Kaolinitic Yellow Latosols in the Brazilian classification system or. Haplustox .... dryer (Perma Pure, Inc., Toms River, New Jersey). .... Statistics and Data Analysis.
GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 13, NO. 1, PAGES 31-46, MARCH
1999
Land use change and biogeochemicalcontrols of nitrogen oxide emissions
from
soils in eastern Amazonia
LouisV. Verchot,•,2,3Eric A. Davidson,•,2 J. HenriqueCattfinio, TMIlse L. Ackerman,•,5 Heather E. Erickson? and Michael Keller6,7 Abstract. The objectivesof thisstudywere (1) to determinethe effectsof landusechangeon N oxidefluxesfrom soil in seasonally dry, easternAmazoniaand(2) evaluatethe "hole-in-the-pipe" modelin a field settingwhereN availabilityvariesamonglandusesandsoilmoisturevaries amongseasons. We measuredN oxideflux from an old-growthforest,a 20-year-oldsecondary forest,an activepasture,anda degradedpasture.We alsomeasuredsoilwatercontent,soil inorganicN stocks,net N mineralizationandnitrificationpotential.To determinethe effectsof pastureageon N oxideflux, we measuredgasfluxesat a chronosequence of pastures(0-13 years).
In thelandusestudy, N20 fluxesfollowed theorder:primaryforest(2.4kgN ha-•yr-•)> secondary forest(0.9 kg N ha4 yr'•) > activepasture(0.3 kg N ha-• yr-•) > degraded pasture(0.1 kg N ha-• yr4), andNO fluxesfollowedthe order:primaryforest(1.5 kg N ha-• yr-•) > degradedpasture(0.7 kg N ha4 yr-•) > activepasture(0.5 kg N ha-• yr4) > secondary forest(0.3 kg N ha-• yr-•). In the chronosequence study,no trendin N oxideemissions with pastureagewasapparent,but emissionsfrom pastureswere lowerthanfrom the forest. Total N oxideflux correlatedwith a labora-
torymeasure of nitrification potential (r2= 0.85). TheratioN20:NOcorrelated withsoilwater content(r2= 0.56). Parameterization of themodelaccounted for variabilityin N oxideemissions acrosslandusesand seasons andthe modelapplicationrevealedthe importanceof studyingboth N oxidegasessimultaneously.Model predictionsfor six independentsitesagreedwell with observedfluxes,suggesting thatthe modelmay be applicableat a broaderscale. The consistently
lowannual emissions ofN20 estimated forall of theAmazonian pastures thatwestudied suggest that conversions of tropicalforeststo cattlepasturesmay not in the longterm causea significant
increase in thecontribution of soilemissions to atmospheric N20 orNO.
Parfi,Maranh•to, MatoGrosso andRond6nia. Recent figures from theBrazilian Space Research Institute, (Instituto Nacional Pesquisas
1. Introduction
Dataon soiltracegasemissions fromhumidtropicalforestsare limited,but emergingtrendssuggestthat for both nitrousoxide
(N20) andnitricoxide(NO), soilemissions arehighin tropical
Espaciais,INPE), estimatedeforestationratesin the mid 1990s
between 15,000and29,000km2yr4 andatotalareaof517,000km2 deforested asofAugust 1996(INPEWebsite athttp://www.inpe.br/ amz-04.htm). Thiscleared landisprimarily usedaspasture, with somedegraded pasture havingbeenabandoned andundergoing secondary succession [Instituto NacionalPesquisasEspaciais, 1996;
forestsandlandusechangeis a potentiallyimportant,butuncertain agentof changingemissionrates.In theBrazilianAmazonregion, forestclearingand pastureestablishment have beenconcentrated etal., 1997].Because bothN20andNOplayimportant alongthe southernand easternflanksof the basinin the statesof Nepstad
rolesinatmospheric chemistry, it isimportant toquantify theeffects of landusechange onthesoilfluxesof thesegases. N20isalong-lived, greenhouse gaswithapermolecule radiative
•Woods Hole Research Center, Woods Hole, Massachusetts.
