Methane production and emission from peat

Methane production and emission from peat

PII: Atmospheric Environment Vol. 32, No. 19, pp. 3239—3245, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–231...

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PII:

Atmospheric Environment Vol. 32, No. 19, pp. 3239—3245, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(97)00501–3 1352—2310/98 $19.00#0.00

METHANE PRODUCTION AND EMISSION FROM PEAT: THE INFLUENCE OF ANIONS (SULPHATE, NITRATE) FROM ACID RAIN ANDREA WATSON and DAVID B. NEDWELL* Department of Biological and Chemical Sciences, University of Essex, Colchester CO4 3SQ, U.K. (First received 10 September and in final form 15 November 1997. Published July 1998) Abstract—The influence of sulphate concentrations on the production and emission of methane in two contrasting peat sites was determined. Seasonal changes in sulphate concentrations appeared to influence the amount of organic carbon oxidised to carbon dioxide by sulphate reduction at both peat sites. For the majority of the year at both sites the amount of carbon mineralised through sulphate reduction exceeded that being transformed to methane by methanogenic bacteria, except when sulphate reduction became sulphate limited. In order to sustain the high sulphate reduction rates measured in the peat sulphide formed from dissimilatory sulphate reduction must be reoxidised rapidly to sulphate within the peat. Laboratory experiments showed that addition of 500 kM sulphate and 100 kM nitrate to peat samples significantly inhibited methanogenesis. Sulphate appeared to be the more important inhibitor of methanogenesis since inhibition of methane formation occurred with additions of sulphate reflecting in situ concentrations. Supplements of either acetate and/or hydrogen in combination with molybdate to peat samples revealed that methanogenesis was hydrogen limited and that the majority of active methanogens were hydrogenutilising methanogens. Methanogenesis in peat samples appeared to be dependant on sulphate reducing bacteria for provision of substrates. Great Dun Fell, receiving the largest sulphate loading, had the lower rates of microbial activity (methane formation and sulphate reduction rates) than Ellergower, which received less than half the annual sulphate deposition of Great Dun Fell. This implied that some other factor—possibly organic matter lability, was limiting microbial rates of methane formation and sulphate reduction at Great Dun Fell. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Peatbog, methane, sulphate, nitrate, acid rain.

INTRODUCTION

Northern wetlands account for approximately 50% of the global wetland area (Fung et al., 1991) with large fluxes of methane reported to the atmosphere of between 100—200 Tg CH yr~1 (IPCC, 1992). Methane 4 is a significant greenhouse gas which has a global warming potential some 11 times that of carbon dioxide (IPCC, 1992). Increasing evidence (Wieder et al., 1991; Nedwell and Watson, 1995) suggests that in peat systems methanogenesis, once thought to be the most important final step in the mineralisation of organic matter, is influenced to a large extent by atmospheric depositions, particularly of sulphate. The interactions of sulphate-reducing bacteria and methanogenic bacteria have not been widely detailed in European peatbogs although some data exists for American wetlands (Wieder et al., 1991) and other freshwater sediments (Conrad et al., 1987; Lovely and Klug, 1983; Winfrey and Zeikus, 1977; Yavitt and Fahey,

* Author to whom correspondence should be addressed.

1993). Our work was undertaken to investigate the dynamics of methane production, oxidation and emission and to establish factors which were regulating these processes in two contrasting U.K. peatland sites. Porewater concentrations have indicated that sulphate and nitrate were the major atmospheric pollutants at Ellergower and Great Dun Fell. Sulphate, unlike nitrate, does not exert a direct inhibitory effect upon methanogenic bacteria but sulphate-reducing bacteria compete more efficiently for mutual substrates such as hydrogen and acetate (Abram and Nedwell 1978a, b; Kristjansson et al., 1982; Lovely and Klug, 1983). This competition for mutual substrates can continue to occur even at freshwater concentrations of sulphate (Lovely and Klug, 1983).

