Carbon Dynamics in Peat Bogs: Insights from Substrate Macromolecular Chemistry Kuder Tomasz Southern Illinois University

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9-2001 Carbon Dynamics in Peat Bogs: Insights From Substrate Macromolecular Chemistry Kuder Tomasz Southern Illinois University

Michael A. Kruge Montclair State University, [email protected]

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Published Citation Kuder, T., & Kruge, M. A. (2001). Carbon dynamics in peat bogs: Insights from substrate macromolecular chemistry. Global Biogeochemical Cycles, 15(3), 721-727. doi:https://doi.org/10.1029/2000GB001293

This Article is brought to you for free and open access by the Department of Earth and Environmental Studies at Montclair State University Digital Commons. It has been accepted for inclusion in Department of Earth and Environmental Studies Faculty Scholarship and Creative Works by an authorized administrator of Montclair State University Digital Commons. For more information, please contact [email protected]. GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 15, NO. 3, PAGES 721-727, SEPTEMBER 2001

Carbon dynamics in peat bogs: Insights from substrate macromolecular chemistry TomaszKuder • and Michael A. Kruge Departmentof Geology,Southern Illinois University,Carbondale, Illinois, USA

Abstract. The macromolecularcompositions of subfossilplants from borealSphagnum bogs and restiadbogs (New Zealand)have been studiedby pyrolysis-gaschromatography/mass spectrometryto evaluatethe extentof degradationin the anoxic zone (catotelm)of a peat bog. Degradationof vascularplant polysaccharideswas apparentonly into the upper catotelm. Sphagnumwas degradedmore slowly than vascularplants, but no cessationof degradationwas observed.The inferredrate of degradationvaried dependingon type of plant, extentof aerobic, precatotelmicdegradation, and mode of litter deposition(rooting versus at the surface). Environmentalforcing on anaerobiccarbon dynamics would potentially be largestif the hydrology was disturbedat a wet and vascularplant-rich site. Peat depositedunder a dry regime would be relativelyinert in anaerobicconditions. Although catotelmic degradation is usuallynot extensive, in somecases, if labile organicmatter is retainedin the aerobicphase (e.g., restiadbogs) a major fractionof peatis degradedin catotelm, potentially resulting in a delayedmajor export of 14C-old methane.

1. Introduction however, these studiesare necessarilylimited to severalyears of observationsand to the relativelyrapid acrotelmicprocesses. Peat About one third of global soil organicmatter (aM) is storedin stratigraphicprofiles, conversely,provide a record of cumulative peatlands[Gorham, 1991]. A low turnover rate, resulting in net degradationsince the initiationof deposition.The major benefit of aM accumulation,is attributedprimarily to watefiogging-induced suchan approachis the ability to detectslow processesoccurring anoxia in the catotelmlayer (in the senseof Ingram [1978]). The in the catotelm,perhaps the least understoodaspect of peatland primarypool of aM is severelydepleted by aerobicdegradation in OM dynamics. the acrotelmlayer (in the senseof Ingram [1978]) in the first years In the presentstudy, we use dataon macromolecularcomposition after deposition,before the permanentonset of anoxia. The rate of peat aM from severalbog siteswith variablevegetation history and extentof catotelmicdegradation are uncertain,but apparently to answer the question of whether and where does catotelmic slow [Ctymo,1983]. Characteristically,peatlands are major sources degradation take place. Interpretation of the result within the of CH4 [e.g., Dise et at., 1993] and dissolvedorganic carbon framework of peatland hydrology and carbon export will be (DaC) emissions[e.g., Schiffet at., 1998]. The informationon aM attempted.The challengeof a studyof peat chemicalcomposition dynamicscomes primarily from ca2, CH4, or Dac flux experi- is the botanical heterogeneityof the material. Analytical data on ments [e.g., Laine et at., 1996; Moore and Datva, 1997; Yavitt et bulk peat, especiallyfrom boreal bogs, must be approachedcau- at., 2000]. On the other hand, the substrateof the degradation tiouslybecause of the dominanceof botanical,rather than diage- productsis rarely defined chemically and botanically, even if netic, signaturesfor bulk Sphagnumpeat [Kuder, 2000]. Moreover, substratequality is a major factor affecting diagenesisrates [cf. rooting results in the introduction of fresh aM into older and Updegraff et at., 1995]. If chemical data are available, they alreadydegraded matrix, resultingin the "dilution" of the actual typically concernbulk peat, which imposescertain limitations on diageneticsignal. To overcomethis problem, fractions of individual interpretationsYavitt et at., 1997; Wittiamset at., 1998; Yavittet at., taxa are collectedfrom the peat matrix and analyzed separately. 