sulfur metabolism in polluted sphagnum peat bogs: a combined 34s–35s–210pb study

SULFUR METABOLISM IN POLLUTED SPHAGNUM PEAT BOGS:A COMBINED 34S–35S–210Pb STUDY

MARTIN NOVÁK1 ∗, MARIE ADAMOVÁ1 and JOVAN MILICIC2

1 Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic;2 Department of Geochemistry, Mineralogy and Mineral Resources, Charles University,

Albertov 6, 128 43 Prague, Czech Republic(∗ author for correspondence, e-mail: [email protected], fax: 420251818748)

(Received 21 August 2001; accepted 23 February 2002)

Abstract. The size and isotopic behavior of sulfur pools in 210Pb-dated peat cores were investigatedto obtain an insight into retention mechanisms of pollutant S in two mountain-top peatlands of theNorthern Czech Republic, Central Europe. The bogs were situated 40 km apart in an area whichbetween the years 1985 and 1995 received as much as 130 kg S ha−1 yr−1 from the atmosphere.Vertical peat accretion was faster at Pod Jelení horou (JH) than at Velký mocál (VM). Organiccarbon-bonded S was the most abundant sulfur pool, constituting 77 and 65 wt. % at JH and VM,respectively. At JH both the S concentration maximum and the highest annual S deposition rate weredisplaced downward by more than 20 years (from 1987 to the 1960s) indicating that the buried Sis vertically mobile. At VM the S concentration was the highest in the topmost 2-cm section eventhough atmospheric S deposition peaked in 1987. Different mechanisms of S isotope redistributionprevailed in the topmost peat layers at JH, where a negative δ34S shift occurred, and at VM, where apositive δ34S shift occurred. Bacterial sulfate reduction was responsible for the negative δ34S shift atJH. One possible explanation of the positive δ34S shift at VM is release of 32S-enriched products ofmineralization during peat diagenesis. There was a strong positive correlation between the abundanceof total and pyrite S along the profiles. The presence of pyrite S at VM (526 ± 60 ppm) suggested thateven at VM bacterial sulfate reduction occurred. An anaerobic incubation of JH peat indicated sulfatereduction rate of 600 nmol g−1 day−1. The turnover times for inorganic S pools were shorter thanfor the organic S pools. Cumulative S contents in the Czech peat bogs were found to be significantlylower than in similar sites in the Northeastern U.S., even though the atmospheric S inputs were morethan three times higher at the Czech sites. Possible causes of such discrepancy are discussed.

Keywords: acid deposition, 210Pb-dating, peat, pollution, 35S, soil, Sphagnum, stable sulfur iso-topes, sulfur metabolism

1. Introduction

Freshwater peat bogs are known as net sinks of pollutant sulfur (S), supplied eitheras atmospheric deposition (Spratt and Morgan, 1990) or acid mine drainage(Wieder, 1989). When anthropogenic S is stored in a wetland in the reduced form(S2−), the acidity of discharging water is lower compared to water entering theecosystem. In contrast, oxidation of reduced S during low water-table periods canincrease the acidity of water output (Lamers et al., 1998). The main S pools inSphagnum-derived peat profiles are inorganic sulfate S, organic carbon-bonded

Water, Air, and Soil Pollution 3: 181–200, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

182 MARTIN NOVAK ET AL.

S, ester sulfate S and reduced inorganic S (FeS and FeS2). Dynamic interconver-sions among these S pools result from sulfur immobilization and mineralization,i.e., microbially mediated redox reactions (Wieder and Lang, 1988; Wieder et al.,1990).

Retention of pollutant S in organic-rich compartments of forest ecosystems isof particular interest in the industrial part of Central Europe, a region characterizedby the highest atmospheric S deposition in the world (up to 130 kg S ha−1 yr−1