METHODS

¹he study sites The field study sites used in the present study were Ellergower Moss, in New Galloway, Dumfriesshire, Scotland (National Grid reference NX 531 790) and Great Dun Fell, in 3239

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Cumbria, England (National Grid reference NY 754 328). The topography of Ellergower peatbog is sub-divided into peat hollows and peat hummocks (Nedwell and Watson, 1995) whilst Great Dun Fell is characterised by blanket bog 2—3.5 m deep with ‘‘wet’’ areas characterised by Narthcium ossifragum, Andromeda polifolia and Eriophorum angustifolium and ‘‘drier’’ areas of peat characterised by Eriophorum vaginatum and Eriophorum angustifolium. The extremes of peat habitat (i.e. hollows and hummocks; wet and dry) were concentrated on at each site. The field study sites were selected for their extremes in sulphate loadings received through acid rain. Ellergower receives a total S load of 0.5—1.0 g S m~2 yr~1 (Downing et al., 1995) most of which is wet deposition (Martin, 1980), whilst Great Dun Fell receives '2 g S m~2 yr~1 as both wet and dry depositions. The field visits encompassed seasonal variations at the sites. Ellergower was visited in Jan—Feb 1993, May 1993, and in Jul—Aug 1993 and Great Dun Fell was visited in March 1995, September 1995 and in May 1996. Unfortunately, winter sampling at Great Dun Fell could not be undertaken due to inaccessibility through snow. Sampling Large peat monoliths (30 cm dia.]40 cm deep) were taken from each site (Nedwell and Watson, 1995) and returned to Colchester, Essex, U.K. where they were maintained in the open air in polystyrene blocks to dampen temperature fluctuations, as would be the case for in situ peat. Water tables in each monolith was kept constant by reference to the water height in an adjacent vertical cylinder connected to the peat by a siphon tube, and by adding distilled deionised water to the peat as necessary. The water tables were kept at the surface of the peat in the case of hollows and wet peat monoliths, and were kept at 17 and 30 cm depths below the surface for hummocks and dry peat areas from Ellergower and Great Dun Fell, respectively. These coincided with typical water table depths measured in situ. Field measurements For each visit a suite of measurements were taken including vertical concentration profiles of methane and oxygen, sulphide concentrations and porewater concentrations of sulphate, nitrate and ammonium (Nedwell and Watson, 1995). Rate measurements included the emission of methane, sulphate reduction rates, methane formation rates and aerobic methane oxidation potentials (Nedwell and Watson, 1995). Experimental Effect of sulphate additions on methane formation rates. Small cores (8 cm dia.]30 cm deep) were taken from triplicate monoliths taken from Ellergower and from triplicate monoliths taken from Great Dun Fell. Triplicate samples of peat were removed in a glove bag under N /CO (80/20 v/v) 2 from the most active depth horizon for2 methanogenesis (8—16 cm for Ellergower and 4—10 cm for Great Dun Fell). Samples were homogenised manually and subsamples of peat (approx. 10 g wet wt) removed and placed in 120 ml glass serum vials. Vials were stoppered with butyl bungs and gassed through with OFN at a flow rate of 15 ml min~1 for a further 10 min to ensure anaerobiosis. Any experimental additions to the peat were made in 0.5 ml degassed water with a sterile plastic syringe with disposable needle. Concentrations of sulphate in Ellergower peat samples were adjusted to 50, 100, 500 and 1000 kM, and in peat samples from Great Dun Fell were 250, 500, 1000 and 5000 kM by addition of 0.5 ml of a suitable concentration of sodium sulphate solution. These ranges of sulphate concentrations reflected those measured in situ at each of the field sites. Controls received the equivalent addition of degassed water only. Autoclaved peat controls were included but significant

methane production was never detected. Sodium molybdate solution (20 mM) final concentration was added to some treatments as an inhibitor of sulphate reduction (Oremland and Taylor, 1978; Nedwell and Banat, 1981). All samples were incubated on a roller in darkness at 25°C. This eliminated any oxygen production from photosynthesis, and maximised equilibration of the headspace gases with the peat sample. Methane concentrations in the headspace of vials were determined by taking samples (100 kl) with a pressure-lok syringe (Alltech, U.K.) and injecting them into a gas chromatograph. (Nedwell and Watson, 1995). The rate of increase of methane in the headspace of samples, usually over 72 h incubation, was calculated using linear regression analysis. Only those samples which showed a statistically significant (P(0.05) rate of increase of methane above zero were considered, otherwise the rate of methanogenesis was regarded as zero. The methane production rate per ml peat was calculated from the slope of the regression line, the volume of headspace for each sample and the wet weight of each peat sample. Effect of nitrate additions on methane formation. Peat cores (8 cm dia.]30 cm deep) were extracted from triplicate hollow monoliths from Ellergower. Subsamples (approx. 10 g wet wt) were removed as described above. Sodium nitrate solution was added to peat samples to give nitrate concentrations of 5, 10, 50 and 100 kM. (Each addition volume was 0.5 ml and control samples received only the relevant addition volume of degassed water.) Each treatment was run in triplicate and samples were rolled in darkness at 25°C. Headspace samples were taken and methane formation rates calculated as outlined above. Effect of molybdate, acetate and hydrogen additions on methane formation rates. A small core was extracted from a single peat monolith from Great Dun Fell and subsamples taken as described above (a shortage of material prevented the taking of triplicate cores from triplicate monoliths). The following treatments (four replicates of each treatment) were added to peat subsamples in 0.5 ml additions; 20 mM sodium molybdate, 20 mM sodium molybdate #20 mM sodium acetate together, and 20 mM sodium acetate only. In controls only degassed water was added. Vials were incubated as described above and headspace methane concentrations determined. After methane formation rates had been determined for all treatments, two vials per treatment were then gassed out with H /CO (80/20 v/v) and two gassed out with N /CO 2 at2a flow rate of 15 ml min~1 for 10 min and 2 vials2 (80/20 v/v) continued to be incubated as before. Methane production rates were calculated as outlined above.

