Ridgwell et al 1999 consumption of atmospheric methane by soils a process based model global biogeoc

GLOBAL BIOGEOCHEMICAL

CYCLES, VOL. 13, NO. 1, PAGES 59-70, MARCH 1999

Consumption of atmosphericmethane by soils: A process-basedmodel

AndyJ.Ridgwell, 1Stewart J.Marshall, andKeithGregson Schoolof BiologicalSciences, Universityof Nottingham,SuttonBoningtonCampus,Loughborough, Leicestershire, England

Abstract.A process-based modelfortheconsumption of atmospheric methane (CH4)by soilswas developed to identifythemostimportantfactorsaffectinguptakeratesandto determinewhetherthe currentuncertainties in theestimated sizeof theglobalsoilsinkmightbe reduced.Descriptions of diffusionandmicrobialoxidation processes, whichtogether determine theCH4 flux,wereincluded.

Theresults suggest thattheglobal sinkstrength lieswithintherange 20-51Tgyr-1CH4,witha preferred valueof38Tgyr'l CH4.Drytropical ecosystems account foralmost a thirdofthistotal.Here microbialactivityratherthandiffusionis limitinguptake.It is alsoin theseareasthatthe impactof any intensification in agriculture will be themostpronounced, with a possiblefuturereductionin uptakein

excess of3 Tgyr-1CH4.Thisisincontrast totheoverall impact ofglobal warming, whichisexpected to leavethesizeof theglobalsoilsinkrelativelyunchanged.

1. Introduction

An alternativeapproachis to constructa "process-based" model.

Here the most importantbiogeochemical processes determining Methaneis currently presentin theatmosphere at a concentration tracegasexchangeare modeledexplicitly,usuallywithina regular of about 1.72 ppmv [Houghtonet al., 1996], contributing an array of cells representingareas of the Earth's surface. Soil, estimated 15% to anthropogenic globalwarming[Rodhe,1990]. ecosystem, and meteorological data are providedby linkingwith CH4 is removedfromtheatmosphere largelythroughreactionwith suitableglobaldatabases.

However,scaleincompatibilities can still hydroxyl molecules (OH) in thetroposphere [Funget al., 1991]and be a problemwith thisapproach.

Descriptions of tracegasprocesses alsoas a resultof microbialoxidationin soils.Althoughrecent are typically developed using single-locationstudies with

estimates of 15-45Tgyr-1CH4 forthesizeoftheglobal soilsink

characteristic scales of the order of a few centimeters. This is in

putit at onlyabout10%of thatdueto reaction withOH [Houghton contrastto their applicationover the entire globe, where et al., 1995],theestimated excess of sources oversinksforCH4, characteristic scales canbe over100km for a 1øby 1øgridnearthe whichaccounts for thepresent rateof atmospheric increase, is of a equator.Processdescriptions mustthereforebe reparameterized so similarmagnitude.

Therefore, changes in the strength of the soil asto be applicable at thecoarser scale,whichwill inevitably result sinkcouldpotentially havea significant effectonfutureatmospheric in a lossof accuracy andwhichmayintroduce significant errorsif CH4 concentrations and thus on the rate and extentof global theprocesses involvedrespond in a nonlinear fashion[Schimel and warming.

Potter,1995].Use of a finergridscalemay reducetheseerrors,but Globalestimates of tracegasfluxesare commonly madeby themaximumresolution thatcanbe employed is ultimatelylimited extrapolation fromfieldandlaboratory studies; pointmeasurements by theresolution of suitable datasetsandalsoby consideration of are scaledup, assuming themto be representative of all locations thecomputational resources available.Thistypeof modelhasbeen sharing somecommon classification suchassoilor ecosystem type successfully appliedto a numberof tracegasprocesses, suchasthe [Matsonet al., 1989; Schimeland Potter, 1995; Steudleret al., production of N20 [Bouwman et al., 1993]andCO2 [Potteret al., 1989]. Global fluxes are then estimatedby multiplyingthe 1993]by soilsandCH4 by wetlands [Caoet al., 1996a]andrice representative flux by the areawithineachclassification [DOff et paddies [Caoet al., 1996b]andtheconsumption of CH4 by soils al., 1993;Ojimaet al., 1993;Potteret al., 1996].However,the [Potteret al., 1996]. assumption that point measurements madeat a limitednumberof

Consumption of atmospheric CH4 by soilshasbeenshown tobe locations andfor a limitedtimeperiodcanreallybe representative the resultof an entirelybiologicaloxidationprocess[Benderand of all locations overanextended timeperiodcanleadto significant Conrad,1994; Whalenand Reeburgh,1990]. The activityof errors[Aselmann, 1989;Matsonet al., 1989].Suchextrapolations methanotrophs (andthusthepotential rateof CH4 oxidation within are valuableas initial estimates and for definingmaximumand the soil)is regulatedby a numberof environmental factorssuchas minimumlimits,but theycannotnecessarily provideinformation temperature, soil moisture,soil nitrogencontent,organicmatter aboutfluxesundernovelconditions, particularly thosearisingasa

content, andpH. However, theprimary control onthefluxof CH4 intosoilsis usually considered to betherateof diffusion of CH4

resultof climaticchange.

withinthetopsoil[Bornet al., 1990;DOrr et al., 1993],whichis

1 Nowat School of Environmental Sciences, University of East Anglia,Norwich,England.

controlled by boththephysicalstructure andmoisturecontentof the

soil.Asa result,in fine-textured and/orrelatively water-logged soils,

Copyright1999by theAmericanGeophysical Union.

lowgaseous diffusivity tends torestrict uptake, andthefluxofCH4

Papernumber 1998GB900004.

intothesoilis notsignificantly affected by thedegreeof microbial activity.Underconditions of highdiffusivity, ontheotherhand,the rateof microbial oxidation is oftenlimiting,especially in extremely

