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The influence of climate on CO2 and CH4 emissions from organic soils
Both carbon dioxide and methane, major contributors to the green ruse effect, are produced by soils as the end-products of’decay of organic matter. In response t lobal temperature changes, processes may increase. creating the potential for a positive feedback mechanism. 4 emissions from two peat sites in Scotland were monitored and rejated to temperature and moisture. Bad 2 Cheo is a deep S~~u‘~9z[f9~1 bog in ‘The ow Country’, Caithness with a small area planted with Sitka spruce and Lodgepole pine. Glensau I is a hill blanket peat at the edge of the Grampian Mountains, the primary vegetation bein,0 heather with a mixture of grasses. A wetter area also contained Sphaynu9lr. CQz emission rates at both sites showed a marked seasonalpattern and para!lelled the ambient soil temperature. Values ranged from 10 mg C m-’ h - ’ in the indigenous bog area at in March to 190 mg C m - ’ h-’ in the adjoining forest area in July. At Glensaugh v dry area tended to be about NI% greater than those in the wet area while at the forest plantation were on average 90% greater than those in the indig effect of temperature was modelled using the Arrhenius equation. The resulting activation energy varied between 8 1 kJ mol - ’ for the Bad B Cheo bog area to I24 k.J mol - ’ for the Glensaugh wet area. Q,, values were estimated as 3.3 and 6.1, respectively. These seemed to be greater than many published values for mineral soils. Significant CH, emissions were on1y found for the Bad g Cheo bog area and ranged from being undetectable in January to I .OS mg C m - ’ h- ’ in September. Linear regression using the Arrhenius model showed that only 23% of the variation in CH, flux could be explained by temperature with a Q,. of 2.8. Using the responses to temperature as determined above it was possible to estimate the increase in carbon dioxide emission from these sites for any given rise in annual mean temperature (AT). For AT = 2.5”C the increase in CO, emission was 36% and 59% for the and Glensaugh wet areas, respectively. For AT = &YC, a doubling of CO, emissions was
* Correspondingauthor.Tel.: + 44-1224-3IS611. Fax: + 44-1225-311556. 0168-1923/96/$15.@O 0 1996ElsevierScienceB.V. All rights reserved SD/ 0168-1923(95)02283-X
206
S.J. Cl~cpnun, M. Thurlorc~/ Agricultud
und Frmst Meteorchgy
79 f 1996) 205-217
predicted which, when extendedover the global range of peat and organic soils, could have a significant effect on the contribution of soil carbon to atmosphericCO,. Repeating the above owever, the main effect on calculationsfor methane gave an increase60% for AT = 45°C. methanewould be due future changesto the moisturecontent. Land use changessuch as drainage and afforestationcan also have a major effect in reducing methaneemissions.
1. Introduction Both carbon dioxide and methaneare radiatively active gasesand are major contributors to the greenhortseeffect, acrounting for approximately 75% and 14% of the predicted changes in radiative forcing, respectively (Wigley and Raper, 1992). gasesare also produced by soils as end-productsof decay of organic matter. Soils, through deforestationand land use changes.contribute an estimated23% of the current total annual increasein atmosphericcarbon dioxide (Bolin, 1986) while 40-50% of the annual release of methane comes from natural and cultivated wetland soils (Tyler,
1991).Organic soils and peats constitute a large reservoir of carbon which is generally consideredto be increasing: net productivity exceeds decomposition losses (Clymo, 1983). However, in responseto global temperaturechanges,decomposition processes may increase such that these soils may become sources rather than sinks of carbon dioxide or, at least, the sink effect will be dim inished (Gorham, 1991). Methane emission from water-loggedorganic soils will also tend to increase with temperature. There exists, then, the potential for a positive feedback mechanismin the greenhouse effect (Schleser, 1981; Kohlmaier et al., 1990; Jenkinson et al., 1991; Waelbroeck, 1993). The aim of the presentproject was to monitor carbon dioxide and methaneemissions at selectedpeat sites in Scotlandand to relate theseto existing temperatureand moisture regimes.This would enablethe responseto temperatureand moisture of these processes to be characterisedand for future predictions of emissions, given a particular climate scenario, to be estimated. An important aspect of land use change for peatlands in Scotlandis afforestationwhich currently accountsfor 9.4% of the total area (Cannel1et al., 1993). This causes a drastic reduction in moisture content through the initial drainagenecessaryfor establishmentand the subsequentincreasein evapotranspiration as the plantation develops.In order to monitor the effects of afforestation management, at one site an indigenousarea was comparedwith one which had been afforested.
2.1. Sites
Two sites were used for the field measurements.The first, a Forestry Commission site at Bad ‘aChco, was situatedon deep blanket peat (5 m deep) in ‘The low Country’, CMhness, and two areas were monitored. One area consisted of the indigenous Sphagnum bog while the other had been planted with a m ixture of Picea sitchensis and
Pinus contorta in
November 1992.
nsaugh, a researc
and November 1992. Fur Table I. 2.2. Measurementof gaseous emissions e determination of C emissions were made using a system. The apparatusconsisted of a diameter 30 cm, height 8 c cm to make a gas tight se k perspex sbeet wh PVC rings were left pe anently in the field in order to disturbance caused by their insertion and were sealed at the ti perspex lid to form a chamber.A leng fixed around the top of the PVC ri effective seal between the PVC ring and the perspex lid. using Vaseline.The chamber was connected to an infra PA404 or c PM2- 13752) y silicon rubber tubing in a closed circuit system for CO, the system’satmosphereat a rate measurement.The gas analyseralso served to circu infra-red analyserwas caiibrated Before CO, flux determination of 800 ml min-‘. using standardcalibration gas mixtures, 329 and 1720 vpm CO, in N, (P tions Ltd.), and zeroed by passing the inflow through a tube of soda lime. was then sealedfor lo-20 min (until a measurableflux rate was observed)and this time CO, concentration of the chamber atmospherewas recorded Initially, two and, from April 1992 onwards, four rings were used in e were spaced about 2-5 m apart On each occasion, dete inations were made in triplicate for each ring. The temperatureof the peat at a dep of 10 cm was recorded. Peat samples were taken back to the laboratory for measurementof moisture content by drying at 115°C overnight. The degree of peat humification was determinedon the Von Post scale (Clymo, 1983).