2000]. The most informative results on the sourcesof methane, Sequencesof peat samples from the four sites were collected, ca2, and Dac were providedby radiocarbondating. CH4 emitted allowing temporalcoverage up to 8000 years B.P. Pyrolysis-gas fromthe bog surface is consistentlyshown to be of recent14C age, chromatography/massspectrometry (Py-GC/MS) was appliedas a indicating production at the expense of recently fixed carbon techniqueallowing a multifacetedcharacterization of macromolec- [Chantonet at., 1995; Charman et at., 1999; Schiff et at., 1998]. ular componentsin submilligramsamples of pure botanicalsub- When old substrateis degraded,the productsmay be retained fractionsof peat matrix [van Smerdijkand Boon, 1987]. within the depositor dischargedinto groundwater,thus remaining undetected[Charman et at., 1993]. An alternativeapproach to peatlandcarbon dynamicsis mon- 2. Methods itoring of temporaltrends of substratedegradation. This approach is widely used in litterbag experiments [e.g., Ctymo, 1983]; 2.1. Locations: Environmental Setting and Sampling Raw data for this papercome from previouslypublished studies 1Nowat Schoolof Geologyand Geophysics, University of Oklahoma, of a Sphagnumbog from Canada,a Sphagnumbog from Poland, Norman, Oklahoma, USA. and two restiadbogs from New Zealand. The Canadiansite ("Carling Lake" bog) is a part of a systemof Copyright2001 by the AmericanGeophysical Union. peatlandsdeveloped in the humid and cold climate of the Hudson Papernumber 2000GB001293. BayLowland (HBL) region(northern Ontario, 51 ø32•N, 81 ø36•W). 0886-6236/01/2000GB001293 $12.00 Thethickness of peatwas 265 cm,and •4C age was 4110 4- 70

721 722 KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS years B.P. at the base [Klinger et al., 1994]. Severallakes occur The samplesfrom the Polish site were also extractedwith a D- next to or within the bog. The topographyof the HBL is flat, with a barrelcorer. Subsamples of •4 cm3 weretaken from a continuous seriesof parallel beach ridges and swales.They representancient seriesof intervalswith distinctvisual humificationor composition coastlinesof a sea retreating because of postglacial isostatic (2-5 cm per intervals with distinct color and fibrosity). The rebound.Peat accumulatedbetween ridges, because of impediment maximum depth of samplingwas •110 cm (a few centimeters of drainageon marine silts and clays and favorableclimate during shortof the bedrock).Plant macrofossilswere handpickedunder a the growing season(May-September). The mean annualtemper- binocular microscopefrom the selectedsubsamples, frozen, and atureis •0øC, growingseason temperature averages 11.3øC, and vacuumdried to preventfurther decomposition. Also, freshtissues mean growing seasonprecipitation is 80.3 mm. Peat depositional of the analyzedtaxa were collectedand preserved.The botanical environmentson the HBL rangefrom saltwatermarshes to fens to composition of peat in Canadian and Polish locations was bogs,a gradientobserved moving inlandfrom the modem shoreof describedto identify the details of stratigraphy.In the caseof the HudsonBay. Accordingly,it is observedthat in bog sitesthe basal New Zealandsite the physicalproperties were usedto differentiate peat was depositedunder the fen regime.Present-day vegetation is peat layers,because of more uniformbotanical characteristics than characteristicof ombrotrophicconditions, consisting of stunted observedin the boreal bogs. bog forests(Picea mariana, Larix laricina) and Sphagnumlawns with a numberof Ericaceae(including species of Andromedaand 2.2. Analytical Procedures Iraccinium).In the stratigraphicrecord few intervalswith abundant All chemical data were obtainedby pyrolysis-gaschromatog- sedges,probably Carex, occur. High amountsof occludedgas raphy/massspectrometry (Py-GC/MS) of peat macrofossils(Can- (CO2 and methane?)occurred below a depth of 200 