sulfur pool sizes and stable isotope ratios in humex peat before and immediately after the onset of...

Environment International, Vol. 18,pp. 545-553, 1992 0160-4120/92 $5.00 +.00 Printed in the U.S.A. All fights reserved. Copyright ©1992 Pergamon Press Ltd.

SULFUR POOL SIZES AND STABLE ISOTOPE RATIOS IN HUMEX PEAT BEFORE AND IMME- DIATELY AFTER THE ONSET OF ACIDIFICATION

Mark D. Morgan Department of Biology and Division of Pinelands Research, Rutgers University Camden, NJ 08102, USA

El 9207-160 M (Received 12 July 1992; accepted 27 August 1992)

Total nonsulfate sulfur (TS), reduced inorganic sulfur (RIS), ester sulfate (ES) and carbon bonded sulfur (CBS) pool sizes, and TS and RIS stable sulfur isotopes were examined in peat cores from control and experimental sites within the Skjervatjern catchment before and 8.5 months after the onset of acidification. The total amount of excess sulfate added to the catchment up to this point was less than expected due to mechanical problems. There have been no observable effects on sulfur cycling within the peat soils. The results from this study, however, provide insight into the natural factors controlling sulfur cycling in this system and suggest that dissimilatory sulfate reduction plays a key role. For instance, the control site, which apparently receives the highest input of sulfate because of its position within the catchment drainage system, has the highest TS, RIS, and CBS pool sizes, the lowest C:S ratios, and the most negative 834S TS ratios. All of these characteristics are consistent with a dissimilatory origin for a substantial portion of the peat sulfur. Thus, a mechanism that can potentially store and neutralize at least some of the acidic sulfate to be added to this system is already in operation. The ultimate ability of dissimilatory reduction to store sulfur will be tested as the acidification of the Skjervatjern catchment proceeds and will provide important information on how other wetland ecosystems respond to excessive acidic sulfate inputs.

INTRODUCTION

Sulfur loading onto watersheds in many parts of the world has increased dramatically in fairly recent times as the result of fossil fuel combustion. Much research has been devoted to understanding the response of terrestrial and clear-water aquatic ecosystems to these excessive inputs (Irving 1991). Only recently, however, has interest been generated in fresh-water wetlands and humic-dominated (brown water) lakes and streams. Wetlands, because of their importance in regulating many biogeochemical cycles, are particularly important because they may play key roles in mediating atmospheric pollution effects on surface water systems (Gorham et al. 1984).

Sulfur may be stored in wetland soils by assimilatory (plant and bacterial nutrient uptake) or dissimilatory (bacterial respiratory sulfate reduction) processes (Gorham et al. 1984). Because assimi!atory sulfur is required in relatively small amounts relative to other limiting nutrients, the potential storage of anthropogenic sulfur in anaerobic soils is primarily regu- lated by dissimilatory reduction. The immediate end product of this process is H2S. If the sulfide is not later reoxidized within the system, two moles of H+ are consumed for each mol of sulfide produced (Kelly et al. 1982).

Dissimilatory sulfate reduction has been shown to play an important role in neutralizing acidic sulfate

545

546 M.D.Morgan

deposition in some lakes (Herlihy and Mills 1985; Rudd et al. 1986; Baker and Brezonick 1988). The importance of this process in fresh-water wetlands is less certain. Though many studies have shown that wetlands have the potential to retain large amounts of sulfate, wetlands have also been shown to release large amounts during dry seasons or droughts (Lazerte and Dillon 1984; Wieder 1985; Bayley et al. 1987). The objective of this study is to assess the ability of peaty wetland soils to permanently remove and buffer acidic sulfates from precipitation before they enter surface waters. Accomplishment of this objective is facilitated by the experimental conditions afforded by the HUMEX Project. Clear indication of permanent nonsulfate storage would be provided by an increase in the nonsulfate sulfur content in peat on the experimental side of the watershed. Unfortunately, the variability of sulfur pool sizes within sediments makes detection of a likely small, but biologically significant, change difficult. A more sensitive measure of sulfur storage can be obtained from examining changes in the stable sulfur isotope ratio.

