
high, averaging about 1500 mm/yr (Sobik and Urban,
2000). This climate and morphological situation provides
excellent conditions for peat bog development.
The studied bog is raised as an asymmetrical dome,
due to extensive plant growth. The majority of the bog’s
surface is currently covered by Pinus mugo. The vegetation in the central part of the dome consists mainly of
peat-forming plants dominated by Sphagnum mosses, with
Carex and Eriophorum species (Wojtuń, 2006). The bog
is surrounded by a sub-alpine spruce forest.
Two moss genera were collected at four different times
(October 12th 2002, May 7th 2003, August 11th 2003 and
October 18th 2003) from two different locations, an Open
bog (OB) and a Spruce forest (SF). These two locations
were 0.45 km from one another at similar altitude: 845
and 840 m, respectively (Fig. 1). Samples of moss species Polytrichum formosum were collected at both locations, with two different species of Sphagnum, S. fallax
and S. girgensohnii, collected on the bog and in the forest, respectively. S. girgensohnii is a shade tolerant species and was not found on the open bog surface. S. fallax
requires light and while abundant on the open bog surface, the species was not observed in the forest setting.
At each collection event, about 10 g (dry mass, 3–5 cm
long capitula tips) of moss was harvested, which mainly
represents the most recent growing period. The collected
samples were frozen the same day.
The surface water samples (2.5 dm3) were collected
from five locations at the same time as the moss collections (i.e., four times). Two additional water samples were
collected on June 1st 2004 and June 4th 2004. Two of
those water sampling points were located at the places of
moss species collection (in distance 1–2 m): point #5,
standing water on open bog surface, point #4, small creek
Jagniecy Potok. Three other points were located downstream in the Jagniecy Potok catchment: #3 from Jagniecy
Potok 10 m below the place where the other creek spur
joins, #2 from Jagniecy Potok, 10 m before Izera River,
and #1 from the Izera River, 10 m up-stream from the
point of junction with Jagniecy Potok (Fig. 1). The Hala
Izerska is covered mostly by peat and the lower section
of Jagniecy Potok downstream to Izera River (below SF
sampling station) is eroding exclusively peatlands. Therefore, the most of sulfur in the lower part of this catchment could be supplied from peat decay and meteoric
precipitation; in contrast more geogenic sulfur from
springs is expected to come to the upper part (Fig. 1).
Stable isotope studies of moss sulfur and sulfate from bog surface waters 483
Fig. 2. The temperature and rainfall during growing periods (2002–2004) recorded by the state weather station at Świeradów
Zdrój (The Institute of Meteorology and Water Management/Poland). Diamonds represent the sampling dates. Each point represents 7-days mean temperature and 7-days sum of precipitation, respectively.
Weather observation and δ 34S in precipitation
The field area is located in the so-called “Black Triangle”, where increasing human industrial activity caused
air pollution and acid rain, which damaged forest stands
during 1980s and the early 1990s in the Izerskie Mts.
(J˛edrysek et al., 2002). In this area, nearly all sulfate ions
in surface water and groundwater originated from acid
precipitation during the 1980s and early 1990s (Kryza et
al., 1994, 1995, Dobrzyński, 1997). Baron and Sobik
(1995) reported that the lowest pH of rain water, an annual mean ~3.6, for Izerskie and Karkonosze Mts. occurred in 1990. Changing environmental policies since
that time have produced rapidly declining concentrations
of atmospheric SO x. In 1996, concentrations were 60–70
µ g/m3, and by 2004 concentrations fell to <10 µg/m 3.
Currently, the annual wet deposition of is relatively low,
and in 2004 deposition varied from 0.3 to 0.7 g/m2/year
of sulfur (Abraham et al., 2005).
Detailed climatic data was acquired from a weather
station at Świeradów Zdrój, maintained by The Institute
of Meteorology and Water Management/Poland (IMGW).