cm. At the ada and Polandsites) and peat matrix (New Zealand). Selectionof moment of samplingthe water table was between 18 and 35 cm plant typesand organswas madeto accountfor the volumetrically below the surface,apparently representing an exceptionallydry dominantones, including Sphagnum,Ericaceae roots, and sedge season[Klinger et al., 1994] andprobably indicating the maximum fibers (the former two virtually dominatingthe studiedboreal bog depthof aerobicprocesses. profiles). Additionally,a surface-depositedfraction that does not The Polish site ("Rownia pod Sniezka" bog) is a small alpine contribute significantly to bulk peat was analyzed, to allow bog (•300 m diameter)situated within a flat water divide of the detectionof diageneticfeatures not apparentotherwise. Conditions main range of the KarkonoszeMountains at an elevationof 1400 of pyrolysis,gas chromatographicprograms, and massspectrom- m. Annual precipitationis in excessof 1500 mm and is supple- eter settingsare given elsewhere[Kuder and Kruge, 1998; Kuder mentedby dew deposition.The freeze-freeseason lasts only •3 et al., 1998; Kuder, 2000]. Py-GC/MS is a degradativetechnique months. Initiation of peat deposition •3500 years B.P. was for characterizationof (primarily) macromolecular/polymeric attributedto a cold and humid climate and a spring contributing organic matter. A pulse of high temperaturein the inert atmos- to waterlogging[Tolpa, 1985]. The peat layer is •100-150 cm phere (Helium carriergas) destroyschemical bonds of a polymer, thick, overlayinggranitic bedrock with pocketsof regolite.Vege- producingGC-amenable components, which are identified,quan- tation consistsof wet Sphagnumlawns with sedges(Trichophorum tified with a MS, and classifiedaccording to the parent biopol- caespitosum,Carex paucifiora, less abundantEriophorum) and ymer. The pyrolytic temperature(here 600øC) is optimizedfor relativelydry hummockswith mosses,Ericaceae, and Eriophorum maximumyield of volatile productsbut is low enoughto prevent vaginatum.A numberof the driesthummocks support dense stands too much damageto the releasedmonomers, rendering them less of Pinus mugo. diagnostic of the parent material (e.g., loss of side chains or The two New Zealand bogs, Kopouatai and Moanatuatua,are functional groups from aromatic ring of lignin monomers).It located on lowlands of North Island. They are isolated bog shouldbe clearly statedthat interpretationof Py-GC/MS data is complexes,of •10,000 ha each and significantpeat thickness basedon relative abundancesof given classesof compounds,not (over 8 m). Peat formationwas attributedto drainageimpediment on absoluteabundances. Summed abundancesof all compounds on clayeysoil, beginning• 11,000-13,000 B.P. [Campbell,1983]. characteristicof a given polymer give the estimateof the abun- Moanatuatuabog has been recently drained for farming. Present dance of the polymer in the analyzed sample(e.g., methoxyphe- vegetationin both Kopouatai and Moanatuatuais dominatedby nols are indicative of lignin). The pyrolytic yields may differ restiads,especially Empodisma minus. Other plantsinclude restiad betweenpolymer types, so that a pyrolyzate-basedestimate of a Sporadanthustraversii, sedges(Baumea, Schoenus),Leptosper- ratio of polymer x to polymer y is not necessarilythe stoichio- mum scoparium (woody angiosperm), and pteridophytes and metric ratio. However, for a seriesof samplesa trend in pyrolytic mosses[Newnham et al., 1995]. Sphagnumis presentas a minor ratio parallelsthat of the samplecomposition. An example of a component.The climate of the New Zealand sitesis warmer and chromatogramwith identified compoundsis given in Figure 1. drier than in the caseof the borealbogs, the mean annualtemper- Details of identificationof compositionof pyrolyzates,including atureis 13.5øC,there are only 46 daysper yearwith frost,and the lists of identifiedcompounds used in quantification,are given in precipitationis 1100-1300 mm. Physiologicaland chemicalprop- the original references. ertiesof the monocotE. minusare consideredcrucial to deposition of thick peat depositsin otherwiseunfavorable conditions [Agnew 2.3. Criteria for Determination of the Degradation et al., 1993; Kuder et al., 1998]. Parameters Details of sample collection and botanical characterizationof peat are given elsewhere[Kuder and Kruge, 1998; Kuder et al., While the researchdesign applied here doesnot accountfor