Biogeochemistry of an Amazonian podzol-ferralsol soil system with white kaolin

16 Pages·2012·0.62 MB·English

Biogeosciences,9,3705–3720,2012
Biogeosciences
www.biogeosciences.net/9/3705/2012/
doi:10.5194/bg-9-3705-2012
©Author(s)2012. CCAttribution3.0License.
Biogeochemistry of an Amazonian podzol-ferralsol soil system with
white kaolin
Y.Lucas1,C.R.Montes2,S.Mounier1,M.LoustauCazalet1,D.Ishida3,*,R.Achard1,**,C.Garnier1,B.Coulomb4,
andA.J.Melfi5
1PROTEE,Universite´ duSudToulon-Var,LaGarde,Toulon,France
2NUPEGEL,CENA,UniversidadedeSa˜oPaulo,Piracicaba,Brazil
3NUPEGEL,IG,UniversidadedeSa˜oPaulo,Sa˜oPaulo,Brazil
4LCE-DMCM,LCP,Aix-MarseilleUniversite´,Marseille,France
5NUPEGEL,ESALQ,UniversidadedeSa˜oPaulo,Piracicaba,Brazil
*nowat:NUPEGEL,CENA,UniversidadedeSa˜oPaulo,Piracicaba,Brazil
**nowat:INERIS,Aix-en-Provence,France
Correspondenceto:Y.Lucas([email protected])
Received:23December2011–PublishedinBiogeosciencesDiscuss.:28February2012
Revised:14August2012–Accepted:28August2012–Published:28September2012
Abstract. The podzol-ferralsol soil systems, which cover Bhsandkaolinbleachingisensuredbyironreductiondueto
great areas of Amazonia and other equatorial regions, are acidity and relatively low E . Fe2+ mobility can be related
H
frequentlyassociatedwithkaolindepositsandstoreandex- tosmallE variationsandenhancedbythesignificantcon-
H
portlargeamountsofcarbon.Althoughnaturalorganicmat- centrationofsmallcarboxylicacids.Thelong-termresultof
ter(NOM)playsakeyroleintheirdynamics,littleisknown theseprocessesisthethickeningofthekaolin,anditcanbe
about their biogeochemistry. In order to assess the specific inferredthatkaolinislikelytooccurwhereactive,giantpod-
role of dissolved organic matter (DOM) on NOM storage zols areclose toa slope gradientsufficient enoughto lower
indeephorizonsandtodeterminepossiblerelationshipsbe- thedeepwatertable.
tweenkaolinformationandDOMproperties,westudiedthe
groundwater composition of a typical podzol-ferralsol soil
catenafromtheAltoRioNegroregion,Brazil.
Groundwater was sampled using tension-free lysimeters 1 Introduction
placedaccordingtosoilmorphology.DOC,E ,pH,anddis-
H
solvedSi,Al3+,Fe2+,andFe3+ wereanalyzedforallsam- Morethan18%oftheAmazonianareaiscoveredbypodzol-
ples and values are given in a database. Quantification of ferralsolsystems(RadamBrasil,1978),whicharecharacter-
other dissolved ions, small carboxylic acids and SUVA ized by the juxtaposition of podzols and ferralsols on the
254
indexandacid-basemicrotitrationwasachievedonselected same landscape units (Lucas et al., 1984). Ferralsols are
samples. usual, climacic soils of equatorial areas where high weath-
Part of the DOM produced by the hydromorphic podzols ering rates and long-time evolution allowed the leaching of
is directly exported to the blackwater streams; another part all major elements except Al, Fe and Ti, the persistence of
percolatesatgreaterdepth,andmorethan90%ofitadsorbs Siaskaoliniteintheupperhorizonsbeingallowedbyplant
in the Bh-Bhs horizons, allowing carbon storage at depth. cycling (Lucas et al., 1993). The podzols appear and de-
Humicsubstancesarepreferentiallyadsorbedwithregardto velopwheredissolvedorganicmatter(DOM)isabletoper-
smallcarboxyliccompounds. colatethroughsoilhorizonsdowntotherivers,allowingAl
Withregardtokaolingenesis,kaoliniteprecipitationisfa- and Fe leaching and thus favoring the dissolution of clay
voredbyAlreleasefromNOMmineralizationwithintheBh- andironoxides(Lucasetal.,1996;Lundstro¨metal.,2000;
Nascimentoetal.,2004).Onceinitiated,thisprocessinduces
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
3706 Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin
a positive feedback for Al and Fe leaching, resulting in a 66°53'30"W 66°53'00"W
progressive replacement of the ferralsols by podzols, even Studied sequence
0°6'10"S
where ferralsols are clayey. Large podzol areas constitute W2
São Gabriel FFeerrrraallssooll W1
thusoneend-memberofequatorialsoilsandlandscapeevo- da Cachoeira aarreeaa Spring 0°6'30"S
lution(DubroeucqandVolkoff,1998)(Fig.1). PPooddzzooll
Thepodzolareashavetheabilitytoreleasegreatamounts aarreeaa500m 0°6'50"S
1°0'0"N
of dissolved organic carbon (DOC) (Leenheer, 1980; Chau-
vel et al., 1996; Benedetti et al., 2003; Patel-Sorrentino et
0°0'0"
al., 2007) in the draining waters, which flow afterwards to
the sea through the river network. Considering the Amazon
1°0'0"S
basin,theyprovideatenthofthe0.13PgCannuallyexported Rio Japurá RioNegro
to the sea (Tardy et al., 2009). They also can store large 2°0'0"S
amountsofcarbonintheupperandthedeepBhandBhshori-
zons(BatjesetDijkshoorn,1999;Bernouxetal.,2002;Veil- 3°0'0"S Rio Solimões
lon and Soria-Solano, 1988; Nascimento et al., 2004). Ac- 0 100 200 km
cording to Montes et al. (2011), hydromorphic podzols can 68°0'0"W 66°0'0"W 64°0'0"W 62°0'0"W
store86.8±7.1kgCm−2,andatleast13.6±1.1PgCisstored
Fig.1.Situationofthestudiedcatenaandextensionofhighlypod-
inAmazonianpodzols.Thesesoilsystemsthereforemakea
zolicareas(ingrey)intheRioNegrobasin.Compilationofhighly
significantcontributiontotheCO cycleattheglobalscale,
2 podzolicareaswasrealizedfromdigitalizedsoilmapsofAmazonia
but the dynamics of DOM transfer and soil organic matter
(IBGE, 2009). The detailed view of the study area is issued from
(SOM)accumulationisstillpoorlyknown. IKONOSimagery.Thedarkerforestedareacorrespondstoclosed,
Podzol-ferralsol systems are also frequently associated low forest over hydromorphic podzols. W2 and W1 are sampling
with kaolin deposits (Montes et al., 2007), which are ge- pointsoutsidethecatena.