The stable sulfur isotope ratio of a material is the amount of 345 tO 328 present in a sample relative to a standard and is expressed as ~34S in parts per thousand (0/00). Materials depleted in 34S (isotopi- tally light) have more negative ~34S numbers. The particular isotope ratio of a substance is dependent on the isotope ratio of the material from which it was made (the source) and any preferential uptake or fractionation of isotopes during the process of formation (Krouse and Tabatabai 1986). If sulfate is not limiting, dissimilatory sulfate reducers preferentially reduce 32S relative to 348 (Chambers and Trudinger 1979). As a result, the reduced products of sulfate reduction become isotopically lighter, or more negative relative to the original sulfate (Nriagu and Soon 1985). Because dissimilatory reduction is the primary biological process causing substantial fractionation of the stable isotope signature (Krouse and Tabatabai 1986), a reduction in the isotope ratio in Skjervatjern catchment peat would be clear evidence for permanent storage of the applied sulfate and consequent neutralization of the added acidity. Further, the total amount of sulfur stored can be calculated by using a simple mixing model (Krouse and Tabatabai 1986).

Normally, stable sulfur isotopes are only useful if sulfate is not limiting as described above. The HUMEX design is advantageous for detecting permanent sulfur storage because the sulfuric acid in the artificial precipitation has a distinctive isotope signature (about 0 o/oo) that is considerably more negative than sul-

fate in normal precipitation at this site. Consequent- ly, if the added sulfate is incorporated into the peat sediments, the stable isotope signature of the peat sulfur fractions will become more negative even if dissimilatory reducers do not discriminate between stable isotopes.

STUDY AREA

Details on site characteristics and the overall experimental design for the HUMEX experiment are discussed elsewhere (Gjessing 1992). The HUMEX Lake (Skjervatjern) catchment (8.9 ha) lies on granitic bedrock and is covered primarily with organic rich soils (up to 2 m). Preliminary preacidification data showed that the stable isotope ratio for total nonsulfate sulfur in peat between 0 and 30 cm was generally uniform at about 13.7 o/oo and that lake water sulfate had a ratio of 12.3. These values are close to sea-water sulfate (21 o/oo), suggesting little anthropogenic sulfur input which tends to be much more negative (Krouse and Tabatabai 1986).

Peat was sampled from one site on the nonacidified (control) portion of the watershed and two sites on the acidified portion (experimental). The control site is located in a boggy area near the shore almost perpendicular to the curtain dividing the lake. One experimental site (upper bog) is located near the watershed boundary just south of the large island. The lower bog site is located down gradient from the upper bog site near the shore. These sites correspond to Vogt's sites A (lower bog), B (upper bog), and E (control) as described by Vogt et al. (1992).

MATERIALS AND METHODS

Skjervatjern catchment peat was sampled in June 1990 before acidification and in June 1991, 8.5 months after the start of acidification (Gjessing 1992). Samples were taken with a 2.5 cm O corer at 0-5, 5-10, 15-20, 20-25, 35-40, and 50-55 cm depths below the living Sphagnum. Immediately after sampling, peat was placed in sealed polyethylene bags with as much air excluded as possible, returned to the field laboratory, resealed under argon, and frozen (-20°C). Samples remained frozen until analysis. Three cores were taken from the control and upper bog experimental site. Four cores were taken from the lower bog site on each sampling occasion. Data analysis by two and three way ANOVA (Analysis of Variance) required that one lower bog core be randomly eliminated from the analysis to balance the design.

Total nonsulfate sulfur (TS) was determined by rinsing and filtering a subsample of wet peat with a

HUMEX peat sulfur chemistry 547

16 m m o l P O 4 "3 solution. This removed all of the inorganic sulfate (Fuller et al. 1986; Wieder and Lang 1988). A portion of this peat was dried, weighed, and ashed to determine the carbon content. Another portion (20-40 mg) was dried and combusted in a Schoniger flask (Bushman et al. 1983). The liberated SO2 was trapped in 20 mL distilled water and con- verted to SO4 "2 which was then measured by ion chromatography.