The station is about 5 km away from the study area, at an
altitude of 550 m. The altitudinal difference between the
weather station and Hala Izerska, which is at 840 m, results in differences in absolute values of temperature, but
the relative temperature differences during the growing
season is supposedly similar for the two locations within
season. The 7-days mean air temperature for the growing
season as well as the 7-days sum of rainfall for sampling
484 G. Skrzypek et al.
years are presented in Fig. 2. The 7-year average air temperature for the growing season from 1998 through 2005
is 14.1°C; the average total precipitation during the growing period is 635 mm. The length of growing periods varied from 170 to 212 days per year, with a mean of 187
days per year. The summer of 2003 (sampling year) was
unusually dry (total precipitation of 440 mm) and warm
(mean = 15.2°C). In contrast, the previous sampling summer (2002) was wet, with the precipitation total of 796
mm exceeding the average (635 mm). Average temperature (14.5°C) was slightly below the 1998–2005 mean.
Most of the rainfall in 2003 occurred within a few days
just prior to the sampling date on October 18th. In contrast, the sampling period on August 11 occurred during
an extreme drought, when no surface water was observed
on the bog (see Fig. 2). The very low water level on the
bog during the summer of 2003 is confirmed by the extremely low discharge levels measured on Izerka River
(at 859 m altitude downstream to Izera River, ~6 km from
sampling place). The discharge varies from 0.08 to 0.37
m3/s (mean 0.14 m3/s) for the period between May 1 and
September 30, 2003, and increased significantly, up to
3.01 m3/s, after heavy rains, which had occurred a few
days prior to sampling on October 18. In contrast, discharge during the wet growing season of 2002 varied from
0.10 to 11.41 m3/s with a few flood events (Izerka State
Monitoring Station of Czech Hydrometeorological Institute).
Currently observed concentrations of SO x ion in pre-
Table 1. Stable isotopic composition ( δ34S, ‰ CDT) and concentration
(wt. %) for sulfur extracted from Sphagnum and Polytrichum mosses collected during four samplings
Location
Date
Sphagnum
Polytrichum
δ S [‰]
S [wt. %]
δ S [‰]
S [wt. %]
34
34
Open bog
Oct. 2002
May 2003
Aug. 2003
Oct. 2003
10.24
7.64
5.20
6.81
0.017
0.030
0.040
0.019
6.48
5.49
4.18
5.30
0.053
0.061
0.097
0.088
Spruce forest
Oct. 2002
May 2003
Aug. 2003
Oct. 2003
5.16
3.99
4.45
4.55
0.030
0.059
0.035
0.054
5.85
6.18
4.41
5.39
0.060
0.052
0.044
0.047
cipitation (Jakuszyce Weather Station/IMGW located ~9
km to the East of the sampling area) are very low, with a
mean of 2.66 mg/dm3 for 2004. However, in the 1980s
and 90s high emissions of pollutants from power plants
produced much higher concentrations (e.g., 10.04 mg/dm3
in 1989, and 7.68 mg/dm 3 in 1996, Twarowski et al.
(1989–1999, 2000–2005)). Unfortunately, there is no permanent station that monitors air δ34S in this region. The
closest station monitoring δ34S of precipitation in located
at the University of Wroc l aw (~150 km NE of the sampling sites), and the δ34S values reported for this site by
J˛edrysek (1999) varies from 1 to 6‰ (1993–95). These
values are close to those (means 5.8‰ and 6.7‰) reported
by Novak et al. (2001) for precipitation at two locations
in the Czech Republic (~200 and 230 km SW to sampling
sites). Jezierski et al. (2006) report δ34S values of ~6‰
for snow in the Rudawy Janowickie Mts., located ~35
km from the sampling sites, and similar values were reported by Szynkiewicz et al. (2008) for Śnieżnik surface
creek waters (~150 km SE of the sampling sites). J˛edrysek
et al. (2002) reported values of 4.5 to 10‰ for sulfur deposited on spruce needle in the Karkonosze Mts., located
~35 km away from Hala Izerska.
Analytical methods
The moss samples for stable isotope analyses were
frozen within a few hours of collection (–20°C) to prevent bacterial activity. In the laboratory, samples were
thawed and species identified. Selected species were separated (~10 g dry mass), and then a washing procedure
using redistilled water was repeated 5 times (soaking in a
2 dm3 beaker for ~1 h and rinsing on sieve) in order to
remove dust and sulfur deposited on the plant surface.