RESULTS

Field results Table 1 shows seasonal changes in the amounts of carbon flow by oxidation to carbon dioxide by sulphate reduction and the amount of carbon converted to methane at Ellergower and Great Dun Fell. At Ellergower carbon flow through sulphate reduction predominated during the winter and spring months (SR to MP ratio 213 and 1000 for hollows and hummocks, respectively) but became progressively less important in summer months (SR-to-MP ratio 0.07 and 0.39 for hollows and hummocks, respectively). At Great Dun Fell, carbon flow through sulphate reduction was greatest during September (SR-to MPratio 106 and 420 for ‘‘wet’’ and ‘‘dry’’ areas, respectively) and lowest during May when carbon flow

!8.48$8.47 0$0 3.14$1.71 !12.06$12.06 !0.28$0.28 !1.28$1.28

0.002 (0.002—0.002) N.D. 1.09$0.96 (at 25°C)

0.003$0.009 0.158$0.079 0.336$0.084

0.003 0.322 5.73

0.135 (0.268—0.0025) N.D. 1.99$0.47 (at 25°C)

0.04$0.01 1.4$0.1 2.3$0.2

CH emission 4 (mmol C m~2 d~1)

0.03 3.70 66.4

CH production 4 (mmol C m~2 d~1)

Note: N.D. Not determined. () Range of duplicates shown. Mean$SE, n"3.

Ellergower hollow Jan—Feb (3.9°C) May (8.4°C) Jul—Aug (14.9°C) Ellergower hummock Jan—Feb (4.1°C) May (8.5°C) Jul—Aug (14.8°C) Great Dun Fell ‘‘wet’’ peat Sept (12.5°C) March (2.2°C) May (3.9°C) Great Dun Fell ‘‘dry’’ peat Sept (12.5°C) March (1.9°C) May (5.2°C)

Month

0.84 (0.72—0.96) 1.20$0.88 0.2 (0.048—0.39)

14.3 (3.2—25.4) 8.8$3.36 0.26 (0.068—0.45)

2.9 (1.9—4.7) 6.7 (3.9—9.5) 2.2 (1.6—2.9)

6.4 (5.4—7.4) 32.2 (21.6—42.2) 4.3 (3.9—4.8)

SO reduction 4 (mmol C m~2 d~1)

36.81 (34.83—38.79) 56.28$7.97 48.77$0.58

198.58 (254.73—142.43) 23.08$2.69 41.23$2.38

20.4$1.03 44.8$2.59 8.23$0.9

11.5$0.52 38.7$1.48 11.2$1.35

SO concn (0—30 cm) 4 (mmols m~2)

0.20

420

0.13

106

1000 20.8 0.39

213 8.7 0.07

SR-to-MP ratio (mol C)

16.8

99.8

11.6

99.1

99.9 95.4 27.7

99.5 86.7 6.1

% C via SO redn. 4

Table 1. Seasonal changes in carbon flow through methanogenesis and sulphate reduction in peat at Ellergower and Great Dun Fell. All rates are expressed in terms of mol C. SR"SO reduction, MP"CH production 4 4 Influence of anions or methane production 3241

