JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 104, NO. D19, PAGES 23,617-23,622, OCTOBER 20, 1999
Methane uptake by a selectionof soilsin Ghana
with
different
land use
Anders
Priem6andSorenChristensen
Departmentof PopulationBiology, CopenhagenUniversity,Copenhagen
Abstract. We measuredthe oxidationof atmospheric
methanein tropicalsoilsin Ghanacovering
a moisturegradientfrom themoistforestzoneto the savannazoneat the onsetof the rainy season.
Landuseat the sitescoveredundisturbed
(forestandsavanna)andcultivatedsoil,including
burning.Generally,themethaneoxidationratesin thetropicalforestandsavannasoilswerelow
(rangefrom 9 to 26 [tg CH4m"•h") comparedto, for exampletemperateforestsoils.In the savanna
soil, annualfire had decreasedsoil methaneoxidationratesto 5 [tg CH4m-2h'• comparedto 9 •tg
CH4m-2h-•at a sitenotsubjected
to fire for 6 years.In pairedsitesof moistforestandarablesoils,
methaneoxidationrateswere lowerby >60% in the arablesoils.Methaneoxidationratesin three
arablesoilsin the savannazonesoilsrangedfrom 7 to 11 [tg CH• m-2h-•beforethe first rain but
increasedto 23-28 •tg CH• m-•h-•aftertherain. Theseratesarecomparableto otherreportsfrom
arablesoilsin tropicalandtemperateregions.Thusarableagricultureand,to a lesserextent,
biomassburningdecreased
methaneoxidationratesby the investigated
soils.
1. Introduction
2. Materials
and Methods
Methaneis an important
greenhouse
gascontributing
to global 2.1. Field Sites
warming[Rohde,1990]. Oxidationof atmospheric
methaneby
Three areaswere selectedfor measurements
of methaneuptake
microorganisms
has been reportedin virtually all investigated rates.In the Guineasavannaarea [Dicksonand Benneh,1988] in
well-drainedsoils.The globalsinkstrength
of thesesoilshasbeen thenorthernregionof Ghana,we workedin thefieldssurrounding
estimatedat 17-23 Tg yr-1 with nearly half of the soil sink for thevillageDalun(9N35', 0W58'), 15 km northof Tamale,andin
methane9ccurringin tropicalecosystems
[Potteret al., 1996].
Mole National Park (NP) (9N15', 1W50'), 100 km west of TaDespitetheir importanceas a sink for methane,tropicalsoils male. Both areasrecieve 1000-1500 mm precipitationannually
have receivedlittle attentioncomparedto temperatesoils.Land [Dicksonand Benneh,1988] and have a distinctrainy season
usechanges
involvingconversion
of forestto agriculture
andvice usuallyextendingfromMay to October.At Mole NP, two paired
versahavebeenshownto have a pronounced
effecton methane
uptakein temperatesoils [Dobbie et al., 1996; Priemd et al.,
1997],but it is not clearif the samelandusechangeshavesimilar
effectson methaneuptakein tropicalsoils.Agriculturalpractices
in mosttropicalcountriesdiffer from temperatezoneagriculture
in termsof, for examplefallow periodsand applicationof fertilizers and pesticides.Also, fire is an integratedpart of agriculturein manypartsof the tropics.
In the Neotropics,
Keller et al. [1990, 1993]and Goreauand
de Mello [1988] foundthatconversion
of foreststo cattlepastures
or arablefields decreased
the soil sink strengthfor atmospheric
methane
or turnedthesoilintoa netsource
of methane.
In tropical
soils, methaneproductionfrom termitesmay affect the sink
strengthfor atmospheric
methane[MacDonaldet al., 1998];and
landusechanges
maychangetermiteactivityaswell asthe soil's
capacityfor oxidationof atmospheric
methane.In this study,we
investigated
the effect of land use (includingfire and fallow
period)on methaneuptakeby a varietyof soilsin Ghana.
experimentalsiteswere selected:an unburnedsite which was
accidentially
burned6 yearsearlieranda siteburnedannuallyfor
the last 10 yearsin Februaryduringthe midstof the dry season.