the 1998; Kuder, 2000]. In brief, the Canadian peat sampleswere total of peat OM, it allowsmore confidentvisualizing of diagenetic collectedfrom a fresh pit as blocks 10-20 cm thick (upper 225 trendsthan results from bulk peat. Qualitativeextrapolation to total cm), and an additional 100 cm of peat and 10 cm of underlying peat is justified by the overall similarityof macromoleculartissue mineralsoil were extractedwith a pistoncorer from 160 to 270 cm. components(lignin, cellulose, etc.). Even with the taxonomic, Before chemicaland botanicalanalyses, blocks/core sections were anatomical,and physiologicvariability of plant tissue and the sawn into slices 2-4 cm thick. At the New Zealand sites, cores of degrading microorganisms,the plant remains are nevertheless •8 m length were extracted with a 6 cm diameter D-barrel accessibleto any degraderpossessing an appropriateenzymatic (Russian)corer. Visual characterizationof macroscopichumifica- apparatus.While the most accessibletissues are degradedprefer- tion was made after collecting the cores, and subsamplesfrom entially,no tissueremains unaffected, since different groups of soil layersof distinctcharacteristics were takenfor more detailedstudy. microorganismsare adapted to utilize various types of tissues KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS 723 724 KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS

A) The changesof vascularplant pyrolyzate compositionthat are indicative of OM loss are (1) relative decreasein abundanceof 20 polysaccharide-derivedversus lignin-derived compounds, (2) rel- •30 • ative depletionof hemicellulose-derivedversus total polysacchar- •0 ide compounds,(3) increaseof abundanceof side-chain-oxidized 0 I I I methoxyphenols(referred to in the followingparagraphs as "lignin oxidationindex"), accompaniedby a decreaseof propenyl-side- chainmethoxyphenols, and (4) relativedecrease in the dimethoxy/ monomethoxyphenolratio (S/G). The two formerindices represent

0.8 polysaccharidedegradation, and the two latter representlignin degradation.The term "lignin" as applied here pertainsto the 0.41.2• - methoxyphenyl-propanoidpolymer (in the sense of Lewis and Yamamoto[1990]). Anaerobic lossesof lignin recordedby some o i i i authorscan be explainedby their imprecisedefinition of "lignin," c) encompassinga wider rangeof phenolicsubstances. As true lignin 0.8 is degradedexclusively in the oxic zone and remains inert in anoxic conditions[Young and Frazer, 1987], the extent of lignin ..• 0.6 alterationscan be used as a proxy for the cumulativeexposure to • 0.4 aerobic degradation.Polysaccharides (PS) are also degradedin anaerobic conditions, and PS versus lignin-derived aromatics • 0.2 depletion has been observed in the catotelm [Kuder, 2000]. 0 i i i ml Compositionof Sphagnumpyrolyzates is not affectedby the extent of tissue degradation;instead, a visual estimationof cell wall damagevia scanningelectron microscopy (SEM) canbe employed. By studyinga sequenceof samples,representing age/depth incre- ments, it is possibleto reconstructa trend of PS depletion.The degradationtrend indicatesan anaerobicprocess if two conditions are satisfied:(1) the trendis apparentbelow a reasonableestimate of the acrotelmlower limit and (2) the trend is not correlatedwith 2 4 6 8 any aerobiclignin degradationindex, which would suggestinstead depth(m) a residual effect of a change in peat surface conditions,not of anaerobicdecay. Lignin, which is not affectedin anoxic environ- Figure 2. Degradationtrends of bulk restiadpeat, New Zealand. ments,is used as a "standard,"against which a depletionof PS is Squaresindicate the Moanatuatuabog, and diamondsindicate the measured. Kopouatai bog. (a) Lignin oxidation index: % Ca acetylatedto total methoxyphenolsratio (see text for explanations).Vinyl 3. Results moieties are not included becauseof a possible affiliation with cinnamicacid fraction,not lignin. (b) Polysaccharideto aromatics 3.1. Restiad Bogs ratio. (c) Anhydrogalactosanto total polysaccharideratio. (d and e) Stratigraphicprofiles of Kopouatai and Moanatuatuabogs (dark Mosses and vascularplants have significantlydifferent diage- tone, sapricpeat; medium tone, hemic peat; light tone, fibtic peat). netic propertiesso that the chemical signatureof bulk peat is predominantlyaffected by botanical content, not the extent of degradation[Kuder, 2000]. At the two studied restiad bogs the dominantpeat