Reduced inorganic sulfur (RIS) and ester sulfate (ES) sulfur fractions were determined by sequential distillation of about 5 g of fresh peat in a Johnson- Nishita apparatus (Wieder et al. 1985). RIS was determined first by the addition of reduced Cr to the reaction flask. The liberated H2S was trapped in zinc acetate and quantified by iodometric titration (APHA 1985). The peat residue following chromium reduction was rinsed thoroughly to remove any free sulfate, filtered, and treated with a hydriodic acid reducing solution in the reaction vessel to reduce ES to H2S (Spratt and Morgan 1990). The H2S was trapped in zinc acetate and quantified as above. Water content was measured by weighing wet and dried subsamples of the peat and was used to convert the RIS and ES values to quantity per unit dry weight. CBS, the only remaining sulfur fraction, was determined by difference.

The error associated with replicates of the direct measures of RIS and ES was about 20%. TS was measured to within 5%. The error associated with the CBS estimate is the sum of those for the estimation of the other fractions. Because CBS accounted for about 90% of TS, this error is likely to be closer to the error for TS.

The procedure for preparation of samples for stable sulfur isotope analysis depended on the sample type. Peat was prepared for TS isotope ratios by following the combustion procedure outlined above for the TS pool size determination. The resulting 20 mL sulfate solution was diluted to 100 mL and precipitated as BaSO4 by adding BaCI2. The RIS and ES component of peat was prepared following the distillation pro- cedures described for the sulfur pools analyses. The zinc sulfide present in the traps was centrifuged down and the clear zinc acetate decanted. The zinc sulfide was then used directly for stable sulfur isotope analysis.

The BaSO4 and ZnS were sent to Dr. Brian Fry at the Marine Biological Laboratory, Woods Hole, MA Stable Isotope Laboratory for analysis. BaSO4 was decomposed to SO2 in quartz tubes at >1600°C (Fry et al. 1982). Zinc sulfide was combusted to SO2 in quartz tubes using V205 as an oxidant. SO2

was then purified with cryogenic and vacuum techniques and analyzed with an isotope ratio mass spectrometer. Results are given in 8 notation where

834S = [(34Rsample/34Rstd) - 1] x 1000;

34R = 34S:32S and values are reported relative to standard troilite S of the Canyon Diablo meteorite.

RESULTS

Total sulfur and carbon to sulfur molar ratios at each site were significantly negatively correlated (r=-0.764, based on TS vs. Log C:S; Fig. 1). Most sediment samples (81%) were highly organic, ex- ceeding 90% C on a dry-weight basis. The correlation of TS to C:S for these highly organic sediments was even stronger (r=-0.980). These data suggest that changes in TS and C:S were primarily caused by incorporation of sulfur into the sediments rather than changes in C or simple compaction. This conclusion is supported by data on the living plant material (primarily Sphagnum) from which the peat was made which was higher in C (97%) but lower in TS (30 ~tmol/g dry weight) than any of the sediment samples.

Figure 2 shows that TS and C:S were the same in 1990 before acidification and in 1991, 8.5 months after the onset of acidification at both the control and experimental sites (P>0.05 based on individual site two-way ANOVA of depth and year). The effect of time was therefore excluded from further analysis of these data.

Table 1 shows both TS and C:S differed from site to site (P<0.002). The average TS concentration was 104.2 ~tmol/g dry weight at the control site, 95.9 at the lower bog, and 84.2 at the upper bog. C:S averaged 682 at the control site, 867 at the lower bog, and 1024 at the upper bog.

C:S was significantly affected by depth, but TS was not. This anomaly is explained by the highly significant interaction between site and depth in the C:S and TS ANOVA (Table 1). From Fig. 2 it is clear that the depth profiles of TS and C:S exhibit different patterns at each site. At the control site, TS generally increased with depth while C:S decreased. In con- trast, at the upper bog site, TS decreased while C:S increased. TS and C:S at the lower bog site changed only slightly with depth.

Pool sizes for the various fractions of TS are presented in Table 2. CBS is clearly the dominant pool, comprising over 90% of the TS. The RIS pool, though small, is about twice as large as the ES pool on most occasions. As was true for TS and C:S, each

548 M.D.Morgan

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Fig. I. Correlation of total nonsulfate sulfur (TS) (panol/g dry w¢) with the carbon ¢o sulfur ratio (C:S) in Skjervatjern catchment peat from all sites. The top panel is for all samples and the bottom panel includes only those samples where the carbon content was >90%.