0886-6236/99/1998GB 900004 $12.00 59

60

RIDGWELL ET AL.: CONSUMPTION OF ATMOSPHERIC METHANE BY SOILS

and Conrad, 1993; Kruse et al., 1996; Schnelland King, 1994], althougha few soilsexhibitoxidationactivitymaximasignificantly deeperthanthis [Reeburghet al., 1997].Away from the maximum will play a role in limitingCH4 consumption [Reeburghet al., 1993].Accuratedetermination of the flux in all possiblesoiltypes thereis typicallya rapidreductionin activitytowardthe surfaceand and environmental conditionswill thereforerequire,in additionto a slightly gentler decline with increasingdepth. All oxidation activitywas thereforeassumedto take placeat a singledepthof 6 an estimationof gaseousdiffusivity, an explicit treatmentof This microbial activity. Despite this, previousregional and global cm in the soil (i.e., as a delta-functionrepresentation). has the advantagethat the flux rate could be estimates havetendedto placemostemphasis on thephysicsof soil parameterization approximatedanalytically,with no need for a computationally diffusivity, whilebiologicalcontrols overCH4 uptakehavetakena minorrole.The aim of thisstudy,then,is to developa mechanistic intensive,multiple-layerscheme. dry soilswhen methanotrophs may experiencea significantwater stress.Betweenthesetwo extremes,liesa regimewherebotheffects

modelfor globalconsumption of CH4 by soils,whichincorporates descriptions of the most importantcontrollingfactors,relatedto bothgaseous diffusionandmicrobialactivity.

Thefluxof atmospheric CH4 intothesoil(JcH4)is calculated on thebasisthatall CH4 consumption takesplaceat a ratedetermined

bya first-order oxidation rateconstant kd (s-z)andat depth zd (centimeters).

In addition,all oxidizedCH4 is assumed to originate

2. Model Implementation 2.1. Calculationof SoilCH 4 Flux

A comprehensive description of theuptakeof atmospheric CH4 by soilsdepends on the abilityto modelthe depthprofileof CH4

fromthe atmosphere, ignoringpossiblemethanogenic productionof CH4 withinthesoil,eitherin anoxicmicrosites [Conrad,1995;Tate andStriegl,1993]or at greatdepth[Strieglet al., 1992; Whalenand Reeburgh,1990]. The modelwill thereforenot be able to account for the effect

on soil flux

of a shift in the balance between

methanogenic productionand methanotrophic consumption within oxidationrate within the soil from all the controllingfactors. the soil in responseto changingenvironmentalconditions,which Unfortunately,not enoughis known aboutthe physiologyof the can be important[Keller et al., 1993; Yavitt et al., 1993]. In the methanotrophs responsible to be able to derivesucha distribution. steadystatethe diffusiveflux into the soil described

by Fick's law Additionally,the soilprofileinformationavailableon a globalbasis (equation(1)) mustbe equalto the rateof microbialoxidationgiven is restrictedto little more than grain size fractions,whereas by knowledgeof soil organicmatter content,inorganicN, and pH JCH4= kd XCdcH4 (2) would alsobe required.A simplificationof the systemis therefore essential.

As a resultof soildiffusivity beingfrequently limitingto CH4 uptake,Potteret al. [1996]deviseda simpleparameterization based onFick'slaw of diffusion[MonteithandUnsworth, 1990]: ACcH4 'JcH4= D CH4

whereCdCH4 is the concentration of CH4 at depthzd. If COCH4 is definedas the atmospheric CH4 concentration (ppmv), setting Az= zd andACcH 4 = COCH4 --CdcH4 in (1) andequating (1) and(2) gives

(CocH4 -- CdCH4) DCH4 = kdCdcH4

(1)

(3)

Zd

whereJCH4is theflux of CH4 intothesoil,DCH4 is thediffusivity

of CH4 in thetopsoil (cm2 s-l), andACcH 4 / Az is a fixed Writing (3) in terms of CdCH4,substitutingback into (1), and concentration gradient, takentobe0.04ppmvcm-•, onthebasis of rearrangingnow gives regression analysisof severalpublishedsoil CH4 concentration profiles[Bornet al., 1990;Whalenet al., 1991].Although Potteret Zd (DcH4+ kdZd) al. [1996] modeledgaseous diffusivityfrom the consideration of F is a factor (616.9mgppmv -• cm-• CH4)chosen to give soiltexture,temperature, andmoisturecontent,no explicitaccount where in unitsof mgm-2 d-1 CH4. Inspection of thisformulation was taken of the influence that microbial responsesto JCH4 confirmsthatsoilCH4 uptakewill notalwaysbe limitedby gaseous environmental forcings mayhaveuponJCH4' An alternative scheme,whichretainsthe abilityto accountfor diffusivityas is often assumed.Instead,to a first approximation, activity, changes in microbialoxidationactivity,is to assume a profileof the uptakedependsontheratioof diffusivityto methanotrophic oxidationrate constantagainstdepthtaken from experimental with uptakedeterminedalmostexclusivelyby diffusivityat high

JCH4-CøcH4DcH411DCH4 /xF (4)

determinations.

However, calculation of theCH4 fluxis nottrivial, rates of microbial activity and by microbial activity at high andin general, therewill benoanalytical solution unless theprofile diffusivities. canbe described by a simplefunctionof depthsuchas a constant Althoughtheatmospheric CH4 concentration is observed to vary [Bornet al., 1990;Striegl,1993]or a negative exponential [D6rr et with both latitudeand season[Fung et al., 1991], for simplicitya al., 1993; Striegl, 1993]. Alternatively,a numericalsolutionfor fixedaverage valuefor COCH4 of 1.72ppmv[Houghton et al., 1996] one-dimensional diffusion with first-order reaction kinetics based on

thepartitioning of the soilprofileintodiscrete layers[Kruseet al., 1996;Moldrupet al., 1996a,b; StrieglandIshii,1989]is possible.

Extensivetestingwith a seriesof compartmentalized models suggested thattherewasno significant advantage to be gainedin termsof theaccuracy of fluxestimation by theuseof a profiletaken

was adopted.This will resultin errorsof no morethan5%, evenat

highlatitudes, whichexhibitthe greatest deviationof COCH4 from the globalmean.

Oxidation

kinetics are assumed to remain first order over all

possiblesoil CH4 concentrations from typicalatmospheric values downto zero.In practice,althoughoxidationis observed to continue

directlyfrom observations over the use of a much simpler at mixingratiosof muchlessthan0.1 ppmv [Benderand Conrad, triangular-or pulse-shaped approximation.