ological or pedological formations rich in kaolinite-type
clays. In the Amazon area, thick, white kaolin horizons
(up to 10m thick) were observed beneath ferralsol-podzol gions.Moregenerally,exceptforthesurfacehorizons,little
systems developed over sedimentary (Lucas et al., 1984; is directly known about percolating soil solution in humid
Chauvel et al., 1987; Costa and Moraes, 1998; Fritsch et tropicsoils.
al.,2009)aswellascrystallinebasementrocks(Dubroeucq In this context, the aim of the paper is two-fold, both re-
and Volkoff, 1998; Montes et al., 2011), raising the ques- latedtonaturalOMdynamics.First,whatisthespecificrole
tionofthegeneticrelationshipsbetweenpodzolicprocesses of DOM with regard to the soil system dynamics, particu-
and kaolin genesis in order to provide guidance for kaolin larlysoilOMaccumulationindeephorizonsandOMtrans-
prospecting. Kaolin genesis is favored where weathering fertowardstherivernetwork?Second,istherearelationship
enhances quartz and Fe-bearing mineral dissolution while betweenkaolinformationandOMproperties,and,ifso,will
maintaining kaolinite stability. As DOM issued from pod- kaolin formation be favored at specific locations within the
zols favors the dissolution of all types of clay minerals be- soil system? Such knowledge would help in predicting the
cause of Al complexation, how can podzolic processes fa- possibility of kaolin ore from remote sensing. The answer
vor kaolin genesis? Montes et al. (2007) hypothesized that tothesequestionshasbeensoughtbystudyinggroundwater
the DOM transferred at depth by podzolic groundwater ad- percolation in a podzol-ferralsol system whose morphology
sorbswithintheBh-Bhshorizonandthatpartofitisminer- andmineralogyhadbeenstudiedinpreviouswork.
alized,whichturnstheacidicpercolatingwatermorereduc-
tive,favoringironleachingandallowingbleachingofkaolin
horizons. Such a hypothesis, however, needs to be assessed 2 Materialandmethods
by studying pH, E , DOM and Fe and Al species in per-
H
colating groundwaters. Several detailed studies of the solid 2.1 Descriptionofthestudyarea
mineralandorganicphaseshavealreadybeenconductedon
Amazonian podzols (Bravard and Righi, 1991; Nascimento ThestudiedsoilcatenaissituatedneartheSa˜oGabrieldaCa-
et al., 2004; Bardy et al., 2008; Fritsch et al., 2009), show- choeiracity,AmazoniaState,Brazil,at0◦602100Sand66◦540
ingthathorizonmorphologyandsolidphasecharacteristics 2200W (Fig. 1), and was described in previous publications
areverysimilarfromonesystemtotheother,buttheproper- (Montes et al., 2007; Ishida, 2010). It was chosen amongst
tiesorcompositionofthecirculatingsolutionswasonlyin- 11 similar catenas that have been studied on the same site,
ferredfromthepropertiesofsolidphases.Fewworks(Cornu therebyensuringitsrepresentativeness.Itcutstheedgeofa
et al., 1997, 1998; Nascimento et al., 2008) have focused plateauwheregiantpodzolsdevelopedfromtheplateaucen-
on sampling and studying the percolation solutions, mainly terattheexpenseofreddishyellow,lowactivityclayferral-
because of difficulties of sampling groundwater in these re- sols,asdescribedelsewhere(Lucasetal.,1988;Dubroeucq
Biogeosciences,9,3705–3720,2012 www.biogeosciences.net/9/3705/2012/
Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin 3707
andVolkoff,1998).Itisrelatedtoarivernetworkthatenters Situation of U-shaped channel Bh-BhOs&A1 P1
theplateaubyregressiveerosion. Tension-free lysimeters
P2
The climate is typically equatorial, with an annual rain- Observation pit Es
fall around 3000mm, without a marked dry season and COebnsteimrveattrioicna lbAolr-enhoodlueles AEf S3-1B601SCB-1S52-16K01 S1-240
with a great interannual variability. Daily rainfall data were Fe-red spots
obtained from the airport station situated 6km from the Water-table (02-2006) B2 S3-380 S2-490 S1-510
Water pathway n°1 P3 K2
study area. The geological substratum is composed of crys-
Water pathway n°2
talline rocks having composition varying between monzo- P4 4 m C1
K3
granitic, syenogranitic and quartz-monzonitic (Dall’Agnol
S4-400 2
B3 S6-80
and Macambira, 1992). The vegetation over the ferralsols K4 S5-380 0
and over well-drained podzols is a typical lowland tropical S6-300 0 20 m
C2
evergreen forest and over the hydromorphic podzols a spe-
Fig.2.Viewofthesoilsystemalongthecatena.Capitallettersof
cific forest called campinarana and characterized by a high
horizonnamesrefertoFAO(1993).O:organic,peat-likehorizon;
densityofsmallertrees(20–30m).Suchdifferencesinvege-
A1:organic-rich,darkbrownhorizon;A:horizonsbrown-colored
tationaretypicalinpodzol-ferralsolsystems(Prance,1978),
byorganicmatter;Es:podzoliceluviatedhorizons;B1:oxicBhori-
and both forest type and soil characteristics result in high
zonsvaryingfromsandyupslopetosandy-clayeydownslope;B2:
DOM differences between ferralsol and podzol groundwa-
oxicsandy-clayBhorizons;B3:pseudogleyBhorizons;K:kaolin
ter (Bravard and Righi, 1991; McClain et al., 1997; Zanchi horizons; Bh-Bhs: horizons with organic matter and Al-Fe accu-
et al., 2011). In ferralsol areas, DOM adsorption and high mulation;Ef:non-podzolicleachedhorizon;C1andC2:saprolitic
decomposing rates account for low DOM concentrations in horizons.P1toP4:situationofmineralogicaldatagiveninTable1.