The two sitesshowedno differencesin the woodyvegetationand
the most commontreesat both siteswerePterocarpuserinaceus
(Fabaceae), Parkia clappertonia (Mimosaceae), Piliostigma
thonningii(Caesalpinaceae),
AJkeliaafricana (Caesalpinaceae),
Detarium microcarpa(Caesalpinaceae),Burkea africana (Caesalpinaceae), Nauclea latifolia (Rubiaceae), Butyrospermum
paradoxurn(Sapotaceae),
Anogeissus
leiocarpus( Combretaceae),
Combretum ghanalense
( Combretaceae), Terminalia
avicennoides (Combretaceae),Annona senegalensis(Annonaceae), Parinaria curatellifolia (Chrysobalanaceae),Parinaria
polyandra (Chrysobalanaceae),
Pseudocedrelakotchyi (Meliaceae),Afrormesialaxifiora (Papilionaceae),Maytenussenegalen-
sisx(Celastraceae),
Lannea
acida(Anacardiaceae),
andHannoa
undalata(Simaroubaceae).
In the southernpart of the country,we
worked in the fields and the primary moist semideciduous
forest
[Dickson and Benneh, 1988] near the village of Odumasi
(5N25'37", 1W41'54"), approximately50 km northof Takoradi.
Here the precipitationis 1500-2000mm annually[Dicksonand
•Nowat Center
forMicrobial
Ecology,
Michigan
StateUniversity,Benneh,1988]. The forestplotshad no logginghistoryand had
EastLansing.
apparentlyneverbeencultivated,and were separated
from fields
Copyright1999 by theAmericanGeophysical
Union.
by a dustroad. Soil temperature
at 2-, 5-, and 12-cmsoil depth
rangedfrom 25ø to 39øC at the sitesat Dalun and Mole NP and
Papernumber1999JD900427.
from from 24 ø to 30øC at the Odumasi sites. An overview of the
0148-0227/99/1999JD900427509.00
sites is found in Table 1.
23,617
23,618
PRIEMI•ANDCHRISTENSEN:
METHANEOXIDATION
BY SOILSIN GHANA
Table 1. Site Characteristics
Location
Land Use
Dalun village
Dalunvillage
Dalunvillage
Dalunvillage
Dalunvillage
DominantVegetation
SoilOrder
SoilMoisture, pH
percentof WHC
Total Soil C, Total SoilN,
percentof dwt percent
of dwt
yamsfield
Dioscoresrotundata
18 yearsfallow grasses
dominated
byAndropogonsp.
25 yearsfallow grasses
dominated
byAndropogonsp.
Oxisol
Oxisol
Oxisol
6.3
12.1
10.1
5.8
5.9
5.7
0.41+0.00
0.43+0.06
0.35+0.05
0.024+0.001
0.027+0.003
0.029+0.003
maizefield
6 yearsfallow
Zea mays
grasses
dominated
byAndropogonsp.
Oxisol
Oxisol
15.3
7.4
5.1
5.1
0.48+0.01
0.40+0.07
0.032+0.001
0.029+0.005
savanna,
seetext
Oxisol
54
7.1
0.77+0.08
0.063+0.007
savanna,burned seetext
Oxisol
63
6.9
1.98+0.14
0.141+0.014
Odumasivillage moistforest
no dominantspecies,
denseundergrowth not known
not known
Odumasivillage 3 yearsfallow
Grasses
not known
Odumasivillage 25 yearsfallow Grasses
79
52
63
5.3
5.1
4.8
1.44+0.05
0.75•-0.02
1.93+0,28
0.107:k0.006
0.073•-0.001
0.167+0.019
no dominantspecies,
denseundergrowth not known
67
89
4.8
4.9
4.33:•0.05
2.00
0.345+0.012
0.178
79
91
4.3
4.7
2.52+0.41
1.86+0.07
0.163+0.015
0.183•-0.005
Mole NP
unburned
Mole NP
Odumasivillage moistforest
Odumasivillage cassava
field
Manihot
Odumasivillage moistforest
nodominant
species,
•
Odumasi
villagecocoa
field
Theobrama
utilissima
not known
undergrowth not known
cacao
not known
Soilmoisture
andpH n=l; totalsoilC andN areaverage
+1 SE(x);n=2.Siteswhicharegrouped
together
werelocated
nextto eachotherorseparated
by a dustroad.WHC iswaterholdingcapacity;
dwtis dryweightsoil.