Samples were then vacuum-dried, and powdered. Weighed
moss samples were explosively combusted under high
pressure of oxygen (2.5 MPa) using the Parr Bomb device (Parr Instrument Company, Moline, Illinois, USA).
The plant in-body sulfur (irremovable sulfur—mainly
consists of organic sulfur plus in-body sulfate ions, etc.)
was recovered by precipitation from hot solution as BaSO4
at pH < 3 by the addition of an excess of BaCl2. The surface water samples were filtered and then SO42– was similarly precipitated as BaSO4. The obtained BaSO4 was
dried overnight at 110°C and weighed to calculate the
dry weight of sulfur in mosses (% wt of organic S) and
SO42– concentration in water (mg/dm3 of SO42–).
All stable isotope analyses were carried out using an
off-line vacuum preparation technique. The BaSO4 was
quantitatively converted to gaseous SO2 during reaction
with V 2 O 5 (Yanagisawa and Sakai, 1983) and then
cryogenically purified and analyzed in a Finnigan-Mat
Delta E/dual inlet isotope ratio mass spectrometer. All
δ34S values were presented relative to an international
scale VCDT (Canon Diablo Troilite), where the δ 34SVCDT
value is defined as the relative difference, in parts per
thousand (‰), between the isotope ratio of the sample
and that of the standard. Analytical uncertainty, estimated
from analyses of laboratory standards, is ±0.20‰.
Water samples for δ13C(DIC) analyses have been preserved in the field using HgCl2, in order to prevent bacterial activity. For the stable isotope analyses of δ13C(DIC),
carbon dioxide was extracted during reaction of 20 ml of
water sample with 0.5 ml of H 3 PO 4 on vacuum line
(Szynkiewicz et al., 2007) and then cryogenically purified for off-line analyses in a Finnigan-Mat Delta E/dual
inlet. All δ13C values were presented relative to an international scale VPDB with ±0.10‰.
RESULTS AND DISCUSSION
Variation of stable isotope composition and concentration of sulfur in mosses versus local water
The sulfur stable isotopic composition of the two studied moss genera, Sphagnum and Polytrichum, varied between 3.99 to 10.24‰ and 4.18 to 6.48‰, respectively
(Table 1). The determined concentrations of sulfur in those
Stable isotope studies of moss sulfur and sulfate from bog surface waters 485
Fig. 3. Relationship between δ34S values and sulfur concentrations (S % wt) of in-body sulfur and surface water sulfates for
Sphagnum and Polytrichum respectively.
mosses are lower than previously reported for polluted
region in Canada (e.g., Nriagu and Glooschenko, 1992)
or in the Czech Republic (e.g., Novak et al., 2001). However, these researchers did not wash plant samples before
the analysis. Therefore, those results represent total sulfur
(organic + inorganic from plant surfaces). Here, our results are only for in-body plant sulfur (mostly organic).
It is important to distinguish what type of sulfur is
analyzed because large difference in stable isotopic composition may occur. For example, J˛edrysek et al. (2002)
reported a difference of more than 3‰ (∆34S) for spruce
needles between organic (plant in-body) and inorganic
sulfur (deposited on needles surface).
According to the previous studies (e.g., Novak et al.,
2001; Bottrell and Coulson, 2003; Coulson et al., 2005),
the plant in-body sulfur stable isotope composition and
concentration reflects stable isotope composition and
concentration in the source of sulfur for moss, which is
mainly sulfate from surface water or rain. In this study,
loose correlations in sulfur isotope ratios were seen between Sphagnum and surface water (n = 7, r2 = 0.66, p =
0.025) and between Polytrichum and surface water (n =
7, r 2 = 0.54, p = 0.059) and also a correlation in sulfur
486 G. Skrzypek et al.
concentrations between Polytrichum and surface water
(n = 7, r2 = 0.67, p = 0.025) (Fig. 3). The δ 34S of in-body
sulfur was generally higher than that of water (Fig. 3).
