JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 104, NO. D9, PAGES 11,577-11,584, MAY 20, 1999
Controlling variables for the uptake of atmospheric carbonyl sulfide by soil J. Kesselmeier, N. Teusch, and U. Kuhn BiogeochemistryDepartment, Max Planck Institute for Chemistry,Mainz, Germany
Abstract. Soil samplesfrom arable land were investigatedfor their exchangeof carbonyl sulfide(COS) with the atmosphereunder controlledconditionsusingdynamiccuvettesin a climate chamber.The investigatedsoil type acted as a significantsink for the trace gas COS. Atmospheric COS mixing ratios, temperature, and soil water content were found to be the physicochemical parameterscontrollingthe uptake. Emissionwas never observed under conditionsrepresentativeof a natural environment.The observedcompensation point (i.e., an ambient concentrationwhere the consumptionand productionbalanceeach other and the net flux is zero) for the uptakewas about 53 partsper trillion. Uptake rates
rangedbetween 1.5and10.3pmolm-2 s-•. Theconsumption of COSbythesoilsample dependedon the physiologicalactivityof the microorganismsin the soil, as indicatedby a clear optimum temperature and by a drasticinhibition in the presenceof the enzyme inhibitor6-ethoxy-2-benzothiazole-2-sulfonamide (EZ), a specificinhibitor for carbonic anhydrase. 1.
background,/lndreaeand Crutzen[1997] recentlyproposedto
Introduction
regardsoilsgloballyasa COSsinkinsteadof asa source,which Carbonylsulfide(COS), the mostabundantvolatilereduced sulfur compoundand nearly inert to photochemicaldecomposition in the troposphere,is an important precursorfor the stratosphericsulfate aerosol layer [Crutzen,1976; Hofmann, 1990;Engeland Schmidt,1994] and thushas an impacton the Earth's radiation budget aswell as on heterogeneousreaction chemistryleading to ozone destruction [Turco et al., 1980; Rocheet al., 1994;Faheyet al., 1993;Solomonet al., 1993].Our understandingof the sinkand sourcerelationshipsof this trace gas is still poor as reflected in imbalancedbudget estimates [Chin and Davis, 1993;Johnsone! al., 1993],which were proposedto be correctedrecentlyby exchangingthe soil source strengthinto a soil sink strength[/lndreaeand Crutzen,1997].
leadsto an obviouslymore balancedglobalbudgetof sinksand sources.However, it should be kept in mind that the troposphericoxidationof dimethylsulfide(DMS) is regardedto act as an additionalsourcefor atmosphericCOS within a range of
0.1-0.28Tg COSyr-• [Barnes e!al., 1994,1996].Furthermore, the anthropogenicinput of reducedsulfurcompounds,including COS, maybe underestimated[Dippelland Jaeschke,1996]. Both exampleswould introduce a new significantimbalance into the global budget estimates. As outlined above, there is a clear need for a better under-
standingof the role of soilsas an importantsinkfor COS and the consequentimpact on the global COS budget. Investigationsof the uptakeof COS by soilsunder controlledlaboratory Vegetation witha sinkstrength of 0.16-0.91Tg COSyr-• is conditionsare a usefulapproachto examiningthe uptake prothe major troposphericsink of COS [/lndreaeand Jaeschke, cesses.Some earlier studieshave already shownthe general 1992; Chin and Davis, 1993; Kesselmeierand Merk, 1993; Kes- uptakecapabilityof soils.However,within thesestudies[Bremselmeieret al., 1997]. Soilswere previouslyconsideredto rep- her and Banwart, 1976; Kluczewskiet al., 1985; Lehmann and resentone of the dominantsourcesof COS [Khalil and Ras- Conrad,1996],very high concentrationsof COS were usedfor mussen, 1984;/lndreae and Jaeschke, 1992; Chin and Davis, flushingthe enclosures,which also generated artificial COS 1993;Johnsone! al., 1993], in additionto photochemicalpro- concentrationgradientsand leadsto similarproblemsin interductionin oceanicsurfacelayersand CS2 oxidation.However, pretation as in the casesof flushingwith COS-free air. Within recently publishedfield studiespresent convincingevidence our studieswe therefore investigatedthe exchangeof