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through methanogenesis became increasingly important (SR-to-MP ratio 0.13 and 0.20 for ‘‘wet’’ and ‘‘dry’’ areas, respectively). The seasonal fluxes of carbon through sulphate reduction appeared to be regulated largely by available sulphate concentration (as well as temperature), since high sulphate pools in the peat corresponded with high sulphate reduction rates. Carbon flow through methanogenesis appeared to be less significant at Great Dun Fell than at Ellergower, since rates of both methanogenesis and methane emissions were lower at Great Dun Fell than at Ellergower. Sulphate concentrations in peat were usually greater at Great Dun Fell, reflecting the higher loading of sulphate at this site, although sulphate reduction rates were not significantly different from those at Ellergower. This suggested that some factor other than sulphate availability may have limited sulphate reduction rates in Great Dun Fell peat and also have contributed to the lower rates of methanogenesis at Great Dun Fell. Experimental results Effect of sulphate additions on methane formation rates. The addition of 50 or 100 kM sulphate to peat samples from hollows at Ellergower did not significantly affect methane formation rates compared to controls (Anovar, P'0.05) during the period of the experiment, although the addition of 500 and 1000 kM sulphate significantly decreased methane formation rates (Anovar, P(0.05; see Fig. 1). The addition of 20 mM sodium molybdate had an inhibitory effect on methanogenesis with significantly lower

methane formation rates (0.1 nmol ml~1 peat h~1) when compared to control samples (1.0 nmol ml~1 peat h~1). All sulphate additions to peat taken from ‘‘wet’’ areas at Great Dun Fell caused a significant inhibition of methane formation (Anovar, P(0.05; see Fig. 2), causing a progressive decrease in methane formation. Methane formation rates were more than halved when 250 kM sulphate was added to peat (0.1 nmol ml~1 peat h~1 cf 0.25 nmol ml~1 peat h~1 in controls). Effect of nitrate additions on methane formation. The addition of '10 kM nitrate to peat also caused a progressive decrease in methane formation (Fig. 3). The addition of 100 kM nitrate caused a statistically significant decrease in methane formation (Anovar, P(0.05) compared to the controls, although additions of 5, 10 and 50 kM nitrate had no statistically significant effect on methane formation rates (Anovar, Tukey post hoc test, P(0.05). Effect of molybdate, acetate and hydrogen additions on methane formation rates. The addition of molybdate alone caused a significant (Anovar, P(0.05) inhibition of methane formation rates (Fig. 4) in peat taken from Great Dun Fell as was also observed in Ellergower peat samples. The addition of 20 mM sodium acetate either alone or with molybdate did not stimulate methanogenesis. Adding H /CO to the 2 2 headspace of half of the treated samples caused a stimulation in methane formation (Fig. 5) compared to samples without H . A significant stimulation 2 of methane formation occurred in all samples with

Fig. 1. (a) Effect of inhibiting sulphate reduction rates and the addition of differing sulphate concentrations on methane formation in Ellergower peat hollow samples. Mean$S.E. (n"3). (b) Effect of sulphate additions on methane formation rates in peat taken from ‘‘wet’’ areas at Great Dun Fell.

Influence of anions or methane production

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Fig. 4. Effect of addition of H /CO on methane formation 2 2 rates in peat from Great Dun Fell. Duplicate samples of each treatment are shown.

DISCUSSION

Fig. 2. Effect of nitrate additions on methane formation rates in peat hollow samples from Ellergower. Mean$S.E. (n"3).

Fig. 3. Effect of molybdate and acetate amendments on methane formation rates in peat from ‘‘wet’’ areas at Great Dun Fell. Mean$S.E. (n"4).

hydrogen, but the effect was most dramatic in the peat where molybdate and acetate were also present (Fig. 5). Most methane formation derived from H /CO but not from acetate. 2 2