2.2. Estimation of Methane Uptake Rates
At eachsite,sixteento eighteen10-cminnerdiametercollars
consistingof PVC cylinderswere pushed5 cm into the soil.
Methane oxidation rates were measured once or twice at each site
duringJuly 1995.Later,a 5-cm-highPVC lid wasfittedintoa
butyl rubberlined groovein eachcollar.Methaneuptakerates
were calculatedfrom the lognormaltransformeddecreasein
chamberheadspace
methaneconcentration
as estimated
fromthe
analysisof 4-mL gassamples
takenafterattaching
the lids and
againafter5, 10, 15, and20 min. The gassamples
weretaken
fromthe chamberheadspace
througha butylrubberstopperusing
a syringeandimmediately
transferred
to 2-mL evacuated
injection
vials crimp sealedwith a blackbutyl rubberstopper.Methane
analyses
weredonewithin 1 monthof sampling,usinga Varian
3400gaschromatograph
(GC) equipped
witha PorapakT column
operatedat 45øC and a flame ionizationdetectoroperatedat
160øC. Also, the GC was equippedwith a Varian 8100 Auto-
remainingflux measurements
showednet methaneemission
rangingfrom 10to 105 gg CH4m-2h-1.
FigureI showsthemethane
uptakeat twopairedsitesof primarymoistforestandagricultural
land(cassava
andcocoa).On
average,
methane
uptakeratesin the agricultural
soilswere63%
lower than in the moist-forest soils. In the savanna and moist
forestareas,methane
uptakeratesweresimilarin pairedfieldand
fallow soilswhichwereinvestigated
on the samedatesandthus
had comparable
water status(datanot shown).Soil methane
uptakeratesin theburnedandunburned
savanna
wererelatively
low. Uptakeratesat the burnedsite were lowerthanat the
unburned
site(Figure2). Methaneuptakein threearablesoilsin
the savannaarea increasedtwofold to threefoldbetween2 days
beforeand2 daysafterthe first heavyrainsof the rainy season
(Figure3).
4. Discussion
samplermodifiedfor injectionof 0.8-mLgassamples.
The injecTheaverage
reduction
in methane
uptakeratesin agricultural
tor temperature
was40øC,and99.9998%purenitrogen
wasused
soilscompared
to forestsoilswas63%, whichagrees
with the
as carriergas. Preliminaryexperimentsshowedthat the vials
average
80%
reduction
following
conversion
of
forest
or
savanna
maintained constant methane con6entrations for at least 2 months
to agriculture
observed
in the tropics(Table2) andof forestor
(we did not testfor longerstoragetimes).
2.3.
Miscellaneous
Analysesfor total soil C and N were performedon a Carlo
ErbaNitrogenAnalyser1500. Soil watercontentwas estimated
afterdryingof soil for 24 hoursat 105øC,pH wasmeasured
in
1:2.5 soil:waterslurries,andsoil temperature
wasmeasured
at 2-,
5-, and 12-cmsoil depth.We did our measurements
duringthe
rainy seasonin the southernpart of Ghana;the rainy seasonwas
delayedand had not startedwhen we initiatedour work in the
northernpart of Ghana.However,the first heavy rain of the
season coincided with our work at Dalun.
