Methane consumption and emission by Taiga

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 5, NO. 3, PAGES 261-273, SEPTEMBER 1991
METHANE
CONSUMFIION
AND EMISSION
BY
TAIGA
S.C. Whalen, W. S. Reeburgh,and K. S. Kizer
Institute of Marine Science,University of Alaska
Fairbanks
Abstract.Taigaor borealforestenvironments
are
a poorlyunderstood
component
of the globalCH4
budget.Resultsfroma 1-yearstudyof CH4fluxes
at a rangeof representative
floodplainandupland
taigasitesin theBonanza
Creeklongtermecological
research area show that soil consumption of
atmospheric CH4 was the dominantprocess.
Methaneemissionoccurredonly sporadicallyin the
earliestsuccessional
stages
in thefloodplainsystem;
all otherfloodplainanduplandsiteswerenet CH4
consumers. Our resultssuggestthat upland and
floodplain
taigasoilsareanatmospheric
CH4 sinkof
upto 0.8 Tg yr-1.Point-source
bogsandfensarethe
onlyimportant
CHn-emitting
sitesin taiga.
INTRODUCTION
Recent observationsof increasingatmospheric
[Adams et al., 1990] and continuesto accumulatein
taigaandcoastalplaintundraenvironments
[Billings,
1987]. Thereis concernthatregionalwarmingand
changesin precipitation
mightmobilizethisreservoir
andmakeit availablefor biogeochemical
conversion
into productslike CH4 and CO 2. The possible
mobilizationand releaseof theseradiatively active
gasesis generallyviewed as capableof causinga
positive climate feedback [Khalil and Rasmussen,
1989].
Tundra and boreal forest environments account for
approximately equal amounts (13% and 14%,
respectively)of the Earth'sterrestrialsoil carbon
[Post et al. 1982]. Boreal forestis a generalterm
describingthe mixed deciduous-coniferous
forest
biome locatedbetweentundraand temperateforest.
Taiga is an equivalentterm used by Soviet and
Alaskanworkers. The taiga is one of the Earth's
largestsingle circum-globa!forest units, covering
CH4 concentration
[Rasmussen
andKhalil, 1984;
some11.1x 1012m2 [Billings,1987]andrangingin
Steele et al., 1987; Blake and Rowland, 1989] have
stimulateda reexaminationof controlson the global
latitude from 70øN in Scandinavia and central Siberia
CH4 budget[Cicerone
andOremland,
1988]. Highlatitude terrestrial environmentsare of particular
interestbecauseof their large storedsoil carbon
contents[Postet al., 1982]. This carbonis believed
to haveaccumulatedsincethe last glacialmaximum
Copyright1991
by theAmericanGeophysical
Union.
to 45øN in eastern Siberia [Kimmins and Wein,
1986]. Tundraenvironments
haverecenfiyreceived
attentionas a CH4 source[Sebacheret al., 1986;
Whalen and Reeburgh,1988], but we are aware of
onlyoneestimateof CI-I4 emissionby taiga [Whalen
andReeburgh,1990a].
The goal of this studywas to obtain systematic
CH4 flux datafrom a rangeof representative
taiga
sitesto determinethe sourcestrengthof taigain the
Papernumber91GB01303.
globalCH4 budget.This paperpresents
CH4 flux
measurements
obtainedduring the first year of a
0886-6236/91/91GB-01303510.00
continuingstudy.
Whalenet al.: MethaneConsumption
andEmission
by Taiga
262
METHODS
BCEF LTER thusprovidesan ideal areafor a study
of taigatracegasfluxes.
Sites
Van Cleve et al. [ 1991] summarizethe statefactors
Our studywas conductedat sitesin the Bonanza
Creek Experimental Forest (BCEF), a 5000-ha
researcharealocated20 km westof theUniversityof
Alaska Fairbankscampus(Figure 1). The BCEF
was establishedin 1963 througha 50-year leaseby
the U.S.
Forest Service from the State of Alaska.