In undisturbed soils, 1992, 1993;Koschorreckand Conrad, 1993; Whalenet al., 1990], oxidationactivitytendsto be concentrated at depthsof 5 - 7 cm kineticsdivergefrom true first orderbelow a thresholdof about0.3 [Adamsen andKing, 1993;Benderand Conrad,1994;Koschorreck ppmv [Benderand Conrad,1993]. Considering that soilsin which

RIDGWELL ET AL.: CONSUMPTION OF ATMOSPHERIC METHANE BY SOILS

61

CdCH4 is likely to fall to belowthisthreshold will be characterized distance of 6 cmis likelytobeanorderof magnitude greater than

by verylow diffusivities andthustendto havea relativelysmall the depthof influenceof wind turbulence.

This effectwill therefore flux, on average, errorsdueto the assumption of purefirst-order playonlya veryminorrolein determining soilflux,andsoit has kineticswill notbe significant.

been omitted.

2.2. Calculationof SoilCH4 Diffusivity

2.3. Calculation of CH4 Oxidation Activity

The gaseous diffusivity of CH4 withinthe soil (DcH4)is

TheCH4 oxidation rateconstant kdisdefined by

described by

DCH4 = GsoilxGr xDocH4

(5)

kd =rN Xrsm x/?xk0

(8)

wherek0 is the baseoxidationrate constantfor an uncultivated

moistsoilat 0øCandrN, rsm , andrr are adjustment factors fortheeffects of aghculture, suboptimal soilmoisture STPof0.196 cm 2s-1[Cusslet, 1984], andGsoil andGr arescalarsaccounting andtemperature, respectively.

ThepH hasbeenshown accounting for the effectsof soil structure and temperature conditions, respectively.

It hasbeensuggested thatthe aqueous diffusion of to be animportant factorat elevated CH4 levels[DunfieM et al., CH4 in soilparticle waterfilmsmayplayanimportant limiting role 1993],witha clearoptimum formethanotrophic activity.

However, at ambientpCH4 are muchlessconclusive inuptake [Castro etal., 1995;Koschorreck andConrad, 1993].For studiesperformed simplicity, liquidphase transport hasnotbeenexplicitly modeled in [Hiitsch etal., 1994;Yavitt etal., 1993],whileD6rretal. [1993] thisstudy,although it is likelythatthequalitative influence of soil could findnosignificant correlation between pH andCH4 uptake moisture onuptakeratedueto aqueous diffusion will be accounted ratein a widerange of soils.

Furthermore, asthereiscurrently no for throughchangesin modeledgaseous diffusion.A distinct global datasetfortopsoil pH, it wasnecessary toomittheinfluence whereDOCH4 is thediffusivity of CH4 in freeair,takinga valueat

advantage of the diffusion-consumption flux calculation scheme ofpH fromthemodeldescription.

Reported peakoxidation rateconstants in nonagricultural soils (equation (4))overthefixedgradient approach (equation (1))isthat fromunder 2 x 10-4s'1toover5.0x 10-3s-1[Adamsen and anyerrorin theestimation of diffusivity willbepartlycompensatedrange for:anoverestimation of DCH4 will leadto a reduced concentration King,1993;Bender andConrad, 1994;Koschorreck andConrad, gradient in thesoil,thereby reducing theresulting overestimation of 1993;Schnel! andKing,1994],withequivalent agricultural soils ß/CH4,andvice versa. typically exhibiting about 30%of this[Bender andConrad, 1994; 2.2.1. Soil structural dependence. A numberof different Koschorreck andConrad,1993].A valuefork0 wasobtained by schemesfor calculatinggaseousdiffusivityin porousmedia,suchas fittingcalculated fluxesgenerated usingequation (4) together with soils,have been proposed.Some requirethe fitting of parameters measuredvalues of gaseousdiffusivityto observedvalues, from experimental studieson individualsoils[Nielsonet al., 1984; correcting for soil temperature (wherereported),as is shownin Troech et al., 1982] and so are unsuitable for global Table1. A k0 value of 8.7x 10-4s-1(R2 = 0.55)wasobtained,

implementation.

Othersdevelopedfor moregeneralapplication are derivedby a "cut-and-random-rejoin" treatmentanduseestimates of total and air-filled porosity together with a tortuosity factor [Millingtonand Quirk, 1961; Millington and Shearer, 1971]. We adoptherethenewparameterization of Moldrupet al. [1996b],with

whichis consistent withthereported rangeof rateconstants.

Useof a fixedgradient scheme resulted in a considerably poorerfit to the

thescalarGsoil definedas

reported temperature responses of atmospheric CH4 consumption bothin situ[Bornet al., 1990;Castroet al., 1995;Crill, 1991;

1.5 + 3/b

Gsoil =(I) 4/3(•)

observed ']CH4 data,withanoptimized gradient of 0.039ppmvcm-•

giving anR2ofonly0.08.

2.3.1. Temperature response. A numberof studieshave

andConrad,1993]andwithsoilsamples incubated in (6)Koschorreck thelaboratory [Cril!et al., 1994;KingandAdamsen, 1992;Whalen

where• is thetotalporevolume (cm 3 cm'3),g is theair-filled and Reeburgh, 1996].The measured temperature dependence of porosity (cm 3 cm-3), andb is a constant thatcanbe empiricallyatmospheric CH4 uptakeis profoundly influenced by thediffusive predicted from soil texture [Saxton et at, 1986]. This parameterizationgives a good prediction of the increasein diffusivitywith soil-aircontent,andit hasbeenfoundto be superior to the commonlyusedschemeof Millingtonand Ouirk [1961] in a numberof soils [Moldrup et al., 1996b; Olesenet aL, 1996]. It is alsomathematically simple,doesnot rely on the use of pore size classes,and has parametersthat may be estimatedfrom widely available soil characteristics.

regime, particularlyfor measurements made in situ and on

undisturbed cores,wheretherecanbe virtuallyno temperature response at low diffusivities anda pronounced response at high diffusivities [Mosier etal., 1996].Anydetermination oftemperature dependence wherediffusionis limitingto somedegreewill therefore notelucidate theunderlying effectuponmethanotrophic behavior.

Increasing microbial activitywithtemperature cantypically be

2.2.2. Temperaturedependence.The diffusivityof CH4 in described by a function of theformQ•0 , where (r/•0) Q•0isthe10ø freeair is a functionof temperature, with Gr definedafterthelinear temperature quotient.