3. Results
Methane uptake (range from <1 to 53 [tg CH4 m-2 h-•) was
observedin 275 out of 295 flux measurements
(93%), while the
prairielandin temperate
climates
[Mosieret al., 1991;Ojimaet
al., 1993;Dobbieet al., 1996].The similarreduction
in tropical
andtemperate
soilsmayseemsurprising
in viewof thelessintensive use in small-scalefarming in most tropicalcountriesof
potentialinhibitors
of methane
uptakeratesincluding
nitrogen
fertilizers
[Hiitschet al., 1994],pesticides
(A. Priem6andF.
Ekelund,Effectsof five pesticides
on soil oxidationof atmos-
phericmethane,
submitted
to Soil Biologyand Biochemistry,
1999),andtractors
andheavyfarmmachinery
[Hansen
et al.,
1993]compared
to European
andNorthAmericanagriculture.
However,thechanges
in soilstructure
andorganicmattercontent
accompanying
arablecultivation
mayalonecausea reduction
in
methane
uptake.
In themoistforestarea,totalsoilC waslowerin
thecultivated
soilscompared
to theforestsoils(Table1), andthis
difference
in organic
mattercontent
andaccompanying
changes
in
soilstructure
mayexplainat leastpartof thereduction
in methane
PRIEMl•ANDCHRISTENSEN:
METHANE
OXIDATION
BYSOILSIN GHANA
40
23,619
40
Forest
soil
Before
rain
Arable
soil
30
'""'":'"'"'"•
Afterrain
T
T
T
20-
20-
r
T
T
10-
T
10-
T
•
0
Site A
Site B
Yam field
18 yr fallow 25 yr fallow
Figure 1. Methaneuptakein soilsat two pairedsitesof primary Figure 3. Methaneuptakein three arablesoils in the savanna
moist forest and agriculturalland near Odumasi village in zone near Dalun village, northernGhana,2 days before and 2
southern Ghana. Error bars denote 1 standard error.
days after the first heavy rains of the rainy season.Error bars
denote I standard error.
uptake rates in the cultivatedsoils. Thus the bulk density is
usuallylargerin agriculturalsoilscomparedto naturalsoils,as a
resultof lossof structurebecauseof decreasing
organicmatter
content.McDonaM et al. [1996] found a negativecorrelation
betweenmethaneuptakeand soil bulk densityin a rangeof temperatesoils,andBall et al. [1997] foundthat relativediffusivity
and air permeabilityin the soil were goodpredictorsof methane
uptake in soils under different land use. Also, the methane-
oxidizingmicroorganisms
may be influencedby changesin the
availabilityof nonmethane
Co substrates
releasedduringorganic
matterdecomposition
[Jensenet al., 1998].The loweredmethane
oxidationrates in tropical arable soils may also be relatedto
changein termite numbersand/or termite speciescomposition
becausetermitesare a significantsourceof atmospheric
methane
[Khalil et al., 1990],andtermiteactivitymay influencethe soils
abilityto absorbmethanefromtheair [MacDonaMet al., 1998].
Workingwith temperatesoils,Priern• et al. [1997] foundthat
it takes decadesor even centuriesfor methaneuptaketo recover
from arableagriculture,while the studyby Keller and Reiners
[1994] indicatesthat methaneuptake in disturbed(secondary
15
forest and abandoned.pasture) systems in Costa Rica were
restoredto predisturbance
levelswithin50 years.The presentdata
on savannaandmoistforestsoilsindicatesomeincreasein uptake
rates in soils left fallow for 3-25 years (Figure 3 and data not
shown), but our data set does not allow us to draw any firm
conclusions
on the recoverytime as we were not able to locate
any pairedsitesof primaryvegetationandlate fallow.
In Africa, the areapotentiallysubmittedto fire is of the order
of 10 million km2 [Delrnaset al., 1991a], andin the WestAfrican
savanna,60% to 80% of the Guineansavannais burnedannually
10-
m
Table 2. MethaneUptakeRatesin UplandSoilsin Different
TropicalEcosystems
Ecosystem
Savanna'
ForesP
Pasture
I
Unburned
Burned
Fields
MethaneUptake,gg CI-I4m-zh-'
37+8 (n=7)
39+5 (n=25)
-8+7 (n--9)
7+5 (n=5)
Ratesareaverage+1 SE(x),andn denotes
numberof sites.The dataare
Figure 2. Methaneuptakein savannasoil in two experimental calculatedfromthereportscompiledin Table 2.
plots at Mole National Park, northernGhana.The burnedsite is
'Savanna includes unburned and burned sites.
burnedannuallyin themidstof thedry season.