COS that soilsact more as a sink than as a sourcefor COS [Castro between a soil and the atmosphereunder more realistic conand Galloway,1991;Fried et al., 1993;DeMello and Hines, 1994; ditionsin the range of natural atmosphericCOS mixingratios. Kuhn e! al., 1999], which suggests that earlier data shouldbe We studiedthe controllingvariablessuchas atmosphericmixcheckedfor artifactsdue to the use of inappropriateair mix- ing ratios, temperature, and soil moisture content. Additionturesfor flushingenclosures.As the ambientconcentrationof ally,we includedenzymaticinvestigationsin order to verify the a giventracegascontrolsthe directionaswell asthe magnitude biologicalcontrol of the uptake. of its flux betweenbiosphereand atmosphere[Kesselmeier and Merk, 1993; Kesselmeieret al., 1993; Conrad, 1994], earlier 2. Material and Methods studies using COS-free air for purging the soil enclosures masked the uptake capabilityof soils, as clearly shownby 2.1. Soil Samples Castroand Galloway[1991] under field conditions.With this Soil samples,consistingof sandyclaywith a low loessconCopyright1999 by the American GeophysicalUnion. tent, were obtainedfrom an agriculturalsite near Mainz, Germany. Sampleswere taken from the top 5 cm after removing Paper number 1999JD900090. 0148-0227/99/1999JD900090509.00 winter barley plants.The sampleswere sievedwith a stainless 11,577
11,578
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ET AL.: UPTAKE
OF ATMOSPHERIC
Table 1. Data on Soil Characteristics Made Availableby the Landesamtfiir Pflanzenschutz (District Office for Plant Protection),Mainz, Germany
7.3 8.8
Humus, wt %
2.22
Ctotal,wt %
1.94
Corganic , wt %
1.36
Stotal,wt % Ntotal,wt %
0.022 0.156
Fieldcapacity, % H20 g-1 DW
SULFIDE
BY SOIL
(1) silicagel(Merck, Darmstadt,Germany), (2) molecular sieve(0.5 am, Merck, Darmstadt),and (3) charcoal(Merck, Darmstadt),3 L each.COS concentrationat the outlet of the
gaspurification system wasfoundto be nearzero( 18 MD cm). mated to be -10%, basedon the variabilityof permeation 2.2. Construction and Performance of Soil Enclosures devicesamplesinjectedon a daily basis(n = 60). The folwere made to calculatethe errorsof the The measurements wereperformedwith twoenclosures (cu- lowingassumptions vettes):one enclosing the soilsampleandthe other servingas COS exchangerates:6% error for the measurementsof the an empty reference.Both cuvetteswere installedinside a cli- COS concentrationsc at the cuvette inlet rrin and cuvette mate chamberunder controlledtemperatureand light condi- outlettrout, 5% errorin thecuvette flowmeasurement rrQ,and tions.Exchangedatawere calculatedasthe differencebetween 0.5% in the dry weight estimationfreq.On the basisof these the two cuvettes. The cuvettes were constructed of Teflon valueswe calculatedthe total error rrF of the COS exchange to the lawof errorpropagation[Doerffel,1984]: (FEP) film asprincipallydescribed byKesselmeier et al. [1996] ratesaccording andKuhnandKesselmeier [1997]andhadthe followingdimen/[(C'nO'in! - • (Cout_O'out) 2 2] sions:ID, 14.5 cm; length, 9 cm; volume, 1.5 L. Total airflow
OF_••/
througheachcuvettewas1 L min-•. All gaslinesdownstream of the cuvetteswere heatedto 30øCto preventwater vapor condensation.Temperatureswere measuredwith thermocouples (0.005", Chrom-Constantan,Omega, Manchester,England).Relativehumidityand temperatureat the cuvetteinlet were determinedwith a Vaisala sensor(model 133Y, Vaisala, Malm6, Sweden).CO2 exchangewasmeasuredwith an infrared gasanalyzer(Licor 6262, Licor, Lincoln,Nebraska)and allowedthe calculationof soil respiration. The influenceof the soilmassper surfacearea (i.e., the soil columnheightinsidethe cuvette)on the exchange of COSwas testedup to 400 g fresh soilweightper cuvettegroundarea.
2.5.
(Coat_ Cin) 2 _.It_ (T•? _.It_ (TA 2 (1)
Data Storage and Handling
All continuously measuredparameterswere storedas5-min averageson a data logger (model 21X, CampbellScientific Inc., Shepshed,England). 2.6.