2Institutode Pesquisa AmbientaldaAmaz6nia,Be16m,Parfi,Brazil. 3Nowat: Instituteof EcosystemStudies,Millbrook,New York. 4Nowat: EmpressaBrasileirade Pesquisa Agropecufiria, Be16m, Parfi, Brazil.
forcing strength 200timesgreater thanCO2 [Shine etal., 1990; 1995]. N20 alsocontributes to stratospheric ozonedestruction [Cicerone,1987]. Recentestimates suggest thatsoilemissions of
N20fromhumidtropical forests account for20-50%of allglobal sources of atmospheric N20 [Pratheret al., 1995;Potteret al., 1996]. An inversemodeling approach, relyingstrictlyon atmospheric dataandassumptions aboutexchange ratesbetween hemi-
SNowat: Departmentof Soil, CropandAtmospheric Sciences, Comell University,Ithaca,New York. 6IntemationalInstituteof TropicalForestry,U.S. Departmentof Agriculture,ForestService,Rio Piedras,PuertoRico. 7Nowat: ComplexSystemsResearch Center,Universityof New Hampshire,Durham. 8Nowat: Departmentof ScienceandTechnology,Universidad Metropolitana,SanJuan,PuertoRico.
spheres, indicated thatthetropical N20source isgrowing [Prinn et al.,1990].Deforestation isaplausible cause ofincreasing emissions fromthetropics, andsomedataexistto support thishypothesis. Increased N20 emissions fromsoilsin pastures following forest clearing havebeenobserved nearManaus[Luizdoet al., 1989;
Copyright1999 by the AmericanGeophysicalUnion. Papernumber 1998GB900019
Matsonet al., 1990].Morerecentworksuggests thatthisincrease
0886-6236/99/1998GB900019512.00
mayonlybetransitory andthattheneteffect oflandusechange over 31
32
VERCHOT ET AL.' NITROGEN OXIDE EMISSIONS FROM EASTERN AMAZONIAN SOILS
thelongtermmaybelowertropicalsoilfluxes.Forexample,Keller
Atmosphere
etal. [ 1993]measured afivetoeightfoldincrease inN20emissions from young pastures((10 yearsold) in CostaRica after forest clearing,butemissions fromoldpastures (10-25yearsold)wereone half to one third those of primary forest. J.M. Melillo et al. (personalcommunication, 1998) alsofounda two fold increasein emissionsin youngpasturesin Rond6nia,but the response was sustainedfor only 2 years,and emissionsin pasturesolderthan2 yearswere 30% lower than in a primary forest. The 1994 IPCC ClimateChangeReport [Prather et al., 1995] acknowledged the conflictingconclusions of thesestudiesandcalledformoreresearch toresolvethequestionof theimpactof landusechangein thetropics
onthesoilsource of N20.
NO
_
N,O
•
and abiologicalrea
• Aqueous Phase ofSoil
'
NO contributes indirectlyto climateforcingthroughitsrolein the
photochemistry of OH-and03. TheglobalNO budgetis poorly constrained, andestimatesof emissions fromsoilsrangebetween5 and 21 Tg N yr-• [Davidsonand Kingerlee,1997;Prather et al., 1995; Yiengerand Levy, 1995]. The analysisby Davidsonand Kingerlee[ 1997]suggested thatsoilNO emissions fromthetropical evergreenforestsare 1.1 Tg N yr4. Thereis goodreasonto believe that atmosphericphotochemistryin theseregionsis particularly sensitiveto NO emitted from soils [Torres and Buchan, 1988; Kaplan et al., 1988;Keller et al., 1991] andthereforesensitiveto landusechange. Fewdataexistontheeffectsof landusechangeonNO fluxes,but
NH.'-•!
)---•N, Nitrification
Denitrification
Figure 1. Hole-in-the-pipe conceptual modelfor soilemissions of
N20andNO [Firestone andDavidson, 1989].TotalN gasproductionisafunction ofN availability, whichisanalogous totheamount
ofNflowing through thepipe.Therelative proportion ofN20orNO isa function of soilwatercontent, whichisanalogous tothesizeof theholesthroughwhichN oxidesleak.
those thathavebeenreported suggest atrendsimilar toN20. Keller et al. [1993] founda five to tenfold increaseinNO fluxesin young pasturesof CostaRica and reducedemissionsin older pastures comparedto old-growthforests. Keller and Reiners[1994] also reportedlowerNO fluxesfrom old, activepastures,comparedto primary forest,for anotherseriesof CostaRican sites. Becauseof the importanceof nitrogenoxidesin atmospheric chemistryandtheatmospheric energybalanceandbecause soilsare a majorsourceof thesegasesbothregionallyandglobally,thereis a needto improveour ability to estimatethe soil sourceof these gases.Thehole-in-the-pipe model[Firestone andDavidson, 1989] providesa conceptualframeworkto explainthe variability of nitrogenoxideemissions, includingtheeffectsof deforestation and landusechange[Davidson,1991]. Thisconceptual modelis both mechanistic in formandapplicableto studiesat a varietyof scales. Themetaphor offluidflowingthroughaleakypipe(Figure1)isused to describetwo levelsof regulationof N oxideemissions fromsoils' (1) Theamountof fluidflowingthroughthepipeisanalogous tothe
eitherNO [dohansson, 1989; Williams etal.,1992;] orN20 [Eichner, 1990]or both[Matsonet al., 1996;Veldkamp andKeller,1997; Veldkamp etal., 1998].Cutting of forests alsogenerally results in anincrease of readilyavailable soilN that,at leasttemporarily, resultsin increased N oxideemissions [Bowdenand Bormann, 1986;KellerandReiners 1994;Matsonetal., 1990;Robertson etal., 1987]. Naturalvariationin soil fertilityin generalandnet N mineralization in particularcorrelatewell with N oxideemissions
across a rangeof humidtropicalforestsites[MatsonandVitousek, 1987;Rileyand Vitousek, 1995].