The concentrationin the chamber typically showed an almost linear mitial increase, decreasingexponentizlly to an asymptotic value. The asymptote is assumedto be the point at which CO, flux from the soil is equalled by CO, lost from the system either throughleakagefrom the chamber itself or by diffusion back through the peat. This time courseof CO, accumulationis describedby the equation: Y=A+BXR’ where Y is CO, concentration, t is time, and A, B and R are constants derived from the analysis (A is asymptotic CO, concentration, B
3.1. Carbon dioxide emissions CO, emission rates at both sites showeda marked seasonalpattern and parallelled the ambient soil temperature(Figs. 1 and 2). At Glensaugh,values ranged from 6.8 mg C m -* h- ’ in the wet areain November to 169 mg C m-* h- ’ in the dry area in August 1991 (Fig. 1). CO, emission from the dry area tended to be, on average, 40% greater than that in the wet area though this was mainly significant in the summer and autumn. At Bad ‘aCheo emissionvalues rangedfrom 10 mg C m-* h-’ in the bog area in March to 187mgCm -* h-‘inthef orest area in July (Fig. 2). Rates in the forest area were,
Fig. I. Seasonalvariation in carbon dioxide flux from two areas, one dry and one wet, on a hill blanket peat (Glensaugh), plotted with soil temperature. The vertical error bars s triplicate analyses on two or four chambers in each area.
on average,90% greater an those in the indigenousbog area. ling occasionexcept in treedhad a noticeable il temperatureh-0 lg. 2). Taken over all
42
A
f
6 4 2
Fig. 2. Seasonalvariation in carbon dioxide flux from two areas, an indigenous bog and a forested area on the bog, on a deep blankeietpeat (Bad ZI Cheo), plotted with soil temperature. The vertical error bars show the standard error of the mean of triplicate analyses on two or four chambers in each area.
210
Xl.
Chapman,
M. Thurlow/
Apkulturul
arrd Forest Meteorology
0 Dry area 0 Wet area
79 (1996) 205-217
tltierl _- - fitted
Fig. 3. An Arrhenius plot of the carbon dioxide flux from the dry and we1 areas at the Glensaugh site. Lines fitted by linear regression, r* = 0.63 (dry) and 0.70 (wet).
were no significant differences between the four sampling rings in the wet area at Glensaughor in the forest area at Bad a Cheo. However, there was significant spatial variability in the dry area at Glensaughand in the bog area at Bad ‘a Cheo where there were factors of 1.7 and 2.1, respectively, between the respiration of the most and least active rings. The effect of temperatureon the CO, flux from the peat was modelled using the Arrhenius equation (Figs. 3 and 4). Linear regressionof tbr: logarithm of the CO, flux (LRESP) against the reciprocal of the absolute temperature(RTEMP) should give a straight line. This model showed that 57-70% of the variation in CO, flux couid be explained by temperature (Table 2). Using multiple linear regression and adding moisture (MOIS) to the above model increasedthe variation explained only slightly for the Glensaughwet area, by 20% for the Bad & Cheo foresi area but not at all for the other two areas (Table 2). (The values for LRESP vs. RTBMP for the multiple linear regressiondiffer slightly from those trying simple linear regressionbecauseof missing values in the moisture data set.) The effect of temperaturemay be quantified from the slope of the Arrhenius plot in the form of the activation energy (or temperature characteristic)which varied between 81 kJ mol- ’ for the Bad & Cheo bog area to 124 kJ mol- ’ for the Glensaughwet area(Table 3). The slopesof the two lines in each of Figs. 3 and 4 iwerenot significantly different giving combined values of 118 kJ mol- ’ and 85 kJ mol- ’ for the Glensaugh and Bad ZI Cheo sites, respectively. The temperature characteristicof the Glensaughwet area was significantly greater than that of the Bad ‘a Cheo bog area. The response to temperature may also be expressed as the more commonly used Q,, value (or temperaturecoefficient), also shown in Table 3.
Fig. 4. An Arrhenius plot of tbe carbon dioxide fhtx from the forest and indigenous bog areas at the Cheo site. Lines fitted by linear regression, r’ = 0.57 (forest) and 0.61 (bog).
Table 2 Percentageof variance in CO2 emission from organic soils in two areas at two sites accounted for by tilting two different models. LRESP, Ln respiration; RTEMP, reciprocal temperature; MOIS, soil moisture content
Glensaugh Bad z1Cheo
DV Wet Bog Forest
Linear regression
Multiple linear regressiona
LRESP vs.
LRESP vs. RTEMP b
LRESP vs.
63.2 70. I 61.4 57.2
59.5 71.0 57.0 54.4
59.0 75.5 c 56.4 75.4 c
a Data set incomplete owing to missing moisture values. b Model corresponds to the Arrhenius equation. ’ Significant improvement on including MOIS.
Table 3 Activation energies (E,, means+SE) and Qlo values for the process of soil CO, emission at two sites calculated from Arrhenius plots Site Glensaugh Glensaugh Glensaugh Bad ‘a Cheo Bad a Cheo Bad B Cheo
Area
E, (kJ mol-‘1
QIO
DV Wet Both areas Bog Forest Both areas
112f8 124+8 118f6 81+8 103 + 10 85f8
5.1 6.i 5.5 3.3 4.4 3.4
1991
1992
Fig. 5. Seasonal variation in methane flux from the indigenous bog area at Bad 8 Cheo, plotted together with soil temperature. The vertical error bars show the standard error of the mean of triplicate analyses on two or four chambers.
3.2. Methane emissions
CH, emissionswere only found at the Bad ‘a Cheo bog area and ranged from being undetectablein January 1991 to a mean of 1.05 mg C m -* h-’ in September1991 and broadly followed a seasonaltrend (Fig. 5). The apparentuptake of methane in January was not significant. Any exchangeof CH, in the forest area or from the Clensaughsite was below the lim it of detection of the presentmethod, i.e. less than 0.17 mg C m -’ h- ‘. The relationshipof CH, emissionto the prevailing temperaturewas not as close as for the CO, flux with a low CH, emissionoccurring in May when the temperaturewas highest(Fig. 5). Linear regressionusing the Arrhenius model (Fig. 6) showed that only 23% of the variation in CH, flux could be explained by temperature. Considering moisture as well as temperaturein a multiple linear regressionincreasedthe variation accounted for to 72% with the methane flux increasing with decreasing moisture content. However, the moisture content within the bog area only varied within narrow lim its (90.1-92.1% fresh weight) and was itself correlated with temperature.Thus the dependenceof CH, flux on moisture content in the bog area is not clear from thesedata. The activation energy for the process was calculated to be 70 (SE 21) kJ mol-’ with Q,()= 2.8.