componentis subaerialroots of monocotyledon Empodisma minus. Whereas boreal bogs contain significant amountsof both Sphagnumand vascularplants, at the studied [Sylvia, 1998]. In the caseof peaty soil the nominal availability of New Zealand sitesSphagnum is presentbut subordinate.For this substrateis overshadowedby the electronacceptor limitations, as reason,it is justifiedto usebulk peat chemicalcomposition without evidencedby actualpreservation of vastamounts of organicmatter. introducinga bias. The two sites differ in the extent of aerobic In anoxic conditionsthe degradationefficiency decreases,and degradation(Figure 2a), estimatedusing the lignin oxidationindex substratebioavailability becomes a dominantlimitation to micro- [cf. Kuder and Kruge, 1998]. A relative decreasein PS contentin bial metabolism.Experiments with glucoseamendment (providing pyrolyzateswith depthhas been observed at both sites(Figure 2b). a readily availablesource of carbon)show that in anoxicconditions Kopouataibog peat retainedmore PS after the acrotelmicdegra- degradationproceeds with maximum possibleefficiency [Bridg- dation,but the ratios of PS to lignin at the two sitesconverged at ham and Richardson,1992; Bergmanet al., 1998, 2000]. Another depthsbetween 3 and 5 m, remainingstable in deeperpeats. Only study showsthat glucoseamendment has no effect in lignin-poor the basal peat (seen at Moanatuatua) was more extensively peat, presumablystill preservingenough labile polysaccharides degraded,showing abnormal dehydroxylation of aromatics,deple- [Yavitt et al., 1997]. Glucosedid not acceleratethe decay rate of tion of hemicellulose,and generationof furan structuresat the peat OM, essentiallyprecluding a chanceof synergisticdiagenetic expenseof PS. A major part of PS depletionat Kopouataiwas effectsbetween various tissue types [Aertsand Toet, 1997]. There- achievedby removalof galactosan-bearinghemicellulose, which is fore, if a temporal accelerationof degradationis detectedin the only a minor component of Moanatuatua peats (Figure 2c). stratigraphicrecord of onetaxon, it is reasonableto expectthat the Pyrolysis of fresh peat-forming plants has shown that the fine other plant remains were similarly exposed to more vigorous rootlets of Empodisma minus are the source of galactosanin degradingactivity, even if the actualextent of the degradationof undegradedpeat [Kuderet al., 1998]. The moreextensive exposure a given tissuedoes depend on its specificchemistry and structure. of Moanatuatuapeat to aerobicdegradation was probably the cause This expectationis supportedby similar directionof degradation of the absenceof major amounts of pristine hemicellulose-rich trends of various botanic units from the same site [Kuder, 2000]. rootletsin peat matrix. KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS 725

40 3.2. Sphagnum Bogs .• 30 Variable content of Sphagnumand vascular plants in boreal .•...• bogs makes it impossible to use bulk peat data. Individual •g20 macrofossilswere analyzed,Sphagnum, Ericaceae roots, conifer needles and sedge shoot fibers. In the case of the Canadian ß• 10 samples,depletion of PS was apparent,but only in the upper 100 0 I I I I I I cm of the profile (Figure 3). In older samplesthe PS contentwas stable. In fact, the depth at which PS depletionwas no longer visible differedbetween individual species and organs:greater in ..o 16 the case of roots (Ericaceae) and lesser for surface-deposited conifer needles.No preferentialdegradation of hemicellulosein = 12 the catotelm was observed. Of the macrofossils from the site in '[ 8

Poland,only sedgevascular fibers were analyzedand were found m 4 to be chemicallyinert in the catotelm.Similarly, as in the restiad bog, basal samplesboth from Canada [Kuder, 2000] and Poland Kuder and Kruge, 1998] have shownevidence of more extensive ½) degradation:dehydroxylation of aromatics,depletion of hemicellulose,and a slight increaseof abundanceof furan structures 0 100 200 30b in pyrolyzates.Conifer needles,which are depositedat the bog depth(cm) surface,and sedgefibers depositedat or closeto the peat surface (and thus most affectedby water table fluctuationsand associated Figure 3. Degradationtrends of vascularplant peatsat Carling changesof biodegradation)were analyzedto determinethe lignin Lake bog, Ontario, Canada. (a) Conifer needles(diamonds and oxidation index. Fluctuationsof lignin oxidation index at both squaresindicate tamarack and black spruce,respectively). Lignin sites suggestedthat the water tables and extent of aerobic oxidation index (percentageof Co• acetylatedto total methox- exposure varied. PS content in macrofossils from Canada yphenolsratio; see text for explanation).