Thesecond levelofregulation addresses therelative importance of NO andN20production. Bothnitrification anddenitrification produce bothgases, butnitrification oftenproduces greater quantitiesof NO relative to N20, anddenitrification usually produces greater quantities ofN20relative toNO [Davidson, 1993].Several
factors have been shown toaffect theratioofN20toNO[Firestone
andDavidson, 1989],butDavidson [ 1993]suggested thatsoilwater rateof N cyclingin generalor, specifically, to ratesof NH4+ content couldbeauseful predictor oftheratioatregional andglobal oxidation bynitrifying bacteria andNO3-reduction bydenitrifying scales. Atwatercontent belowfieldcapacity (fieldcapacity isoften bacteria.(2) Theamountof N that"leaks"outofthepipeasgaseous operationally definedas watercontentat 0.010MPa tension), N oxides, through one"hole" forNO andanother "hole" forN20, is nitrification is oftenthepredominant gasproducing processes, so determined by several soilproperties, butismostcommonly and NOpredominates. In wetsoils, denitrification increases as02 moststronglydeterminedby soilwatercontent.This effectof soil diffusion decreases, andassoils become more anaerobic, N20from water content,and in some casesacidity or other soil factors, denitrification becomesthe predominant N oxide. The water determines the relative rates of nitrification and denitrification and
content effect isacontinuum, although theresponse oftheN20:NO
hencethe relativeproportionsof gaseousendproductsof these ratiotosoilwatercontent maynotbelinear.Experimental evidence processes.The firstlevel of regulationdetermines thetotalamount andfieldstudies existthatsupport thishypothesized relationship of •C•NI.N + NI2v/,N• while level of [Davidson,1002; D,•,,i,4... t ,,I, • oo•. v',,n.... ,4D,,;.... Inn. .... NI nxicleqnrndlleed t- ...... .................. the qeenncl 1995]. regulation determines therelative importance ofNOandN20asthe Rileyand Vitousek, gaseousendproductsof theseprocesses. Ourspecificobjectives in thisstudywere:(1) to determine the This mechanisticmodelis based,first, ontheideathatemissions of N oxidesincreasewith increasingN fertility. This notion is supportedin a generalsensefrom studiesthatshowa relationship betweenagronomicuseof N fertilizersandhighsoilemissions of
effects oflandusechange onN oxidefluxesfromsoilinseasonally dry eastern Amazonia; and(2) to evaluate thehole-in-the-pipe modelin a fieldsetting whereN availability variesamong landuses andsoilmoisturevariesamongseasons.
VERCHOT ET AL.: NITROGEN OXIDE EMISSIONS FROM EASTERN AMAZONIAN
SOILS
33
2. Methods
theBarreiras formation, although redclaysoftheBelterraformation wereobserved at somesites.Rainfalldepthandseasonality were
2.1. Site Description
similar to Fazenda Vit6ria.
We conductedthis studyat two ranchesin easternAmazonia,
Fazenda Vit6ria(VictoryRanch)andFazenda AguaParada (StoppedWater Ranch),near Paragominas (2ø 59' S, 47ø 31' W) in the Brazilianstateof Parfi.Meanannualprecipitation is 1850mm,with lessthan20% of theannualtotalfallingbetweenJuneandDecember [Jippet at., 1998]. Despiteseasonal waterstress, deeppenetrating roots(> 8m) allowtheforests ofthisregiontoretaintheirleafcanopy year-round[Nepstadet al., 1994]. Soilsin theregiondeveloped onPleistocene terraces cutintothe BelterraclayandTertiaryBarreirasformations [Sombroek, 1966; Clapperton,1993]. Thesesediments consist primarilyofkaolinite, quartzandhematiteandarewidespread atelevations below200m in the Amazonbasin[Clapperton,1993]. Soilsareclassifiedas KaoliniticYellow Latosolsin theBrazilianclassification systemor
We measuredgas fluxes in pasturesthat were on their first rotationafterclearing.Forcomparison, wemeasured oneforestsite
thathadbeenselectively logged6-9 monthsbeforewe began measurements,andwe measuredin an areawith no down or broken
trees.Therewerenounloggedforestsaccessible tousatthisranch.