Resultsfrom researchperformedwithin BCEF are
summarizedby Van Cleve et al. [1986]. The 1983
Rosie Creek Fire burned some 3500 ha and involved
approximatelyhalf of the south-facing
uplandsin the
BCEF [Juday and Dyrness, 1985]. A long term
ecologicalresearch(LTER) program was initiated
within BCEF in 1987. The activities of the Bonanza
Creek LTER program focus on two successional
sequences:
primary succession
on a fiver floodplain
and secondarysuccessionin an upland region
following fire. This focushasled to establishment
of a rangeof representative
andwell-understood
sites
where ancillarydataare collectedcontinuously;the
controllingtaigaforestsuccession.
The floodplain
successionis controlled by river erosion and
depositionandbeginswith baresandbars.Theseare
populatedwith grassesand shrubsand the soilsare
frequently salt affected (gypsumand calcite) as a
resultof evaporationof soil pore water following
capillaryrise. Fluvial depositionof silt and sand
during high fiver stagesleadsto developmentof
terraces,which are populatedby alder and willow.
This stageis followed by developmentof balsam
poplar standswith an understoryof white spruce.
The longer-lived evergreenspruceovertopsthe
poplar, and an insulating feather moss layer
develops. Shading by the evergreenforest and
insulationprovidedby the moss surfacelead to
developmentof permafrost. The climax in this
successional
sequence
is a blackspruce
community,
whichdevelopsinto a bog. Soil pH decreases,
and
soil carbon and nitrogen increase during the
148ø20 '
1
10 /
:-F
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......
,,•.':':"':".'
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..........
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•
::-'.::::"i::::,:i•
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...:i:?'
....
0
I
N -'•
1I
2I miles
/
I
148010 '
Fig. 1. Sitelocations,
BonanzaCreekExperimental
Forest,Alaska.Hatchedareashowsextentof
1983 Rosie Creek Fire.
Whalenet al.: MethaneConsumption
andEmissionby Taiga
succession.
River
erosion
or
fire
resets
the
successional
sequence.
The upland successional sequence is firedominated,and progresses
on micaceousloesssoils
through herb-shrub, hardwood, and evergreen
conifer stages, all governed by longevity and
overtopping.North-facingslopesreceivemuchless
radiation and are dominatedby black spruceand
permafrost.
The sitesat which we measuredCH4 fluxes are
representativeof each stagein the floodplain and
upland successional
sequences,so they cover the
rangeof conditionsencounteredin taiga. Our flux
chambers were permanently deployed in the
vegetationcontrolsite adjacentthe primaryLTER
sites. A variety of data are collectedat thesesites,
including incident radiation, air and soil
temperatures,
precipitation,and water table depth.
Complete vegetation and soil descriptions are
availablefor each site. Table 1, which is largely
adaptedfrom Van Cleve et al. [1991], summarizes
the sites involved in this study and contains
information on the biota, soils, and disturbance
historyat similarsites.
Flux Measurements
263
collected from each chamber over a 0.75-hour time
courseandwereanalyzedfor CH4 in thelaboratory
by flameionizationdetectiongaschromatography.
Our CH4 standards
arerelatableto NationalBureau
of Standardsstandards.The net CH4 flux was
calculatedfrom a leastsquaresfit of concentration
changeversustime within the chamber. Details are
given by Whalen and Reeburgh [1988]. The
minimum detectableflux dependedon chamber
area:volume
ratios,but wasusually_-_4-0.2
mg CH4
m -2 d-1. Flux values below the detection limit are
reportedas zero.
Additional Measurements
A soiltemperature
profilewasmeasured
adjacent
to one chamber at each site with a portable
multithermistor
probe(2-cmintervals),andthemean
soil temperatureto 15 cm was calculated. Soil
moisture
(w/w, oven-dried
at 105øC)[Black,1965]
was determined on 2.8 cm diameter x 10 cm soil
corescollected1 m fromeachchamber.Soilorganic
content(w/w; losson ignitionat 550øCof oven-dried
sample) and pH [Black, 1965] were measuredon
similarcorescollectedfromeachcontrolplotin late
September, 1990. These data are summarizedin
Table 2.