(b) Ericaceaeroots and respondedto bog water table fluctuations(Figure 3). In the ratio of polysaccharideto methoxyphenols(lignin) ratio. Methox- upper half of the profile, in the zone where excessPS has not yphenols, not total aromatics, are used to avoid bias due to yet been removedby anaerobes,low lignin oxidationcould be degradationof labile tannins. (c) Stratigraphicprofile. (Dark correlatedwith high PS content and vice versa. Sphagnumdid shading indicates Sphagnum/Ericaceaepeat, medium shading not show any chemical features indicative of the extent of indicatesmoss/sedge peat, and light shadingindicates Sphagnum degradation,but SEM of stem cell walls showedthat damage peat; in the lower part of the profile, sedgeis accompaniedby increasedwith depth and also fluctuated as a function of the Drepanocladusrather than Sphagnum.) degreeof aerobicexposure, similar to the PS contentof vascular plants [Kuder, 2000]. efficient enzymaticdegradation in the acrotelmwould generate DOC relatively enrichedin aromatic/aliphaticmoieties, which is less energeticallyattractive (the statementis speculative,as no 4. Discussion relevant molecular data on bog dissolvedorganic matter are 4.1. Sources of Pore Water Gases and Dissolved available). The main implicationof different compositionsfor Organic Matter DOC generatedin the catotelmand acrotelmwould be that mass balancecalculations, e.g., basedon radiocarbonage, would not The environmentalsetting of peatbogs (anoxia and oligotrophy) necessarilyreflect bioavailability. The DOC fractionflushed down constrainsmicrobial activities by limiting aerobicrespiration to the from the acrotelm,relatively enriched in recalcitrantcompounds, acrotelm. In anoxic conditions, fermentative bacteria are able to would be metabolizedless efficiently than DOC generatedin situ. mineralizemacromolecular materials, such as cellulose[Deming Additionally,DOC would be radiometricallyyounger than dis- and Baross,1993]. For the sakeof thermodynamicefficiency, free solved gas. Such age discrepancyhas been noticed [Charman et hydrogen(a side product of fermentation)needs to be removed al., 1999] but hypotheticallyexplained by more efficient down- from the system.In the semistagnantpore watersof a peat deposit, ward transportof DOC than CH4 or CO2. in the absenceof alternativehydrogen consumers, methanogens are Degradationproducts collected near to the peat surfaceyield a necessarycomponent of an anaerobic microbial consortium recent radiocarbonages [Chanton et al., 1995; Charman et al., [Demingand Baross, 1993]. Sulfatereducers outcompete metha- 1999; Schiff et al., 1998]. Exported CO2, CH4, and DOC form nogensif hydrogenis a limiting factor, but the phenomenonis apparentlyat the expenseof recentlyfixed carbon.CH 4 formed at observedonly in the topmostlayer of bog peat where sulfateis the very top of the catotelmdominates over the flux from deep suppliedvia precipitation[Watson and Nedwell, 1998]. Therefore peat.Pore water gases and DOC collectedfrom deeplayers of peat an inferencecan be made that the extentof anaerobicdegradation bogs show radiocarbonages younger than the adjacentpeat by is proportionalto methaneproduction. severalhundreds of yearsto over 2000 years[Chanton et al., 1995; Potential carbon substrates for microbial metabolism in the Charman et al., 1993, 1999; Schiff et al., 1998]. Ages of pore catotelm are peat OM and DOC flushed from the acrotelm water carbon,which are intermediatebetween recently fixed OM [Chantonet al., 1995]. The immediateproducts of OM degrada- and old peat, suggestmixing of modem and generatedin situ tion/depolymerizationcontribute to the pool of DOC. Chemical materials.The increaseof pore water carbonage terminatesor at compositionof DOC is a functionof the substratecomposition. leastslows down in deeperpeat layers [Chanton et al., 1995;Schiff More PS-rich substratesyield PS-enrichedDOC and vice versa et al., 1998]. Theseresults seem to agreewith substratechemistry [Nierop and Buurman, 1998]. The resultspresented in this paper data:An initial distinctage increaseis a resultof in situ methane point to the PS-rich