in the period 1985–1995; Novák et al., 2000). We studied the metabolism of S inpeat cores taken in two extremely polluted mountain-top peat bogs in the NorthernCzech Republic. To obtain an insight into historical changes in S retention in thesepeat bogs, we chose a combination of three geochemical tools, 210Pb dating, stableisotope determinations (δ34S) and radiolabelling with 35S, along with chemical spe-ciation. Previously, these tools have been used separately to study selected aspectsof S cycling in peat (Morgan, 1998; Novák et al., 1994, 1999, 2001a; Groscheováet al., 2000; Alewell and Novák, 2000). The objectives of this study were: (i)to evaluate the relative abundances and turnover times of individual S pools inpolluted freshwater peat, (ii) to relate changes in δ34S values to changes in annualS accumulation rates along vertical peat profiles, and (iii) to compare S retention inhighly polluted Central European peat bogs and in pristine to moderately pollutedpeat bogs in the United States. Six Sphagnum peat bogs located in the Northeast-ern U.S., which are included in this comparison, have been studied previously(Novák and Wieder, 1992; Novák et al., 1994) using the same methodology asthat used in this paper. Specifically, we hypothesized that S retention in the highlypolluted Czech peat bogs should exceed that in the considerably less polluted NorthAmerican peat bogs.

2. Study Sites

Pod Jelení horou (JH) and Velký Mocál (VM) are ombrotrophic peat bogs located40 km apart on the mountain plateau of the Krušné hory Mts. near the Czechborder with Germany. The principal characteristics of the two bogs are given inTable I. The region was known in the 1980s as the Black Triangle because ofspruce die-back related to heavy industrialization and high degree of soil and wateracidification (Novák and Prechová, 1995). All Norway spruce stands in the vicinityof JH were defoliated due to air pollution at the time of peat sampling (1994) andawaiting clear-cutting, whereas spruce surrounding VM was only moderately dam-aged with slight symptoms of crown thinning. The health status of trees reflectedthe distance from the main source of S emissions, the cluster of North Bohemiancoal-burning power plants. The power plants are situated down-wind (south tosouth-east) from both JH and VM, with JH being closer to these S sources (12 km).Boggy areas underneath spruce canopy in the Krušné hory Mts. receive 3 to 8 timesmore S via atmospheric deposition relative to nearby clearings due to scavenging

S METABOLISM IN 210Pb-DATED PEAT CORES 183

TABLE I

Study site characteristics

Pod Jelenı horou (JH) Velky Mocal (VM)

Location 50◦31′ N, 13◦10′ E 50◦28′ N, 13◦7′ E

Elevation (m) 875–895 920

Bedrock Orthogneiss Granite

Peat thickness (m) 6.3 6.7

Annual precipitation (mm) 1290 1100

Mean annual temperature (◦C) 4.1 4.0

Sulfur deposition in clearings (kg S ha−1 yr−1) 14.9 11.6

Spruce canopy througfall (kg S ha−1 yr−1) 120 100

Prevailing wind direction North-west North-west

of anthropogenic SO2 by spruce needles (Novák et al., 1996, 2001b; Groscheováet al., 1998). Both JH and VM are 100-percent Sphagnum covered and underlain byS-defficient crystalline rocks (< 0.01 wt. % of S). The following Sphagnum specieswere identified at the study sites: S. riparium Angstr., S. fallax v. Klinggr. and S.girgensonii Russ. The herbaceous layer was dominated by Deschampsia flexuosaand Calamagrostis villosa. The surface area of JH is 306 ha, while that of VM is51 ha (Dohnal, 1965).

3. Sampling

One peat core, 10 cm in diameter, was taken from JH (June 1994) and VM (April1995), avoiding hummocks and hollows. A PVC cylinder with a sharpened bottomedge was used to collect the cores. A cylindrical incision through the surface layerof the peat lawn was made by a serrated knife and the corer was inserted intothe substrate with no compaction of the entrained peat. Partial excavation of thepeat outside the corer made it possible to cover its bottom by hand so the entirecore could be lifted. The cores were transported to the laboratory, frozen, slicedinto 2-cm sections, weighed, air-dried at 25 ◦C (Wieder et al., 1996), reweighed,homogenized by grinding, and each section divided into four aliquots. One bulksample (100 g wet mass) was collected at JH from the depth of 30 ± 5 cm for the35S incubation study, transported to the laboratory and frozen.


4. Analytical Techniques

Each of the 2-cm thick peat sections was dated by α-spectrometric measurementsof residual activities of 210Po (a daughter isotope of 210Pb) and 208Po, which hadbeen added into the sample in a known amount (15 dpm) as a chemical yield tracer.The extraction method was based on an unpublished procedure developed at theUniversity of Pittsburgh, Pa, U.S.A. (W. R. Schell, personal communication), citedby Adamová and Novák (1998). The radioisotopes were extracted by a three-stepdigestion with concentrated HCl and HNO3 and plated out on 4 cm2 silver discsat 60◦ C (cf. also Vile et al., 2000). Polonium activities were measured on a Silenaanalyser with Si barrier detector Canberra (JH), and an ORTEC 576 analyser (VM).The constant rate of supply model of Appleby and Oldfield (1978) was used tocalculate the dates.