groundwater and streams (white or clear waters), with low
C/N ratio and aliphatic, poorly aromatic fulvic and humic
acids(FAandHA).Inpodzolareas,thelackofDOMadsorp- darker,duetoanincreaseoforganicmattercontent;(2)Bhs
tionandlowdecomposingratesaccountforhighDOMcon- – there is an irregular, finger-like transition towards a hard-
centrationsingroundwaterandstreams(blackwaters),with ened, sandy-clay horizon, brown-colored (10YR 5/2) (col-
highC/Nratio,poorlyhydrolysableN,poorlyaliphaticand ors are given according to Munsell, 1990) with dark brown
highly aromatic FA and HA. The soils were studied using (7.5YR5/6)featuresformedbyorganicmatterandFe-oxides
a structural analysis approach (Boulet et al., 1982; Delarue accumulation in cracks and tubular pores; the gibbsite con-
et al., 2009). Macromorphological features were observed tentishigher,andthequartzsandcontentdiminishesquickly
throughhandaugerboreholesandpitdescriptions.Mineral- indepth,togetherwiththeincreaseofkaolinite,andthehy-
ogywasdeterminedbyX-raydiffractiononpowdersamples, draulicconductivityturnslower.ThekaolinK1andK2hori-
diffusereflectancespectroscopyandthermogravimetricanal- zonsconsistmainlyofkaolinite,withgibbsitecontentaround
ysis. Only the data necessary to understand the water geo- 8% at the upper part of K1 and decreasing in depth to be
chemistryaregivenhere;detailedmethodology,mineralogy lowerthan2%atthelowerpartofK2(Fig.3).Theupperpart
andgeochemistryaregiveninIshida(2010). ofthekaolinisaclayey,whitishmaterialwithsomeorange-
Thesoilcatena(length200m,differenceinaltitude15m) colored coatings in tubular pores (K1 horizon). These coat-
issketchedinFig.2.Twomainsetsofhorizonscanbedis- ingsprogressivelydiminishindepth,givingplacetoahomo-
tinguished:(i)thehorizonsofthewell-expressedpodzols,at geneous,white,clayey,butter-likematerial(K2horizon).At
theupperpartofthecatena,and(ii)theoxichorizonscolored depth,theC1horizonisasilt-claysaprolitewithmuscovite
by Fe-oxideson the slopes. Thickkaolin horizonswere ob- andweatheredfeldspar.
servedbelowbothpodzolicandoxichorizons.Mineralogical Goingdownslope,podzolichorizonsarereplacedbyoxic
dataaresummarizedinTable1. horizons colored by Fe-oxides: A horizons, on the upper
Thepodzolshaveatypicalverticalsuccessionofhorizons: partoftheprofiles;B1horizons,whichprogressivelychange
O, A1, Es, Bh, Bhs. The organic O horizons are peat-like, from sandy to sandy-clay in the downslope direction; B2
with a thickness varying from a few centimeters to more sandy-clayhorizons;B3horizonsinthedownslopeposition
than 50. The more water-logged the topsoil throughout the havingwater-loggingfeatures.BelowtheB1horizon,asilty
year,i.e.farfromtheplateauslopes,thethickertheOhori- clayloam,paleyellowhorizonwithcoarsequartzgrains(Ef)
zons are. The humic A1 horizons are well-developed; they wasobserved.Itindicatesaperchedwatertableandisasso-
consist of clean, white quartz sand and organic matter par- ciated with U-shaped channels (2 to 4m wide and 1 to 2m
ticles.TheeluviatedsandyEshorizonshavethin(1–2mm), depth) whose flat bottom is at the same altitude as the Ef
darkgreymicro-horizonscoloredbyorganicmatterparticles; horizon, the channels forming a hydrographical network on
they have traces of kaolinite and gibbsite and high porosity theslopes.Indepth,thethickkaolinhorizons,theclayeyK1
and hydraulic conductivity. The transition between Es hori- and K2 horizons extend downslope. The colors are yellow
zons and the underlying kaolin shows the following verti- (10YR 8/6) and white (5Y 8/1 to 5Y 8/2), for K1 and K2
cal sequence: (1) Bh – the Es horizon turns progressively horizons,respectively.AK3horizonhavingsomeremaining
www.biogeosciences.net/9/3705/2012/ Biogeosciences,9,3705–3720,2012
3708 Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin
Table 1. Minerals identified in the soil material. Pits and horizon are located on Fig. 2. A: anatase, M: muscovite, V: vermiculite, Mc:
microcline,Mg:magnetite,I:illite,R:rutile,Go:goethite,H:hematite,ε:verysmallamounts.