fractionof vascularplants (in the boreal site generation,diluted by carbontransported with descendingground- only in the initial phaseof degradation)and Sphagnumas sub- water. A cessationof anaerobicdegradation of vascular plant stratesavailable for anaerobes.Formation of carbohydrate-rich tissues,as describedin section4.2, would limit the in situ methane DOC in the catotelmcould be expected.On the otherhand, more generationto the slower processof Sphagnumdegradation (or 726 KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS othermosses, perhaps Drepanoctadus, similar in their resistanceto aerobes.Sphagnum withstands acrotelmic degradation relatively degradation;cf. Kuder [2000]). In suchcase the age of methanein well [cf. Johnsonand Damman, 1991; van der Heijden, 1994]. In the lowerparts of the profile would be only slightlygreater than the the catotelm,slow degradationproceeds even in peats in which age at the depthwhere PS was actively depleted. vascularplant OM is inert.Degradation of vascularplants leads to selectiveremoval of PS-richand syringyl lignin-rich cell walls [van der Heijden, 1994]. In anoxicconditions, the energeticexpense of 4.2. Controls of Degradation and Effects of Environmental residuedegradation exceed its nutritionalvalue [Postet at., 1995]. Change on C Emission from Peatlands Sphagnumtissues degrade without introducing changes in chemical The extent and rate of catotelmicdegradation depend on the composition[van der Heijden, 1994; Kuder, 2000], and the inhib- quality of plant litter after it is buried beneaththe acrotelm.Bog ition doesnot occur.Shielding of the nominallylabile components hydrologyregulates oxygen supply and acrotelmthickness. High (e.g., cellulose)by resistantbiopolymers (e.g., lignin or polyali- peat surface wetness(PSW) results in a thin acrotelm, limited phatic cuticularpolymers) makes them even less attractiveas a aerobicdegradation, and a significantportion of labile OM left for substrate,especially in anoxicconditions, unsuitable for oxidative anaerobes in the catotelm. Low PSW results in a thick acrotelm and enzymes.PS of sedgefibers with their compacttexture and high mineralization of labile OM before it becomes available to anae- lignin contentare likely to be protectedthis way. Another good robes.Peaks of high polysaccharidecontent apparent in the upper illustrationof sucha mechanismis degradationof spruceneedles, part of the Canadianprofile disturbthe polysaccharidedepletion resultingin a relative loss of lignin rather than cellulose,since a trend and coincidewith high PSW. The effectsof peatlandwater portionof the latteris apparentlyprotected within the frameworkof table movementson CH4 and CO2 productionand emissionwere polyaliphaticand resistantpolymer on the cuticle, whereasligno- describedby a numberof authors[Dise et at., 1993; Laine et at., celluloseof the vesselsis freely accessibleto the microbialattack in 1996; Sitvota et at., 1996; Moore and Datva, 1997; Dautat and oxic conditions[Kuder, 2000]. Thereforethe presenceof labile, or Ctymo, 1998]. Given other parametersare kept constant,produc- "nominally labile," compoundsin peat cannot be equatedwith tion of CH4 increasesat high watertable, but CO2 decreases.Apart bioavailability. from the CH4 productionincrease, thinning of the acrotelmshrinks The effect of substrateon the rate of decompositionwas best the habitat for methylotrophicbacteria [e.g., Dautat and Ctymo, observedfor restiadbogs. At the Kopouataibog the depletionof 1998], and anoxiafluctuations impose stress on methanogenicand polysaccharidein the catotelmalone resulted in a net lossof over (to a lesserextent) methanotrophic bacteria [Whaten and Reeburgh, 50% of the original lignocellulose,as estimatedfrom pyrolyzate 2000]. The effect of water table drawdowndepends on the extent composition.At Moanatuatua,where there was more acrotelmic of peat decomposition,having more impact on relatively well- degradation,the losswas smaller(and its rate was slower),so that preservedpeats [Hogg et at., 1992]. DOC exportapparently is not peat compositionat both sitesultimately converged. directlyaffected by the water table fluctuationsbut dependson the amountof lateral dischargealong the acrotelmand resultingflux out of the system[Moore et at., 1998; Schiffet at., 1998]. On the 5. Summary other hand, water table fluctuations,which produce significant changesof hydraulichead, may