Determinations
of netCH4 fluxweremadeusinga
staticchambertechnique
[WhalenandReeburgh,
1988]. Each chamberconsistsof a skirtedaluminum
base,which is permanentlyseatedin the soil, and
lucite vertical sections and lids that utilize a waterfilled channel for a seal. Flux chamber bases were
permanentlydeployedin or near the 15 m x !5 m
vegetationcontrolplotwithineachof thefloodplain
and uplandsites(Table 1) duringSeptember1989.
Baseswere deployedin triplicateat all floodplain
sitesandat uplandsiteUP3A. Basesat siteUP1A
were deployedin duplicatein areaswhere ground
coverwaspredominantly
Calamagrostis(bluestem)
or Equisetum(horsetail). Experimentalplots (also
15 m x 15 m) adjacentall otheruplandsites(AS2,
SB1, NB2 and BS2) have been fertilized (1:1
mixtureof ureaandNH4804) twiceper year since
1988atratesof 5 and20 g N m-2yr-1.Thelow-N
treatmentrepresentsa doublingof the natural N
input; the high-N treatment was intended to
overwhelm microbiological contributions(F. S.
Chapin, personal communication, 1991). Two
chamber bases were deployed in the vegetation
controland low-N plotswithin thesesitesto assess
theeffectof N fertilizationon CH4 flux.
Methaneflux measurements
were madeweekly at
each upland and floodplain site from late May
through September1990. Syringe sampleswere
Soil CH4 depthdistributionswere measuredat
uplandsitesNB2, SB1, BS2 andUP3A on samples
obtainedby insertinga steeltubeprobeto knownsoil
depthsandpumpingsoilgasintoTedlarsamplebags
[Born et al., 1990] with a battery-powered
diaphragmpump. The bags were sampledand
analyzedfor CH4 in thelaboratory.
Methaneconsumptionthresholdsand capacities
were studiedat theabovefour sitesusingchambers
with ambient and amendedatmospheres. The
ambientatmosphere
experiments
involvedallowing
chambers
withfreeairatmospheres
(-1.7 ppmCH4)
to "randown"withperiodicsamplingovera 24 hour
period. The amendedatmosphereexperiments
involvedadjustinginitial chamberatmospheres
to
concentrations
of ~20 ppmCH4andsampling
overa
24 hourperiod.
Methane
fluxes were often below
the limit
of
detection. We reportthe medianand interquartile
range(IQR; rangeof central50% of data)of theCH4
flux dataasmeasures
of centralvalueandvariability.
This procedure
is recommended
by Helsel[1990] for
analysis of censored(i.e., many values below
detection limit) data. Nonparametricstatistics
[Conover, 1980; Zar, 1984] are usedbecauseof the
largenumberof valuesbelow the detectionlimit and
becauseno singledatatransformation
consistently
homogenizedvariances. Statisticalanalyseswere
264
Wha!eneta!.: Methane
Consumption
andEmission
byTaiga
,
,
,
,
++
0 0 0 0 •0
<<<<<
<
Whalenet al.' MethaneConsumption
andEmissionby Taiga
i
i
265
i
i
i
i
i
i
266
Whalenet al.' MethaneConsumption
andEmission
byTaiga
m-2d-1,witha median
of-0.23 mgCH4 m-2d-1for
performedat a significancelevel of a--0.05 in all
cases.
the entiredata set. Overall, CH4 fluxeswere low,
with 43% of the observations below the detection
RESULTS
limit.
Fluxesfrom the floodplainsitesand uplandsites
are are presentedin Figures2 and 3, respectively.
Table 2 givessummarystatisticsfor the CH4 flux
measurements.Both positive(net CH4 emission)
and negative(net CH4 consumption)fluxes were
observed.Fluxesvariedfrom -1.81 to 8.37 mg CH4
floodplain sites,but there were clear differencesin
fluxes among sites (Figure 2). A transitionfrom
FP 1A -Sandbar
No seasonal
signalfor CH4flux wasevidentin the
CH 4 productionto consumptionwith advancing
plantcommunitysuccession
is evidentin Figure2.
Methaneemissionwas frequentlyobservedin the
earlieststageof succession
(sandbar,Figure2a), and
a
- Alder-Poplar
8
b
ß
,r
4
o•
2
hi' 0
.......