Total S was determined gravimetrically using the Eschka procedure (Chakra-barti, 1978) and the resulting BaSO4 was preserved for mass spectrometric de-termination of S isotope composition. The sum of free and adsorbed inorganicsulfate was determined by liquid chromatography (SHIMADZU LC 6A) followingextraction with 16 mM NaH2PO4. H2O (Wieder et al., 1987). Separation of three Sfractions was carried out using Johnson-Nishita distallation (Johnson and Nishita,1952). In this procedure, different chemical forms of sulfur are specifically re-duced to H2S, trapped in zinc acetate as ZnS and quantified by iodometric titration(Greenberg et al., 1981). Acid volatiles (mainly Fe monosulfides) was released byanaerobic introduction of 12M HCl into the reaction flask. Chromium-reducibleS (FeS2 was relased by Cr2+ freshly prepared by passing CrCl3· 6H2O through acolumn filled with amalgamated Zn (Wieder et al., 1987). Hydriodic-acid reducibleS (ester sulfate) was released by a 4:2:1 mixture of 50% hydrophosporous acid,90% formic acid and 48% hydriodic acid. The conversion of total S to SO2 for massspectrometry was performed in a vacuum line using the method of Yanagisawaand Sakai (1983). Sulfur dioxide was then analysed on a FINNIGAN MAT 251mass spectrometer with the reproducibility better than ± 0.3�. The results wereexpressed in the usual notation as a per mil deviation of the 34S/32S ratio in thesample from the CDT standard.

For the incubation study, six approximately 8 g wet peat subsamples fromJH (depth 30 cm below surface) were placed on separate pieces of aluminiumfoil under a stream of O2-free N2. Each subsample was injected with 30 µL ofNa2

35SO4 solution which had an activity of 174 kBq. The foil was then foldedaround the injected sample. The foil packages were placed in glass bottles thatwere stoppered and purged with O2-free N2. The bottles were incubated for 1 or14 days at 8◦ C. At the end of each incubation three replicates were frozen to ter-minate biological activity. Each sample was then thawed, pressure-filtered and thefiltrate retained for the determination of sulfate concentration and 35S activity. Thepeat was then weighed and the fate of added 35SO2−

4 determined using Johnson-Nishita distillation (sequential Cr2+ and HI reduction; Wieder and Lang, 1988).


Prior to iodometric titration, duplicate 1 mL aliquots from each solution trap weremixed with 10 mL of Aquasure solution. 35S activities were measured by liquidscintillation on a BECKMAN LS 9800 spectrometer. The amount and activity oforganic C-bonded S was determined upon Eschka’s digestion of the sample filteredfollowing HI reduction. The 35S incorporation rate into individual S pools wascalculated from the measured 35S activities of each S pool at time t (1 or 14 days),total 35S activity added at t = 0 and pool size (cf., Houle et al., 2001). The turnovertime for each S pool (sulfate, FeS2, ester sulfate, C-bonded S) was calculated bydividing the pool size by 35S incorporation rate.

5. Results and Discussion

5.1. CHRONOLOGY OF PEAT ACCRETION

The deepest peat section datable by the 210Pb technique is usually less than 200years old because of the relatively short half-life of 210Pb (22.3 years) and 210Pbcounting errors. In our study, the deepest dated section was 120 ± 33 yrs old at JHand 135 ± 20 yrs old at VM (Figure 1). These peat increments, found at a depthof 30 and 26 cm below current peat surface, originated around 1877 and 1860 atJH and VM, respectively. The investigated time span captures the entire period ofindustrialization, since local soft coal mining started in 1866 (Novák et al., 2000).As seen in Figure 1, vertical peat accretion was faster at JH (solid circles) comparedto VM (solid diamonds). At each particular depth, the substrate was younger at JHthan at VM. For example, at the depth of 26 cm the substrate was 52 years youngerat JH than at VM.