Errorbarsrepre-
•Forestincludesprimaryforests,secondary
forests,disturbed
forests,and
sent 1 standard error.
clearcut forests.
23,620
PRIEMI•ANDCHRISTENSEN'
METHANE
OXIDATION
BY SOILSIN GHANA
Table 3. MethaneFlux Ratesby Predominantly
AerobicSoilsin TropicalEcosystems
Ecosystem
Location
Description
Methaneflux, Citation
•tg CH4 m-2h-1
Savanna
SouthAfrica
dry period
Savanna
Mali
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Savanna
Congo
Congo
Brazil
Brazil
Brazil
Venezuela
Venezuela
Brazil
Brazil
Brazil
SouthAfrica
SouthAfrica
Savanna
India
Savanna
Savanna
Ghana
Ghana
unburned,dry season
burned,dry season
Forest
Forest
Brazil
Brazil
rain forest
rain forest
Forest
Brazil
clearcut, rain forest area
Forest
Ecuador
rain Ibrest
Forest
Forest
Forest
Ecuador
PuertoRico
PuertoRico
secondary
rain forest
secondaryrain forest
mahoganyplantation
34
24
12
Forest
Puerto Rico
wet Ibrest
21
Forest
Puerto Rico
clearcut, wet forest
10
Forest
Brazil
rain forest
48
Forest
Brazil
disturbed forest, rain thrust area
45
Forest
Brazil
rain Ibrest
72
Forest
Forest
Forest
Forest
Forest
Forest
Congo
Malaysia
Malaysia
Malaysia
Venezuela
Panama
Evergreen
rain tbrest
old secondaryrain Ibrest
youngsecondary
rain lbrest
semideciduous,
rainy season
semideciduous,
rainy season
42
56
50
39
48
24
Forest
Forest
Costa Rica
Costa Rica
Semideciduous
Semideciduous
51
53
Forest
Forest
Forest
Forest
CostaRica
Cameroon
Cameroon
Cameroon
semideciduous,
secondaryforest
nearprimarymoist
secondarymoist
youngplantation
50
75
49
18
Forest
Central Afi'ica
rain forest
Forest
Forest
India
Ghana
seasonallydry
moistsemi-deciduous,
rainy season
Pasture
Pasture
Brazil
Brazil
rain lbrest area
rain lbrest area
52
3
unburned
burned
burned17 daysbefore,dry season
burned45 daysbefore,dry season
bumed20 yearsbefore,dry season
dry season
rainy season
unbumed,dry season
bumed2 daysbefore,dry season
burned30 daysbefore,dry season
unburned,dry season
burned,dry season
26
26
40
40
73
-41a
-34a
-30b
182b
850b
-68-+40
-56-+9
480c
9d
5d
15
16
12
-26
81
450c
18d
11
-41
Pasture
Pasture
Panama
Panama
burned,rainy season
unburned,rainy season
Pasture
Pasture
Costa Rica
Costa Rica
semideciduous forest area
semideciduous forest area
-27
-9
8
6
Pasture
Costa Rica
abandoned,semideciduous
-38
Pasture
Puerto Rico
Pasture
PuertoRico
8
differentagriculturalpractices
8
Fallow, weeded Cameroon
mosit forest area
17
Fields
Fields
rain tbrest area
rain forest area
-3
-5
Ecuador
Brazil
Field
Panama
semideciduous
forestarea,rainyseason
Field
Fields
Fields
Congo
Ghana
Ghana
savannaarea
moistthrustarea,rainy season
savannaarea,dry season
Fields
Fields
Ghana
Venezuela
savannaarea,onsetof rainyseason
savannaarea,dry and rainy season
Field
India
savanna area
9
19
14d
8d
26d
-47a
240c
Seiler et al. [ 1984]
Delrnaset al. [ 1991b]
Delrnaset al. [ 1991b]
Delmaset al. [1991b]
Andersonand Poth [1998]
Andersonand Poth [1998]
Andersonand Poth [1998]
Hao et al. [1988]
Scharffeet al. [ 1990]
Poth et al. [ 1995]
Poth et al. [1995]
Poth et al. [1995]
Zeppet al. [1996]
Zepp et al. [ 1996]