Horizon Depth Texture Mineralogy
(m)
Quartz Kaolinite Gibbsite Muscovite Otherminerals
(%) (%) (%) (%)
PitP1–Podzol
O&A1 0.0–0.8 Sand 88–95 2–12 ε–5 0–ε A,R,Go,H
Es 0.8–1,9 Sand 100 ε ε 0–ε A,R,Go,H
Bh-Bhs 1.9–2.3 Sandyloam 90–94 6–10 ε ε A,R,Go,H
K1 2.3–4.0 Siltyclayloam ε–3 92–96 8–4 ε–1 A,M,Go,H
K2 4.0–10.0 Siltyclayloam ε–3 96–97 4–2 ε–1 A,M,Go,H
PitP2–Ferralsol
A 0.0–0.3 Sandyclayloam 59–70 18–26 2–3 ε A,V,Go
B1 0.3–1.3 Sandyclayloam/Clayloam 36–68 14–66 1–3 ε A,V,R,Go
Ef 1.3–1.6 Siltyclayloam 37 60 3 ε–1 Go
K1 1.6–3.0 Siltyloam 6 94 ε 3–4 A,R,Go,H
K2 3.0–6.6 Siltyloam 10 90 ε 2–6 A,R,Go,H
PitP3–Ferralsol
A 0.0–0.1 Sandyclayloam 62 34 4 ε–1 A,R,Go
B2 0.1–1.0 Sandyclay 8–65 32–86 3–6 ε–1 A,R,Go
K3 1.0–1.3 Clay 10 86 4 1 A,Go,H
K1 1.3–4.0 Clay 2–9 86–92 5–8 1–2 A,R,Go,H
K4 4.0–5.6 Clay/Siltyclay 2 96 2 2–13 A,R,Mc,Mg,Go,H
PitS1–Gleysol
A 0.0–0.1 Sandyloam 59 24 1 3 A,R,Go,H
B3 0.1–1.4 Sandyloam 60–71 9–13 ε–1 3–7 A,R,Mc,Go,H
K4 1.4–2.7 Loam 42 15 ε 15 A,V,R,Mc,I,Go,H
0 20 40 60 80 100 quartzgrainsandsomeredandyellownodulesappearsatthe
0
O andA mid-slope at the upper part of the kaolin body. Downslope,
0.5 TOC horizons the K1 and K2 horizons give place to the K4 horizon char-
acterized by more abundant unweathered muscovite and Fe
1.0 E red spots at its upper part. Gibbsitic nodules are located at
horizons the downslope half of the catena as indicated in the figure.
1.5 Indepth,C1andC2horizonsaresaprolitichorizonshaving
m)
h ( manymuscoviteandweatheredfeldspar.Atthelowerpartof
pt 2.0 Kaolinite Bh and Bhs thecatena,thewatertablehadthetypicalodorofsulphurs.
De horizons
2.5
2.2 Watersamplingandanalysis
Gibbsite K
horizons
3.0
2.2.1 Lysimeters
3.5
Itappearedunnecessaryforthepurposeofthepresentwork
Fig. 3. Solid soil phase: kaolinite, gibbsite and total organic car- to collect and analyze rain openfall, throughfall and stem-
bon(TOC)inthepodzolfromtheupperpartofthecatena(Pit1). flow.Suchastudyisdifficultbecauseofhighlyspatialvari-
Horizontalaxisisin%forkaoliniteandgibbsiteandin‰forTOC. ability under forest cover and had already been conducted
inasimilarpodzol-ferralsolsystemthroughoutawholeyear
(Cornu et al., 1998). The results showed that the precipi-
tation input was negligible with regard to mineral-solution
Biogeosciences,9,3705–3720,2012 www.biogeosciences.net/9/3705/2012/
Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin 3709
equilibriuminsoilhorizons.Meanconcentrationsandstan- (pointsS1-510,S2-490,S3-380,S4-400,S5-380,S6-300)in
dard deviation in openfall were 0.02±0.01, 0.03±0.02 horizonswheregibbsiteandironoxideprecipitationoccurs.
and 0.04±0.02mgl−1 for dissolved Si, Al and Fe, respec- Moreshallowsamplingpointswerechoseninordertosam-
tively;inthroughfall,theywere0.08±0.04,0.06±0.04and plegroundwaterfromaperchedwatertablelikelycirculating
0.07±0.05mgl−1, respectively. Stemflow fluxes were in- intheEfferralsolhorizon(S2-160,SCB-15,S3-160)andex-
significant. pectedtooverflowinanU-shapedchannel(SCB-15).
Regarding groundwater, 13 zero-tension lysimeters were
installed inside drilling holes at different depths, 2 points 2.2.3 Samplingperiodsandanalysis
(W1-20andW2-150)withinthepodzoliccampinaranaarea
(Fig. 1) and 11 points along the soil catena (Fig. 2). Af- Groundwaterwassampledduringthreeperiodsindicatedin
ter installing the lysimeters, each drilling hole was tamped Fig.4.Thefirstone(27to30July2007)wastypicaloflow
by filling with the previously extracted soil material at the rainfall periods, the second one (29 January to 4 February
corresponding depth. Each lysimeter was made of a 50ml 2008) intermediate, and the third one (19 to 25 May 2008)
polypropylene bottle bored with 5mm diameter holes all typical of high rainfall periods, allowing sampling of high-
around. A 2mm diameter capillary PTFE tube was inserted levelwatertables.The10-daycumulatedrainfallsbeforethe
through the bottle cap in order to permit extracting water samplingperiodswere53,88and178mmforthefirst,sec-
from the topsoil with a manual vacuum pump. To prevent ond and third sampling periods, respectively. Complemen-
cloggingofthecapillarytubebysoilparticles,acylindrical tarysamplingforacid-basemicrotitrationandDOCcharac-
filtermadeofaSEFARNITEX64µmpolyamideopenmesh terizationwasrealizedon9February2009forpointsSpring,
was put inside the bottle. All lysimeters were acid-washed W2-150,S1-240,S1-510,S3-380andS4-400.