H
-- •
----•
-------'
3I
-2
,
i
, ,.
June
May
,
i
,l
July
"
,
Sept
Aug
,
.
May
ß
June
199O
,
i
July
i
Aug
,
Sept
1990
,
FP4A- WhiteSpruce
FP3A - Poplar
d
1
1
E
•
0
0
0
E
-1-2,
'i
May
,
June
ß
July
-2
•i
Aug
i
May
Sept
i
June
'
July
'i
'
Aug
11
Sept
1990
199O
FP5A - BlackSpruce
i
May
i
i
June
i
iI
July
e
i
Aug
Sept
199O
Fig.2. Methane
fluxes
(mgm'2d'1)fromBonanza
Creek
Experimental
Forest
floodplain
sites,
May 24 to September
25, 1990.(a) FP1A, sandbar
(notethatflux scalediffersfromothersites).
(b) FP2A, alderto poplar.(c) FP3A, poplarwith spruceunderstory.
(d) FP4A, whitespruce.(e)
FP5A, black spruce.
267
Whalenet al.: MethaneConsumption
andEmission
by Taiga
2
UP1A-
AS2 - S. Aspen
Burn Site
b
1
E -1
-2
i
i
i
i
i
i
June
May
iii
May
Sept
Aug
i
i
i
July
i
,
i
June
July
,
i
Aug
Sept
1990
1990
i
2
,
SB1 - S. Birch
i i i
i
c
i
i
,
UP3A- White Spruce
d
1
E
•.0
0
E -1
-2
-2
May
June
July
Aug
June
May
Sept
July
Aug
Sept
1990
1990
2
NB2 - N. Birch
i
,,-
1
i
i
BS2- N. BlackSpruce
e
f
1'
'E 0......
O
-c,•.,,
'--•- .....
•['% ',,•,.,.
"'•"
•
•.."&" .....
'
r'.; :
•'-1
-2
i
May
I
i
June
i
July
i
ii• ,
Aug
,. ,
Sept
1990
-2
,
May
.
,
June
.
.
July
l
Aug
Sept
1990
Fig.3. Methane
fluxes
(mgm'2d'l) fromBonanza
Creek
Experimental
Forest
upland
sites,
May
22- September
27,1990.DatafromtheN-fertilized
plots
atsites
AS2,SB1,NB2,andBS2are
plotted
astriangles.
(a)UP1A,burn
site(Calamagrostis,
triangles;
Equisetum,
open
symbols).
(b)AS2,aspen.
(c)SB1,birch.
(d)UP3A,whitespruce.
(e)NB2,north
birch.
(f) BS2,north
black spruce.
sporadicallyseenin middlestagesof succession
(alder-poplar,
Figure2b; poplar,Figure2c). The
magnitudeand frequencyof emissiondecreased
along the succession
for thesesiteswith some
stationsshowingconsumption
aswell as emission,
depending
on date. The late stagesof succession,
whitespruce(Figure2d) andblackspruce(Figure
2e) showedonly CH4 consumption
when fluxes
were detectable. Differencesin CH4 flux among
siteswere statisticallysignificant(Fa-uskal-Wallis
one-way analysis of variance or ANOVA). A
posteriofianalysis
by Dunn'snonparametric
multiple
comparisonclearly showedthat the sandbarsite
(Figure4a) had the highestCH4 flux (dataranked
lowestto highest,i.e., greatestrate of consumption
to greatestrate of emission). The remainingdata
showedthreesetsof overlapping
similarities(Figure
4a). The meanranksfollow an order that showsthe
268
Whalenet al.' MethaneConsumption
andEmissionby Taiga
(a) Floodplain
Site
FP4A
FP5A
FP3A
FP2A
FP1A
n
45
45
42
48
48
Mean Rank
57.4
89.9
104.4
139.9
174.6
,
,
(b) Upland
Site
BS2
UP3A
SB 1
AS2
NB2
n
38
55
38
38
38
Mean rank
60.8
95.8
128.9
129.8
152.9
, ,
UP1A
76
223.2
•
Fig. 4. Multiplecomparisons
of rankedmethane
fluxmeasurements
forBCEF(a) floodplainand
(b) uplandsitesby Dunifsnonparametric
procedure.Meanranksfor thesitetypesarearrangedin
increasing
order. Sitesunderscored
by thesameline haveno significant
differences;
sitesnot
underscored
by the san• line showsignificantly
differentfluxes.