4.1. Carbon dioxide emission rates
Our rates of carbon dioxide emission tend to be greater than many published values, probably due in part to differences in temperature and methodology. Rather smaller
Fig. 6. An Arrhenius plot of the methane flux from the indigenous bog area at the Bad B Cheo site. Line fitted by linear regression, rz = 0.23.
values (4.6-28.5 mg C m-’ h- ‘1 over a similar temperaturerange were fottnd for a subarctic mire by Svensson(1980) and again by Svenssonand Rosswall ( 19843.Smaher values were also found by Clymo and Moore and Knowles ( 1987) for a Canadi becauseclear c ever, similar values to ours were found by Uaviti et al. (1988) (42- 108 mg C mm2 h- ‘) and by Silvola (1986) for Finnis m-* h- ‘). Silvola (1986) also re drainage as found here when c However, it is likely that a significant fraction of the C 2 emission from under the trees was due to root respiration though it was not possible to quantify this. It has been estimatedthat a third of the C0, emission from a forest litter layer may come from live root respiration(Bowden et al., 1993). 4.2. Sensitivity of CO, emissions to tenperatwe
emission are The temperaturecoefficients (Q,,) found for e peat soils for usually in the larger than those commonly found for mineral soi and litters which range 1.5-3.0 (Drobnik, 1962;Wiant, 1967; Witkamp, 1969; Clark and Gilmour, 1983). Schleser(1981), in summarisinga number of studieson various soils, found Q,, valuer in the range 2-5 but indicated that these tended to increaseat lower temperatures.Using ,os for the laboratory incubations, we have earlier shown (unpublished data) that bog and forest peats at Bad ‘a Cheo were 3.5 and 2.9, respectively. A sample from Glensaugh showed a Q,, of 2.3. However, these and other peat samples showed a
214
S.J. Chapmarr. M. Thurlow
/ Agricultural
and Forest Mrteordoyy
79 (1996) 205-217
marked increasein Q,, at low temperatures(under 5°C). Svenss for CO, emission from peat from a subarctic m ire to vary increasing with decreasingtemperatureand increasing moisture content. At the mea moisture content and at 5°C the value was 3.4, increasingto 4.7 at 2.8”C. Similar high valueshave been found for other tundra sites (Flanaganand Veum, 1374). It is possible that this increasedtemperaturesensitivity is characteristicof acidic, highly organic soils at low temperatures.Whether this is due to the low pH per se or due to the particular m icrobial population found under these conditions is not clear. 4.3. Methane emissionrates The methaneemissionrates from the bog area at Ba B Cheo are at the lower end of the range of values summarisedby Crill et al. (1991) but neverthelesscharacteristicof high latitude ombrotrophicbog sites. Svensson(1980) found lower emission ratesfor the subarctic m ire in Sweden with mean values in the range 0.02-0.17 mg C m -’ h- ‘. Occasionally, our values were as high as 3.6 mg C m -* h- ‘. The great variation in methane emission rates over space and time has been emphasisedby I-Iarriss et al. (1985) who reported a lognormal distribution for fluxes from M innesota peatlands.A very similar range of values (0.13-0.94 mg C m -* h- ’) were previously reported for the UK by Clymo and Reddaway(197 1) while Bubier et al. (1993) recently also found a similar summer mean emission rate of 0.5 mg C m -* h-! for the lawn area of a Canadianpeat bog. 4.4. Sensitivity of CH, emissionsto temperature Few determ inationsof Q,, for methaneemission from peats have been made and thosethat have show a wide range. A. meanvalue of 1.6 may be calculatedfrom the data of W illiams and Crawford (1984) for peat profiles down to 90 cm while the data of Grill et al. (1991) for three different wetland habitats gave values of 9- 11. Svenssonand Rosswall (1984) also gave high values (5.5-8.0) but only for wet areas. Svensson (1980) earlier found no relationship between methane fluxes and temperature.Chris-. tensen(1993) concludedthat methaneemissionfrom wet sites is controlled by tempera-. ture while at dry sites it is controlled by moisture. Where both parametersare changing during the season,as in this study, it becomes difficult to determ ine the individual effects of these two abiotic variables. 4.5. CO2 emissionsin a j&w-e climate Using the responsesto temperatureas determ inedabove it was possible to estimate the increase in carbon dioxide emission from these sites for any given rise in annual mean temperature (AT). It was assumed, for simplicity, that AT applied equally throughout the year. In the absenceof detailed temperaturedata for both sites, the monthly mean 30 cm soil temperaturescorrespondingto each area (Scotland E and Scotland N) were obtained from the Meteorological Office (averagesfor 1951-1980). These were then corrected for the altitude of the sites ( - 0.6”C for Bad ‘a Cheo and
Table 4 ~re~~cte~ ratwe
emissions
temperature parentheses
AT (“0
of
scenarios.
show
carbm
dioxide
Values
senarios.
a
e percentage
amd methane cakulat
e model increase
from
@ rn-?
meam
king
gaseous
present
daily
year-
' ) PromI
the
study
sites
given
fmtmrc
te~~e~at~~~s,
emission
mtcs
to
values
Carbon dioxide GIensaugh
0 1.5 2.5 4.5
(g
from
463 597 (29) 705 (52) 981 (111)
Bad i Cheo 354 470 (32) 565 (59) 816(129)
310 372 (20) 420 (36) 534 (72)
783 986 4261 1148 (47) 1553 (98)
3.65 4.27 ( 171 4.74 (30) 5.82 (60)
fitted to a cosine f~~cti~~ to give daily mean ause someerror in affected. The predicted ual emissionsof carbondioxide and the effect of in annual mean temperatureare shown in emission varied between 20 and 32% for (the least and most tern corresponding increases respectively. This doubling of C increase,if it can be justifiably ex wouid have a significant effect on well as having a significant effect on This effect would be mitigated by which would eventually tend to takes no account of any increase effect. For these reasons,the inc generally for soils by Jenkinsone based on current and future vegetation patterns,have recently predicted an equilibrium state of increasedcarbon storagebut indicate that there would be a transient the next 50- 100 years. Kohlmaier et al. (1990), using a modelling app includesthe responseof both soil respiration and net prim suggest that Q,, values for the former are greater than for soil respiration Q,, = 2.0, the positive feedbackfrom soil CO, may be overcome by the negative feedback from CO, fertilisation, dependingon effect. However, when the Q,O is increased to 2.5 dominates in most cases. Peatlandsare likely to show increasedmicrobial respiration than gain through increa and may make a considerablecontribution to global soil c on loss (Go&am, 1991).