Figure 1 also shows the results of 210Pb dating of JH peat performed two yearsearlier (i.e., in 1992) by Vile et al. (2000; open circles, dashed line). The distancebetween the two JH peat cores was 100 meters. The chronology of the topmost6 peat sections (back to 1960) was virtually identical in both cores, however, indeeper sections the profile of Vile et al. (2000) exhibited much slower accretion,approaching the year 1850 at the depth of mere 20 cm. It follows that a single depthprofile per peat bog may seriously underrepresent the typical history of verticalpeat accretion. As a rule, hummocks and hollows are avoided during peat samplingbecause peat accumulation rates differ between microtopographic elevations anddepressions (Appleby and Oldfield, 1992). Even though both cores at JH werecollected in a flat Sphagnum lawn, the assumption that no hummocks and hollowsexisted in this area apparently may not be valid for the whole period of the past 150years.

All measured depth sections contained < 4 wt. % of silicate admixture. There-fore, the vertical patterns in bulk peat density were controlled mainly by the densityof organic matter. Figure 2a shows that peat density at both JH and VM remainedmore or less constant throughout the profiles, with the exception of the top 4 to


Figure 1. Depth-year representation of the 210Pb chronology of peat from JH and VM, CzechRepublic. The vertical bars associated with each data point are error terms which comprise both210Pb counting error and propagated error associated with fitting the CRS model to the 210Pb data.

6 cm where transition from living plants to necromass caused a slight increase inbulk density. The lack of a clear-cut vertical density gradient in Figure 2a is ratheruncommon among Sphagnum-dominated bogs. Out of 29 peat cores studied so farin our laboratory (7 cores from the U.S., 12 from the British Isles and 10 from theCzech Republic; Novák and Wieder, 1992; Bottrell and Novák, 1997; Novák et al.,1999, 2001a), constant density was observed only in two cores in addition to JHand VM. The most common pattern was that of sharply increasing density withdepth (16 cores), the second most common pattern was that of a subsurface peak(9 cores). In general, increasing bulk density at greater depth should be expectedas a result of substrate compaction following burial. During peat maturation indeeper layers (> 15 cm below current surface) decomposition of organic mattermay cause a reversal to lower density. This concept is based on removal (flushing,volatilization) of biogenic elements from more decomposed substrate (Applebyand Oldfield, 1992).

Despite the monotonous vertical density pattern at JH and VM, the number ofyears represented by individual 2-cm sections changed with depth (Figure 2b). Thenumbers of years per section at JH increased throughout the depth profile from 2to 15, with one exception at a depth of 11 cm (3 years). In contrast, the number ofyears per section at VM exhibited a mid-depth maximum (13 years). Deeper than11 cm at VM the number of years per section decreased with increasing depth. Weconclude that at both sites diagenetic changes did affect peat substrate even thoughsuch changes were not see when examining bulk densities alone.


Figure 2. Peat density (a) and the number of years per 2-cm peat section (b).


5.2. SULFUR SPECIATION

Novák and Wieder (1992) have shown for Sphagnum-dominated peat bogs thatthe horizontal variability in S concentrations is usually smaller than the verticalvariability. Using data from 5 different freshwater peat bogs from North Americaand Europe they argued that a single peat core per site may be sufficient to studyvertical trends in S storage. We have tested this assumption at JH, where threelevels below peat surface (0, 15 and 60 cm) were sampled in triplicate (distancesof ca. 60 m). The total S concentration averaged 2300, 6370 and 4630 ppm at 0,15 and 60 cm, respectively. The within-bog horizontal variability in S contents wassmall, the coefficients of variation were 3.6, 20.6 and 20.4% at 0, 15 and 60 cmbelow peat surface, respectively (unpublished data by Novák). We conclude that asingle peat core per site may represent a relatively cost-effective sampling strategyfor the determination of temporal changes in the amount of buried S.