Calculation of the r value was based on log transformed C:S data.

sulfur fraction failed to show any effect of time in an ANOVA. Ignoring time effects, the ANOVA showed that depth had no significant impact on the size of any of these pools (Table 1). RIS and CBS were significantly different at the different sites. The pattern of the site differences was the same as for TS, where RIS and CBS were highest at the control site, lowest at the upper bog site, and intermediate at the lower bog site.

Stable sulfur isotope ratios were determined for TS and RIS on composite samples from the control and lower bog sites only (Fig. 3). As with p size analysis discussed above, TS and RIS 534S- did notsignificantly change with time at either site. TS 534S was affected by depth at the control site where the most positive ratios tended to be found at intermediate depths (Table 3; Fig. 3). In both 1990 and 1991, the control site had significantly more ncga-


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Fig. 2. Plot of total nonsulfate sulfur (TS) (~tmol/g dry wt) and carbon to sulfur ratios (C:S) (x 100) with depth at each site over the two years of this study. Individual points represent the average of three to four cores per site.

tive TS 834S ratios than the lower bog (1990 mean 834S=13.5 control and 16.2 experimental; 1991 mean 834S=12.8 control and 15.3 experimental). The stable isotope ratio for RIS did not significantly differ between sites.

The stable isotope ratio of TS was consistently more positive than RIS (mean 634S for TS=14.3; RIS=8.9, P<0.0001). There was, however, a significant tendency for RIS and TS ~34S ratios to change in the same direction (r=0.586, P<0.01) (Fig. 3). This pattern was especially strong at the control site.

DISCUSSION

The total sulfur content of Skjervatjern catchment peat was higher than expected for an area receiving little anthropogenic sulfur in deposition. For instance, the total sulfur content of peat from wetlands in West Virginia and New Jersey, U.S.A. ranges from about 100-200 gmol/g dry wt (Wieder and Lang 1988; Spratt and Morgan 1990). These concentrations are not much higher than those observed in this study, even though the concentration of sulfate in precipitation is about four to five times greater at the U.S. sites. The high sulfur content of Skjervatjern catchment peat is probably

550 M.D.Morgan

Table I. ANOVA of the various sulfur pool sizes and the C:S ratio from Skjervatjern catchment peat based on three cores per site. Data from both years are combined for this analysis. Statistically significant F values are indicated by *

F Statistic

Source df TS C:S ES RIS CBS

Site 2 7.9*** 36.5*** 1.3 10.8"** 6.2**

Depth 5 1.4 2.9* 0.6 1.9 1.2

SxD 10 2.0* 5.3*** 1.4 1.2 1.6

*P<0.05; **P<0.01; ***P<0.001

Table 2. Peat sulfur pool sizes from Skjervatjern catchment cores over the two year study period. Values are the average from three to four cores depending on the site; and units are ~tmol/g dry wt. Samples were collected in June of each year.

Control Upper Bog Lower Bog

Depth RIS ES CBS RIS ES CBS RIS ES CBS

1990

0-5 2.2 1.5 89.2 3.2 1.0 93.4 1.9 1.5 95.6

5-10 4.4 1.7 102.8 4.4 4.2 88.5 3.7 3.2 86.4

15-20 6.7 1.5 104.6 3.9 1.9 81.7 2.2 2.0 96.7

20-25 3.6 3.2 66.6 1.1 1.4 70.7 3.2 3.8 88.0

35-40 4.5 2.0 89.8 0.6 0.2 60.6 3.5 2.7 85.4

50-55 4.4 1.5 103.3 3.6 1.2 61.5 2.7 1.0 82.2

Mean 4.3 1.9 92.1 2.8 1.6 76.1 2.9 2.4 89.9

1991

0-5 3.2 2.1 84.8 3.9 1.1 100.1 5.0 3.4 102.2

5-10 5.0 1.5 89.1 2.8 1.1 97.9 3.3 3.0 100.5

15-20 6.3 5.6 112.3 2.2 3.4 85.3 3.5 1.5 85.2

20-25 3.8 1.3 102.2 2.0 2.0 84.2 2.2 1.6 86.4

35-40 5.3 1.5 108.3 3.1 0.2 65.7 2.4 4.1 80.3

50-55 7.8 5.8 110.9 1.4 1.5 69.6 2.6 0.2 95.0

Mean 5.2 3.0 101.4 2.5 1.5 83.0 3.2 2.5 82.0

1990

0 0

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Pig. 3. Plot of total nonsulfate sulfur (TS) and reduced inorganic sulfur (R.IS) stable sulfur isotope ratios (5~'S) with depth at the con trol and lower bog sites. Data are based on a composite sample for each site.