One reason for the weak correlation may be due to the
difficulty in sampling of representative water for the
mosses, which would possess the average isotope signature of the whole growing period, whereas water samples
represent momentary δ34S values at a time of sampling.
The concentration-weighted averages of the data of specimens sampled over a year are δ34S = 5.40‰ for Sphagnum (n = 8), 5.32‰ for Polytrichum (n = 8) and δ34S
=7.18‰ (n = 7) for sulfate in water. The difference between δ 34 S of mosses and water equal –1.78‰ and
–1.86‰, respectively, which is similar to the reported by
others fractionation involved in the assimilation of sulfate
ion by Sphagnum (–1.5‰ by Trust and Fry, 1992 and
–0.7‰ by Novak et al., 2001).
Correlation between stable isotope composition and concentration for in-body sulfur in Sphagnum
A statistically significant correlation between the inbody sulfur concentration and the in-body isotopic composition was observed for Sphagnum (r2 = 0.73; p < 0.01)
Fig. 4. Relationship between δ34S and concentration of in-body
sulfur for Sphagnum from both locations: Open bog and Spruce
forest (see Fig. 1 for location details). The presented regression best-fit model curve for all points is matching the Rayleigh
models when, we use the Rayleigh model parameters listed in
Table 3: S (wt %.), δ 34S of sulfates, ∆34S (plant-sulfates).
(Fig. 4); but not for Polytrichum (r2 = 0.18; p = 0.29).
This observed relationship suggests that higher δ34S of
in-body plant sulfur is associated with lower in-body
sulfur concentrations. This concave-upward relationship
is well-known in the case of the Rayleigh distillation
models. If we consider that the concentration of sulfur in
plants is proportional to that in water, which is likely in
the case of Sphagnum because of its passive uptake, the
relationship can be interpreted as the relationship between
the sulfur concentration in water and the isotope ratio of
plants. The conversion factor from the plant sulfur concentration to water concentration was estimated as 28000
mg/dm3 by using the average concentration in water during growing seasons (9.94 mg/dm3) and the average concentration in Sphagnum (0.036%).
We obviously need to introduce one important assumption in order to consider the two data sets for the two
different sites using a unique curve, i.e., both the locations (Open bog and Spruce forest) have ultimately an
identical sulfur source with an identical primary isotope
signature and two different species of Sphagnum behave
similarly with respect to sulfur uptake. The main sulfur
source in this area is precipitation but any difference between these locations may occur due to canopy throughfall at the Spruce forest. This effect is rather negligible
because of frequent precipitation and probably low sulfur
retention in the canopy (J˛edrysek et al., 2002). The best-
Fig. 5. Polytrichum versus Sphagnum—in-body sulfur isotopic
composition from Open bog sampling station (see Fig. 1).
fit regression curve describing this relationship for Sphagnum (all data from both location Fig. 1) is expressed by
the equation y = 0.57x–0.677, where x and y are the in-body
plant sulfur concentration and the plant sulfur stable isotope composition, respectively. We have three unknown
parameters: the isotope ratio of unfractionated original
sulfur in precipitation and its concentration and mass
fractionation factor of the reactions responsible for the
sulfate removal from the water. To reduce the number of
unknown parameters, the isotope ratio of unfractionated
original sulfur supplied to the sites has been assumed.
The mean δ 34S values of the precipitation during 1993
and 1995 supplied to Wroc l aw (~150 from our location)
were 1.76 and 4.26‰ for the two separate periods
(J˛edrysek, 1999); Jezierski et al. (2006) reported the value
around 6‰ for Sudety Mts. near the studied sites (~35
km). We have assumed three values of 2, 4, 6‰ for the
unfractionated original sulfur for the purpose of the exercise. If this sulfur is assimilated by Sphagnum, this would
correspond roughly to 0, 2, 4‰ in-body sulfur, respectively, by employing a ∆ value of –1.78‰ involved in
sulfur uptake by Sphagnum, estimated earlier in this study.
The two unknown parameters in the Rayleigh distillation model were estimated and values that produce
curves consistent with the best-fit regression curve were
determined. From the residues in the estimation of inbody ∆34S, the errors of the parameters were estimated.