Rates of sulphate reduction measured in the field show that carbon flow from organic matter in the peat to carbon dioxide via sulphate reduction vastly exceeded carbon flow via methane production during winter at Ellergower (SR-to-MP ratio 189 : 1 compared to spring when ratio was 8.7 : 1) and during autumn at Great Dun Fell (SR-to-MP ratio 106 : 1 compared to spring when ratio was 0.13 : 1). Sulphate concentrations largely determined the sulphate reduction rates at both Ellergower and Great Dun Fell. At Ellergower sulphate reduction rates were greatest in spring when sulphate concentrations were highest. Similarly, at Great Dun Fell sulphate reduction rates were highest when sulphate concentrations were high (i.e. in September for wet areas). The sulphur deposition rate in the Ellergower region is 0.5—1.0 g S m~2 yr~1 (Downing et al., 1995) and for Great Dun Fell is '2 g S m~2 yr~1 and it might be expected that sulphate reduction rates may have been higher at Great Dun Fell. However, sulphate reduction rates did not increase at Great Dun Fell in relation to the higher sulphate loadings as was expected (cf. sulphate reduction rates for Ellergower and Great Dun Fell). Some factor other than sulphate concentration appeared to limit sulphate reduction rates and other rates of microbial activity (i.e. methanogenesis) at Great Dun Fell. Organic matter ability could have limited microbial rates in Great Dun Fell peat since molar C : N ratios measured for the most active methanogenic zone were 30 : 1 compared to 26 : 1 for Ellergower and Great Dun Fell, respectively. Temperature differences between the sites could also have lowered microbial activity at Great Dun Fell compared to rates measured at Ellergower since Great Dun Fell is higher in altitude and windier than Ellergower. (The annual average temperature

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for Great Dun Fell is 6°C compared to 12°C for Ellergower.) Methane production has been shown to be inhibited by active sulphate reducing bacteria (Abram and Nedwell, 1978a, b). Consequently, when carbon flow to carbon dioxide through sulphate reduction was high it was low through methane production, and vice versa, as in other peats (Wieder et al., 1990; Sinke et al., 1992). Methane production rates were generally greater from hollows and wet areas than from hummocks and drier areas of peat from Ellergower and Great Dun Fell, respectively. Rates of methane emission clearly increased with seasonal temperature at Ellergower, but were more variable at Great Dun Fell. Seasonal cycles of methane emissions have been reported before (Crill et al., 1988; Pulliam, 1993) and represent a net flux of methane from the surface. However, a large percentage of the methane produced in the peat is oxidised prior to its emission from the peat surface (Nedwell and Watson, 1995), the fraction oxidised often increasing with temperature. In the extreme case, the dry areas of peat at Great Dun Fell usually showed a net uptake of methane from the atmosphere. This implied an active methane oxidation zone over the deeper oxic layer in this dry peat, combined with the smaller rate of methane formation from apparently relatively refractory organic matter. Experimental data corroborated the inhibitory effects of sulphate reduction on methanogenesis. Additions of 500 and 1000 kM sulphate to Ellergower peat samples (additions reflecting those sulphate concentrations measured in the field) caused a significant inhibition in methanogenesis. Similarly, methanogenesis in Great Dun Fell peat was inhibited at sulphate concentrations as low as 250 kM. Significant inhibition of methanogenesis may occur at even lower sulphate concentrations in the field, over longer time scales than was used in our experiments. Nitrate was also found to inhibit methanogenesis at concentrations of 100 kM which was generally much higher than nitrate concentrations measured in the field. At in situ concentrations (10 kM) nitrate was considered less likely to inhibit methanogenesis since methanogenesis was not significantly inhibited with additions of 10—50 kM nitrate added to peat samples. Sulphate appeared a more important regulator of methanogenesis. The addition of molybdate to both Great Dun Fell and Ellergower peat significantly inhibited methanogenesis, instead of stimulating it as might be expected by suppressing the sulphate reducing bacteria. This implied that sulphate reducing bacteria could be involved in providing substrates for methanogens. Other work has discussed the implications for hydrogen-dependant methanogenic bacteria, where the available hydrogen pool depends on the development of hydrogen-syntrophic associations between methanogenic bacteria and fermentative bacteria, and even sulphate reducing bacteria (Conrad et al., 1987). At