Enzyme Inhibition
The enzymecarbonicanhydrase(CA) was inhibited by 6-ethoxy-2-benzothiazole-2-sulfonamide (EZ). Owing to its abilityto penetratebiologicalmembranes,this specificinhibitor is able to inhibit extracellular
as well as intracellular
CA
Thiscorresponds to 2.42g cm-2 and2.6-3.1cmsoilcolumn [Moroheyet al., 1985]. Accordingto Andersonand Domsch height.We found a linear correlationof COS exchangeand [1975],EZ was mixedthoroughlywith the soil sample(0.5 soilmassupto 200g soilpercuvette(1.21g cm-2 and1.6-2.1 g/100g soil) and kept at room temperaturefor 2 hoursbefore cm soil height),which shiftedto a saturation-likeexchange behaviorwith increasingsoil massesbetween200 and 400 g. All furtherexperiments wereperformedwith 200 g of soilper cuvette.All resultsare generallyrelatedto the appliedmassof soil.For comparisonof the obtainedvalueswith surfacearearelatedfield data, the laboratorydata were adjustedto a soil massof 400g, exhibitingan exchange saturationcomparable to natural
2.3.
soil surfaces.
Configuration of Gas Mixtures
Compressedair was purifiedby passingit througha threestagegaspurificationsystemconsistingof three columnswith
measurements
were taken.
2.7. Algorithmic Description of the COS Uptake
An algorithmtakinginto accountan enzymaticbehaviorfor processdescription[Sharpeand de Michelle,1977],simplified by Guentheret al. [1991, 1993] to describeisopreneemission from higher plants,was in this caseempiricallyadaptedto outlinethe COS exchange dependence on temperatureaswell as soilwater content.Thus the exchangeat a giventemperature F r or givensoilwater contentF w is describedas a function of a standardexchangefactorF o multipliedby a temper-
ature ½(T) or soil water content ½(W) correctionfactor,
KESSELMEIER
ET AL.' UPTAKE OF ATMOSPHERIC
CARBONYL
SULFIDE
BY SOIL
11,579
respectively(equations(2) and (3)). Empirical coefficients by the enclosedsoil in the well-mixed cuvetteatmosphere,the were fitted by iterative adaptationof the least squarederrors. mixingratio measuredat the outlet droppedto lower values. Standardtemperaturewas defined as 25øC,and standardsoil Accordingto Winnerand Greitner[1989] these outlet values water content was defined as 13.5%. represent the effective mixing ratios for the incubated soil samples.Therefore we related all exchangedata to the COS r0(r) (2) concentrationsat the cuvetteoutlet. The data are compiledin withFr, COSexchange rateat temperature T (pmolg DW-• Figure 1 and showan increaseof COS uptakewith increasing h-•); F o, COS exchange rate at standardtemperatureTs ambient COS mixing ratios. This increaseappearslinear for (pmolgDW-• h-•); andq0(T),empirical correction factorfor the range between 0 and 1200 ppt. Under higher concentrationsa saturation-likeuptakebehavioris visible(upperinset). temperature. Only below mixing ratios of 50 ppt was a significant(confi(3) dencelevel equal to 95%) emissionof COS found. The obwith Fw, COS exchangerate at water content W (pmol g servedlinear correlationbetweenthe uptake of COS and the DW-• h-•); F o, COSexchange rateat standard watercontent ambient concentrationsin the range of 0-1200 ppt made it Ws(pmolg DW-• h-•); andq0(W),empirical correction fac- possibleto normalizeall other data obtainedin the courseof this studyto virtual fumigationconditionsof 500 ppt COS (see tor for water content.The parameterq0(T) is definedby equation(6) below).
kn(T(oc)- Ts(oc))
exp RTs(i•)T(i•)
qo(r) -kT2(r(oc )_kT3) 1+ exp RTx(K)T(K)
(4)
with T(øC) or T(K), actualtemperature(øC or K); Ts(øC)or Ts(K), standardtemperature(øCor K); R, molargasconstant
(8.314)(J K-• mol-•);kr•, empirical coefficient (116,781)(J K mol-• øC-•); kr2, empiricalcoefficient (312,219.8)(J K mol-• øC-•);andkr3, empirical coefficient (18.449)(øC).The parameterq0(W) is definedby kw•(W-
Ws)
exp RWWs
=
(5) 1+ exp RWsW
with W, actualwater content(%); Ws, standardwater content
(%); R, molargasconstant (8.314)(J K-• mol-•); kr•, empiricalcoefficient (165.627) (J(%) K-• mol-•);kw2, empirical coefficient (1,028.005) (J (%) K-• mol-•); andkw3, empirical coefficient(15.071) (%). 3.