4.4. CH, emissions in u future climate
Repeating the above calculations for future temperaturescenariosfor methane gave emission increasesof 17% and 60% for AT = 1S”C and 4S”C, respectively (Table 4). However, these values are only tentative since the main effect on methane was due to the moisture content. Future predictions of precipitation are very uncertain and even a small decreasein water table levels can inhibit methane generation or give sufficient spacefor methanegeneratedat depth to be oxidised by methanotrophs(Corham, 1991). Burke et al. (19901, assuming a Q10= 5, predicted a much greater increase in global methane emissions: for AT = 3°C the emission more than doubled. The absence of detectablemethaneemissionsfrom the forest area at Bad 5 Cheo indicates that land use changessuch as drainage and afforestation can also have a major effect in reducing methaneemissions. This has implications for calculating the net greenhouseeffect of afforestationon peatlands(Cannel1et al., 1993).
We acknowledge the funding from the Scottish Office Agriculture and Fisheries Department.We thank Mitchell Davidson and Angela Norrie for their technical assistance. We are also grateful to the Meteorological Office, Bracknell for supplying the temperaturedata.
eferences Bolin, B.. 1986. How much CO, will remain in the atmosphere? The carbon cycle and projections for the future. In: B. Bolin. B.R. Does, J. Jager and R.A. Warrick (Editors). The Greenhouse Effect, Climate Change ‘and Ecosystems. Wiley and Sons, New York, pp. 9% 155. Bowdcn, R.D., Nadelhoffer, K.J., Boone, R.D., Melillo, J.M. and Garrison, J.B., 1993. Contributions of aboveground litter, belowground litter, and root respiration to total soil respiration in a temperature mixed hardwood forest. Can. J. For. Res., 23: 1402- 1407. Bubier, J., Costello, A., Moore, T.R., Roulct, N.T. and Savage, K., 1993. Microtopography and methane flux in boreal peatlands. Northern Ontario, Canada. Can. J. Bot., 71: 1056-1063. Burke, M.K., Houghton, R.A. and Woodwell, GM., 1990. Progress towards predicting the potential for increased emissions of CH, from wetlands as a consequence of global warming. In: A.F. Bouwman (Editor), Soils and the Greenhouse Effect. Wiley, Chichester, pp. 45 I --455. Cannell, M.G.R., Dewar, R.C. and Pyatt, D.G., 1993. Conifer plantations on drained peatlands in Britain: a net gain or loss of carbon? Forestry, 66: 353-369. Christensen, T.R., 1993. Methane emission from Arctic tundra. Biogeochemistry, 21: 117-139. Clark, M.D. and Gilmour, J.T., 1983. The effect of temperature on decomposition at optimum andstandard water contents. Soil Sci. Sot. Am. J., 47: 927-929. Clymo, R.S.. 1983. Peat. In: A.J.P. Gore (Editor), Ecosystems of the World 3r.. Mires: Swamp, Bog, Fen and Moor. Elsevier. Amsterdam, pp. 159-224. Clymo. R.S. and Reddaway, E.J.F., 1971. Productivity of Sphagnum (bog-moss) and peat accumulation. Hidrobiologia, 12: I8 I - 192. Grill, P.M., Harris, R.C. and Bartlett, K.B., 1991. Methane fluxes from terrestrial wetland environments. In: J.E. Rogers and W.B. Whitman (Editors), Microbial Production and Consumption of Greenhouse Gases:
ane, Ni1rogen ashington, DC. np. 91-109. robnil, J . 1962. effect of temperatureon soil respiration. Folia robid., 7: B32- ! 40. anagan, P.W. an urn, AK., PY73.Tbc influence of tempcrmturc llloistuhc cm dec~a~p~sit~~n ra(cs an tundra. In: A.J. olding. 0.W. Heal, S.F. adcan and P.W. Flanagan (Editors). Soil Organisms and Decomposition in Tundra. I Tundra Biome Steering Committee. Stockholm. pp. 239-278. Gorham, E., 19Y$.Northm pr ids: de in tbC Carhn CyCPSillad probable responses to climatic w;~.ming. Ecol. Appt., 1: 182-195. Harriss, R.C., Gorham. E., Sebacher,D.1.. Bartlett, K.E. and Flebbe, .A., 1985. Methane peatlands.Nature, 3 15: 652-654. Jenliinson, D.S., 1977. Studies on the decomposition of lant material in soil. V, e effects of plant cover an soil type on the loss of carbon from ‘“C labelled ryegrassdecomposingunder field conditions. J. Soit Sci.. 28: 424-434. Jenkinson. D.S., Adams, D.E. and W ild, A., 1991. Model estimates of CO, @miSSifJnS from soil in responseto global warming. Nature, 35 1: 304-306. Kohlmaier, G.H., Janecek, A. and Kindermann. J., 1990. Positive and negative feedback loops witbin the vegetation/soil system in responseto a CO2 greenhousewamiing. Iln: A.F. Bouwman (Editor), Soils and the GreenhouseEffect. W iley, Chichester, pp. 415-422. Moore, T.R. and Knowles, R.. 1987. Methane and carbon dioxide evolution from subarctic fens. Can. J. Soit Sci., 47: 77-81. Schleser, C.H., 1981. The responseof CO, evolution from soils to global temperaturechanges. Z. Natueforscbung, 37a: 287-291. Silvola, U.. 1986. Carbon dioxide dynamics in mire:. reclaimed for forestry in eastern Finland. Ann. Bot. Fenn., 23: 59-67. Smith, T.M. and Shugart, H.H., 1993. The potential responseof global terrestrial carbon storage to a climate change. Water, Air Soil Pollut., 70: 624-642. Svensson, B.H., 1980. Carbon dioxide and methane fluxes from the ombrotrophic parts of a subarctic mire. Ecol. Bull. Stockholm, 30: 235-250. Svensson.B.H. and Rosswall. T.. 1984. In situ metbane production from acid peat in plant communities with different moisture regimes in a subarctic mire. Oikos. 43: 341-350. Tyler, S.C., 1991. The global methane budget. In: J.E. Rogers and WB. Whitman (Editors), Microbial Production and Consumption of Greenhouse Gases: Mctbanc, Nitrogen Oxides, and Halomethancs. American Society for Microbiology, Washington, DC, pp. 7-38. Waelbroech. C., 1993. Climate soil processesin the presenceof permafrost--A systems nrodelhng approach. Ecol. Modelling, 69: 185225. W iant. H.V.. 1967. Influence of temperatureon the rate of soil respiration. 9. For.. 65: 4X9-490. W igley, T.M.L. and Raper, S.C.B., 1992. Implications for climate and sea level of revised BPCC emission scenarios.Nature, 357: 293-300. W illiams, R.T. and Crawford, R.L., 1984. Methane production in Minnesota peatlands. Appt. Environ. Microbial., 47: l266- I27 I. W itkamp, M., 1969. Cycles of temperatureand carbon dioxide evolution from litter and soil. Eco!ogy, SO: 922-924. Yavitt, J.B., Lang, GE. and Downey, D.M., 1988. Potential methaneproduction and methaneoxidation rates in peatland ecosystemsof the Appalachian Mountains, United States. Global Biogeochemical Cycles, 2: 256-268.