priortoinstallation. Sampling was done by applying a continuous suction of
25mmHgtothelysimeters.Thefirst20mlofeachsampling
2.2.2 Samplingpoints wasdiscardedinordertoavoiddeadvolumeandtorinsethe
sampling equipment. Groundwater was then sampled until
The studies already conducted on Amazonian podzol- 250ml was reached or air was entering the system. pH and
ferralsolsystems(seeintroduction)haveshownthathorizon E weremeasuredimmediatelyaftersamplingwithaEutech
H
morphologiesandsolidphasecharacteristicsareverysimilar pH310instrumentafterstabilizationwithgentleshake(cali-
from one system to the other: soil age and high weathering brationofthepHelectrodewasdoneeveryday).Eachsam-
rateshaveresultedinobliterationofmostsmall-scalespatial plewasthenfilteredusinganinline0.22µmcellulosemem-
variabilityduetoparentrockheterogeneity.Asinmanyold, brane filter (Nalgene surfactant-free cellulose acetate) and
deeplyweatheredsoilcovers,soilvariationsarespatiallyex- separatedinfouraliquotsforimmediateanalysisofdissolved
plicit and depend on soil forming processes closely related H SiO ,Al3+,Fe2+andFe3+,separately.Afifthaliquotwas
4 4
to water pathway and residence time (Boulet et al., 1982). filteredusinga0.7µmfiberglassfilter,poisonedwithsodium
In such a context, sampling points were selected based on azide,storedin10mlvacuumglassflasks(Vacutainer)then
themorphologyofthesoilcoverandonthewaterpathways keptatlowtemperature(around4◦C)forlaboratoryanalysis
thatcouldbeinferredfromsoilpatternsandtopography;the ofDOCandotherdissolvedspecies.
continuity and consistency of observed variations along the AnalysesofdissolvedH SiO ,Al3+andFe2+/Fe3+were
4 4
waterpathwayswilldeterminethevalidityofthedata. performedwithinonehouraftersamplingbycolorimetryus-
Three points were located in the podzolic Es horizons, ingaLangeDR2800Spectrophotometerwithanalyticalkits
where the DOM-rich water table perched over the Bh-Bhs LCK301,LCW028andLCK320,respectively.Themeasure-
horizonscirculates.The“W1-20”pointwassituatedat20cm mentrangeswere0.01–0.60mgl−1,0.005–1.000mgl−1and
depth in the podzolic campinarana area, and the “Spring” 0.05–6.0mgl−1 and the absorbencies measured at 620nm,
point was situated at the very beginning of a spring flow- 695nmand485nmforAl3+,H SiO andFe2+,respectively.
4 4
ingfromthecenterofthepodzoliccampinaranaarea.Both Thecolorimetricmethodwaschosenbecauseitallowedmea-
pointsallowedsamplingtheupperpartofthewatertablecir- surements immediately after sampling and because it mea-
culating in the sandy podzol horizons. The “W2-150” point sures the free or displaceable species able to participate to
wassituated150cmindepthwithinthewhitesandEshori- solution-minerals equilibria. To ensure that no oxidation of
zon,50cmovertheBhhorizon.The“S1-240”pointwassit- Fe2+ was occurring between sampling and analysis, Fe2+
uatedat240cmindepthwithinthesandy-clayBhshorizons, measurements were performed 10 minutes and 6 hours af-
whereOMtransferredfromthesoilsurfaceaccumulates.The ter water sampling. No significant differences between the
remainingpointswerechosenalongthelineofgreatestslope results were observed. As the colored DOM absorbency
inordertoobtainasequenceofsamplesdescribingchanges exponentially decreases from 220 to 650nm (Schwartz et
in the groundwater properties when it passes in depth from al., 2002), DOM contribution is low for Al3+ and H SiO
4 4
theBhhorizonstothekaolinhorizons,wherebleachingand measurements realized at 620 and 695nm, respectively, but
kaoliniteprecipitationoccur,thenflowslaterallydownslope canbesignificantforFe2+ measurementat485nm.Inorder
www.biogeosciences.net/9/3705/2012/ Biogeosciences,9,3705–3720,2012
3710 Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin
80 pH was then corrected by using the empirical relationship
70 [1pH = 0.508−0.0047×DOC] where DOC is expressed
Rainfall (mm) 2345600000 iuenxnaimtmsg;plcl−eo,1ra.resIcntthimorenoeswHta+csaashreoeswe1xecvpheHrannwgeaecsdesbdseuatrrwiynegfeongrib0mb.3soidateenldhiny0gd..r5oFplyoH-r
10
0 sis,ifpHincreases0.5units,thesaturationindexwithregard
June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March Apr. May
2007 2008 togibbsitewillincrease1.5.
In order to evaluate the behavior of the studied solutions
Fig.4.Dailyrainfallinthearea.Greyverticalbarsindicatethesam- with regard to H+, acid-base logarithmic scale microtitra-
plingperiods.
tions of selected samples were performed using the proce-
dure describe by Garnier et al. (2004a). Briefly, sample so-
lutions were acidified by 0.2moll−1 HNO additions until
3
to limit possible interferences due to the DOM color of the pH2.5,N purged,andthentitratedusing0.1moll−1 KOH
2
samples,absorbanceofthefilteredsamplebeforeanyreagent untilpH11.5.Acid-basetitrationswereperformedusingtwo
addition was subtracted from absorbance of the final mea- Titrino 716 apparatus controlled by a TiNet 2.4 software
surement. (Metrohm). The combined pH-micro-electrode used (Met-
Laboratory analyses were performed between 10 and 45 tler, Inlab422, reference: Ag/AgCl/KCl 3.0moll−1) was
days after sampling. DOC analyses were performed with calibrateddailybythetitrationofaHNO 0.01moll−1solu-
3
a Shimadzu TOC-V Analyzer calibrated with a 10ppm tionfollowedbytheoreticalfittingofthetitrationcurve.