3.1.
Results
Uptake of COS Under Varying COS Mixing Ratios
Accordingto a simpleempiricalconceptproposedby Conrad [1994] and Lehmann and Conrad [1996] to describethe dependenceof the COS flux rate on the ambient concentration, the net exchangewithin a certain biotic systemis interpreted as the result of simultaneously operatingproduction and consumptionprocesses. The consumptionrate is assumed to be a functionof trace gasconcentration,whereasthe production rate is not. This implies the existenceof a so-called compensationpoint, that is, an ambient concentrationwhere the consumptionand productionbalanceeach other and the net flux is zero. The productionrate, the depositionvelocity, and the compensationpoint concentrationare determinedby the linear regressionof observedfluxesversusconcentrations, and are found as the ordinate intercept, the slope, and the abscissaintercept of the regressionline, respectively.At ambient concentrationsbelow the compensationpoint, net emissionis observed,while concentrations abovethe compensation point are associatedwith net deposition.Within our experimentswe varied the COS mixingratiosin the air flushingthe cuvettebetween0 and 2700 ppt. Owing to the uptake of COS
3.2.
Uptake of COS Under Varying Temperatures
Temperaturehas a significantinfluenceon chemicalaswell as biological reactions. We therefore determined the COS uptakeby the soil undervaryingtemperatures(0ø to 30øC)as a function of COS mixing ratios between0 and 1400 ppt. In accordancewith the uptake of COS under varyingmixing ratios at 17øC as describedin the previous section, a linear responsewasalsofound betweenthe exchangeratesand COS mixingratiosunder all investigatedtemperatureregimes(Figure 2). The data obtainedfrom the experimentswith COS concentrations around 600 (_+130ppt), and hence closeto natural ambientconditions(500 _+100 ppt), were usedto describethe dependenceof COS uptake on temperature.The linear responsecurvesalloweda normalizationof the COS exchange ratesto a uniformCOS mixingratio of 500ppt accordingto (6) and thus alloweda comparisonand discussion of the temperature dependency.
Fn=(Fex p- Fp)COSnorm COSexp +Fp
(6)
withFn,normalized netfluxofCOS(molgDW-• h-•);Fexp, measured net flux under experimental conditions (mol g
DW-1 h-l); COSexp, COSmixing ratiounderexperimental conditions(partsper trillion, ppt); COS..... COS mixingratio
undernormalized conditions (ppt);andFp,y axisintersection of regressionline (e.g., Figure 2) representingthe production rate of COS in the soilat the giventemperature[Lehmannand
Conrad,1996](molg DW-• h-•). Figure 3 showsthe normalizedCOS exchangedata in relation to the temperature regimes.The uptake increaseswith temperatureup to an optimumbetween16 and 20øC,followed by a sharpdecreaseat higher temperatures.Using the empirical model of Conrad[1994] and Lehmannand Conrad[1996] and the data presentedin Figure 2, the COS depositionvelocity and the productionrate can alsobe describedas a function of the incubationtemperature(Figure 4). The temperature dependencyof the depositionvelocity,derivedfrom the slopes of the regressionlines,showeda similarpattern as compared
to the fluxdataof the naturalCOS concentrations range(Figure 3). The productionrates,which were derived from the ordinateinterceptsof Figure 2, were very low and resultedin high relative errors. Thus, although a similar pattern for the productionrate is expected,it is not as obviousin Figure 4.
11,580
KESSELMEIER ET AL.: UPTAKE OF ATMOSPHERIC CARBONYL SULFIDE BY SOIL
- -6
•
A
[:12 =0,99
Y=0,006x -0,32
-4••/ Y=-2E-06x 2+0,0092x -0,5911 R2=0,996 n=29
_-2
, 500
1000
ß
1500
_
_
_
•
•
lOO
300
0
0
200
400
600
800
1000
1200
1400
COS mixing ratio [ppt] Figure 1. COS exchange correlatedto the ambientmixingratioinsidethe cuvette(positivesignis uptake). For eachcondition,two differentsoilsamples weremeasuredat leastfour times.Air temperaturewithinthe cuvetteswas held constantat 17 _ iøC. The left, upper inset givesthe exchangedata over all ambient concentrationranges.The right, lower insetexhibitsthe rangebetween0 and 400 ppt includingthe 95% confidencelevel. The compensation point, reflectingthe compensation betweensimultaneously operating productionand consumption processes, wascalculatedto be about53 ppt COS.