Singhet al. [1997]
thisstudy
this study
Keller et al. [1983]
Keller et al. [1986]
Keller et al. [1986]
Keller et al. [1986]
Keller et al. [ 1986]
Keller et al. [1986]
Keller et al. [1986]
Steudleret al. [ 1991]
Steudleret al. [1991]
J. A. MacDonaldet al. (1999)½
MacDonaldet al. (manuscriptin preparation,1999)
Steudleret al. [ 1996]
Delmaset al. [ 1992]
MacDonaldet al. (manuscriptin preparation,1999)
MacDonaldet al. (manuscript
in preparation,1999)
MacDonaldet al. (manuscript
in preparation,1999)
Scharffeet al. [1990]
Keller et al. [ 1990]
Keller et al. [1993]
Keller and Reiners[ 1994]
Keller and Reiners[1994]
MacDonaldet al. [1998, manuscript
in preparation,1999]
MacDonaldet al. [1998, manuscript
in preparation,1999]
MacDonald et al. [ 1998]
Tathyet al. [ 1992]
$ingh et al. [1997]
this study
Goreauand de Mello [1988]
$teudleret al. [1996]
Keller et al. [ 1990]
Keller et al. [ 1990]
Keller et al. [1993]
Keller and Reiners[1994]
Keller and Reiners[1994]
Mosier and Delgado [1997]
Mosier et al. [1998]
MacDonald et al. [1998]
Keller et al. [1986]
Goreauand de Mello [1988]
Keller et al. [1990]
Delrnaset al. [ 1991b]
this study
this study
this study
$anhuezaet al. [ 1994]
Singhet al. [1998]
Negativeratesdenoteemissionof methanefrom soil to atmosphere.
The flux ratesfrom naturalecosystems
in Singhet al. [1998] are not includedin the tableastheyare a repetitionof datareportedby Singhet al. [1997].References
tbr footnotesa-d arenot includedin the calculations
of averageuptakeratesfor the differentecosystems
in Table3 (seemaintextfor furtherexplanations).
•Methaneemissionwaspossiblydueto emissionfi'omunderground
gasreservoirs.
bHighuptakerateswereduetonearbybiomass
burningcausing
elevated
methane
concentration
in theair.
cUptakeratesare unusuallyhigh.
dMeasurement
periodis tooshortto allowextrapolation
of data.
eSourceis J. A. MacDonaldet al. (The effect of ternritebiomassand anthropogenic
disturbances
on the CH4 budgetsof tropicalforestsin
Cameroonand Borneo,manuscriptin preparation,1999). Hereinafterreferredto as MacDonaldet al., manuscriptin preparation,1999.
PRIEMI•ANDCHRISTENSEN:
METHANE
OXIDATION
BYSOILSIN GHANA
[Menautet al., 1991]. Savannafire may havepronounced
effect
on soil microbialcommunitiesbecauseof for examplethe concomitantchangesin plant cover, the releaseof nutrients,the
biocidaleffectof burning,andchangesin soilmoisture.We found
that fire decreased
methaneuptake(Figure2). Soil water content
at the burnedsitewas somewhathigherthan at the unburnedsite
(63% versus54% of water holdingcapacity)which may partly
explainthe differencein methaneuptakerates,as Gulledgeand
Schirnel[ 1998]foundthatmethaneoxidationratesdecreased
with
increasingsoil moistureaboveapproximately
40% waterholding
capacity.The differencein soil water contentmay be due to
changesin plant covercausedby fire. Other workersalsoreport
smallor no effectof biomassburningon the soil sinkfor methane
[Keller et al., 1990; Delmas et al., 1991b; Zepp et al., 1996;
Andersonand Poth, 1998]. It shouldbe notedthat Zepp et al.