transitionfrom CH4 emissionto consumption
with
increasing ecosystem maturity. There is no
difference in CH4 consumptionbetween white
spruce(FP4A) and black spruce(FP5A) or between
black spruceandpoplar(FP3A), but consumption
is
greaterin white sprucethan in poplar and in black
sprucethanin thealder-poplar(FP2A) stand.
Methane
emission
was never
observed
at the
othersitescouldnotbe clearlyclassified.However,
ranksandoverlapping
similaritiestogetherindicated
thatwhitespruce(UP3A)wasmostcloselyrelatedto
blackspruce(latesuccessional
stage)andnorthbirch
(NB2) was most closely related to other midsuccessional
(southbirch, SB1 and aspen,AS2)
sites.The lack of a significant
differencein CH4
consumptionbetween south birch and north birch
(Figure4b) indicatesthataspecthadno influenceon
CH4 flUXfor thisplantcommunity.Net CH4 fluxes
were significantly
lower(i.e., higherrate of CH4
uplandsites(Figure3; Table 2). As in the floodplain
sites,there was no clear seasonalsignal for CH4
flux. The earliest successionalstage (bum site,
Figure 3a) showedthe smallestflux, with only 4%
(3 of 76) of the flux determinations above the
detectable
level. The latersuccessional
stages,white
spruce(Figure 3d) and black spruce(Figure 3f),
exhibitedhigherratesof CH4 consumption
thanthe
middle stagesof succession
(aspen,Figure3b; and
birch,Figures3c and3e). Differencesin CH4 flux
at control stationsamong sites were significant
(ANOVA), and multiple comparisonof fluxes
(Dunn's test) suggeststhat consumptionincreases
floodplainsiteswhenthe entiredata were pooled
(Mann-WhimeyU test).
The soilpH at thestudysitesrangedfrom slightly
basicto acidic(Table1). The highestvaluesof pH
were observedin the first stage of succession
with advancing
plantcommunity
succession
(Figure
4b). A significant
increase
in CH4consumption
rate
wasobservedat blackspruce(BS2) relativeto the
bum site(UP1A). The CH4 consumption
rateat all
floodplainanduplandsites,respectively
(Table1),
withthelowestvaluefor eachsequence
occurring
in
thefirstsuccessional
stage.
The soiltemperature
duringtheobservation
period
consumption) in the upland sites than in the
(FP1A, UP1A), and the lowestvalueswere found at
theblacksprucesitesin boththefloodplain(FP5A)
and upland(BS2) areas. The soil organiccontent
varied from 2 to 45% and from 9 to 37% in the
Whalenetal.' Methane
Consumption
andEmission
byTaiga
variedfrom about2ø to 21o C in boththe floodplain
anduplandsites(Table 2). Meansvariedfrom 10.5ø
to 15.7 øCfor floodplainsitesand from 8.9 to 12.9
*C for uplandsites.The meansoil moisturecontent
showeda higherrangefor uplandsites(112%) than
for floodplainsites(51%), dueto thehighmeansoil
moisture at BS2 (Table 2). The effect of soil
temperatureand moistureon CH4 flux at eachsite
was assessedby correlation analysis (Table 3).
Correlations
werenot statistically
significantin many
cases. However, statisticallysignificantdata show
threetrends.First,CH4 flux is negativelycorrelated
with soil temperature,indicating that soil CH4
consumption (negative flux) increases with
increasing temperature. Second, CH 4 flux is
positively correlated with soil moisture at sites
showing no CH4 emission(Figures 2 and 3),
indicatingthat CH4 consumption
decreasedwith
increasingsoil moisture. Third, increasingCH4
emissionis correlatedwith decreasingsoil moisture
at the only site showingCH4 emissionand no
consumption
(FP1A, sandbar).