The data showa zero exchangeat a water contentbelow6%. Higher water contentsbetween10 and 15% resultedin a maxThe water content of soilsis a critical parameter for the exchangeof tracegases[Conrad,1995;Meixneret al., 1997],as imum of COS uptake,but further increasesin the water conit influences chemical, physical, and biological processes. tent led to a decreaseof exchange. Thereforewe investigated the exchangeof COS by varyingthe 3.4. PhysiologicalBackground for the Uptake of COS soilwater contentbetween5 and 42% (dry weight).TemperBoth biologicaland physicochemical parameterscanpotenature was held constantnear the temperatureoptimum(see Figure3) at 15øC(_+0.6).This coolenvironmentallowedsev- tially explainthe revealedCOS exchangebehaviorin relation to the soil water content and ambient COS concentration. The eral repetitionsof the measurements with a negligiblewater lossof 0.2-0.5% during the experiment.Figure 5 showsthe observedoptimumin relationto temperature,however,points catalyzedprocess[RadmerandKok, 1979]. COS exchangenormalizedto an ambientmixingratio of 500 to an enzymatically ppt COS (seesection3.1) in relationto the soilwatercontent. An enzymeincreasesthe turnover with increasingtemperature, but thistrend is superimposed by a decreaseof activityif 3.3. Uptake of COS Under Varying Soil Water Content
30oC
¸
o
•
10
15
20
25
30
35
Temperature [øC]
Figure 3. COS uptakenormalizedto ambientmixingratios of 500ppt in relationto the temperatureregimes(n = 4; error bars are 1 tr). Only measurements applyingCOS concentraFigure 2. RelationbetweenCOS uptakeand ambientmixing tionscloseto naturalambientconditions(600 _+130ppt) were ratiosat different temperatures.Three groupsof three to four used.The plottedline givesthe mathematicalapproximation sampleseach were taken for every temperaturerange. All accordingto the algorithmF r = Foq>(T) as describedin section2.7. Someerror barsare smallerthan the symbolwidth. regression linescalculated showed r2 > 0.95. 0
200
400
600
800
1000
COS mixing ratio [ppt]
1200
1400
KESSELMEIER
,--
ET AL.' UPTAKE OF ATMOSPHERIC
CARBONYL
SULFIDE
BY SOIL
11,581
0.8
E 0.6 o o
0.4
o
0.2
t
o
•
.......
0
12E
•
:
- 8 ._o -
•
4.o
5
15
10
20
25
o
Temperature [øC]
Figure4. COSdeposition velocities (mms-•) andgross production ratespersurface area(pmolm-2 s-1) asa function of theincubation temperature determined bytheslopes andtheordinateintercepts, respectively, of the regression linesin Figure2 (normalized to ambientmixingratiosof 500ppt). Someerrorbarsare smallerthanthesymbol width.Dryweight-based production ratesweretransformed intosurface-based rates. Forcomparison of theobtained values withsurface area-related fielddata,thelaboratory datawereadjusted to a soilmassof 400g, exhibiting anexchange saturation comparable to naturalsoilsurfaces (for details,see section2.2).
the temperature rangeexceeds a certainvalueowingto reorganization and/ordenaturation of the enzymestructures. The key enzymefor the uptakeof COS by differentbiological organisms hasbeenidentifiedto be carbonicanhydrase (CA) [Chengelis andNeal, 1979,1980;Milleret al., 1989;Badgerand Price,1990;Protoschill-Krebs andKesselmeier, 1992;ProtoschillKrebset al., 1995, 1996;Kuhn, 1997].Accordingto Moroneyet
al. [1985]thisenzymecanbe specifically inhibitedby 6-ethoxy2-benzothiazole-2-sulfonamide (EZ). We thereforecompared theuptakeof thesoilsample withandwithoutinhibitionof the specific enzymeCA. Resultsareshownin Figure6. The addition of the inhibitorEZ to the soilsampleresultedin a highly
significant (p < 0.001) reductionof the COS uptakeof more than 50%, showingCA to be the dominantfactor for the consumption of COS by the investigated soil. 4.