The results of the calculation are summarized in Table 3.
For example, in the case of δ34S source = 6.00‰, Wsource =
0.033 ± 0.003% and ∆34Ssource-plant = 3.9 ± 0.7‰ ( α =
Stable isotope studies of moss sulfur and sulfate from bog surface waters 487
Table 2. Stable isotopic composition (δ 34S,‰ CDT) and sulfur concentration (wt. %)—sulfates from surface water collected at
five places during six samplings and δ13C of DIC collected on July 2004
Date
1 River
δ34S
2 Creek
δ34S
SO 42−
3
3
[mg/dm ]
Oct. 2002
May 2003
Aug. 2003
Oct. 2003
June 2004
July 2004
8.41
9.23
3.72
8.24
ls
8.37
July 2004 δ13C (DIC)
13.7
6.8
7.0
7.5
7.2
6.8
−9.38
3 Creek
δ34S
SO 42−
3
[mg/dm ]
8.48
7.92
5.05
7.55
8.10
8.02
6.8
7.4
7.4
8.4
6.5
7.3
−12.74
4 Creek
δ34S
SO 42−
3
[mg/dm ]
8.35
7.70
4.68
7.75
7.64
8.31
13.4
7.2
6.7
8.9
6.2
6.5
−12.65
5 Bog
δ34S
SO 42−
[mg/dm3]
[mg/dm ]
8.11
7.27
4.26
7.35
7.28
7.48
12.2
6.0
7.3
9.1
7.5
7.0
−14.63
SO 42−
20.26
7.74
nw
6.31
6.45
6.99
2.6
1.4
nw
31.0
0.9
2.1
−19.53
VPDB]
ls, lost sample; nw, no water on the bog surface.
Fig. 6. δ34S versus concentration—sulfates from surface water
from five locations in Jagniecy Potok catchment. Two characteristic ranges are indicated on the plot: 5 - bog (water from a
pool on the bog surface), August 2003 (drought during summer). No data for August 2003—bog water was not present.
0.9961 ± 0.0007) were obtained. It should be noted that
similar ∆34Ssource-plant values around 4‰ were obtained
irrespective of the choice of δ34S values for the source.
The estimated mass fractionation factor is 4 ± 0.7‰,
which is more than the factor involved in sulfur assimilation standalone. Bacterial reduction of sulfate could be
considered as the other important factor responsible for
the decrease of sulfate concentration in water as well as
increase of δ34S (Groscheova et al., 2000; Chambers and
Trudinger, 1978).
488 G. Skrzypek et al.
Fig. 7. Concentration (top) and sulfur isotopic composition
(bottom) both in surface water versus date of sampling. Each
line represents variation in one point over time. No data for
August 2003—bog water was not present.
The concentration of sulfur (S wt. %) in the original
water, from which no sulfur is removed, the other unknown in the model, was estimated in the range between
0.032 and 0.088% (for the range of δ34S 2 to 6‰). This
corresponds to 9 and 25 mg/dm3 of SO42– in the original
water, which is compatible with the concentration level
of the local precipitation, endorsing the validity of our
discussion.
Table 3. The regression best fit models
(y = 0.57x–0.677 refer to Fig. 4) are matching Rayleigh’s models when the following values of listed parameter are used
δ3 4 Sso u rce
[‰]
W so u rce
S [wt. %]
∆3 4 Sso u rce-p lan t
[‰]
2.00
4.00
6.00
0.089 ± 0.009
0.054 ± 0.005
0.033 ± 0.003
4.0 ± 0.3
4.0 ± 0.3
3.9 ± 0.7
Wsource, concentration of sulfur in source for moss assimilation (sulfates
in a surface water); δ 34Ssource, the δ -value of sulfur in source for moss
assimilation; ∆ 34Ssource-plant, difference due to fractionation between the
original δ -value in the source and the moss δ -value.