potassium phthalate standard solution. Blank signal for fil- Small carboxylic acids were identified and quantified in
tered ultra-pure MQ water was about 0.1ppm. Na+, NH+, selected samples by ionic chromatography (Dionex ICS-
4
K+, Mg2+, Ca2+, F−, Cl−, NO−, NO−, PO2−, and SO2− 3000) using AG11-HC guard column, an IonPac ICE-
2 3 4 4
ions were determined by ion chromatography (Dionex DX AS11 column, for elution NaOH with a gradient from 1 to
120), using 9mmoll−1 NaHCO for cation elution and 5mmoll−1 and a CD25 pulse electrochemical detector. On
3
10mmoll−1methanesulfonicacidforanionelution. thesamesamples,theSUVA indexwasmeasuredusinga
254
Someauthors(Menziesetal.,1992)observedthatcolori- ShimadzuUV-1800spectrophotometer.Thisindexiscalcu-
metric methods discriminate against micro-particulates and latedastheratioofUVabsorbanceat254nmtotheproduct
donotmeasureallAlpresent,assolubleorganicforms,and ofDOCandUVcellpathlength;itisconsideredascharac-
thusunderestimatetotalsolubleAl.Inordertocheckifsome teristicofthehydrophobicityandaromaticityoftheconsid-
metal species were so strongly bonded to DOM that they eredDOC(Chinetal.,1994).
were not displaced during the on-site colorimetric analysis, Thermodynamic modeling of the solution-minerals inter-
Al3+, Fe2+ and Fe3+ were measured for a selected set of actionswasperformedbyowncalculationandcheckedusing
samples,beforeandafterUV-oxidationoftheDOM(Patel- thePHREEQC(2.11)softwarewiththeWATEQ4Fdatabase
Sorrentino et al., 2004). Oxidation was performed in cylin- (BallandNordstrom,1991).
drical quartz vessels using a 450watt Hanovia Hg lamp;
complete DOM oxidation was checked by DOC determina-
tionafterirradiation.Resultsgavenosignificantdifferences 3 Results
in concentrations before and after UV-oxidation, so that we
mayconcludethatthecolorimetricmethodgavethesumof 3.1 Waterpathwaysalongthecatena
freeplusOM-bondedspeciesforAlandFe.
During data modeling, the pH values seemed underesti- During the first sampling period, no samples were avail-
matedincomparisontofieldobservations,sothatspecialat- able in the S2-160 and S3-160 points because of the lack
tentionwaspaidtopHmeasurements.pHelectrodecalibra- of perched water table in the Ef horizon due to insufficient
tionatfieldwasrealizedwithastandardelectrodeandstan- rainfall. During the second sampling period, the point S2-
dard reference solutions having a ionic strength high when 160gaveasampleonlyonesamplingday(31January2008)
compared to the studied solutions, which can give a mea- when the S3-160 point gave a sample three sampling days
surement error. To test this hypothesis, pH measurements (29 January 2008, 31 January 2008 and 2 February 2008).
were achieved on seven selected samples, using the same During the third sampling period, both S2-160 and S3-160
electrode and adjusting the ionic strength (I) to 10−4, 10−3 pointsgavesampleseverysamplingday.Thisbehaviorofthe
and 10−2 M with KCl. The pH increased with increasing samplingpointsatteststhefastlevelchangesoftheperched
ionicstrength,anasymptotebeingreachedwhenI=10−2M water table after heavy rains. When sufficiently high, a wa-
(Fig. S1). The difference 1pH between pH measured be- tertableperchedoverthepodzolicBh-Bhshorizonlaterally
foreadjustmentandpHmeasuredatI=10−2Mwasdepen- overflowstowardstheEfhorizon,thenpercolatesdownslope
dent on the DOC content, which indicates that the DOM is along this horizon or overflows out in the U-shaped chan-
actually charged and participates to the ionic strength. The nels.Suchoverflowallowsadirectexportofthegroundwater
Biogeosciences,9,3705–3720,2012 www.biogeosciences.net/9/3705/2012/
Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin 3711
downtothestreamsandrivers,besideotherpathwayssuchas lopefrompointS1-510(average4.5)topointS6-300(aver-
the usual overflowing of the groundwater in the lower parts age5.0).ThepHslightlychangedfromonesamplingperiod
of the landscape or pipes as observed elsewhere (Lucas et totheother,but,consideringeachsamplingperiodindividu-
al., 1996). The water sampled at the SCB-15 point during ally,thepHincreaseinthedownslopedirectionwasalways
thefirstsamplingperiodcamenecessarilyfromrecentrain- progressiveandsignificant(Fig.S3):t-testvaluesofthedif-
fall percolating the topsoil horizons, because of the lack of ferencebetweenpointsseparatedbyatleast2otherpointsin
perchedwatertable,whenduringthesecondandthirdsam- thesequencewereallbuttwohigherthan2.pHvaluesover
plingthewaterscouldcomefrombothrecentrainfallsperco- 5.2 were only observed for the three points situated at the
latingtopsoilhorizonsandoverflowingoftheperchedwater lowerpartoftheslope(S4-400,S5-380,S6-300).
table.ExceptpointS3-380anddownslopepointS6-80,other DOCconcentrations–thewatersfromtheupperhorizons
sampling points of the catena gave samples every sampling ofthepodzolicarea(W1-20)hadhighDOCconcentrations;
day. the averages were 46, 36 and 27mgl−1 for the first, sec-
Two water pathways can therefore be defined along the ondandthirdperiods,respectively.Thedecreasefromape-
catena (Fig. 2). A deep, permanent water table flows more riod to the other was likely a dilution effect due to higher
thantwometersdeepwithinthekaolinhorizons.Thelateral rainfall. The water sampled at 150cm in depth in the white
flow of its upper part corresponds to pathway No. 1 and is sand (W2-150) had a more buffered composition through-
describedbythefollowingsuccessionofsamples:S1-510– out the three periods (37, 37 and 43mgl−1, respectively).