Pastures atthebeginning of thestudywere0, 1,3, 6, and13years old. TheO-year pasture hadbeencutandburned approximately 6 months beforemeasurements began,butpasture grasses wereseeded only3 months priortothefirstmeasurement. All pastures andthe logged forest werelocated onrelatively flatland( 20 cm,andabovegroundbiomass in theforestwas264 Mg ha4. The secondary forestwasregenerating naturallyfroma pasture that had been abandoned in 1976. The area had been used with
attheInstituto Brasileiro deGeografia eEstatistica (IBGE)Reserve nearBrasilia.NearSantana doAraguaia, wemeasured twoprimary forestsitesat two differentranches,FazendaS•.oSebasti•.oand
FazendaS•o Jos•anda recentlyformedpasture,plantedto B. brizantha,at S•o Sebasti•o.At theIBGE Reserve, we measured threesiteswith densescrubvegetation (cerrado).Soilsat the Santana sites were classified as Yellow Latosols in the Brazilian
classification system (Haplustox according to USDAtaxonomy), andat theIBGE Reserve soilswereclassified asdystrophic Red Latosols intheBrazilian classification system (Haplustox according to USDA taxonomy).Measurements weremadeduringthedry season: in July 1996at Santana doAraguaia,andin October1996 at the IBGE Reserve.
2.2. N20 andNO Fluxes
moderate intensityduringthe1970sasa pasture(1-3 headof cattle per hectare)andhad beenburnedperiodically.At the time of measurement, standheightwaspatchy,with someareasashighas 13-16m, andtheabovegroundbiomass was50 Mg ha4. Thisforest is floristicallymuchsimplerthanthe primaryforestwith 75 tree speciesfoundon 12 10 m x 10 m plots [W. Stanley,personal
Surface fluxesofN20 andNO weremadeusingchamber techniques. Chambers consisted ofapolyvinylchloride (PVC)ring(20-
communication,1998]
fluxesduetoinsertion ofthering,butthese bursts lasted only15to 20min. Therefore, nomeasurements weremadeduringthefirst30
The degradedpasturewas first clearedin 1969andhadbeen plantedto Panicurnmaximumandlaterto Brachiariahumidicota. The pasturewasheavilygrazeduntiltheearly1980s(2-3 headof cattleperhectare)andthengrazedonlyintermittently untilit was abandoned in 1990. Much of thepastureis coveredwith shrubby regeneration, distributedin a patchymanner. Fires frequently escapeinto this pasture,andit bumsalmostannually. The active pasturewasa "reformed" pasture, meaningthatit hadbeenthrough a similarlandusehistoryto thatof thedegraded pasture until 1987, whenitwascleared, burned,disked,fertilizedwithphosphorous and plantedtotheforagegrass Brachiariabrizantha.Theactivepasture presentlyhasfew woodyinvaders. 2.1.2. FazendaAgua Parada. Thisranchis locatedapproximately 20 km north of FazendaVit6ria, on the Bel•m-Brasilia highway.Thissiteis locatedin thesamegeologicformationbutat
a lowerelevation,andit appears thatsoilswereformeddirectlyon
cmdiameterx 10-cmheight)anda ventedPVC covermadefroman
endcapofa20-cmdiameter PVCpipe.PVCringswerepushed into thesoilto a depthof 2-3 cmto makethebaseof thechamber.Tests in forestandpastureecosystems showedshort-lived increased NO
min aftersettingringsintothe soil.