Incubation studies at elevated CH4 mixing approximation of Potteret al. [ 1996]:

Gr = 1.0+ 0.0055T

ratios withmainly landfill coversoilshavefound Q•0values over (7)

temperatures in the range0ø - 20øCof 1.8 - 2.0 [Boeckxet al., 1996],1.4- 2.1 [Dunfield et al., 1993],and1.8- 2.1 [Whalen etal.,

whereT isthetemperature in degrees Celsius.

1990].Although Crilletal. [1994]found a Q•0valueofonly1.3in incubation experiments at 10ppmv, theirsamples wereconsiderably in increasing therateof mass transfer of CH4 isunlikely toextend largerandmoremoist,soit seemsquitelikelythatdiffusion was anydeeper than1 or2 cm,evenin verycoarse soils[Farrelletal., stilllimiting.

A Q10valueof around 2.0 is consistent withtypical 2.2.3. Surface air turbulence effects. The effectof wind action

1966; Scotteret al., 1967]. For most other soilsthe diffusion values forenzymatic activity[AtlasandBartha,1987]andalsowith

62

RIDGWELL ET AL.' CONSUMPTION OF ATMOSPHERIC METHANE BY SOILS

Table1. Measured SoilMethane Fluxes JCH4 Compared WithValuesCalculated Withthe Diffusion-Consumption Scheme UsingExperimentally Determined Valuesof CH4 DiffusivityDCH4 and Temperature T andanOptimizedValuefortheBaseOxidation RateConstant k0 DCH4,

T,

JCH4 Observed,

JCH4 Calculated,

cm 2s-1

øC

mgm-2d'l CH4

mgm-2d-•CH4

0.064 0.032 0.058 0.0105 0.0025 0.036

12.5a 12.5a 12.5a 12.5a 12.5a 0.0a

0.069b 0.043 0.029

0.041b 0.036b 0.095b 0.048b

Reference

2.9 1.5 2.9 0.35 0.23 0.7

1.79 1.54 1.76 1.02 0.36 0.80

DOrr et al. [1993] DOrr et al. [1993] D6rr et al. [1993] DOrr et al. [1993] DOrr et al. [1993] DOrr et al. [1993]

10.0a

1.7

1.57

Striegl[1993]

14.0 23.0

0.9½ 2.0

1.78 2.08

Koschorreck andConrad[ 1993] Koschorreckand Conrad[ 1993]

4.5 4.1 1.5 2.7

0.55 0.22 0.62 0.55

1.06 1.01 0.96 0.97

Whalen etal. [1992] Whalen etal. [1992] Whalen etal. [1992] Whalen etal. [1992]

Diffusion-consumption scheme isgiven byequation (4).Value forthebase oxidation rate constant k0was 8.7x 10-4S-1. aEstimated temperature.

bEstimated fromtopsoil air-filled porosity. ½Average flux.

frozen, whenCH4uptake virtually ceases [Castro etal.,1995;Crill, 1991;KingandAdamsen, 1992;Koschorreck andConrad,1993]. whenmonthly meantemperatures fallbelow0øC,the 1995]. Despite there being no reliable Q10 resultsfor In thisstudy, layers of thesoilareassumed tofreeze, sothatrr isthenset methanotrophic activityunderatmospheric CH4 concentrations,surface considering thata Q10valueof 2.0 appears to havesuchgeneral to zero.

Thescalar rr istherefore defined as microbiological applicability [Schimel and Potter,1995],it was adopted here.Aftera maximum hasbeenreached in theregionof rr = 0 r < 0øC (9a)

valuesadopted in othertracegasfluxmodeling studies [Caoet al.,

1996a;Potteret al., 1993;Partonet al., 1996;SchimelandPotter,

25ø - 31øC,activityrapidlydeclines[Boeckxand van Cleemput, 1996;Boeckxet al., 1996; Whalenet al., 1990; Whalenand

Reeburgh, 1996].Thecomplete temperature response isthusof the form shownin Figure 1.

Some studiesreportsignificantCH4 oxidationOCCUlTing at

temperatures downtoa couple of degrees belowzero[Castro etal., 1995;Crill et al., 1994;KingandAdamsen, 1992],unless thesoilis

T_> 0øC

(9b)

whereT isthetemperature in degrees Celcius.

Thefunction for T >_ 0øCwaschosen to havea Q10response of 2.0 overthe interval

0ø-15øC and to produce an approximate fit to experimental measurements [Dunfield et al., 1993;Whalen etal., 1990;Whalen andReeburgh,1996].

2.3.2. Cultivation response.Nitrogen inputto thesoilasa

result of agricultural fertilizer application, particularly whenin an inorganic form,hasbeen found tohaveaprofound inhibitory effect

ß ß

ß

4.0

• = exp[0.0693T - 8.56x10-7T4]

onCH4oxidation activity [Castro etal.,1995; Hiitsch etal.,1993; KingandSchnell, 1994;Schnell andKing,1994;Steudler et al.,

_

_

1989],although themechanism by whichthisinhibition operates hasstillnotbeenfullydetermined.

Typically, it canreduce soil

..

uptake bytheorder of 60%,butthere iswidevariability, witha complete cessation ofuptake being reported insome cases [Hiitsch etal.,1993].

Theunderlying microbial response toN application is difficultto deducefrom the variousstudiesto date,primarily

because of thelikelydifferences inthedegrees of diffusive control onuptake between studies.

Reduction inmethanotroph activity asa resultof landconversion foragriculture, dueto boththeapplication 0

10

20

30

40

Temperature(øC) ß DunfieMet al. [1993]

ß Whalenetal. [1990]

of N-based fertilizers and reduced soil organic matter content

[Hiitsch et al., 1994],is therefore parameterized rathersimply, according totheintensity of cultivation.

Oxidation activity in land

under100%cultivation is assigned a multiplication factorof 0.25, chosen to enablethe modelto reproduce the rangeof reported Figure 1. Scalarfunctionrr for the effectof temperature on cultivation-related reductions in consumption whentypicalsoil methanotrophic CH4 oxidation activity(equation (9b)).Alsoshown diffusivities are takeninto account.