S2-490–S3-380–S4-400–S5-380–S6-300.Atemporary, In the Bhs horizon (S1-240), the DOC concentration var-
closer to the surface water table circulates in the Es and Ef iedfrom12to19mgl−1,15mgl−1 onaverage.Inthedeep
horizons only after heavy rainfalls and can overflow in the clayed horizons (points S1-510 to S6-300), the DOC con-
U-shaped channels. It corresponds to pathway No. 2 and is centrationswerelowerbutnon-negligible,varyingfrom1.5
describedbythefollowingsuccessionofsamples:S1-240– to9.7mgl−1,4.0mgl−1onaverage,withoutsignificantvari-
S2-160–S3-160. ationsalongthecatena.
SiandAlconcentrations–thewatersfromtheupperhori-
3.2 Compositionofthepercolatingwaters zons of the podzolic area (W1-20) always had very low Si
andAlconcentrations;therespectivemaximumvalueswere
The composition of the groundwater from each sampling 0.1 and 0.14mgl−1 and the respective average values were
pointandforeachsamplingdayfordissolvedSi,Al3+,Fe2+, 0.05and0.03mgl−1.Ontheotherhand,thewatersfromthe
Fe3+,pH,EH andDOCisgiveninSupplementaryTableS1 Bhshorizon(pointS1-240)hadthehighestSiandAlvalues
andsummaryfromthesedataisgiveninTable2.Somekey observedduringthethreesamplingperiods:Sirangedfrom
points,representingthedifferentkindsofcirculatingwaters 0.29to0.59mgl−1 andAlrangedfrom0.37to0.63mgl−1.
(Spring, W1-20, W2-150, S1-240, S1-510, S2-490, S4-400 On every sampling day, the Si and Al values were close to-
andS6-300),werechosenforanalyzingothersolutes(Na+, gether(Fig.S2)andtheaverageoverthethreesamplingpe-
NH+, K+, Mg2+, Ca2+, Cl−, NO−, NO−, SO2−, S2− and riods was 0.48mgl−1 for both. The waters from point W2-
4 2 3 4
F−). Results are given in Table 3, except for S2− and F− 150 had values intermediate between those of points W1-
whichremainednegligible. 20 and S1-240. From the Bhs horizon down to the kaolin
The chemistry of the collected waters is hereafter de- horizon,i.e.frompointS1-240topointS1-510,theSicon-
scribedanddiscussedfollowingthetwowaterpathwaysde- centrations underwent a decrease by more than half, down
finedabove.Overallaveragecompositionsofcirculatingwa- to 0.13mgl−1 on average, when the Al values suffered a
tersaregiveninFig.5forbothpathways.Additionalinfor- slighter decrease, down to 0.37mgl−1 on average. Going
mationcanbefoundinFigs.S2andS3thatgiveaverageby downslopefrompointS1-510,theSivaluesprogressivelyin-
samplingperiodsforpathwayNo.1andNo.2,respectively. creasedagainuptovaluesaround0.5mgl−1.TheAlvalues
Formostsamplingpoints,thechemistryofthecollectedwa- continueddecreasingprogressively.
tersshowedlowvariationswithinasamplingperiodaswell Feconcentrationsand EH –forallpointsexceptS3-380,
asbetweensamplingperiods,asshownbythestandarddevi- Fe2+ concentrations were higher than Fe3+ concentrations.
ationsinFigs.5,S2andS3. Fe3+ concentration had, however, similar behavior to that
of Fe2+ along the whole catena except the last downslope
3.2.1 Fromthepodzolicsandsdownslopefollowingthe point. From points W1-20 to S5-380, Fe3+ and Fe2+ con-
deepwatertable(pathwayNo.1) centrations were frequently below the quantification limit
andnoindividualsamplingsexceeded0.20and0.46mgl−1
pH –thepHwasacidicforallsamples,rangingfrom3.4to for Fe3+ and Fe2+, respectively. The points situated ups-
5.5. The minimum values were observed for the waters cir- lopeinthewhitesand(W1-20andW2-150)andintheBhs
culatinginthewhitesand(pointsW1-20andW2-150)andin horizon (S1-240) had values more variable and higher than
theunderlyingBhs(pointS1-240)(average4.1).Regarding the points situated midslope in the deep kaolin (S2-490 to
thedeepwatertable,thepHprogressivelyincreaseddowns- S4-400). Those Fe3+ and Fe2+ values corresponded to E
H
www.biogeosciences.net/9/3705/2012/ Biogeosciences,9,3705–3720,2012
3712 Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin
Table2.Summaryofthecompositionofthegroundwaterforeachsamplingpoint.