NO fluxesweremeasured usinga dynamicchamber technique similarto Davidsonet at. [1991]. At thetimeof measurement, a ventedcoverwasplacedoverthe base,makinga chamberwith approximatelya 4 L headspacevolume. Air wascirculatedin a
closed loopbetween aScintrex LMA-3NO2analyzer (Scintrex, Inc., Concord,Ontario,Canada)andthechamber throughteflontubing usinga battery-operated pumpat a rateof 0.5 L min4. Insidethe
instrument, NOwasoxidized toNO2byreaction withCrO3, andthe NO2wasthenmixedwithLuminol© solution toproduce a lumines-
centreaction directlyproportional to themixingratioof NO2. Because of problems withhumidity wetting theCrO3catalyst, we driedtheairstream entering theanalyzer usingaNationgassample dryer(PermaPure,Inc.,TomsRiver,New Jersey).NO concentrationswererecorded at 5-sintervalsovera periodof 3-4 minusing
34
VERCHOT
ET AL.: NITROGEN
OXIDE
EMISSIONS
FROM EASTERN
AMAZONIAN
SOILS
a datalogger.Fluxeswerecalculated fromtherateof increase ofNO concentration usingthe steepest linearportionof theaccumulation curve.The averagelengthof timeusedfor thecalculation of fluxes was 1.9 min. The instrumentwas calibrated2-3 timesdaily in the field by mixingvaryingamountsofa 1 ppmNO standard withNO-
approximately25, 50, 75, 100, 300, 500 and 800 cm depth(for detailsseeDavidsonand Trurnbore,[1995]). Sampleswerecollected3 timesduringthecourse of thestudy:May 1995,September 1995, andFebruary1996.
andNO2-free air. At thelowsample flowrate,theresponse wasnot
2.5.
linear,andwe fit the calibrationcurveaccordingly.
N20 fluxesweremeasured with a staticchamber technique [Matsonet al., 1990],usingthe samechamberbasesasthoseused for theNO measurement.At the time of measurement,a PVC cover
(20-cmPVC endcap)wasplacedoverthebasemakinga chamber with a headspacevolumeof approximately5.5 L. Four 20-mL headspace samples werewithdrawnat 10-minintervalsandreturned to the laboratoryfor analysiswith a gaschromatograph fittedwith an electroncapturedetector.
N20 fluxeswerecalculated fromtherateof concentration in-
Soil Water
Content
Soil water content was measured in associationwith each flux
measurement usingTDR probes(30 cm long),inserted vertically into the soil surfaceto measurethe dielectric constantofthe soil. The dielectric constant was converted to volumetric soil water content
usingcalibration curvesderivedfromlaboratory analysis of intact soilcoresof thesamesoil[Jippetal., 1998].Then,usingaverage bulk densityvaluesfor soilsat eachsite,volumetricsoilwaterwas
converted topercent waterfilledporespace (%WFPS);[seeDavidson,1993]. All measures of soilwatercontent will beexpressed in unitsof %WFPS in thispaper.
crease,determinedby linearregression basedon thefour samples. Occasionally,andparticularlyfor very high fluxes,the accumula2.6. N Availability Indices tion curveappearednonlinear,probablydueto thereductionin the concentration gradientbetweenthe soil atmosphere and the head N availabilitywasdetermined 4 timesbetween1995and1997, space[Hutchinsonand Livingston,1993]. In thesecases,only twiceduringthedry season (July 1995andJuly1996)andtwice pointsrepresenting the linearportionof the accumulation curve duringthewetseason (January 1996andFebruary1997).Wemade
wereused.In almost allcases, NO andN20fluxmeasurements for a varietyof measurements to characterize N poolsandN turnover a particularsiteweremadeon the sameday andwithin 90 minutes of each other.
2.3. Sampling Design
2.3.1. FazendaVit6ria. Eightchamberbaseswereplaced in a circularpatternin eachof two studysitesin eachlanduse,and measured monthlyfromFebruary1995to May 1996. Eachstudy sitewaslocatedin thevicinityof a soilpit instrumented with time domainreflectometry (TDR) probes[Nepstadet al., 1994;Jippet al., 1998] and soil gastubes[Davidsonand Trumbore,1995]. Chamberbaseswere left in place throughoutthe courseof the experimenton all sitesexceptthe activecattlepastures.Because thesesiteswereactivelygrazed,ringswerewithdrawnattheendof eachsamplingandreplacedin thesamegeneralareaforthefollowing sampling. Occasionally, becauseof useor root ingrowth,a permanently installedringwouldbecomeloose.In thiscase,thering wasremovedandresetwithin 2-3 m of its originalplacement. 2.3.2. Intensive Sampling. To determinethe accuracyof estimating sitefluxeswitheightchamber measurements persite,we sampledgasfluxesintensivelyin a primaryforestand an active pasture,oncein the wet season(April) andoncein the dry season (November). In the primary forest,we chosean area with no apparentdisturbance, andin thepasture,we choseanareathatwas activelygrazed.Neitherareahadanypriorresearch activity.In each area,we sampled36 chambers ona 5 m x 6 m gridin a 750m2area, usingthemeasurement techniques citedabove. 2.3.3. FazendaAgua Pararia. Thesesitesweresampled 5 times betweenApril 1995andMay 1996;however,thelastsamplingwas incompletedue to instrumentproblemsand flooding. For each sampling,eight chamberswere measuredper site. Becausethe pastureswere activelygrazed,ringswerewithdrawnat the endof each samplingand replacedin the samegeneralarea for each sampling.
rates.We collectedeightsoilsamples to a depthof 10 cmpersite andper samplingdate. Samples weretransported on ice to the laboratory wheretheywererefrigerated untilextraction or incubation. After returningto thelab, all soilsamples werethoroughly mixed;coarserootsand coarseorganicmatterwere removedby hand.