Total S concentration in peat, when averaged across all depths, was slightlyhigher at JH than at VM (3500 ± 860 and 3200 ± 590 ppm, respectively; mean± standard deviation). As seen in Table I, JH was the site with the slightly higheratmospheric S deposition. Organic C-bonded S was the largest S pool throughoutthe profiles, constituting 77 and 69% of total S at JH and VM, respectively. Theconcentration of individual S species at JH decreased in the order of C-bondedS > sulfate > pyrite > ester > FeS. In contrast, VM exhibited a sequence of C-bonded S > pyrite > ester > sulfate > FeS. Figure 3 shows several vertical gradientsin the concentration of individual S species. At JH, total S concentration sharplyincreased from 0 to 9–11 cm below surface and then decreased (Figure 3). Suchpattern containing a mid-depth S concentration maximum was previously found ata variety of freshwater peatlands (Novák and Wieder, 1992). It is easier to interpretthe subsurface increase in S concentration above the maximum (downward migra-tion of deposited sulfate to the anaerobic zone of bacterial sulfate reduction) than tointerpret the decrease in S concentration below the maximum. This deeper decreasein S concentration is not a simple reflection of less pollution in older times, becausethe concentration maximum is displaced by 25 to 30 years downward (most sulfurat JH is at the depths corresponding to the years 1955–1967, while the actual atmo-spheric S loads peaked in 1987; Novák et al., 2001b). Sulfur profiles in peat resultfrom vertical movement of S, probably in an aqueous form.

Total S concentration at VM (Figure 3 top right) was characterized by a nar-rower range of values compared to JH (3000–4500 versus 1500–4800 ppm), anddecreased with increasing depth in the top 5 cm. Between 0 and 28 cm, total Sconcentration at VM decreased in three segments (noticeably in six adjacent depthintervals between 7 and 19 cm) and was off-set back to higher values twice. Suchpattern is rather unusual (Novák and Wieder, 1992; Novák et al., 1994; Morgan,1998) and will be further discussed in connection with its equally unusual stable Sisotope signal (see next page).


Figure 3. Sulfur speciation at JH (left column) and VM (right column).


JH and VM differed in the relative abundance of inorganic (free + adsorbed)and organic (ester-bonded) sulfate-S (Figure 3 middle). The concentration of inor-ganic sulfate S was higher than ester-sulfate S at JH and lower at VM. Ester sulfateis not produced by plants, its entire content in peat is a result of microbial synthesis(McGill and Cole, 1981). Our data suggest that the ‘older’ (i.e., more slowly accu-mulating) profile (VM) tended to have higher ester sulfate concentration. However,we cannot decide whether this trend is site-specific or typical for S cycling in avariety of waterlogged peat bogs. Groscheová et al. (2000) investigated in detailthe metabolism of free sulfate in Sphagnum peat. Using stable isotopes they wereable to show that free sulfate is replenished from the ester sulfate pool when freesulfate is needed to fuel bacterial sulfate reduction.

Pyrite S was more abundant in the ‘older’ profile (VM), while exhibiting strik-ingly different gradients in both profiles (Figure 3 bottom). A 10-fold maximumwas present at JH at 13 cm relative to the topmost peat sections, whereas a distinctminimum was present at VM at 19 cm. We have previously observed a decreasein pyrite S abundance below a maximum in total S and pyrite S concentration ata number of different sites (Novák and Wieder, 1992). The data presented hereare consistent with the suggested general occurence of such trend. At both JH andVM, there was a strong positive correlation between the concentrations of total andpyrite S (correlation coefficient R = 0.70 and 0.63, respectively). The maxima ofpyrite S concentration fall into the anaerobic zone at both sites (throughout the yearonly tips of living Sphagnum are situated above the water table), and therefore theexistence of these maxima can be ascribed to horizons with high rates of bacterialsulfate reduction. Intuitively, we would expect further build-up of pyrite contentsin deeper, more mature peat (soft coal in Central Europe contains 3 wt. % of S,of which over two-thirds are sulfidic S; Mach et al., 1999). However such wasnot the case, at least during the first ca. 120 years after moss burial (Figure 3bottom). Iron monosulfide S was present in both profiles in minute amounts only,with no vertical gradient (Figure 3 bottom). Iron monosulfide has been shown tobe a precursor of FeS2 which forms more slowly than FeS upon bacterial sulfatereduction (Giblin and Wieder, 1992). On the other hand, FeS2 can form quicklyand directly, by-passing the FeS phase.