Discussion
The data showthat the investigatedsoil type is a significant sinkfor the tracegasCOS. The consumption of COS by this soildepends on physiologically activemicroorganisms, asindicatedby the drasticinhibitionin the presenceof the enzyme inhibitorEZ. Physiologically activemicroorganisms, responsible for the consumption of COS by soils,were alreadyindicatedby experiments of Bremnerand Banwart[1976].However,evennow thereis still onlyrare informationaboutthese
Water-filled pore space[% ]
4
210
i
6•0
i
1•00 i 3
'7.
3 2 m
0
•
-1
E
0
10
20
30
40
•
50
1
Soil water content [ % ]
Figure 5. COS uptakein relationto the soilwater content. COS uptakeratesare normalizedto ambientmixingratiosof 500pptCOS(n - 4; errorbarsare1 tr).Theplottedlinegives the mathematicalapproximation accordingto the algorithm F,• = Foq>(W ) asdescribed in section2.7.Someerrorbarsare smallerthan the symbolwidth. Soil water contentwas measuredasdescribed in section2.1. Water-filledporespace(up-
0
0
Figure 6. Inhibitionof soil COS uptakeby the carbonicanhydrase(CA) specificinhibitor 6-ethoxy-2-benzothiazole-2sulfonamide (EZ); n - 8, p < 0.001. Inhibitionby nucleo-
philicattackat the activecenter(Zn) of CA. Applicationis perx axis)wasestimated according to Hillel [1980],assuming 0.5 g EZ/100 g soil with 2 hoursof incubationbeforefirst [Anderson andDomsch,1975].Standarddeviaa bulkdensity for clayeysoilsof 1.25g cm-3 anda general measurements
particle density of 2.65g cm-3.
tion is +_1
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KESSELMEIER
ET AL.: UPTAKE OF ATMOSPHERIC
CARBONYL
SULFIDE BY SOIL
within the investigatedsoil specimensis of minor importance. We calculateda COS productionin the rangeof 0.33-1.8 pmol
gDW-1 h-l, witha maximum at25øC. Thesesmallnumbers as
Increase (at
i Decrease (a, low ambient COS)
Figure 7. Modelingsoil COS uptakein relationto temperature and soil water content.AtmosphericCOS concentration influencesthe amplitudeof the uptake.
comparedto the measuredCOS uptake in combinationwith the compensation point observedaround50 ppt (95% confidencelevel, Figure 1) clearly showthat the investigatedsoil type will hardly emit COS in nature. These results are in contrastto the investigations of Lehmannand Conrad[1996], who calculateda muchhighercompensation point, reflectinga higher probabilityof COS emissionby soils.However, those studieswere done with a different soil and a COS fumigation well above500 ppt. Thus they may not reflect natural conditionsand uptakebehavior,as our studiespoint to a nonlinear pattern under fumigationwith high COS concentrations.The observedcompensationpoint of 53 ppt is very similar to data obtainedduringfield measurements on a different soil type in California [Kuhn et al., 1999] showinga compensationpoint below 100 ppt. Similarvaluesof 90-150 ppt were also found for the exchangeof COS betweenplantsand the atmosphere [Kesselmeier and Merk, 1993]. The resultsof this studysupportdata obtainedwithin field experiments[Castroand Galloway,1991; Fried et al., 1993; DeMelloandHines,1994;Kuhnet al., 1999]whichsuggestthat soilsact as COS sinksinsteadof sources.The uptake rates reported here for the arable soil samplesrange between 1.5
COS-consumingsoil microorganisms. To our knowledge,only Kelly et al. [1994] succeededin showingThiobacillusspecies consumingCOS. It would be of great interest to expandthe and10.3(pmolm-2 s-1) andfitwellintotherangereported by studiesto more soil microorganisms, includingfungi, in order Castroand Galloway[1991],who found a net COS deposition to get a better understandingof the COS depositionto soils.