The scalarrN is therefore ß WhalenandReeburgh[1996]

are experimental datafor CH4 oxidationrate,scaledto a valueof unityat T = 0øC,fromDunfieldet al. [1993], Whalenet al. [1990], and WhalenandReeburgh[1996].

defined as

5v= 1.0-(0.75X/cult )

(10)

RIDGWELL ET AL.' CONSUMPTION OF ATMOSPHERIC METHANE BY SOILS

where-/cult is the fractionalintensityof cultivation[Matthews,1983].

Reductionin CH4 uptakedue to physicalchangesin soil structure arisingout of land conversion[Dobbieet aL, 1996;Hfitsch,1996; Keller et aL, 1993] is implicit within this factor.No attempthas beenmadeto accountfor the effect of anthropogenically enhanced atmosphericnitrogen deposition,which has been estimatedto reduceCH4 fluxesby about30% in urbanareas[Goldmanet aL, 1995],asitsglobaldistribution is notwell known.

2.3.3. Soil moistureresponse.Low moisturecontenthasbeen

observed to suppress CH4 uptakeby soils[Mosieret aL, 1996; Nesbit and Breitenbeck,1992; Torn and Harte, 1996; Whalenet aL, 1990].However,the lack of sufficientsoiltexturalandhydrological

informationin many of thesestudiesmakesit difficultto compare the variousquotedthresholdsof soil moisturecontentin termsof waterpotential.In addition,asthe soilwaterbalancesubmodel used heremakesno attemptto predictsoil dryingbeyondwilting point, when inhibition would be greatest,a general scheme for soil microbialactivity was adopted.The functiondescribingthe soil

moisture scalarrsmwasadapted fromPotteret al. [1993]: rsm = 1.0

(P+ SM)/ETp > 1.0

(11a)

5'm=(P+ SM)/ETp

(P+ SM)/ETp _<1.0

(11b)

whereP is the monthlyprecipitation, SM is the soilmoisturestored to a depthof 30 cm (or to the rootingdepthif less),andETp is the

!

potentialevapotranspiration.

Some studiesalso limit soil microbial activityat highsoilmoisturecontents[Partonet aL, 1996;Potteret al., 1993], when oxygensupplywould be restricted.However,the

microaerophilic natureof methanotrophic bacteria[Hanson,1980] makesthisadditionunnecessary.

2.4. Global Application

To estimatethe size of the global soil sink for CH4, the diffusion-consumption submodelwas appliedto a 1ø by 1ø grid. A maskwas constructed for areaswhereconsumption was certainto be negligible,initially definedby areasof water (includingocean and lakes)and land ice [Matthews,1983]. Areasdefinedas desert were alsomaskedout [Matthews,1983],assuming the consumption

of atmospheric CH4 to be associated with the production of CO2 within the soil [DOrr et al., 1993] and thuswith the presenceof vegetation.Althoughwetland soils can becomenet consumersof atmospheric CH4 whenthewatertablefallsin dry seasons [Harriss and Sebacher,1982] or by landdrainage[Martikainenet al., 1995], consumption of atmospheric CH4 is generallya propertyof aerobic soils.For thisreason,areasof permanentwetlandwere maskedout accordingto their distributionand coverage[Matthewsand Fung, 1987].

Averagemonthlyprecipitationand near-surface air temperature weretakenfromLegatesand Willmort[1992]. Soil temperature at a

.

Rooting Depth

I Potential Evapo-

transpiration

Soil Water Balance Submodel 'r

Derived Soil

Soil Water

Storage Capacity

Properties

Soil Water Status

Methane

Oxidation

Activity

63

Evapotranspiration

Soil Methane

Diffusivity

I I

Diffusion-Consumption Submodel

Figure 2. Schematicrepresentation of the globalatmospheric methanesoil consumption model.Shadedandunshaded boxes represent input data sets and calculation processes,respectively. The soil water balance and diffusion-consumption submodels aredelineated by dashedlines.

64

RIDGWELLET AL.' CONSUMPTION OFATMOSPHERIC METHANEBY SOILS

depthof 6 cm was assumedto follow air temperature.

This is a reasonable assumption giventhatthe dampingdepthfor annualair temperature cycles,evenin poorlyconducting peatysoils,is greater than 1 m [Monteithand Unsworth,1990] and gives a minimum 95% of the surfaceamplitudeat the depthwhereoxidationactivity

approach.

However,our schemeperformedvery poorlyagainstthe secondtime series,althoughonce measurements for which there was evidenceof a significantinhibitionof methanotrophic activity throughwaterstress wereremoved,andallowancewasmadefor the

is concentrated.

valueof 0.05 wasobtained.This wasstill an improvement overthe fixedgradientmethod.

The structure of the complete global model is shown schematically in Figure2.

2.4.1. Derivation of soil properties. Soil physicalproperties requiredby the water balanceand gaseousdiffusivitycalculations were estimatedfrom soil particlesize fractionsafter Saxtonet al.

[1986]. Soil particlesize informationconsistedof two setsof data, describing percentsandandclaycontent,bothderivedfroma global soilpropertiesdatabase[Webbet al., 1992].Sincethe averagesoil propertiesoverthe depthaffectingdiffusionwill not necessarily be representative of thoseoverthe entirerootingdepth,onesetof data was averagedover the diffusiondepth(6 cm) for use in gaseous diffusivitycalculations, while anotherwasaveragedover 150cm for use in the soil water balance submodel.

2.4.2.

Soil water

balance.

The soil water balance submodel

wasbasedon the widelyused"bucket"formulation, wherechanges in stored moisture arise out of imbalancesbetween gains (precipitation) andlosses(evapotranspiration andsurfacerunoff)to the system[Bouwmanet al., 1993;Potter et al., 1993; V6r6smarty et al., 1989]. The schemeusedhereis similarto thatof Potteret al.

[1993] and so will not be discussed in detail.The main differences were the use of a simpler soil drying curve (representingthe resistance of an alreadydryingsoilto furthermoistureloss)andthe estimationof rootingdepth(for usein calculatingsoilwaterstorage capacity),whichwasinitiallyassigned followinga schemebasedon vegetationtype [Webb et al., 1991] and modified accordingto cultivationintensity[Matthews,1983] and soil profile thickness [Webbetal., 1992].Any precipitation addedto the systemin excess of the soil'sfield capacitywasassumed to be lostasranoffwith no transferof moisturebetweengrid cells.This is likely to resultin an overestimation of CH4 uptakeduringperiodsof intenserainfall, when soil water contentmay exceed field capacity,restricting gaseous diffusivityto nearzero.