low sulphate concentrations sulphate reducers can act as proton reducers for H -scavenging meth2 anogens. Supplements of either acetate and/or hydrogen in combination with molybdate revealed that methanogenesis in peat samples from Great Dun Fell was hydrogen-limited, and that the majority of active methanogens were hydrogenophilic methanogens rather than acetoclastic methanogens (Indeed, the addition of acetate to the peat apparently decreased the rate of methane formation (Fig. 5) for which we can at the moment provide no explanation.) The predominance of hydrogen-utilising methanogens in natural soils and sediments has been documented before (Jones et al., 1982; Winfrey and Zeikus, 1979). The data from both Great Dun Fell and Ellergower supported the hypothesis that acid deposition reduced the emission of methane by inhibiting methanogenesis within the peat. However, other factors also influenced methane formation so that there was no simple relationship between acid deposition load and methane output at the two sites. Great Dun Fell, with significantly greater acid loads than Ellergower, did not exhibit higher sulphate reduction rates. In contrast, it exhibited slower rates in virtually all of the processes measured. This seemed to be related to a more refractory quality of organic matter in the peat at Great Dun Fell, limiting the amounts of available organic substrates in comparison to Ellergower. This was probably related to the different types of plant material contributing to the peat at the two sites. Therefore, while acid deposition loads may play an important role in regulating methane formation and emission in different wetlands, it will be only one factor. Differences in the quality and refractility of the peat organic matter may be even more important. Acknowledgements—This work was supported by a research grant (GST/02/616) to D. B. Nedwell under the TIGER programme of the Natural Environment Research Council, U.K. We acknowledge the technical assistance and support of John Green; and the collaboration of other members of our TIGER consortium.

REFERENCES

Abram, J. W. and Nedwell, D. B. (1978a) Inhibition of methanogenesis by sulphate reducing bacteria competing for transferred hydrogen. Archives of Microbiology 117, 89—92. Abram, J. W. and Nedwell, D. B. (1978b) Hydrogen as a substrate for methanogenesis and sulphate reduction in anaerobic saltmarsh sediment. Archives of Microbiology 117, 93—97. Conrad, R., Lupton, F. S. and Zeikus, J. G. (1987) Hydrogen metabolism and sulfate-dependant inhibition of methanogenesis in a eutrophic lake sediment (Lake Mendota). FEMS Microbial. Ecology 45, 107—115. Downing, C. E. H., Vincent, K. J., Campbell, G. W., Fowler, D. and Smith, R. I. (1995) Trends in wet and dry deposition of sulphur in the United Kingdom. ¼ater, Soil and Air Pollution 85, 659—664.

Influence of anions or methane production Fung, I., John, J., Lerner, J., Matthews, E., Prather, M., Steele, L. P. and Fraser, P. J. (1991) Three-dimensional model synthesis of the global methane cycle. Journal of Geophysical Research 96, 13,033—13,065. Intergovernmental Panel on Climate Change (1992) Climate Change 1992, eds J. T. Houghton, B. A. Callander and S. K. Varney, Cambridge university Press, Cambridge. Jones, G., Simon, B. M. and Gardener, S. (1982) Factors affecting methanogenesis and associated anaerobic processes in the sediments of a stratified eutrophic lake. Journal of General Microbiology 128, 1—11. Kristjansson, J. K., Scho¨nheit, P. and Thauer, R. K. (1982) Different Ks values for hydrogen of methanogenic bacteria and sulfate reducing bacteria. Archives of Microbiology 131, 278—282. Lovely, D. R. and Klug, M. A. (1983) Sulfate reducers can outcompete methanogens at freshwater sulfate concentrations. Applied Environmental Microbiology 45 (1), 187—192. Martin, A. (1980) Sulphur in the air and deposited from air and rain over Great Britain and Ireland. Environmental Pollution 1, 177—193. Nedwell, D. B. and Banat, I. M. (1981) Hydrogen as an electron donor for sulfate-reducing bacteria in slurries of salt marsh sediment. Microbial Ecology 7, 305—313.

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Nedwell, D. B. and Watson, A. (1995) CH production, 4 oxidation and emission in a U.K. ombrotrophic peat bog: influence of SO from acid rain. Soil Biology and Biochem4 istry 27 (7), 893—903. Oremland, R. S. and Taylor, B. F. (1978) Sulfate reduction and methanogenesis in marine sediments. Geochimica Cosmochimica Acta 44, 209—214. Wieder, R. K., Yavitt, J. B. and Lang, G. E. (1990) Methane production and sulfate reduction in two Appalachian peatlands. Biogeochemistry 10, 81—104. Winfrey, M. R. and Zeikus, J. G. (1977) Effect of sulfate on carbon and electron flow during microbial methanogenesis in freshwater sediments. Applied Environmental Microbiology 33, 275—281. Winfrey, M. R. and Zeikus, J. G. (1979) Anaerobic metabolism of immediate methane precursors in Lake Mendota. Applied Environmental Microbiology 37, 244—253. Yavitt, J. B. and Fahey, T. J. (1993) Production of methane and nitrous oxide in organic soils within a northern hardwood forest ecosystem. In Biogeochemistry of Global Change, ed by R. S. Oremland, pp 261—277. Academic Press, London.