Correlation of sulfur stable isotope composition between
Sphagnum and Polytrichum
Although no relationship was observed between the
isotopic composition and concentration of sulfur for
Polytrichum, a significant correlation has been found
between δ34S values of Polytrichum and Sphagnum sample pairs collected from the Open bog location (r2 = 0.96,
p = 0.02, Fig. 5). This correlation can be a mere coincidence, considering the different length of time samples
represent, and growth rate of each species, but can also
represent the results of simplicity of the sulfur trophic
system in the Open bog. The correlation between both
genera appears high because one major source (precipitation) supplies nutrients to both genera. On average, the
δ34S difference of Sphagnum (max. value–min. value =
5.0‰) is greater than that of Polytrichum (2.3‰) and also
the value of Sphagnum is higher than that of Polytrichum.
One reason for this difference may be that Sphagnum
grows in more reducing conditions or in more sulfurlimited conditions better than Polytrichum. Either condition corresponds to higher δ 34S of water.
δ34S and concentration of sulfate in surface water
The stable isotopic composition and the concentration
of sulfates in the surface water in the investigated area,
vary over a wide range, with δ 34S ranging from 3.72 to
9.23‰ (mean 7.29‰) and concentration levels varying
from 0.90 to 13.66 mg/dm3 (mean 7.06 mg/dm3) (Table
2). Two samples, both collected on the Open bog (point
#5) in October 2003 (concentration = 31.03 mg/dm3) and
October 2002 (δ 34S = 20.26‰) are outliers from the general pattern (Figs. 6 and 7). While both samples were collected at the end of the growing season, the climatic conditions for these two years were very different. The summer of 2003 was dry with only 440 mm of precipitation
occurring during the growing period, well below the 7
year mean of 635 mm. During sampling in August 2003
Fig. 8. The correlation (LSD) between δ 13CDIC (Dissolved Inorganic Carbon) and δ34S of sulfates from water in Jagniecy
Potok catchment. Number at the points: 1, Izera River; 2, 3,
and 4, Jagniecy Potok Creek; 5, pool on the bog surface (see
Fig. 1 for locations on the map).
temporary stop of moss growth was observed at point #5
(Open bog) due to insufficient water. The prolonged dry
conditions are confirmed by low (≥0.10 dm3/s) water discharge at Izerka River. Therefore, during that summer
(2003) bacterial activity was limited because no standing
water was observed on bog surface. This would produce
increased oxidation in the upper layers of peat. Probably,
most of the sulfate originated from decomposition of the
peat due to a low water table (e.g., Braekke, 1981;
Mandernack et al., 2000; Bottrell et al., 2004). Rainfalls
during September and October 2003 (148 mm) increased
the water level on the bog, but that increase was accompanied by a rapid decrease in air temperature (from 15°C
on September 20, to 6.7°C on October 12) producing conditions that were not favorable for either vegetation
growth or bacterial activity (Fig. 2). Although the precipitation was rather limited, it increased water levels significantly and dissolved the sulfates accumulated during
the summer, resulting in high concentration of SO 42–
(31.03 mg/dm3) in the surface bog water. Due to limited
bacterial activity, which causes limited isotopic
fractionation, the δ 34S value (6.31‰) remained close to
the long-term mean observed at this location (7.29‰).
The climatic situation during the summer of 2002 was
essentially the opposite of the 2003 summer pattern. During the summer of 2002, the total amount of rainfalls was
higher than usual (~796 mm) and the bog was water-
Stable isotope studies of moss sulfur and sulfate from bog surface waters 489
logged. Therefore, the concentration of SO42– was maintained at low levels (2.58 mg/dm 3). The relatively warm
summer and high water level provided good conditions
for bacterial activity, resulting in 34S enrichment of the
sulfates in the surface water. However, the high sulfur
enrichment (δ34S = 20.26‰) is difficult to attribute solely
to increased bacterial activity.
There is no clear observed correlation between δ34S
and SO42– concentration for water samples. In general,
the seasonal variation pattern is similar at each sampling
site (Fig. 7). However two groups of points specific to
certain locations and periods could be distinguished based
on the δ34S–SO42– relation (Fig. 6). Accordingly, water
on the bog surface (point #5) is characterized by low concentration of sulfur (0.9–2.6% wt, except 10/2003). This
is probably due to high biological activity and limited
sulfur inputs (Table 2). The second group (August 2003,
values for points #1–4, creek and river) is characterized
by lower than usual values of δ34S (from 3.72 to 5.05‰)
and a narrow range of concentrations (6.70 to 7.43 mg/
dm3). During August 2003 the water level in the river
and creek was low due to drought. Runoff from the bog
was limited and most water in the creek probably came
from spring outflows providing geogenic from upper parts
of catchment only.