alter the flow systemin a bog and The following points summarizeour conclusions. indirectly affect DOC export. An excessof labile OM preserved uponhigh PSW will ultimatelybe degradedin the catotelm,so that 1. Macromolecularcomposition of peat macrofossilsand matrix old peats with variable extent of aerobic degradation(variable from boreal Sphagnumbogs and New Zealand restiadbogs has PSW) have uniform chemicalcomposition (Figure 3). It hasbeen been studiedby pyrolysis-gaschromatography/mass spectrometry proposedthat CH4 generatedin catotelmmay be migratingdirectly to determinedegradation trends. to groundwaterand so remain unaccountedfor in flux studies 2. Degradationof vascularplant remainswas most noticeablein [Charmanet at., 1993]. Accumulationof CH4 in pore watersin the the upperpart of the catotelm.In the lower catotelmthe vascular deeper part of peat bogs has been reported in several papers plant organicmatter was apparentlyinert. Sphagnum was degraded [Romanowiczet at., 1995; Brown, 1998]. A confining layer of more slowly than vascularplants, but no cessationof degradation occludedbubbles was proposedto explain such a phenomenon was observed. [Romanowiczet al., 1995], and indeed it has often been observed 3. The fraction of mass loss and duration of catotelmic in peat sections,including the Canadianbog (between180 and 200 degradationvaried dependingon the following factors:(1) type cm). Only sporadically,when hydrostaticpressure is for some of plant, (2) extentof aerobic,precatotelmic degradation, and (3) reason lowered (e.g., by a prolongeddrought and water table methodof litter deposition. drawdown),accumulated CH4 is removedby ebullition [Romano- 4. The extent of the aerobic degradationdetermines whether wicz et at., 1995]. Samplesfrom New Zealandsuggest that a major labile OM reachesthe catotelm.It dependson peatsurface wetness, part of the fixed OM was mineralized in the catotelm over a i.e., on the water table height.Wet peat retainsa high fractionof prolongedperiod of time, very likely resulting in generationof polysaccharides,which is later used by anaerobes,potentially largeamounts of deep,•4C-old methane (although the age of pore resultingin generationof greenhousegases. water CH4 was never studiedthere). 5. Publishedradiocarbon ages of DOC, CH4, and CO2 from peat Within a given set of hydrologic/climaticconditions, substrate pore waters are generally in agreementwith the data presented qualityis a limitingfactor for anaerobes.Highly variablediagenetic here. The differencesin age between peat, DOC, and dissolved responsesof plant parts (e.g., apparentlyinert sedgefibers versus gasesmay be explainedby mixing of modem signatures(DOC ericaceousroots versus Sphagnum) highlight the potential impact of transportedfrom the acrotelm)with DOC generatedin situ, but the botanicalcomposition of peat on bioavailabilityof deposited also by differentialbioavailability of the two DOC pools. OM. Sensitivityto environmentalforcing would be higherfor peat 6. Effectsof an environmentalchange on carbondynamics may rich in vascularplant remains.Unlike degradationof moss,degra- be potentiallygreatest if the hydrologywere disturbedat a wet and dation of vascularplant OM may producean inert residue(thick vascularplant-rich site. Peat depositedunder a dry regime would acrotelmand intensedegradation) or relatively pristine,polysac- be relatively inert under anaerobicconditions, having alreadylost charide-richOM (thin acrotelm).Subsequent methane generation much of its polysaccharides. potentialwould vary accordingly.The impactof low PSW may be 7. In certain cases(e.g., restiad bog from New Zealand) it is in part compensatedwith deeper rooting, protectingPS against possible that a major fraction of peat is degraded in the KUDER AND KRUGE: PEAT BOG CARBON DYNAMICS 727 catotelm,potentially resulting in a large export of radiometrically Laine, J., J. Silvola, K. Tolonen, J. Alm, H. Nykfinen, H. Vasander,T. old CH4. Sallantaus,I. Savolainen, J. Sinisalo, and P. J. Martikainen, Effects of water-leveldrawdown on global climaticwarming: Northern peatlands, Ambio, 25, 179-184, 1996. Lewis, N. G., and E. Yamamoto,Lignin: Occurrence,biogenesis and biode- Acknowledgments. We thank I. P. Martini, G. Skrzypek, M. 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(ReceivedApril 27, 2000; revised March 26, 2001; Coal Geol., 3 7, 3-27, 1998. acceptedApril 13, 2001.)