Table 3. ANOVA of 8uS ratios from Skjervatjern catchment peat at the control and lower bog experimental sites in 1990 and 1991. Data were from pooled cores at each site, so there is no replication. Statistically significant F values are indicated by *.

F Statistic

Control Lower Bog

Year Depth Year Depth Site

1990 1991

Depth Site Depth

df 1 5 1 5 1 5 1 5

TS 6.6 20.0*** 2.8 2.6 31.3"* 3.0 95.6** 9.6*

RIS 0.8 3.8 0.1 2.5 2.4 3.6 0.0 3.0

*P<0.05; **P<O.01; ***P<O.O01

552 M.D.Morgan

related to the high rainfall (about two times greater than at the U.S. sites). Because of the high rainfall, total deposition of sulfate is relatively high (about 17 kg/ha.y; Gjessing 1991). The large mass of sulfate entering the catchment probably stimulates sulfur accumulation in the peat, much as anthropogenic deposition does at the U.S. sites (which receive about 28 kg/ha.y).

The domination of the peat total sulfur by CBS in Skjervatjern catchment appears typical of other acidic freshwater wetlands (e.g., Brown 1986; Wieder et al. 1985; Spratt and Morgan 1990). ES and RIS fractions also tend to be low at other locations.

Sea water sulfate has a stable isotope ratio of 21 o/oo (Krouse and Tabatabai 1986). Sulfur in lake sediments and wetland soils from regions impacted by anthropogenic sulfate typically have much more negative stable isotope ratios. For example, the stable isotope ratios for two Finnish lakes presently affected by excess acidic sulfur deposition showed an increase from 3 o/oo at the sediment surface to 11-14 o/oo at depth for preindustrial sediments (Kokkonen and Tolonen 1987). Similarly, the isotope ratios for peat from New Jersey wetlands impacted by sulfur deposition ranged from 5 to -20 o/oo (Morgan et al. 1992; Morgan et al. 1993). The closeness of the TS isotope ratios from Skjervatjern catchment peat (I 1 to 18 o/oo) to sea water values appears to confirm that little anthropogenic sulfur naturally enters this catchment.

After 8.5 months into acidification of the catchment, there were no demonstrable effects of the added sulfate on peat sulfur pool sizes or stable sulfur isotope ratios. Gjessing (1992) reported that for the first year of acidification at the HUMEX site, only about half of the expected sulfate was added to the catchment due to mechanical problems. Thus, the system has not yet been severely stressed and it is not surprising to find a lack of response after only 8.5 months. In fact, the relative consistency of the results from year to year when relatively little sulfate was added to the catchment indicates that natural variability in sulfur pool sizes and stable isotope ratios is fairly small. Consequently, these data suggest that as acidification continues, it should be possible to detect storage of sulfur within the peat using the methods described here if in fact it occurs.

Although there were no significant differences between years, the effect of site and depth on sulfur pool sizes and stable isotopes provides some insight into how sulfur is processed within the peat under natural conditions. The relatively high levels of TS and low C:S ratios at all sites suggest that S is being

actively incorporated into the sediments. Further, the fact that RIS and CBS were higher in the areas with higher TS suggests that the main process accounting for the higher sulfur content is dissimilatory reduction, since both RIS and CBS are endproducts of this process (Wieder and Lang 1988; Spratt and Morgan 1990). ES, on the other hand, is produced only by assimilatory processes.

Site to site differences in TS, C:S, RIS, CBS pool sizes, and stable isotopes provide additional strong evidence for the importance of dissimilatory processes in controlling peat sulfur content. The upper bog is near the high point in the drainage. As a conse- quence, available sulfate is derived only from precipitation falling directly on the site. The lower bog and control sites are low within their respective drainage systems and acquire sulfate not only from direct precipitation, but from drainage water higher up in the catchment.