We determined theinorganic N poolsizesbyextracting NO3-N andNHn-Nfroma 15g subsamples offield-moist soilwith100mL of 2 M KC1. The soil-KC1 solution was shaken for an hour on an
orbitalshakerandallowedto settleovernight.A 20-mL aliquotof thesupernatant wasremoved,filteredthrougha 45 gmpolysulfone membraneandfrozenfor lateranalysis.Analysiswasdoneon an Alpkem(Wilsonville,Oregon)autoanalyzer usingamodifiedGriess-
Illosvay procedure fordetermination ofNO3-N+NO2-N, which was reported asNO3-N[Bundy andMeisinger,1994]anda salicylatehypochlorite procedure forNHn-N[Kemper andZweers, 1986]. Net mineralizationandnet nitrificationweredetermined using theaerobicincubation procedure described by Hart et al. [ 1994]. For eachsampledescribedabove,a 15-g subsample of field-moist soilwasplacedin a 120-mLspecimencup,whichwasthenclosed with a perforatedplasticcapto allowgasexchange whileminimizing evaporationloss. Soils were incubatedfor 7 days at room
temperature (approximately 24øC).NHn-NandNO3-Nconcentrationsweredetermined by extractinginorganicN fromsubsamples with 100 mL of 2 M KC1 solution before and after incubation and
analyzingasdescribedabove.Net mineralizationratesweredeterminedfromthedifferencebetweeninorganig N atthebeginning and endof theincubation,andresultswereexpressed ona basisof mean daily inorganigN production. Likewise, net nitrificationwas
determined fromthedifference inNO3-Natthebeginning andend of the incubation,andresultswere expressed in similarunits.We
encountered problems withNH4+contamination of ourKC1andas a resultthe net mineralizationdatafrom July 1996 andFebruary
1997werelost.Wealsoencountered problems withexcessive NO32.4. N:O Profilesin the SoilAtmosphere
loss(> 2.5 gg N g-soil4 d4) in incubations of primaryforestsoil collectedin January1996perhapsdueto anaerobic conditions in the
At FazendaVit6ria, soilgasesweresampledfromstainless steel tubes(3 mm OD) previouslyinstalledinto thewalls of eachpit at
primary forests.
soils. Therefore we have no wet season net mineralization data for
VERCHOT ET AL.: NITROGEN
OXIDE EMISSIONS FROM EASTERN AMAZONIAN
Nitrification potentials weredetermined usingtheshaken slurry methodof Hart et al. [ 1994]. Thismethodassesses themaximum
rate(Vmax) of nitrification fora soilsample.A 15g subsample was takenfromeachsoilsampleandmixedwith 100mL of a solution
2.8.
Model
Parameterization
SOILS
35
and Evaluation
Modelparameters weregenerated bylinearregression analysis of therelationships between N availability andthecombined N oxide
andbetween %WFPS andtheratioofN20:NO.Atthesites containing 1.5m3//NH4 +and1•PO4 3-ina 250-mLErlenmeyer fluxes near Santana do Araguaia and attheIGBEReserve, wemeasured gas flaskto makea slurry.Slurrieswereshakenon anorbitalshakerat flux, %WFPS and nitrification potential following the methods 180rpmto maintainaerobicconditions.At 2 hoursand24 hourswe described in sections2.2, 2.5 and2.6. Thesedatawerethenusedin
withdrew a 15mLsample fromeach slurry foranalysis forNO3-N. themodeltopredicttheN oxideflux. Predictions werecompared Because ofdispersion ofclayparticles, it wasimpossible tofilterthe solutions, andwedidnothaveaccess to a centrifuge withenough powerto rapidlysettlethesolids.Therefore wemixedtheslurry sampleswith 15mL of 4 MKC1 to flocculatethecolloidsandletthe
solutionsettleovernightat4øC.Thisresultedin a solutionwithan approximate final concentration of 2 M KC1. Afterpipetting the supematant into samplevials, the sampleswere frozenfor later analysis asdescribed above.Nitrificationpotential wascalculated
fromtherateof increase inNO3-Nin theslurry,andresults were expressed on a daily basis.
2.7. Statisticsand Data Analysis
with observed values.