* Correspondingauthor.Tel.: + 44-1224-3IS611. Fax: + 44-1225-311556. 0168-1923/96/$15.@O 0 1996ElsevierScienceB.V. All rights reserved SD/ 0168-1923(95)02283-X
206
S.J. Cl~cpnun, M. Thurlorc~/ Agricultud
und Frmst Meteorchgy
79 f 1996) 205-217
predicted which, when extendedover the global range of peat and organic soils, could have a significant effect on the contribution of soil carbon to atmosphericCO,. Repeating the above owever, the main effect on calculationsfor methane gave an increase60% for AT = 45°C. methanewould be due future changesto the moisturecontent. Land use changessuch as drainage and afforestationcan also have a major effect in reducing methaneemissions.
1. Introduction Both carbon dioxide and methaneare radiatively active gasesand are major contributors to the greenhortseeffect, acrounting for approximately 75% and 14% of the predicted changes in radiative forcing, respectively (Wigley and Raper, 1992). gasesare also produced by soils as end-productsof decay of organic matter. Soils, through deforestationand land use changes.contribute an estimated23% of the current total annual increasein atmosphericcarbon dioxide (Bolin, 1986) while 40-50% of the annual release of methane comes from natural and cultivated wetland soils (Tyler,
1991).Organic soils and peats constitute a large reservoir of carbon which is generally consideredto be increasing: net productivity exceeds decomposition losses (Clymo, 1983). However, in responseto global temperaturechanges,decomposition processes may increase such that these soils may become sources rather than sinks of carbon dioxide or, at least, the sink effect will be dim inished (Gorham, 1991). Methane emission from water-loggedorganic soils will also tend to increase with temperature. There exists, then, the potential for a positive feedback mechanismin the greenhouse effect (Schleser, 1981; Kohlmaier et al., 1990; Jenkinson et al., 1991; Waelbroeck, 1993). The aim of the presentproject was to monitor carbon dioxide and methaneemissions at selectedpeat sites in Scotlandand to relate theseto existing temperatureand moisture regimes.This would enablethe responseto temperatureand moisture of these processes to be characterisedand for future predictions of emissions, given a particular climate scenario, to be estimated. An important aspect of land use change for peatlands in Scotlandis afforestationwhich currently accountsfor 9.4% of the total area (Cannel1et al., 1993). This causes a drastic reduction in moisture content through the initial drainagenecessaryfor establishmentand the subsequentincreasein evapotranspiration as the plantation develops.In order to monitor the effects of afforestation management, at one site an indigenousarea was comparedwith one which had been afforested.
2.1. Sites
Two sites were used for the field measurements.The first, a Forestry Commission site at Bad ‘aChco, was situatedon deep blanket peat (5 m deep) in ‘The low Country’, CMhness, and two areas were monitored. One area consisted of the indigenous Sphagnum bog while the other had been planted with a m ixture of Picea sitchensis and
Pinus contorta in
November 1992.
nsaugh, a researc
and November 1992. Fur Table I. 2.2. Measurementof gaseous emissions e determination of C emissions were made using a system. The apparatusconsisted of a diameter 30 cm, height 8 c cm to make a gas tight se k perspex sbeet wh PVC rings were left pe anently in the field in order to disturbance caused by their insertion and were sealed at the ti perspex lid to form a chamber.A leng fixed around the top of the PVC ri effective seal between the PVC ring and the perspex lid. using Vaseline.The chamber was connected to an infra PA404 or c PM2- 13752) y silicon rubber tubing in a closed circuit system for CO, the system’satmosphereat a rate measurement.The gas analyseralso served to circu infra-red analyserwas caiibrated Before CO, flux determination of 800 ml min-‘. using standardcalibration gas mixtures, 329 and 1720 vpm CO, in N, (P tions Ltd.), and zeroed by passing the inflow through a tube of soda lime. was then sealedfor lo-20 min (until a measurableflux rate was observed)and this time CO, concentration of the chamber atmospherewas recorded Initially, two and, from April 1992 onwards, four rings were used in e were spaced about 2-5 m apart On each occasion, dete inations were made in triplicate for each ring. The temperatureof the peat at a dep of 10 cm was recorded. Peat samples were taken back to the laboratory for measurementof moisture content by drying at 115°C overnight. The degree of peat humification was determinedon the Von Post scale (Clymo, 1983).
The concentrationin the chamber typically showed an almost linear mitial increase, decreasingexponentizlly to an asymptotic value. The asymptote is assumedto be the point at which CO, flux from the soil is equalled by CO, lost from the system either throughleakagefrom the chamber itself or by diffusion back through the peat. This time courseof CO, accumulationis describedby the equation: Y=A+BXR’ where Y is CO, concentration, t is time, and A, B and R are constants derived from the analysis (A is asymptotic CO, concentration, B
3.1. Carbon dioxide emissions CO, emission rates at both sites showeda marked seasonalpattern and parallelled the ambient soil temperature(Figs. 1 and 2). At Glensaugh,values ranged from 6.8 mg C m -* h- ’ in the wet areain November to 169 mg C m-* h- ’ in the dry area in August 1991 (Fig. 1). CO, emission from the dry area tended to be, on average, 40% greater than that in the wet area though this was mainly significant in the summer and autumn. At Bad ‘aCheo emissionvalues rangedfrom 10 mg C m-* h-’ in the bog area in March to 187mgCm -* h-‘inthef orest area in July (Fig. 2). Rates in the forest area were,
Fig. I. Seasonalvariation in carbon dioxide flux from two areas, one dry and one wet, on a hill blanket peat (Glensaugh), plotted with soil temperature. The vertical error bars s triplicate analyses on two or four chambers in each area.
on average,90% greater an those in the indigenousbog area. ling occasionexcept in treedhad a noticeable il temperatureh-0 lg. 2). Taken over all
42
A
f
6 4 2
Fig. 2. Seasonalvariation in carbon dioxide flux from two areas, an indigenous bog and a forested area on the bog, on a deep blankeietpeat (Bad ZI Cheo), plotted with soil temperature. The vertical error bars show the standard error of the mean of triplicate analyses on two or four chambers in each area.