Nitrogenfertilizationhadno consistent
effecton
269
CHn flux in the uplandsites. Neitherstationwithin
the fertilized plot at SB1 (Figure 3c) showed
detectableCHn consumption
duringthe entirefield
season.Comparison
of N-fertilizedandcontrolplots
within other upland sites (Mann-Whitney U test)
showedsignificantlyincreasedconsumptionin the
fertilizedplot at AS2 (Figure 3b) and no difference
betweenplotsat BS2 (Figure 3f) and NB2 (Figure
3e).
The CH4 concentrationin soil atmospheres
decreased
rapidlywith increasingdepthat upland
sitesNB2, SB1, BS2, andUP3A. Figure5a shows
resultsfrom NB2, which are typical of the other
sites. Methanewas essentiallydepletedbelow a
depthof 40 cm, andconcentrations
between40 to 60
cm wereconsistently
at or belowthedetectionlimit.
Soils at upland sitesshoweda low thresholdand
high capacity for CH4 consumption. Methane
concentrations in unamended chambers at NB2,
SB 1, BS2, and UP3A decreasedto a thresholdvalue
varyingfrom below the limit of detection(0.1 ppm)
to 0.87 ppm over 24 hours;resultsfrom NB2 are
shown in Figure 5b. Methane concentrationsin
TABLE 3. Kendall'sRank CorrelationCoefficientx for CorrelationsBetweenCH4 Flux
andSoil TemperatureandMoisture
Kendall',s•:
Soil Moisture
Soil Temperature
Site
, Station 1
, ,
Station!
Station2
-0.59
FP1A
Station3,
-0.52
Station4
-
FP2A
FP3A
FP4A
-O.43
0.53
0.49
0.63
FP5A
UP1A
AS2
-O.57
O.70
SB1
-0.38
0.47
NB2
-0.60
0.46
UP3A
0.62
0.39
0.46
0.41
0.79
BS2
The numberof observations
at a stationfor eachx variedasfollows: Floodplain
temperature,
14-16;Floodplain
moisture,10-11;Uplandtemperature,
19;Upland
moisture,15. Stations1 and2 arewithinthecontrolplotat uplandsites;stations3 and4
arewithintheadjacent
N-fertilizedplot. Onlystatistically
significant
correlations
are
reported.A dash(-) indicates
nodata;a blankindicates
nosignificant
correlation.
Whalenet al.: MethaneConsumption
andEmission
byTaiga
270
CH4 (ppm)
0.5
1
chamberswith atmospheres
amendedto-20 ppm
decreased
to valuesvaryingfrom belowthedetection
1.5
limitto 1.08ppmwithin24 hours.Figure5csh6ws
resultsfroman amendment
experiment
at NB2.
10
2O
DISCUSSION
3O
Resultsof this studyclearly indicatethat welldrainedtaigasoilsarea CH4 sink. Seasonal
studies
havealsoreportednet CH4 consumption
for other
undisturbed, moist forest soils. Temperate
deciduous
andevergreenforestsshowedmeanfluxes
4050-
60.
NB2
4 Oct 9O
of-4.15 and-3.51 mg CH4 m-2d-1 [Steudleret
70
1.
a1.,1990], and tropical forest soils had average
fluxes of-0.14 [Keller et a1.,1986], -0.57 (dry
season) [Goreau and de Mello, 1988] and-0.8
2t.
NB2
27
uly90
,
[Kelleret al., 1990]mg CH4 m-2d4. Ourmedian
flux of-0.23 mgCH4m'2d'1 is closest
to thelower
meanreportedfor tropicalsofts.
The significantdifferencein CH4 flux between
E
O.
,
0
5
10
15
20
25
Time (h)
25
floodplain and upland sites may be related to
drainageand water table level. Early successional
stagesat floodplainsites(lowestterraceelevations,
Table 1) showedepisodicCH4 emission(Figure2a
to 2c) following heavy rains. Sites at higher
floodplain terrace elevations (late successional
stages)andall uplandsitesconsumed
CH4 or had
fluxes
below
the detection
limit.