The waterbalancesubmodelwasinitializedby settingthe soilto a uniform moisture ratio of 50% for all months. It was then mn

repeatedly untiltheaverageyearlysoilmoistureconverged to within 0.01% betweensuccessive years.

3. Results and Discussion 3.1. Model

Validation

3.1.1. Diffusion-consumption submodel. The full rangeof

predicted gaseous diffusivity (0.001-0.04 cm2s-l) andoxidation rate constant (<0.0001-0.0036 s-1)allows fora range inCH4 uptake of <0.01-2.45 mgm-2 d-1 CH4. Thiscovers thebulkof typical

factthatCH4oxidation atthissiteappeared tobeN-inhibited, anR2 3.1.2.

Global

model.

Global models where the characteristic

spatialandtemporalscalesare substantially differentfromthoseof experimentalmeasurements are virtually impossibleto validate satisfactorily.However, as discussedin section3.2, the model predictionsare in broad agreementwith global and regional estimates madeby othermethods.

3.2. Basic(Biological)Model

The size of the globalsoil sink of atmospheric CH4 (Jtot)is

estimated at 37.8Tg yr-1 CH4 by ourmodel.

Thisfigurelies comfortablywithinthe Intergovernmental Panelon ClimateChange

(IPCC) estimate of15-45 Tgyr-1CH4 [Houghton etal.,1996].

It is alsoconsistent withtherange ofpossible values of5.6-58.2 Tgyr-1 CH4 proposed

by Bornet al. [1990],andwiththebestestimate of

DSrretal.[1993] of28.7Tgyr-1CH4(range 9.0-55.9 Tgyr-1CH4) madeon thebasisof a soiltextureparameterization derivedfroman extensivesurveyof mainly Central Europeansoils.The estimate presentedhere does, however, differ significantly from the

model-derived estimate ofPotter etal. [1996] of 17Tgyr-1CH4.

Theprimaryreasonfor thisis theirassumption of a fixedsoilCH4 concentration gradientset at a relativelylow value of 0.04 ppmv

cm-1.Astheypointout,thisisa veryconservative estimate which

cannot explain fluxes larger than1.0mgm-2d-1CH4,which are widely observedin soils.Using a higher gradientof 0.07 ppmv

cm-1,theyreported a figure of nearly 30 Tgyr-1CH4,which is closerto the findingsof thispresentstudy.

The globaldistribution of CH4 uptakeis shownin Plate la.

Uptake averagedby aggregatedHoldridge life zone [Leemarts, 1992] is detailed in Table 2. Dry tropical ecosystemstend to dominateas a resultof the high diffusivityof the dry sandysoils characteristic of theseareas,in associationwith high temperature driven microbial activity. However, this is in contrast to experimental studiesthatsuggest thatannualfluxesin tropicalareas aregenerallymuchlower [Kelleret al., 1993].This is becauseonly

theoxidation of CH4 in soilsis considered, whichwill predicta low rate of consumption duringperiodsof soil waterlogging owingto restrictedgaseousdiffusivity. Flux measurements, on the other

hand,can registera net emissionof CH4 if the soil becomes anaerobic.Annualuptakebasedon flux measurements may thenbe quitesmallor evennegative,with net emissionduringwet seasons offsettingconsumption at othertimesof theyear.

3.2.1.

Effects

of microbial

water

stress.

The effect of the

schemefor the estimationof microbialwater stressadoptedhere

wastosuppress '/tot byover16%fromamaximum of45.2Tgyr-1 have reported fluxes inexcess of2.5mgm-2d'l CH4[Adamsen and CH4. Themodelis thereforerelativelysensitive to thischoice,and

measurements [D6rr et al., 1993], althougha numberof studies

King, 1993; Crill, 1991; Steudleret al., 1989], which cannotbe explained by thecurrentmodelconfiguration.

The diffusion-consumption scheme(equation(4)) wasvalidated

theuseof a considerably morecomplexsurfaceschemethatis able to predictthe dryingof surfacelayersbeyondthepermanent wilting pointwouldprobablybejustified.

3.2.2. Effectsof agriculture. The effectof landconversionfor against twoof theweeklyaverage CH4 uptaketimeseries presented andparticularlythe useof inorganicnitrogenfertilizers

by Mosieret al. [1996],fromwhichsoiltemperature, watercontent, agriculture andflux dataweresampled(N = 30). For the firstof the Mosieret wastoreduce '/tot by3.7Tgyr-1CH4fromapotential 41.5Tgyr'l

al.timeseries, ourscheme gaveanR2valueof0.35,whereas none CH4.Potteretal. [1996]estimated theglobaleffectof agriculture at of the observedvariancecouldbe explainedusinga fixed gradient

around 13%, which is not dissimilar from the 9% reduction

RIDGWELL

0

ET AL.: CONSUMPTION

1O0

200

OF ATMOSPHERIC

300

400

METHANE

500

BY SOILS

600

mgCH4m'2yr-â&#x20AC;˘ Plate1. Globaldistribution of annual uptake of atmospheric CH4 by soilspredicted by (a)thebasicmodeland(b)the

fixed0.087ppmv cm-1soilgradient model.

65

66

RIDGWELL ET AL.: CONSUMPTION OF ATMOSPHERIC METHANE BY SOILS

Table 2. MethaneConsumption

by Soils,Averagedby Aggregated HoldridgeLife Zone,for BoththeReference Caseanda No AgricultureScenario, TogetherWith Percentage Reduction in UptakeDueto Cultivation AggregatedHoldridgeLife Zone

Annual Sink, Tgyr-1CH4

FluxRate, mgm-2yr-1CH4

No

No

Class

Description

Reference Agriculture

Area,106km2

Cultivation-Related

Reference Agriculture Reduction in Uptake,%

1

Tundra

100.2

100.3

10.5

1.05

1.05

0.0

2 3

Cold Parklands Forest Tundra

207.0 161.0

210.3 161.8

2.8 8.9

0.59

0.59

0.0

1.44

1.45

0.7

4

Boreal Forest

187.3

196.4

15.2

5

Cool Desert

265.0

274.2

4.0

2.85 1.07

2.99 1.11

4.7 3.6

6 7

Steppe TemperateForest

255.9 240.6

316.9 297.3

7.4 10.0

1.91

2.36

19.1

2.41

2.98

19.1

8

Hot Desert

179.2

183.4

20.9

9 10 11 12 13 14

Chapparal Warm TemperateForest TropicalSemiarid TropicalDry Forest TropicalSeasonalForest TropicalRain Forest