210
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0 Dry area 0 Wet area
79 (1996) 205-217
tltierl _- - fitted
Fig. 3. An Arrhenius plot of the carbon dioxide flux from the dry and we1 areas at the Glensaugh site. Lines fitted by linear regression, r* = 0.63 (dry) and 0.70 (wet).
were no significant differences between the four sampling rings in the wet area at Glensaughor in the forest area at Bad a Cheo. However, there was significant spatial variability in the dry area at Glensaughand in the bog area at Bad ‘a Cheo where there were factors of 1.7 and 2.1, respectively, between the respiration of the most and least active rings. The effect of temperatureon the CO, flux from the peat was modelled using the Arrhenius equation (Figs. 3 and 4). Linear regressionof tbr: logarithm of the CO, flux (LRESP) against the reciprocal of the absolute temperature(RTEMP) should give a straight line. This model showed that 57-70% of the variation in CO, flux couid be explained by temperature (Table 2). Using multiple linear regression and adding moisture (MOIS) to the above model increasedthe variation explained only slightly for the Glensaughwet area, by 20% for the Bad & Cheo foresi area but not at all for the other two areas (Table 2). (The values for LRESP vs. RTBMP for the multiple linear regressiondiffer slightly from those trying simple linear regressionbecauseof missing values in the moisture data set.) The effect of temperaturemay be quantified from the slope of the Arrhenius plot in the form of the activation energy (or temperature characteristic)which varied between 81 kJ mol- ’ for the Bad & Cheo bog area to 124 kJ mol- ’ for the Glensaughwet area(Table 3). The slopesof the two lines in each of Figs. 3 and 4 iwerenot significantly different giving combined values of 118 kJ mol- ’ and 85 kJ mol- ’ for the Glensaugh and Bad ZI Cheo sites, respectively. The temperature characteristicof the Glensaughwet area was significantly greater than that of the Bad ‘a Cheo bog area. The response to temperature may also be expressed as the more commonly used Q,, value (or temperaturecoefficient), also shown in Table 3.
Fig. 4. An Arrhenius plot of tbe carbon dioxide fhtx from the forest and indigenous bog areas at the Cheo site. Lines fitted by linear regression, r’ = 0.57 (forest) and 0.61 (bog).
Table 2 Percentageof variance in CO2 emission from organic soils in two areas at two sites accounted for by tilting two different models. LRESP, Ln respiration; RTEMP, reciprocal temperature; MOIS, soil moisture content
Glensaugh Bad z1Cheo
DV Wet Bog Forest
Linear regression
Multiple linear regressiona
LRESP vs.
LRESP vs. RTEMP b
LRESP vs.
63.2 70. I 61.4 57.2
59.5 71.0 57.0 54.4
59.0 75.5 c 56.4 75.4 c
a Data set incomplete owing to missing moisture values. b Model corresponds to the Arrhenius equation. ’ Significant improvement on including MOIS.
Table 3 Activation energies (E,, means+SE) and Qlo values for the process of soil CO, emission at two sites calculated from Arrhenius plots Site Glensaugh Glensaugh Glensaugh Bad ‘a Cheo Bad a Cheo Bad B Cheo
Area
E, (kJ mol-‘1
QIO
DV Wet Both areas Bog Forest Both areas
112f8 124+8 118f6 81+8 103 + 10 85f8
5.1 6.i 5.5 3.3 4.4 3.4
1991
1992
Fig. 5. Seasonal variation in methane flux from the indigenous bog area at Bad 8 Cheo, plotted together with soil temperature. The vertical error bars show the standard error of the mean of triplicate analyses on two or four chambers.
3.2. Methane emissions
CH, emissionswere only found at the Bad ‘a Cheo bog area and ranged from being undetectablein January 1991 to a mean of 1.05 mg C m -* h-’ in September1991 and broadly followed a seasonaltrend (Fig. 5). The apparentuptake of methane in January was not significant. Any exchangeof CH, in the forest area or from the Clensaughsite was below the lim it of detection of the presentmethod, i.e. less than 0.17 mg C m -’ h- ‘. The relationshipof CH, emissionto the prevailing temperaturewas not as close as for the CO, flux with a low CH, emissionoccurring in May when the temperaturewas highest(Fig. 5). Linear regressionusing the Arrhenius model (Fig. 6) showed that only 23% of the variation in CH, flux could be explained by temperature. Considering moisture as well as temperaturein a multiple linear regressionincreasedthe variation accounted for to 72% with the methane flux increasing with decreasing moisture content. However, the moisture content within the bog area only varied within narrow lim its (90.1-92.1% fresh weight) and was itself correlated with temperature.Thus the dependenceof CH, flux on moisture content in the bog area is not clear from thesedata. The activation energy for the process was calculated to be 70 (SE 21) kJ mol-’ with Q,()= 2.8.