Soils in the
floodplainanduplandsitesalsodiffered; floodplain
soilswerea sandyor siltyalluvium,whiletheupland
soils consistedof a micaceousloess [Van Cleve et
al., 1991].
,,
20
Nitrogenfertilizationhad no influenceon CH4
consumption
at ouruplandtaigasitesbut resultedin
significantly lower CH4 consumptionrates in
temperatehardwoodand softwoodforests[Steudler
•
15
et al., 1990]. The studies differed in form of N
-,-
10,
NB2 10 Aug 90
E
.
Time (h)
Fig. 5. Methanedepthdistributionandconsumption
at siteNB2. (a) Depthdistribution,
October4, 1990.
(b) Chamber methane concentrationvs. time with
ambient (-1.7 ppm) initial atmosphere,July 27,
1990. (c) Chambermethaneconcentrationvs. time
with amended(-20 ppm)initial atmosphere,August
10, 1990.
added, relative N loading, and frequency of
application, so their results are not directly
comparable.
Soil CH4 profiles(Figure5a) stronglysuggest
that
soils at deciduousand coniferousupland sitesare
well aeratedat leastto 60 cm. The organichorizonin
thesesoilsis generallylimitedto depthslessthan20
cm. Thus it is likely that methanogenesisis
nonexistent
or confined
to anaerobic
microzones
[SmithandArah, 1985]andtheimportantbiological
processinfluencingthe soil CH4 distributionis
oxidation. Keller et al. [1990] and Born et al. [1990]
found similardepthdistributionsfor CH4 in moist
soilsfrom tropicalandtemperateforests,whichalso
suggestsCH4 consumptiononly. Yavitt et al.
[ 1990]reportCH4 oxidationin theupper0 to 20 cm
Whalenet al.: MethaneConsumption
andEmissionby Taiga
and productionbelow 20 cm in CH4 profiles for
temperate mesophytic and spruce forest soils.
Collectively, these data indicate that forest soils
supporta community of methanotrophsor other
CH4-oxidizingbacteria(e.g. nitriflers[Bedardand
Knowles, 1989]) whose CH4 sourceis largely
atmospheric.
Our taiga sitesshowedno strongseasonalsignal
for CH4 consumption(Figures2 and 3). Keller et
al. [1983] found no seasonaltrend in CH4
consumption
in soilsof a temperatehardwoodforest,
and no seasonalvariationsare evidentin the May
throughOctoberrecordat othertemperatehardwood
sites [Steudler et al., 1990]. These reports are
consistentwith our observationthat the atmosphere
is the majorCH4 sourcefor microbialoxidationin
many forest soils. The moist surface zone of
microbialactivitycanbe expectedto thawandwarm
quickly in the spring. in contrast,the gradual
thawing and warming of saturated,biologically
activetundrasoilsunderlainby permafrost
resultsin
a pronouncedseasonalsignal for CH4 emission
[WhalenandReeburgh,1988].
The low thresholdand high capacityfor CH4
oxidationshownin theCH4 amendment
experiment
(Figure 5½) is in agreementwith our previous
observationson end-member CH4 oxidizing
environments,
namelywell-drainedsubarctictundra
[WhalenandReeburgh,1990b]andthe surfacesoil
of a retired landfill [Whalen et a1.,1990]. These
resultsindicatethepresence
of a ubiquitous
bacterial
communitycapableof rapidly oxidizing CH4 at
concentrations
well aboveand below atmospheric
levels. Theseresultsalsosuggestthat the ratesof
CH4 consumption
reportedabovefor forestsoilsare
governedby physicalsoilconditions(morphology,
porosity,moisturecontent,and temperature)that
regulateCH4 diffusionto microbes,ratherthan
intrinsicbiologicalfactors. Further supportfor
physicalcontrolof CH4 consumption
by forestsoils
is shownin Table 3; increased
CH4 consumption
(negativeflux) was correlatedwith increasingsoil
temperature and decreasingsoil moisture when
significant values of z were found for sites that
showedCH4 consumption
only. It is likely thatsoil
moisture content exerts the greatest physical
influence
onCH4 oxidation
ratesin forestandother
soils. We demonstrated
experimentallythat CH4
oxidation rates in moist soils from a retired landfill
were reducednearly ten fold when saturatedwith
water [Whalen et al., 1990], and $teudler et al.