None of these characteristics would matter if the peat sulfur were derived solely from accre- tion of plant-derived sulfate (assimilatory processes). On the other hand, dissimilatory reduction is often limited by sulfate, particularly in fresh water (Lovley and Klug 1986), and it is usually stimu- lated by higher sulfate. The control site then would be expected to exhibit the highest degree of dissimilatory activity, followed by the lower bog and upper bog site. Similarly, because RIS and CBS are the end products of dissimilatory activity, the largest RIS and CBS pool sizes would be expected at the control site, with decreasing levels at the lower bog and the upper bog site, if dissimilatory sulfur were being stored. This is exactly the pattern observed. Further, the only sulfur fraction not following this pattern was ES, which is not derived from dissimilatory activity.

The stable sulfur isotope ratios also suggest that dissimilatory activity is controlling peat sulfur content. Sulfate reducing bacteria preferentially take up 32S relative to 34S when sulfate is not limiting and the stable isotope signature of the end products be- comes more negative (Krouse and Tabatabai 1986). The control site apparently had higher sulfate input than the lower bog because of the drainage characteristics described above, so sulfate at the control site should be less limiting. Consequently, the most negative TS 834S ratios would be expected at control site if dissimilatory activity controlled peat sulfur content. This is exactly what was observed.

Site drainage characteristics and dissimilatory reduction probably also account for the observed effect of depth on TS pool sizes and C:S ratios. For instance,


TS was highest near the surface at the upper bog site and declined with depth. If sulfate is derived only from precipitation, dissimilatory sulfur accumulation would be greatest near the surface, where sulfate concentrations would be highest, than at depth. The control site, on the other hand, receives much of its sulfate from higher up in the catchment as subsurface flow (there is no surface flow to the lake). Thus, dissimilatory processes and storage of sulfur would be high at the surface and also at depth.

This discussion has focused on the natural processes regulating sulfur biogeochemistry of Skjervatjern catchment peat before heavy artificial acidification. The data strongly suggest that dissimilatory sulfur reduction plays a key role in controlling sulfur content and distribution in the peat. Thus, a mechanism that can potentially store and neutralize at least some of the acidic sulfate to be added to this system is already in operation. The ultimate ability of dissimilatory reduction to store sulfur will be tested as the acidification of the Skjervatjern catchment proceeds and will provide important information on how other wetland ecosystems respond to excessive acidic sulfate inputs.

Acknowledgment - - I thank L. Lynch and D. Gray for help with the sample analysis. E. Gjessing helped enormously with field work and logistical support. Funding was provided by the National Science Foundation (BSR 90 19883), the Norwegian Institute for Water Research (NIVA) and Rutgers University, Camden Provost Office. Contribution No. 92-47, Institute for Marine and Coastal Sciences.

REFERENCES

APHA (American Public Health Association). Standard methods for the examination of water and wastewater. 16th edition. American Public Health Association, Washington, D.C.; 1985.

Baker, L.A.; Brezonik, P.L. Dynamic model of in-lake alkalinity generation. Water Resour. Res. 24: 65-74; 1988.

Bayley, S.E.; Vitt, D.H.; Newbury, R.W.; Beaty, K.G.; Behr, R; Miller, C. Experimental acidification of a Sphagnum-dominated peatland: First year results. Can. J. Fish. Aquat. Sci. 44: 194- 205; 1987.

Brown, K.A. Formation of organic sulphur in anaerobic peat. Soft Biol. Biochem. 18: 131-140; 1986.

Bushman, L.M.; Dick, R.P.; Tabatabai, M.A. Determination of total sulfur and chlorine in plant materials by ion chromatography. Soil Sci. Soc. Am. J. 47: 1167-1170; 1983.

Chambers, L.A.; Trudinger, P.A. Microbiological fractionation of stable sulfur isotopes: A review and critique. Geomicrobiol. J. 1: 249-293; 1979.

Fry, B.; Scanlan, R.S.; Winters, J.K.; Parker, P.L. Sulphur uptake by saltgrass, mangroves and seagrasses in anaerobic sediments. Geochim. Cosmochim. Acta 46: 1121-1124; 1982.

Fuller, R.D.; MitcheLl, M.J.; Krouse, H.R.; Wyskowski, B.J.; Dris- coil, C.T. Stable sulfur isotope ratios as a tool for interpreting

ecosystem sulfur dynamics. Water Air Soil Pollut. 28: 163-171; 1986.