4.1. Carbon dioxide emission rates
Our rates of carbon dioxide emission tend to be greater than many published values, probably due in part to differences in temperature and methodology. Rather smaller
Fig. 6. An Arrhenius plot of the methane flux from the indigenous bog area at the Bad B Cheo site. Line fitted by linear regression, rz = 0.23.
values (4.6-28.5 mg C m-’ h- ‘1 over a similar temperaturerange were fottnd for a subarctic mire by Svensson(1980) and again by Svenssonand Rosswall ( 19843.Smaher values were also found by Clymo and Moore and Knowles ( 1987) for a Canadi becauseclear c ever, similar values to ours were found by Uaviti et al. (1988) (42- 108 mg C mm2 h- ‘) and by Silvola (1986) for Finnis m-* h- ‘). Silvola (1986) also re drainage as found here when c However, it is likely that a significant fraction of the C 2 emission from under the trees was due to root respiration though it was not possible to quantify this. It has been estimatedthat a third of the C0, emission from a forest litter layer may come from live root respiration(Bowden et al., 1993). 4.2. Sensitivity of CO, emissions to tenperatwe
emission are The temperaturecoefficients (Q,,) found for e peat soils for usually in the larger than those commonly found for mineral soi and litters which range 1.5-3.0 (Drobnik, 1962;Wiant, 1967; Witkamp, 1969; Clark and Gilmour, 1983). Schleser(1981), in summarisinga number of studieson various soils, found Q,, valuer in the range 2-5 but indicated that these tended to increaseat lower temperatures.Using ,os for the laboratory incubations, we have earlier shown (unpublished data) that bog and forest peats at Bad ‘a Cheo were 3.5 and 2.9, respectively. A sample from Glensaugh showed a Q,, of 2.3. However, these and other peat samples showed a
214
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and Forest Mrteordoyy
79 (1996) 205-217
marked increasein Q,, at low temperatures(under 5°C). Svenss for CO, emission from peat from a subarctic m ire to vary increasing with decreasingtemperatureand increasing moisture content. At the mea moisture content and at 5°C the value was 3.4, increasingto 4.7 at 2.8”C. Similar high valueshave been found for other tundra sites (Flanaganand Veum, 1374). It is possible that this increasedtemperaturesensitivity is characteristicof acidic, highly organic soils at low temperatures.Whether this is due to the low pH per se or due to the particular m icrobial population found under these conditions is not clear. 4.3. Methane emissionrates The methaneemissionrates from the bog area at Ba B Cheo are at the lower end of the range of values summarisedby Crill et al. (1991) but neverthelesscharacteristicof high latitude ombrotrophicbog sites. Svensson(1980) found lower emission ratesfor the subarctic m ire in Sweden with mean values in the range 0.02-0.17 mg C m -’ h- ‘. Occasionally, our values were as high as 3.6 mg C m -* h- ‘. The great variation in methane emission rates over space and time has been emphasisedby I-Iarriss et al. (1985) who reported a lognormal distribution for fluxes from M innesota peatlands.A very similar range of values (0.13-0.94 mg C m -* h- ’) were previously reported for the UK by Clymo and Reddaway(197 1) while Bubier et al. (1993) recently also found a similar summer mean emission rate of 0.5 mg C m -* h-! for the lawn area of a Canadianpeat bog. 4.4. Sensitivity of CH, emissionsto temperature Few determ inationsof Q,, for methaneemission from peats have been made and thosethat have show a wide range. A. meanvalue of 1.6 may be calculatedfrom the data of W illiams and Crawford (1984) for peat profiles down to 90 cm while the data of Grill et al. (1991) for three different wetland habitats gave values of 9- 11. Svenssonand Rosswall (1984) also gave high values (5.5-8.0) but only for wet areas. Svensson (1980) earlier found no relationship between methane fluxes and temperature.Chris-. tensen(1993) concludedthat methaneemissionfrom wet sites is controlled by tempera-. ture while at dry sites it is controlled by moisture. Where both parametersare changing during the season,as in this study, it becomes difficult to determ ine the individual effects of these two abiotic variables. 4.5. CO2 emissionsin a j&w-e climate Using the responsesto temperatureas determ inedabove it was possible to estimate the increase in carbon dioxide emission from these sites for any given rise in annual mean temperature (AT). It was assumed, for simplicity, that AT applied equally throughout the year. In the absenceof detailed temperaturedata for both sites, the monthly mean 30 cm soil temperaturescorrespondingto each area (Scotland E and Scotland N) were obtained from the Meteorological Office (averagesfor 1951-1980). These were then corrected for the altitude of the sites ( - 0.6”C for Bad ‘a Cheo and
Table 4 ~re~~cte~ ratwe
emissions
temperature parentheses
AT (“0
of
scenarios.
show
carbm
dioxide
Values
senarios.
a
e percentage
amd methane cakulat
e model increase
from
@ rn-?
meam
king
gaseous
present
daily
year-
' ) PromI
the
study
sites
given
fmtmrc
te~~e~at~~~s,
emission
mtcs
to
values
Carbon dioxide GIensaugh
0 1.5 2.5 4.5
(g
from
463 597 (29) 705 (52) 981 (111)
Bad i Cheo 354 470 (32) 565 (59) 816(129)
310 372 (20) 420 (36) 534 (72)
783 986 4261 1148 (47) 1553 (98)
3.65 4.27 ( 171 4.74 (30) 5.82 (60)
fitted to a cosine f~~cti~~ to give daily mean ause someerror in affected. The predicted ual emissionsof carbondioxide and the effect of in annual mean temperatureare shown in emission varied between 20 and 32% for (the least and most tern corresponding increases respectively. This doubling of C increase,if it can be justifiably ex wouid have a significant effect on well as having a significant effect on This effect would be mitigated by which would eventually tend to takes no account of any increase effect. For these reasons,the inc generally for soils by Jenkinsone based on current and future vegetation patterns,have recently predicted an equilibrium state of increasedcarbon storagebut indicate that there would be a transient the next 50- 100 years. Kohlmaier et al. (1990), using a modelling app includesthe responseof both soil respiration and net prim suggest that Q,, values for the former are greater than for soil respiration Q,, = 2.0, the positive feedbackfrom soil CO, may be overcome by the negative feedback from CO, fertilisation, dependingon effect. However, when the Q,O is increased to 2.5 dominates in most cases. Peatlandsare likely to show increasedmicrobial respiration than gain through increa and may make a considerablecontribution to global soil c on loss (Go&am, 1991).
4.4. CH, emissions in u future climate
Repeating the above calculations for future temperaturescenariosfor methane gave emission increasesof 17% and 60% for AT = 1S”C and 4S”C, respectively (Table 4). However, these values are only tentative since the main effect on methane was due to the moisture content. Future predictions of precipitation are very uncertain and even a small decreasein water table levels can inhibit methane generation or give sufficient spacefor methanegeneratedat depth to be oxidised by methanotrophs(Corham, 1991). Burke et al. (19901, assuming a Q10= 5, predicted a much greater increase in global methane emissions: for AT = 3°C the emission more than doubled. The absence of detectablemethaneemissionsfrom the forest area at Bad 5 Cheo indicates that land use changessuch as drainage and afforestation can also have a major effect in reducing methaneemissions. This has implications for calculating the net greenhouseeffect of afforestationon peatlands(Cannel1et al., 1993).