of 1.4-8.4 (pmolm-2 s-1) for unvegetated forestsoils.Fur-
Carbonic anhydrase(CA) activity responsiblefor the consumptionof COS wasshownin severalmicroorganisms already [Chengelis and Neal, 1979,1980;Miller et al., 1989;Badgerand Price, 1990; Protoschill-Krebs et al., 1995]. Accordingto our studiesthe enzymecarbonicanhydrasecanbe regardedas the dominant factor for the consumptionof COS by the investigated soil. AtmosphericCOS mixing ratios, temperature,and soilwater contentare the physicochemical parameterscontrolling the uptake. An existingalgorithm was adapted to the temperatureas well as soil water contentdependencies(see Figures3 and 4). Combiningthe environmentalparameters, controllingthe uptake of COS, we come up with an uptake algorithmwhere the actualflux F is achievedby multiplyinga standarduptakerate F o by correctionfactorsfor water content q0(W), ambientCOS concentrationq0(C), and temperature
thermore, the rates are similar to those observed with intact
F = Fo•p(W)•p(C)q(T)
(7)
The combined influences are illustrated by the threedimensionalplot in Figure 7. The observedtemperatureand soil water contentdependenciesresult in a peak-shapedoptimum whose amplitude is influenced by the applied atmosphericmixingratio. Emission
of COS was not observed
under a natural
set of
peatsoilranging between1.25and4.2(pmolm-2 s-l) [Fried etal., 1993]and1.0-15.3(pmolm-2 s-•) [DeMello andHines, 1994] or to those observedat the soil surface in an open Californianoak woodlandrangingbetween8.8 and 13.3 (pmol
m-2 s-i), thoughthesevaluesmaybe slightlyoverestimated [Kuhnet al., 1999].Also, the calculatedCOS depositionvelocities fit into the data setsas found in the literature for plant surfaces[Kluczewski et al., 1985;Tayloret al., 1983;Goldanet al., 1988;Kesselmeierand Merk, 1993;Huber, 1994;Kuhn et al.,
1999] as well as soil surfaces[Kuhn et al., 1999]. These similarities give reasonto assumethat soilsin generalshouldbe consideredassinksand that an adaptationof the globalbudget by changingthe sourcestrengthinto a sink strength[Andreae and Crutzen,1997] is justified, althoughsoil types and soil redox conditionsexistwhichwill lead to a significantproduction and emissionin some specialcasessuch as salt marsh sediments[Devai and Delaune, 1995] or waterloggedpaddy soils[Yanget al., 1996]. In addition, thoseresultshave to be discussedin light of knowledgeabout the influence of atmosphericCOS mixing ratios and compensationpoints.Further measurementsof soil/atmospherefluxesof COS under natural conditionsare urgently needed and should be incorporated into future estimatesof the global COS budget to reduce the existinguncertaintiesand imbalances.
experimentalconditions.Accordingto Remdeet al. [1989]and Conrad[1994] the gasexchangecan be discussed as a resultof simultaneouslyoperating productionand consumptionproAcknowledgments. This work was supportedby the Max Planck cesses.The applicabilityof this concepthas been empirically Society.We gratefully acknowledgethe help of Hubert Stenner for confirmed for exchangeof various biogenic trace gaseswith providingsoil samplesand the staff of the Landesamtf/Jr Pflanzenschutz,Mainz, for providingdata on elementalanalysisand soil qualsoils[Remdeet al., 1989;Conrad,1994;Ludwig,1994],for COS ities. We thank T. Biesenthalfor his help with the manuscript. in plants [Goldan et al., 1988; Kesselmeier and Merk, 1993; Kesselmeier et al., 1993], and lichens[Kuhn and Kesselmeier, References 1997;Kuhn, 1997].However,the extremelylow mixingratiosof Anderson, J.P., and K. H. Domsch, Measurements of bacterial and COS leading to a measurablerelease of this gasfrom the soil samples(Figure 1) show that the productionterm of COS
fungalcontributionsto respirationof selectedagriculturaland forest soils, Can. J. Microbiol., 21,314-322,
1975.
KESSELMEIER
ET AL.: UPTAKE OF ATMOSPHERIC
Andreae, M. O., and P. J. Crutzen, Atmospheric aerosols:Biogeochemicalsourcesand role in atmosphericchemistry,Science,176, 1052-1058, 1997.
CARBONYL
SULFIDE BY SOIL
11,583
as an oxidant scavenger,Atmos. Environ., Part A, 26, 2445-2449, 1992.
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