5.3. δ34S PROFILES

Openness of the bog system toward buried sulfur often results in redistribution ofthe light isotope 32S and the heavy isotope 34S along the vertical profile (Nováket al., 1994). A variety of isotope-selective processes can in principle occur in peatprofiles: differential downward diffusion of either isotope along a concentrationgradient, bacterial sulfate reduction, S assimilation by plants, S mineralization(Novák et al., 2001a). From theory alone, it would be difficult to predict whichS isotope effects would be quantitatively most important. It appears that a differentmechanism of S isotope redistribution prevailed at each JH and VM: In the topmost


sections we observed a 2.3�negative δ34S shift at JH, and a 5.1�positive δ34Sshift at VM (Figure 4). Dissimilatory bacterial sulfate reduction may result in alarge fractionation of S stable isotopes (Canfield, 2001). The produced isotopicallylight S2− is either incorporated into the organic matter, forming C-bonded S, orreacts with Fe2+ forming Fe monosulfide and FeS2. Both types of products tend toremain in situ, specifically in the upper segment of the anaerobic zone. In contrast,the residual isotopically heavy sulfate S is flushed out of the zone of reduction. Theresult is simultaneous occurence of lower δ34S values and higher total (organic andpyrite) sulfur with an increasing depth, such as the one observed at JH (0–7 cm).The same near-surface pattern was described at all ten sites studied previously(Novák et al., 1994; Novák et al., 1999). The magnitude of the negative δ34S shift(2.3�) fits well in the range reported before (1.9–11.7�). In this perspective,the positive near-surface δ34S shift at VM is quite unusual. VM is also uniquein that there was a significant negative correlation between total S concentrationand δ34Stot (R = 0.79; Figures 4 bottom and 5). One mechanism that could ex-plain greater δ34S values at greater depth is mineralization of organic S, leaving34S-enriched organic residues in situ and enabling escape of 32S-enriched gaseousproducts of oxidation (Gebauer et al., 1994; Novák et al., 1994, 1996). At VM, thepositive δ34S shift is accompanied by a sizable decrease in total S concentration(mainly seen at 9–13 cm below surface; Figure 4 bottom). Such decrease in Sconcentration toward older and deeper layers would, indeed, be expected to resultfrom mineralization-related S removal. This decrease in total S, however, is notan unequivocal diagnostic feature of mineralization, since it could also be causedby lower atmospheric S inputs in earlier stages of industrialization. We concludethat JH fits well the previously suggested scenario of mid-depth enrichment intotal, C-bonded and pyrite S, together with low δ34S values, due to high rates ofsulfate reduction (Novák et al., 1994, 1999). In contrast, VM lacks these symp-toms of bacterial sulfate reduction. The more slowly accumulating peat of VMexhibits relatively high pyrite contents and possibly also isotope signs of substratemineralization (years 1900–1950 in Figure 4 bottom).

5.4. SULFUR ACCUMULATION RATES

Figure 6 gives annual S accumulation rates at both sites. JH, the site located closerto point sources of atmospheric S pollution, exhibited generally higher S accumula-tion rates. They formed a distinct maximum (44 µg S cm−2 yr−1) around the year1964. Since this is more than 20 years earlier than the time of actual maximumin atmospheric S deposition (1987), we conclude that the data in Figure 6 mustbe viewed as apparent annual S accumulation rates. The actual rates were alteredby sulfate migration to the anaerobic zone caused by a concentration gradient dueto on-going bacterial sulfate reduction. VM, the site characterized by greater peatdensities, exhibited lower S accumulation rates, with one localized maximum at23 cm (around the year 1880).


Figure 4. Sulfur isotope composition of bulk peat from JH (top) and VM (bottom).


Figure 5. Relationship between total S concentration and δ34S value for VM.

Figure 6. Apparent annual sulfur accumulation rates.


5.5. COMPARISON OF THE CZECH AND U.S. PEAT BOGS

Figure 7 depicts cumulative S content beneath the peat surface. Sites located inthe Czech Republic are marked in black, whereas sites located in the NortheasternU.S. (Minnesota, Pennsylvania, West Virginia, Maryland; Novák et al., 1994) aremarked with open symbols. Czech sites had lower cumulative S contents than theU.S. sites, despite the fact that atmospheric S deposition in the Northern CzechRepublic reached much higher values (130 kg ha−1 yr−1) than in the NortheasternU.S. (36 kg ha−1 yr−1 or less).