343.9 331.6 420.3 452.5 411.9 333.8

429.9 385.5 462.9 504.3 439.7 345.5

5.6 3.2 9.5 14.9 15.1 8.5

3.74 1.94 1.07 4.01

3.83 2.42 1.24 4.42

TOTAL

136.7

6.73

7.50

6.22

6.64

2.83

2.92

37.84

41.50

2.3 19.8

13.7 9.3 10.3 6.3 3.1

Aggregated Holdridgelife zoneis fromLeemarts[ 1992].

intensity>_20%[Matthews, predicted here.The effectonJtotof thedegreeto whichagricultural all areasdefinedashavinga cultivation 1983]aresetto 100%(representing globalapplication of intensive land use reducesCH4 oxidationactivityin the soil is shownin Figure3, and the effectsof cultivationby ecosystem classification are shownin Table 2. Dobbie et al. [ 1996] estimateda reductionof

agriculture), then alto t falls byalmost 5Tgyr-•CH4to33.2Tgyr-• CH4, ofwhich over 3Tgyr-1CH4occurs indrytropical regions.

2.3 Tg yr-1 CH4 dueto landconversion to agriculture in the

3.2.3. Seasonalvariationsin CH4 consumption.Figure4 showsthe seasonality in CH4 consumption averaged by latitude

temperatezone,while Ojima et al. [ 1993] estimatedthat it couldbe

of climaticfactors ashighas6 Tgyr-1CH4.Thispresent study puts thefigure rather zone.As wouldbe expectedfromconsideration alone,zonesnearestthe equator(+30ø) showverylittleseasonality lower at 1.7Tgyr-1CH4,some 19%ofthepotential (undisturbed) soilsinkin thetemperateregion. in CH4 uptake,whereashigherlatitudeshave a muchmore response, withanalmost totalcessation of consumption Future intensification of agriculture,particularlyin currently pronounced duringcoldwintermonths in thenorthern hemisphere.

Contrasting nonintensively cultivateddry tropicalregions,can be expectedto this with the seasonalityin consumption calculatedwith the have a significantimpact. The effect of the suppression of temperature dependence of methanotrophic activityremoved(not methanotrophicactivity in temperateareas by cultivation is shown),suggests thatmuchof theseasonal controlon CH4 uptake restrictedby the predominantlydiffusion-limitednature of the at midlatitudes andhighlatitudes is dueto changes in diffusivity processes in thesesoils.In much warmerand drier regionswhere diffusionmay not generallybe limiting,the full impacton uptakeof a reductionin methanotrophic activitycan potentiallybe realized.

1.4 An equivalentintensityof cultivationwould thereforebe likely to producefar morethanthe average19% temperatezonereduction.If

1.2

42

--,

>•

•-

-

1.0

4O

0.8

0.6

38

0.4

36

0.2 0

20

40

60

80

100

% Inhibition Factor

0.0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3. Sensitivityof the annualglobaluptakeof atmospheric

ß 30øN-0øN

ß 60øN-30øN

CH4 by soils(dtot)to the degreeof inhibitionof methanotrophic

o 0øS-30øS

[] 30øS-60os

oxidation activity resulting from the conversionof land for agriculture.

ß 90øN-60øN

Figure4. Seasonality of themonthlyuptakeof atmospheric CH4 by soils,averagedby 30ø latitudezones.

RIDGWELL

ET AL.' CONSUMPTION

OF ATMOSPHERIC

METHANE

BY SOILS

67

confidence, is thebaseoxidation rateconstant k0. Thesensitivity of 50

globaluptaketo thevalueof thisconstant is shownin Figure5. It is apparent thatalthough Jtotis relativelysensitive to k0 at low values, in general,the choiceof k0 will be significantly lesscriticalthan would be the choice of AC/Az in a fixed gradientcalculation

?

30

scheme, whereJtotis directlyproportional to AC / Az.Assuming the confidence limitsin k0 to be the maximumand minimumvalues

•' 20

whichstillachieve a fit to thefluxdataof Table1 withR2 > 0, a

likely range forJtot is20- 51Tgyr-1CH4.

3.4.2. Choice of gaseousdiffusivity scheme. Use of the Millington and Shearer[1971] gaseousdiffusivityschemeresulted

10

ina slightly higher estimate forJtot of42.4Tgyr-1CH4.However,

0

0.0

0.0005

0.0010

0.0015

BaseOxidationRate,k0(s-1)

the Millington [1959] and Millington and Quirk [1961] schemes

both resulted inmuch lower estimates of24.4and25.0Tgyr-1CH4,

respectively,which is not unexpectedgiven their tendencyto Figure 5. Sensitivityof the annualglobaluptakeof atmospheric underpredictgaseousdiffusivity [Davidsonand Trumbore,1995; CH4 by soils(Jtot)to the choiceof valuefor baseoxidation rate Moldrup et al., 1996b].A degreeof caremustthereforebe takenin constant k0.

the choiceof scheme.One deficiencyin the couplingof the gaseous diffusivityand soil waterbalanceschemesis that soil diffusivities

within the soil arising from changing soil moisture content.

However,temperature-driven variationsin microbialactivityappear to restrictthepotentialsummermaximumat highnorthernlatitudes (90ø - 60øN) by over 25%, and they significantlyenhancethe corresponding trendin the response of southernmidlatitudes(30ø 60øS).

3.3. FixedSoilCH 4 GradientModel As a comparisonof flux estimationschemes,our model was

adjusted to usea fixedCH4 gradientin the soil.A valueof 0.087

much greater than0.04cm2s-1arenotpredicted, although theyhave beenobservedin the field [DOff et al., 1993].The reasonfor this is likely to be the artificialrestrictionof the minimumsoil water potentialto wilting point. It is alsopossiblethat the presenceof macropores, such as worm and root channelsand cracksfrom wetting-dryingcycles, are importantin generatingthe highest observeddiffusivities,althoughthe incorporation of theseeffects wouldthenrequireconsideration of the macroporestructureof the soil,whichwouldbe extremelycomplex.