[ 1990]reportedsignificantdecreases
in ratesof CH4
consumption by temperate forest soils under
conditions of increased moisture.
271
Theincrease
in CH4 consumption
withincreasing
ecosystem
maturity(Figures2 and3) alsopointsto
physicalcontrolof the CH4 consumption
rate. As
the taigamatures,mineralsoilbecomescoveredwith
decaying
organicmatteranda thickcarpetof feather
moss [Van Cleve et al., 1991]. We observedthe
highestratesof CH4 oxidationandgasdiffusionin
experiments
involvingbothchambersandsoilcores
frommossyenvironments
[WhalenandReeburgh
1990b; also unpublisheddata, 1990], and attribute
them to rapid gas phasediffusion in the pillowy,
moist surface matrix.
Taiga has been considereda net CH4 source
[Sebacheret al., 1986;MatthewsandFung, 1987]of
~15 Tg yr-1 [WhalenandReeburgh,1990a],largely
becauseof high ratesof CH4 emissionfrom point
sources(bogsand fens) [Crill et al., 1988;Hatrisset
al., 1985; Moore et al., 1990; Moore and Knowles,
1990] distributed throughoutthis environment.
Usingtheoverallflux resultsfrom thisstudy(IQR of
0.0 to -0.42 mg CH4 m-2 d-1, Table2), an active
periodof 150 days,and a globaltaiga coverageof
11.lx106 km2, we estimatethat uplandand
floodplaintaigahavea CH4 sinkstrength
of 0 - 0.8
Tg yr-1. This is smallerthanthe previousboreal
forestsoilconsumption
estimates
of- 1 to -15 Tg yr-1
[Born et al., 1990] and -0.3 to -5.1 Tg CH4 yr-1
[Steudler et al., 1990], which were based on fewer
data. Resolvingthe contributionsof point sources
within taiga environmentswill require a high
resolutiondatabasefor theglobalextentof eachsite
type.
The results from this study extend beyond
establishinga firmly based estimate for CH4
consumptionby taiga. The data clearly show
quantitativeconsumption
of atmosphericCH4 by
moisttaigasoilsandsupportcontrolof oxidationby
physicalprocesses.Recentstudieshave indicated
that moist soils in both natural and managed
ecosystemsare net CH4 sinks (summarized by
Steudleret al. [1990] and Born et al. [1990]) and
suggestCH4 consumption
equivalentto 1 to 11% of
the540Tg CH4 yr-1emittedto theatmosphere
from
all sources [Cicerone and Oremland, 1988].
Consumption of atmospheric CH4 by these
ecosystemscould provide a significantnegative
feedbackto increases
in atmospheric
CH4. Changes
in soil physical characteristicsresulting from
temperatureand precipitationchangesare likely to
influenceratesof CH4 consumption.Water table
andtemperature
manipulation
experiments
(1 to 10
m2 scale)are an approachto understanding
these
relationships.
272
Whalenet al.: MethaneConsumption
andEmissionby Taiga
Acknowledgments.Keith Van Cleve (University
of Alaska), Leslie A. Viereck (U.S. Forest Service),
and C. T. Dyrness(U.S. ForestService)provided
the opportunityto work within the BonanzaCreek
Experimental
ForestLongT-ermEcologicalResearch
program. Advice on site selectionand logistic
supportin thefloodpl• studieswasprovidedby the
aforementioned
individualsandby Bob Schlentner.
Terry ChapinandEric Vanceadvisedon uplandsite
selection
andmadetheirN-fertilizexl
plotsavailableto
this study. Glenn Juday provided Figure 1.
UndergraduatesummerassistantHeatherAnderin
assisted
withfield samplingandlaboratory
analyses.
We are •ateful for all of thesecontributions.This
work was supportedby NASA grantIDP-88-032;
the BonanzaCreekL.T.E.R. is supportedby NSF
•ant BSR 8702629.
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