3. Results 3.1.
Fazenda
Vit6ria
Land
Use Measurements
3.1.1. Generalsoilcharacteristics.A summary of physicaland chemical characteristics of the surface 10 cm of soils in each
ecosystem is presented in Table 1. Pasturesoilshadhigherbulk densities thantheprimaryforestsoils,butthesecondary foresthad returnedto abulkdensityin thesurface10cmthatwassimilartothat oftheprimaryforest.SoilpHwas4.4intheprimaryforestand> 5.4 in thepastures andsecondary forest.TotalN andC stocksin this superficiallayer were highestin pastureecosystems, lowestin primaryforests,andintermediate in secondary forests.Because of
Normaldistribution ofthedatawasdetermined byagoodness of fit testusingthe Kolomogorov-Smimov D statistic[Sokaland Rohlf,1981]. Gasflux datawerenotnormallydistributed, butN availability indexdatawerenormal.N availability indexdatawere soilcompaction in thepastures, however, thisapproach compares of soil,andthestocksappearto increase signifianalyzed usingstandard parametric analyses (analysis of variance differentmasses [Trumbore etal., 1995].If we andt-test).Forthegasfluxdata,weusedtheBox-Coxprocedure cantlyasaresultof forestconversion [SokalandRohlf,1981] to estimate thebesttransformation, butdid comparethe soilson a commonmassbasis,we notethatinventories by onlyabout5% (1.2 Mg ha-1)for C and16%(0.4 Mg notfinda transformation thatprovided a satisfactory distribution. increased whicharewithintheuncertainties of these Therefore all statistical testsweremadeusingnon-parametric pro- ha-l) forN in thepastures, TheC:N ratiowassimilaracross all ecosystems. cedures. We usedtheNPAR1WAY procedure in SAS[SASInsti- measurements. 3.1.2. Precipitation and soil water content. Annual rainfall tute,1992]to calculate Wilcoxonscores fortheMannWhitneyU testtocompare twomeansorfortheKruskal-Wallis testtocompare wasgreaterthanthe22-yearmean(1850mm)duringthisstudy; morethantwo means.Because of unequalobservations between rainfallwas1905mmand2380mmfor1995and1996,respectively. categories, no separation testswereperformedfor gasflux results. A distinctdry seasoncan be observedfrom Juneto December
We usednonoverlapping standard errorintervals to inferprobable (Figure2) xvhere precipitation madeuplessthan15%of theannual significantdifferences. To evaluatethe intensivemeasurement results,we used a
total for 1995.
resampling procedure to determine theprobability thata sample withn = 8 wouldestimate thetruemean,asdefinedbytheuniverse of 36samples, usingResampling Statssoftware [Simon,1992].We drew1000randomsamples ofeight,withreplacement, fromthefull datasetof 36 observations andtalliedthemeanof eachsampleto
Followingthecessation of rainsin June,WFPSin thesurface30 cm
Soil watercontentvariedin phasewith rainfall(Figure2).
ofsoildeclined untilreaching moreorlessconstant levelsinAugust. Forest soilshadconsistently higherWFPSthanpasture soilsduring
thedryseason.WFPSincreased morerapidlyin pasture soils following theonset ofrains inJanuary, duetoadelay indevelopment determinethe frequencydistribution of themean. We thendeter- ofevapotranspiration capacity inthepasture grasses. Bythemiddle minedtheprobabilitythata randomsampleof eightfell withina of thewetseason, WFPSin thepastures wassimilarto thatin the certaindistance ofthemean,based uponthisfrequency distribution. forests.
Table 1. Soil Characteristics for the the Surface 10 cm of Soil in the
DifferentEcosystems Studiedat FazendaVit6ria Ecosystem
BulkDensity pH- H20 gcm-3
Primaryforest Secondaryforest Active pasture Degradedpasture
0.99 0.96 1.25 1.22
4.4 5.5 5.7 5.4
TotalN
TotalC
Mg ha4
Mg ha4
2.2 2.6 3.6 3.2
24.5 29.0 31.7 32.1
Sourceis D. Markewitz andE.A. Davidson,unpublisheddata(1998).
C:N
11.1 11.2 8.8 10.0
36
VERCHOT
ET AL.' NITROGEN
OXIDE EMISSIONS
FROM EASTERN AMAZONIAN
SOILS
800
pasturesburnedin Octoberwhen a fire escapedfrom a nearby
600
sawmill, butthisdidnotresultin increased N20fluxes.