3.4.3. Climatic change. The effect of changesin global

ppmv cm-1waschosen togenerate a similar value forJtot (38.0Tg temperatures on Jtotwas initiallyinvestigated in a rathercrude yr-t CH4)asthebasicmodel.

Theresultof thiswasa global mannerby uniformly incrementingall of the monthly average distribution as shownin Plate lb. The verypronounced contrastin temperature valuesby between0.5ø and 5.0øC.Soil moisturewas uptakeat similar latitudesis due primarily to variationsin soil notallowedto respond to thischange.

Theresponse of Jtotis shown texture,and in general,low CH4 uptakecorrelates stronglywith in Figure6. Initially,increases at highlatitudesdominatethe global regionsof high-percent claysoils(notshown). response (not shown),but beyondabout1ø of warming,declining The differences in flux distribution betweenthe biologicaland uptakeratesat low latitudesstartto determinethe overallresponse, fixedgradientmodels(i.e., Platesl a and lb, respectively) ariseout which is a resultof the steepdeclinein microbialactivitybeyond of the solecontroloverCH4 uptakeof gaseous diffusivityin the fixed gradient model. This highlights the regions where environmental conditions are suchthatmicrobialoxidationactivity 41.0 is stronglylimitingto uptake.The impactof agriculturallanduse (mainly through the inhibition of methanotrophic activity by N-basedfertilizers)can be seen, particularlyin central North 40.0 America.The effectsof microbialwaterstressin restricting uptake in regionsborderingareasof permanent desertare alsoapparent.

It shouldbe notedthattheseareareaswheresoilsarerelativelycoarse, -7• 39.0 and thussoil diffusivityis not limiting.Conversely, thereis very littleeffectonuptakeof theintensive cultivation in muchof Europe, • 38.0 wheresoilstend to be relativelyfine, and thusuptakeis largely limitedby the resultinglow soil diffusivity.Also apparentin the spatial contrast between the two models is the substantial 37.0 underestimation by the fixed gradient model of uptake in low-diffusivityclay soils(e.g., SouthAmerica,equatorialAfrica, 36.0 and Southeast Asia). As a result,the importance of soil texturein ).0 1.0 2.0 3.0 4.0 5.0 determining the spatialflux distribution appearsto be muchlessin thebiologicalmodel.

TemperatureIncrease(øC) 3.4. SensitivityAnalysis

3.4.1. Model parameters. Probablythe mostcriticalsingle parameterin the model, and the one for which there is the least

Figure6. Sensitivity of theannual globaluptakeof atmospheric CH4 by soils(Jtot)to a uniformincrease in average monthly temperatures across theglobe,withfixed(solidcircles) andvariable (solidsquares) monthlysoilmoisture.

68

RIDGWELL

ET AL.: CONSUMPTION

OF ATMOSPHERIC

thepeakat 25ø - 30øC(Figure1). Although theresponse of Jtot couldbe expected to be differentwith a morerealisticglobal distribution of temperature increase and/oran alternative microbial temperature response function, it seems likelythat2to t is relatively insensitive to thedirecteffectsof temperature change duringglobal

METHANE

BY SOILS

theconsumption of atmospheric CH4, withnoaccount takenof CH4 production withinthe soil.In a similarmanner,previous emission modelshavetakenno accountof thepossibilityof net consumption

of atmospheric CH4. An integratedCH4 consumption and production modelis thereforerequiredin orderto addressthe

of soilsas a whole(bothaerobicandanaerobic) to the warming.

If average monthly soilmoisture content is nowallowed contribution CH4 budget.In fact, there is no masonwhy a to decrease in response to increasing air temperature (stillwithno atmospheric soil scheme,ableto accountfor both precipitation changes accounted for),thenJtotisunlikely to exceed sufficientlycomprehensive

about 41.5Tgyr-1OH4(Figure 6).Obviously, thehighly simplified aerobicand anaerobicconditionsand processes,could not be utilizedto estimate the atmospheric exchange of CO2 andN20 in to CH4, all aspartof a singleglobalmodel.However,for changeforcingappliedmeanthatlittleweightcanbe attached to addition suchestimates.

Despite this,anincrease in globalOH4 sinkstrength this to be achievable,substantialprogressis requiredin the

natureof the waterbalanceschemetogetherwith the crudeclimate

withglobal warming ofabout 1 Tgyr-1øC-1 OH4 canprobably be takenasa reliableupperlimit.

construction of datasetsof globalsoilproperties (bothphysicaland chemical)at an increased verticalandhorizontalresolution.

An alternativemethodfor estimating the possibleimpactsof climatechange is to hybridize process-based andfluxextrapolation Acknowledgment. This researchwas in part fundedby the Natural approaches.

In thiscase,average modeled fluxesforeachHoldridge EnvironmentalResearchCouncilin the form of a grantto AJR to studyfor theM.Sc. in EnvironmentalScienceat NottinghamUniversity. life zone[Leemarts, 1992]arekeptconstant, whilethechange in the areaof eachzoneundera globalwarmingis considered.

Average zonalfluxeswerethereforeappliedto the estimated areaof each Holdridgelife zoneundera x2 CO2 scenario[Leemarts, 1990],

giving aslight increase inJtot of1.7to39.5Tgyr-•CH4.

4. Conclusions

Theestimate of37.8Tgyr-• CH4presented hereforthesizeof the globalatmospheric methanesoil sinkis consistent with most previous estimates.

Thisstudysupports thecurrent IPCCrangeof

15 - 45 Tg yr-• CH4 [Houghton et al., 1996]butoffersno

justification foranydownward revision ofthisto15- 30Tgyr-• CH4,asproposed byPotteretal. [1996].Thesizeoftheglobal soil sink appearssurprisingly insensitive to the effectsof global warming.

It islikelythattheimpactof a x2 CO2 scenario will result

ina change insinkstrength within therange -1to+3Tgyr'l CH4.

This is a smaller changethan that which might arise as a consequence of anysignificant futureintensification of agriculture.

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(ReceivedJanuary2, 1998; revisedJune5, 1998; acceptedSeptember11, 1998.)