Figure 7 includes an additional site located in the Northern Czech Republic,Boží Dar Bog, previously studied by Novák et al. (1994). This site (located be-tween JH and VM) also showed extremely low cumulative S contents. In fact,the cumulative S contents are very similar at all Czech sites (the fourth Czechsite in Figure 7, Jezerní slat’, is situated in the pristine southern part of the CzechRepublic at a distance of 160 km from both JH and VM; atmospheric depositionof 15 kg S ha−1 yr−1). Regardless of the large range of pollution levels within theCzech Republic (15 to 130 kg S ha−1 yr−1), all Czech sites accumulate sulfur ina fashion similar to Marcell Bog located in remote pristine Northern Minnesota,(atmospheric deposition of 15 kg S ha−1 yr−1). Sofar, we have no explanation tooffer with respect to the geographical data separation in Figure 7.

Site inspection at Boží Dar Bog revealed little or no living Sphagnum on thesubstrate surface. Therefore we had hypothesized that lower-than-expected cumu-lative S contents in the Boží Dar profile could be related to discontinued peataccumulation (Novák et al., 1994). However, the two new sites, JH and VM, arecharacterized by continuing Sphagnum growth. Their lower-than-expected S accu-mulation cannot be ascribed to discontinued peat accumulation. We suggest thatthe only major uncertainty is related to the rate of spruce retreat from individualpeat bogs. As mentioned before, needles of conifers efficiently scavenge air-borneSO2, which leads to higher atmospheric S deposition under tree canopies (Fowleret al., 1989; Novák et al., 1996, 2000, 2001a, b). It follows that peat sampled in anopen area received less S than in the adjacent forests. However, spruce throughfallin upland portions of each Czech watershed did supply elevated amounts of S tothe peat coring sites. The actual input at the peat coring site (typically 10 to 30meters from the spruce stand receiving as much as 130 kg S ha−1 yr−1) is difficultto quantify directly. At the same time, it is not possible to analyze surface bog waterfor sulfate content and view such data as a proxy for local atmospheric S inputs.The reason is that both bacterial sulfate reduction and evapotranspiration changebog-water sulfate contents in a site-specific manner (see detailed study by Bottrelland Novák, 1997).

5.6. 35S INCUBATIONS

Of the total activity of 174 kBq added to the JH peat samples, 66.2 and 90.2%were recovered after 1 and 14 days, respectively. The most abundant 35S pool


Figure 7. Cumulative S content beneath the peat surface. Solid symbols – sites in the Czech Republic,open symbols – sites in the Northeastern U.S. from Novak et al. (1994).

was the residual inorganic sulfate (Figure 8). The greatest amount of 35S carriedoriginally by 35SO2−

4 was incorporated into the organic C-bonded S pool, followedby the inorganic reduced S pool (FeS + FeS2) and the ester sulfate S pool (Fig-ure 8). Such sequence is in agreement with previous studies (Behr, 1985; Brownand MacQueen, 1985; Brown, 1985; Wieder and Lang, 1988). The relationshipsbetween pool sizes, S reduction/incorporation rates and turnover times for thelonger incubation (14 days; higher 35S recovery) are summarized in Table II. Thesulfate reduction rate was 600 nmol g−1 day−1. The sulfur incorporation rate intothe C-bonded form, FeS + FeS2, and ester sulfate was 239, 0.7 and 0.3 nmol g−1

day−1, respectively. The shortest turnover time was found for inorganic sulfate (183days), followed by reduced inorganic S (FeS + FeS2; 2.6 years). Turnover times


TABLE II

Results of 14-day incubation of peat from JH spiked with Na235SO4. Means of

triplicate treatments given with standard errors

S pool Pool size S reduction rate Turnover time

(µmol g−1)a (µmol g−1 day−1) (days)

Sulfate 104 ± 37 0.6 ± 0.27 183 ± 57

S pool Pool size S incorporation rate Turnover time

(nmol g−1)b (nmol g−1 day−1) (years)

C-bonded S 13000 ± 200 239 ± 214 5.4 ± 4.1

FeS + FeS2 784 ± 163 0.7 ± 0.31 2.6 ± 0.95

Ester sulfate 595 ± 128 0.3 ± 0.22 20.7 ± 5.2

aWet mass; bdry mass.

were longer for the organic S forms (5.4 years for C-S and 20.7 years for estersulfate). Apparently, the reduced inorganic S pool at JH turns over rapidly with thereates of reduced S oxidation similar to rates of S reduction. The newly depositedSO2−

4 is rapidly converted to reduced inorganic S, and later a portion of this sulfideis incorporated into organic forms, which turn over more slowly. This result issimilar to Big Run Bog which was studied using 35S by Wieder and Lang (1988).We conclude that our 35S study at JH revealed no qualitative difference from BigRun Bog, one of the U.S. sites included in Figure 7. Big Run Bog exhibited higherlong-term S retention under lower S inputs as compared to JH. We note that we mayhave underestimated the sulfate reduction rates due to relatively long incubationtimes (i.e., days) chosen for our 35S study (cf., Fossing, 1995).