[1996] worked on soilswhich were showinga net emissionof
methaneto the atmosphere
dueto biogenicmethaneproduction.
Soil methaneuptakemaybe restrictedat low soilwatercontent
by desiccationand at high soil water contentby diminished
methanetransportinto the soil [Czepiel et al., 1995;Bowdenet
al., 1998].After a prolonged
dry season
with littlerain,the agricultural soils in the savanna area had a moisture content of less
than 16% of the waterholdingcapacity(Table 1), andmethane
uptakeratesweresmall.The firstheavyrain afterthedry season
had a pronounced
effecton methaneuptakein the arablesavanna
soilswhichincreased
nearlythreefoldat threesites(Figure3).
This observation
is in accordance
with Gulledgeand Schirnel
[1998] observingan increasein methaneoxidationrateswith soil
moistureup to about30% of the water-holdingcapacityandwith
Strieglet al. [1992],who foundthatuptakeof methaneby desert
soilswasenhancedby rainfall.
Table 3 contains a compilation of methane uptake rates
reportedfor tropicalsoils.From thesedata,we calculatedaverage
methaneuptakeratesfor differenttropicalecosystems
(Table 2).
However, for different reasons,someof the values in Table 3 are
not includedin the calculationof the ecosystemuptakerates.We
excludedthe very high uptakerates(up to 850 [xgCH4 m-2 h-l)
reportedby Poth et al. [1995] from bumed tropical savannain
Brazil. The highrateswere causedby activefiresandsmouldering
combustionfrom earlier fires in the immediatestudyareawhich
resultedin elevatedbackgroundlevelsof methanein somecases
by a factorof 10 (M. Poth,personalcommunication,
1998). Also,
someVenezuelansoilsare not includedastheyarenet emittersof
methanebecauseof biogenicmethaneproductionand possibly
also leakagefrom undergroundgasreservoirs.The ratesreported
by Singhet al. [1997, 1998] are not includedas they are much
higherthanany otherratesreportedin any ecosystem
andneedto
be supported
by additionalmeasurements.
The averagemethaneuptake rates in tropical savannaand
forestsoils(Table 2) are of the sameorderas soilsin temperate
naturalan'dseminatural
ecosystems
[e.g.,Mosieret al., 1991;
Dobbie et al., 1996; Priern• et al., 1997]. If extrapolatingthe
methaneuptake rates in tropical savannasto tropical semiarid
steppeand usingthe land areadatagivenby Potter et al. [1996],
then the total sink strengthfor tropicalsoils(excludingarable
soils)is 17 Tg yr-I whichis higherthanthe 8 Tg yr-I reportedby
Potter et al. [ 1996].
In conclusion,
the observed
ratesof methaneuptakein tropical
forestandsavanna
soilsweresomewhat
lowerthanin othertropical and temperateforest soils while the observedrates in arable
soilswerecomparable
to otherreportsfromarablesoilsin tropical
and temperate
regions.We foundthat arableagricultureand,to a
23,621
lesserextent,biomassburningdecreased
methaneoxidationrates
by the investigated
soils.
Acknowledgments. We acknowledgethe financial supportfor this
work from the Commisionof the EuropeanUnion throughEnvironment
Program contract EV5V-CT91-0052 and from the Danish Intemational
DevelopmentAgency (DANIDA).
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S. Christensen,Departmentof PopulationBiology, Copenhagen
University,Universitetsparken
15, DK-2100 Copenhagen
O, Denmark.
(schristensen•zi.ku.dk)
A. Priem&Centerfor MicrobialEcology,540 PlantandSoil Science,
MichiganStateUniversity,EastLansing,M148824.
(prieme•pilot.msu.
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(ReceivedFebruary4, 1999;revisedJune15, 1999;
accepted
June17, 1999.)