We acknowledge the funding from the Scottish Office Agriculture and Fisheries Department.We thank Mitchell Davidson and Angela Norrie for their technical assistance. We are also grateful to the Meteorological Office, Bracknell for supplying the temperaturedata.
eferences Bolin, B.. 1986. How much CO, will remain in the atmosphere? The carbon cycle and projections for the future. In: B. Bolin. B.R. Does, J. Jager and R.A. Warrick (Editors). The Greenhouse Effect, Climate Change ‘and Ecosystems. Wiley and Sons, New York, pp. 9% 155. Bowdcn, R.D., Nadelhoffer, K.J., Boone, R.D., Melillo, J.M. and Garrison, J.B., 1993. Contributions of aboveground litter, belowground litter, and root respiration to total soil respiration in a temperature mixed hardwood forest. Can. J. For. Res., 23: 1402- 1407. Bubier, J., Costello, A., Moore, T.R., Roulct, N.T. and Savage, K., 1993. Microtopography and methane flux in boreal peatlands. Northern Ontario, Canada. Can. J. Bot., 71: 1056-1063. Burke, M.K., Houghton, R.A. and Woodwell, GM., 1990. Progress towards predicting the potential for increased emissions of CH, from wetlands as a consequence of global warming. In: A.F. Bouwman (Editor), Soils and the Greenhouse Effect. Wiley, Chichester, pp. 45 I --455. Cannell, M.G.R., Dewar, R.C. and Pyatt, D.G., 1993. Conifer plantations on drained peatlands in Britain: a net gain or loss of carbon? Forestry, 66: 353-369. Christensen, T.R., 1993. Methane emission from Arctic tundra. Biogeochemistry, 21: 117-139. Clark, M.D. and Gilmour, J.T., 1983. The effect of temperature on decomposition at optimum andstandard water contents. Soil Sci. Sot. Am. J., 47: 927-929. Clymo, R.S.. 1983. Peat. In: A.J.P. Gore (Editor), Ecosystems of the World 3r.. Mires: Swamp, Bog, Fen and Moor. Elsevier. Amsterdam, pp. 159-224. Clymo. R.S. and Reddaway, E.J.F., 1971. Productivity of Sphagnum (bog-moss) and peat accumulation. Hidrobiologia, 12: I8 I - 192. Grill, P.M., Harris, R.C. and Bartlett, K.B., 1991. Methane fluxes from terrestrial wetland environments. In: J.E. Rogers and W.B. Whitman (Editors), Microbial Production and Consumption of Greenhouse Gases:
ane, Ni1rogen ashington, DC. np. 91-109. robnil, J . 1962. effect of temperatureon soil respiration. Folia robid., 7: B32- ! 40. anagan, P.W. an urn, AK., PY73.Tbc influence of tempcrmturc llloistuhc cm dec~a~p~sit~~n ra(cs an tundra. In: A.J. olding. 0.W. Heal, S.F. adcan and P.W. Flanagan (Editors). Soil Organisms and Decomposition in Tundra. I Tundra Biome Steering Committee. Stockholm. pp. 239-278. Gorham, E., 19Y$.Northm pr ids: de in tbC Carhn CyCPSillad probable responses to climatic w;~.ming. Ecol. Appt., 1: 182-195. Harriss, R.C., Gorham. E., Sebacher,D.1.. Bartlett, K.E. and Flebbe, .A., 1985. Methane peatlands.Nature, 3 15: 652-654. Jenliinson, D.S., 1977. Studies on the decomposition of lant material in soil. V, e effects of plant cover an soil type on the loss of carbon from ‘“C labelled ryegrassdecomposingunder field conditions. J. Soit Sci.. 28: 424-434. Jenkinson. D.S., Adams, D.E. and W ild, A., 1991. Model estimates of CO, @miSSifJnS from soil in responseto global warming. Nature, 35 1: 304-306. Kohlmaier, G.H., Janecek, A. and Kindermann. J., 1990. Positive and negative feedback loops witbin the vegetation/soil system in responseto a CO2 greenhousewamiing. Iln: A.F. Bouwman (Editor), Soils and the GreenhouseEffect. W iley, Chichester, pp. 415-422. Moore, T.R. and Knowles, R.. 1987. Methane and carbon dioxide evolution from subarctic fens. Can. J. Soit Sci., 47: 77-81. Schleser, C.H., 1981. The responseof CO, evolution from soils to global temperaturechanges. Z. Natueforscbung, 37a: 287-291. Silvola, U.. 1986. Carbon dioxide dynamics in mire:. reclaimed for forestry in eastern Finland. Ann. Bot. Fenn., 23: 59-67. Smith, T.M. and Shugart, H.H., 1993. The potential responseof global terrestrial carbon storage to a climate change. Water, Air Soil Pollut., 70: 624-642. Svensson, B.H., 1980. Carbon dioxide and methane fluxes from the ombrotrophic parts of a subarctic mire. Ecol. Bull. Stockholm, 30: 235-250. Svensson.B.H. and Rosswall. T.. 1984. In situ metbane production from acid peat in plant communities with different moisture regimes in a subarctic mire. Oikos. 43: 341-350. Tyler, S.C., 1991. The global methane budget. In: J.E. Rogers and WB. Whitman (Editors), Microbial Production and Consumption of Greenhouse Gases: Mctbanc, Nitrogen Oxides, and Halomethancs. American Society for Microbiology, Washington, DC, pp. 7-38. Waelbroech. C., 1993. Climate soil processesin the presenceof permafrost--A systems nrodelhng approach. Ecol. Modelling, 69: 185225. W iant. H.V.. 1967. Influence of temperatureon the rate of soil respiration. 9. For.. 65: 4X9-490. W igley, T.M.L. and Raper, S.C.B., 1992. Implications for climate and sea level of revised BPCC emission scenarios.Nature, 357: 293-300. W illiams, R.T. and Crawford, R.L., 1984. Methane production in Minnesota peatlands. Appt. Environ. Microbial., 47: l266- I27 I. W itkamp, M., 1969. Cycles of temperatureand carbon dioxide evolution from litter and soil. Eco!ogy, SO: 922-924. Yavitt, J.B., Lang, GE. and Downey, D.M., 1988. Potential methaneproduction and methaneoxidation rates in peatland ecosystemsof the Appalachian Mountains, United States. Global Biogeochemical Cycles, 2: 256-268.