6. Conclusions

For a long time, freshwater wetlands were believed to have sulfate concentrationstoo low to initiate bacterial sulfate reduction (Nedwell, 1984). With the adventof acid rain and elevated atmospheric S deposition, S cycling in peat bogs receivedmore attention (Gorham et al., 1984). Bacterial sulfate reduction was reported frompractically all peatlands under study (reviewed by Giblin and Wieder, 1992). Anear-surface negative δ34S shift was systematically found at Sphagnum peat bogsand viewed as a diagnostic feature of bacterial sulfate reduction (Novák et al.,1994). In this study, sulfur at one site (JH) behaved in a predictable fashion, i.e.,exhibited the netative δ34S shift accompanied by increasing C-bonded and pyriteS content toward greater depth. However, the other site (VM), lacked these char-acteristics in a profile whose undisturbed character was documented by its 210Pb


Figure 8. Activity of 35S recovered from anaerobically incubated peat from JH after 1 and 14 days.Initially, 174 kBq were added in the form of Na2

35SO4 to each replicate (n = 3 per treatment).Means plus standard errors given.

activities (a disturbed profile would not be datable by 210Pb). One possible inter-pretation would be that VM lacked bacterial sulfate reduction. We are reluctant toaccept this conclusion because of the considerable amount of pyrite sulfur in theVM profile (526 ± 60 ppm; Figure 3) and the close correlation between the con-centration of total and pyrite S. In a coupled design, with nearby profiles throughaerobic forest soil and peat, Alewell and Novák (2001) showed that aerated soillacks both bacterial sulfate reduction and any traces of pyrite. The pyrite sulfurat VM must come from bacterial sulfate reduction. Such conclusion may still beconsistent with the absence of lower δ34S toward the anaerobic zone, because the


magnitude of the isotope effect is sensitive to the rate of the bacterial sulfate re-duction, which, in turn, is site-specific (�δ34S decreases with increasing reductionrate; Kaplan and Rittenberg, 1964).

Along the vertical peat profiles, the maximum S accumulation rates were dis-placed with respect to the actual atmospheric input by more than 20 years, indic-ating that bacterial activity releases aqueous S species that are mobile. One of ourmotivations to carry out the present study was the unexplained low cumulative Scontent in another peat bog in the Northern Czech Republic, Boží Dar Bog (3 timesless polluted than the U.S. sites, Figure 7; Novák et al. 1994). The two new sites, JHand VM, were as polluted as the nearby Boží Dar, and yet, the found cumulativeS contents again were extremely low compared to the U.S. sites (Figure 7). Bystudying S speciation and 35S-derived turnover times of S pools we were not able toidentify any major differences from the U.S. sites. We suggest that in industrializedareas the estimates of atmospheric S inputs into peat bogs are burdened with alarge uncertainty related to advection of sulfate-rich water from spruce stands tothe open segments of the peatlands. Total S deposition in a forest is the sum ofwet deposition, dry deposition of gases, vapors and particles, and interception ofcloud water, fog and mist droplets. Bogs receive less dry deposition and cloud/fogdroplets than forests due to their lower surface roughness. While peat cores areusually taken in open locations to avoid the presence of tree roots in the profiles,care should be taken to sample far (> hundreds of meters) from conifers whichintercept sizeable amounts of industrial SO2 in a gaseous form.

Acknowledgements

Dr. Mike Tobin of the University Pittsburgh is thanked for his help with 210Pbdating of the VM peat profile. This project was supported by the Czech GeologicalSurvey (Grant no. 3300 to MN). Later portions of this work were supported by theCzech Grant Agency (Grant no. 205/02/1060 to MN).

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sulfur metabolism in polluted sphagnum peat bogs: a combined 34s–35s–210pb study

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