Sulfur stable isotope variation along creek
High correlations of δ34S between all combinations
of sampling point pairs (δ 34S 1–2, 1–3, 1–4, 2–3, 2–4, and 3–4)
along Jagniecy Potok creek are observed for all samplings
2002–2004 (5 samplings). The r2 values range from 0.89
to 0.98 (p ≤ 0.02). These correlations reflect a slight trend
with δ34S values of SO42– becoming progressively higher
(more 34S) downstream. This process can be explained
by preferential assimilation by vegetation and bacteria of
the lighter isotope of sulfur (32S) and increasing oxidation of organic sulfur downstream that would lead to 34S
enrichment of the remaining water sulfates (Chambers and
Trudinger, 1979). A similar trend has been observed for
δ 13CDIC (Dissolved Inorganic Carbon), which has been
analyzed for July of 2004 sampling only (13C enrichment
downstream). The very high correlation between
δ 13C(DIC) and δ34S (SO42–) values (r 2 = 0.88, p = 0.02),
and subsequent increase of both δ-values downstream
(from the Open bog through Jagniecy Potok Creek to Izera
River, Fig. 8) also confirms that increased assimilation
of lighter isotopes of sulfur and carbon ( 32S and 12C) increase downstream in the catchment.
CONCLUSIONS
The significant correlation between the plant in-body
sulfur concentration and the isotopic composition was
observed for Sphagnum but not for Polytrichum. Based
490 G. Skrzypek et al.
on the theoretical Rayleigh distillation model and the regression curve, the possible isotope fractionation during
the sulfur assimilation by Sphagnum including sulfate
reduction prior to or during assimilation can be about 4‰.
The calculated Rayleigh model matches the regression
curve when sulfur concentration in the source for assimilation varies from 0.033 to 0.089% for the assumed δ34S
value of the source between 2 and 6‰. The estimated
isotope fractionation is greater than that which can be
attributed to sulfate assimilation by mosses alone and
probably reflects the mixed effect of assimilation and
sulfate reduction. In general, sulfate reduction can be accompanied by significant fractionation. The presented
study indicates that this reduction could be the one of the
most important processes, besides intake by Sphagnum,
which removes sulfate from bog water; based on estimated
mass fractionation it may contribute in fractionation up
to 10‰. The sulfate isotopic composition of mosses may
be so vulnerable to the reducing reaction that the sources
of sulfate as well as its concentration are hardly understood only from the isotope ratio of sulfur and the concentration data. Sphagnum seems to record the average
sulfate concentration of water fairly well but differs
greatly point-to-point suggesting that the reduction and
assimilation varies depending on its local conditions.
The strong correlation between δ34S (SO42– ) and δ13C
(DIC) along the creek discharging the bog and the simultaneous increase of δ34S and δ13 C values downstream
suggest that lighter isotopes of carbon and sulfur (12C and
32
S) are preferentially removed, probably solely due to
biological assimilation processes, unlike bog water.
Acknowledgments—The authors are grateful to Dr. Anna
Szynkiewcz for the DIC analyses and Dr. Anna BaranowskaK˛acka for help during plants botanical identification. Appreciation is also expressed to Dr. Raymond P. Mauldin from the
University of Texas at San Antonio for his critical reading of
the text and valuable remarks. The authors are grateful to The
Institute of Meteorology and Water Management (IMGW
Wroc l aw Poland) for access to weather and SOx data and to
The Czech Hydrometeorological Institute for hydrological data.
This study was supported by the University of Wroc l aw grants
(2022/W/ING and 1017/S/ING). We also wish to thank, the reviewer Dr. Simon Bottrell and anonymous reviewer whose comments and suggestions contributed to the improvement of the
manuscript.
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