Gjessing, E. Humic lake acidification experiment: status Septem- ber 1990, one week before start of treatment. Report No. 1/91 from HUMEX. Norwegian Institute for Water Research, Oslo; 1991.

Gjessing, E. The water chemistry of Lake Skjervatjern's separated basins two years before and one year after start of treatment. HUMOR/HUMEX Newsletter (Norwegian Institute for Water Research, Oslo) 2; 1992.

Gjessing, E. The Humex Project: Experimental acidification of a catchment and its humic lake. Environ. Int. 18: 535-543; 1992.

Gorham, E.; Bayley, S.E.; Schindler, D.W. Ecological effects of acid deposition upon peatlands: a neglected field in "acid rain" research. Can. J. Fish. Aquat. Sci. 41: 1256-1268; 1984.

Herlihy, A.T.; Mills, A.L. Sulfate reduction in freshwater sediment receiving acid mine drainage. App. Environ. Microbiol. 49: 179-186; 1985.

Irving, P., ed. Acidic deposition: state of science and technology, Vnl. H and III. National Acid Precipitation Assessment Pro- gram, Washington, D.C.; 1991. Available from: Superintendent of Documents, Government Printing Office, Washington, DC (20402-9325).

Kelly, C.A.; Rudd, J.W.M.; Cook, R.B.; Schindler, D.W. The potential importance of bacterial processes in regulating rate of lake acidification. Limnol. Oceanogr. 27: 868-882; 1982.

Kokkonen, P.; Tolonen, K. Analysis of organic and inorganic sulfur and 34S-isotopes in dated sediments of forest lakes in southern Finland. Water Air Soil Polint. 35: 157-170; 1987.

Krouse, H.R.; Tabatabai, M.A. Stable sulfur isotopes. In: Sulfur in agriculture. Agronomy monogr, no. 27. Madison, WI: Soil Science Society Am.; 1986.

Lazerte, B.D.; Dillon, P.J. Relative importance of anthropogenic versus natural sources of acidity in lakes and streams in central Ontario. Can. J. Fish. Aquat. Sci. 41: 1664-1672; 1984.

Lovley, D.R.; Klug, MJ. Model for the distribution of sulfate reduction and methanogenesis in freshwater sediments. Geo- chim. Cosmochim. Acta 50: 11-18; 1986.

Morgan, M.D.; Lynch, L.; Diegmann, M.; Spratt, H. Hydrologic control of sulfur biogeochemistry in an acidic freshwater wetland. Verh. Internat. Verein. Limnol. 25; 1993. (In press)

Morgan, M.D.; Diegmann; M.H.; Spratt, H.G., Jr. Fate of anthropogenic sulfur in a cedar dominated wetland. Mitt. Internat. Verein. Limnol.; 1992. (In press)

Nriagu, J.O.; Soon, Y.K. Distribution and isotopic composition of sulfur in lake sediments of northern Ontario. Geochim. Cos- mochim. Acta 49: 823-834; 1985.

Rudd, J.M.W.; Kelly, C.A.; Furutani, A. The role of sulfate reduction in long term accumulation of organic and inorganic sulfur in lake sediments. Limnol. Oceenogr. 31: 1281-1291; 1986.

Spratt, H.G., Jr.; Morgan, M.D. Sulfur cycling over an annual cycle in a cedar dominated freshwater wetland. Limnol. Oceanogr. 35: 1586-1593; 1990.

Vest, R.D., Seip, H.M.; Rannklev, S. Soil and soilwater studies at the HUMEX site. Environ. Int. 18: 555-564; 1992.

Wieder, R.K. Peat and water chemistry at Big Run Bog, a peatland in the Appalachian mountains of West Virginia, USA. Biogeochemistry 1: 277-302; 1985.

Wieder, R.K.; Lang, G.E. Cycling of inorganic and organic sulfur in peat from Big Run Bog, West Virginia. Biogeochemistry 5: 221-242; 1988.

Wieder, R.K.; Lang, G.E.; Granus, V.A. An evaluation of wet chemical methods for quantifying sulfur fractions in freshwater wetland peat. Limnol. Oceanogr. 30: 1109-1115; 1985.