Differences between ecosystemswere highly significant (Kruskall-Wallistest,ct= 0.0001)in boththewet anddry seasons. We couldnot computemultiplerangeseparation statistics because samplesizeswereunequal,sononoverlapping standard errorinteriO0 valswereusedto indicateprobabledifferences (Table2). Fluxes from primary forest were greaterthan either active pastureor degraded pasturefluxesand,20 yearsfollowingabandonment, the 60secondaryforesthad not fully recoveredemissionlevelsequalto thoseof theprimaryforest(Table2). 40A few negativefluxes (uptake)were observedfor individual chambers in forestecosystems duringbothseasons, butthesemade 20 • Pdma•Forest • A•ive Pasture up 1-4% of the observationsin each ecosystemand were not significantlydifferentfromzero(ct= 0.05). In theforestecosystems, F MAM J J A S ON D J F M A M 10-35%of thepositivefluxeswerenot significantlydifferentfrom zero(ct= 0.05). Negativefluxesweremorecommonlyobservedin 1996 1995 pastureecosystems andmadeup 30-40%of theindividualchamber Month observations duringthewet season and55-65%of theobservations Figure2. (Top)Monthlyrainfallfor thestudyperiodand(bottom) duringthedry season.Of theseobservations, 10-20%weresignifisoilwatercontentexpressed aspercentwaterfilled porespace(% cantlydifferentfromzero(ct= 0.05). In thepastureecosystems, 45WFPS) in thetop 30 cm of the soilprofile.. 97% of thepositivefluxeswerenotsignificantly differentfromzero (ct: 0.05). We calculated theminimumdetectable fluxfollowingHutchinson 3.1.3. N20 fluxes.Duringallfluxmeasurements in 1995-1996, weobserved production ofN20byforestsoils(Figure 3). Fluxes andLivingston[ 1993]. For eachindividualchamberflux measureweregreatest inthewetseason (January-May) andlowestduringthe ment, we computedthe 95% confidenceinterval, groupedthe by incrementsof 0.1 ng cm-2 h-1, and definedthe dry season(June-December). Pasturesoilsshowednetuptakeor observations zero fluxesduringthe dry seasonand emissionduringthe wet minimum detectable flux as that flux at which > 67% of the season, butthisseasonal variationwasmuchlesspronounced than confidence intervalsdid not includezero(i.e., a 2:1 signalto noise in forestecosystems. Seasonal differences weresignificant at ct= ratio). The minimumdetectableflux was0.6 ng cm-2h-•. 0.01 (Mann Whitney U test) for all ecosystems.The degraded 3.1.4. NO fluxes. During all flux measurements in 1995-1996
400
80-
•
N20
•
NO
10 Primary Forest 6 (.>
0
4 5
Active Pasture
Degraded PastUre 3
2 -1
F MAM
J J A S ON
1995
D J F MAM
F MAM
1996
J J A S ON
1995
D J F MAM
1996
Month Figure3. Monthlyfluxratesof N20andNO in thedifferent ecosystems atFazenda Vit6ria.Errorbarsare+ 1 SE.
VERCHOT ET AL.: NITROGEN OXIDE EMISSIONS FROM EASTERN AMAZONIAN
SOILS
37
Table2. Seasonal andAnnualN20 andNO FluxesatFazenda Vit6ria N20 Ecosystem
NO
Wet
Dry
Dry
Season,
Annual Total,
Wet
Season,
Season,
Season,
Annual Total,
(ng N cm-2h'l)
(ng N cm'2h'l)
(kg N ha'l yr-l)
(ng N cm'2h'l)
(ng N cm-2h4)
(kg N ha-I yr-l)
Primary forest
5.23 + 0.40 (135)
1.04 + 0.08 (120)
2.43
1.18 + 0.15 (136)
2.13 + 0.26 (112)
1.52
Secondaryforest
1.62 + 0.13 (128)
0.69 + 0.11 (128)
0.94
0.44 + 0.11 (136)
0.34 + 0.03 (109)
0.33
Active pasture
0.96 4- 0.23 (118)
-0.20 4- 0.05 (103)
0.25
0.62 4- 0.11 (127)
0.54 4- 0.12 (87)
0.50
Degradedpasture
0.18 4- 0.05 (136)
-0.00 4- 0.03 (112)
0.06
0.59 4- 0.20 (107)
0.92 4- 0.13 (95)
0.69
Annual totalswere calculatedby stratifyingthe year into wet season(January- May) and dry season(June- December)and multiplyingthe meanflux for the seasonby thenumberof daysin therespectiveseason.Valuesaremean4-SE; the numberof observationsare given beneatheachmeanin parentheses.
significantly differentfromzero(ct= 0.05). The majorityof the fluxeswerepositiveand