Si(mgl−1) Al3+(mgl−1) Fe3+(mgl−1) Fe2+(mgl−1)
av max min av max min av max min av max min
Spring 0.05 0.07 0.02 0.02 0.05 <0.01 0.01 0.05 0.00 <0.05 0.10 <0.05
W1-20 0.05 0.10 0.02 0.03 0.14 <0.01 0.03 0.07 0.00 0.06 0.13 <0.05
W2-150 0.27 0.48 0.19 0.18 0.24 0.14 0.04 0.15 0.00 0.10 0.38 <0.05
S1-240 0.48 0.59 0.29 0.48 0.63 0.37 0.06 0.15 0.00 0.10 0.19 <0.05
S1-510 0.13 0.20 0.10 0.37 0.41 0.33 0.02 0.03 0.00 0.07 0.20 <0.05
SCB-15 1.86 2.44 1.19 0.68 0.81 0.57 0.79 1.65 0.39 1.58 2.52 0.42
S2-160 0.46 0.57 0.31 0.20 0.27 0.17 0.04 0.08 0.02 0.21 0.63 <0.05
S2-490 0.21 0.41 0.11 0.14 0.18 0.07 0.03 0.04 0.02 <0.05 0.09 <0.05
S3-160 0.30 0.40 0.21 0.06 0.09 0.04 0.01 0.04 0.00 0.05 0.15 <0.05
S3-380 0.19 0.24 0.12 0.07 0.14 0.01 0.02 0.05 0.00 <0.05 0.06 <0.05
S4-400 0.49 0.71 0.25 0.03 0.07 <0.01 0.07 0.46 0.01 <0.05 0.07 <0.05
S5-380 0.55 0.69 0.38 <0.01 0.01 <0.01 0.03 0.07 0.01 0.08 0.15 <0.05
S6-80 0.29 0.56 0.15 0.63 0.76 0.50 0.06 0.11 0.01 0.15 0.48 <0.05
S6-300 0.48 0.57 0.24 0.09 0.11 0.08 0.06 0.13 0.03 0.74 1.11 0.52
pH EH(mV) DOC(mgl−1)
av max min av max min av max min
Spring 4.0 4.3 3.4 521 542 503 37 45 29
W1-20 4.1 4.5 3.6 473 495 451 34 55 24
W2-150 4.2 4.5 3.9 512 563 472 40 52 35
S1-240 4.1 4.4 3.5 501 567 410 15 19 12
S1-510 4.5 5.1 3.8 514 575 468 2 3 2
SCB-15 4.6 5.1 4.0 372 415 325 32 38 27
S2-160 4.6 5.1 3.8 399 484 296 7 9 6
S2-490 4.6 5.1 3.9 499 543 444 4 7 2
S3-160 4.9 5.2 4.7 482 519 448 5 10 4
S3-380 4.9 5.2 4.6 494 532 467 4 6 2
S4-400 4.8 5.3 4.1 494 520 482 5 9 2
S5-380 4.9 5.4 4.3 460 478 437 3 5 2
S6-80 4.5 5.1 4.1 456 510 365 16 19 13
S6-300 5.0 5.5 4.4 172 191 147 5.4 9.7 3.5
valuesvaryingfrom410to575mV,withoutsignificantcor- The point SCB-15 had pH values similar to those of points
relation between these variables. Downslope, the behaviors S2-160andS3-160(4.6onaverage),butDOCvaluesalways
of Fe3+ and Fe2+ concentrations diverged: when the Fe3+ higherthan27mgl−1(32mgl−1onaverage).
concentrations remained low, the Fe2+ concentrations in- Si and Al concentrations – the Si concentrations slightly
creased slightly on point S5-380 then drastically on point decreased when passing from point S1-240 (average
S6-300witha0.57mgl−1averagevalue,thisincreasebeing 0.48mgl−1)topointS2-160(average0.46mgl−1),thende-
relatedtoadecreaseoftheE below253mV. creased markedly on point S3-160 (average 0.30mgl−1).
H
The waters from the Spring point had pH and chemical Alconcentrationsalreadydecreasedmarkedlywhenpassing
characteristicsverysimilartothoseobservedfortheW1-20 frompointS1-240(average0.48mgl−1)topointS2-160(av-
point, except regarding the E which was higher (521±15 erage 0.30mgl−1), then continued decreasing on point S3-
H
versus473±17mV). 160(average0.06mgl−1).Thisresultedinaprogressivein-
creaseoftheSi/Almolarratiobasedonaveragevalues:1.0
3.2.2 Fromthepodzolicsandsdownslopefollowingthe atpointS1-240,2.3atpointS2-160and4.8atpointS3-160.
perchedwatertable(pathwayNo.2) The waters sampled from point SCB-15 were characterized
byhighSiandAlconcentrationswhencomparedtotheother
pHandDOCconcentrations–thetransitionfromthepoint samplingpoints,onaverage1.86and0.68mgl−1 forSiand
S1-240 to the points S2-160 then S3-160 came with a pH
Al, respectively, with a Si/Al molar ratio varying between
increase between 0.5 and 1pH unit (4.1, 4.6 and 4.9 on av-
1.8and4.1(2.7fortheaverageofratios).
erage, respectively) and a progressive decrease of the DOC
concentrations(15,5and4mgl−1onaverage,respectively).
Biogeosciences,9,3705–3720,2012 www.biogeosciences.net/9/3705/2012/
Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin 3713
Table3.Majorionscharge,DOCandpH(average±standarddeviation).
Sampling Number of Majorionscharge
point samples
Na+ NH+ K+ Mg2+ Ca2+ Ala Fe2+
4
(µeql−1) (µeql−1) (µeql−1) (µeql−1) (µeql−1) (µeql−1) (µeql−1)
Spring 4 24.7±2.5 9.7±4.5 8.9±2.0 10.3±3.0 103.0±26.4 2.2±2.6 1.0±0.5
W1-20 1 26.4 3.8 36.5 12.6 82.8 3.3 2.2
W2-150 1 34.2 64.5 35.1 15.8 88.6 19.5 3.6
S1-240 4 30.2±13.7 13.4±19.1 19.7±15.5 7.8±2.7 46.4±15.6 52.8±4.5 3.6±3.8
S1-510 4 14.8±2.1 4.8±6.1 5.0±1.3 4.7±1.4 34.7±8.5 39.6±3.4 2.2±1.1
S2-490 4 27.6±13.2 33.6±28.2 8.7±6.9 3.9±1.4 53.2±12.7 15.0±5.5 1.4±0.8
S4-400 4 36.9±10.4 31.4±19.4 43.2±6.6 9.9±1.4 73.9±12.4 3.3±2.3 1.4±0.9
S6-300 4 18.1±2.7 16.4±10.0 30.4±1.7 4.9±0.5 61.6±8.4 9.0±5.2 26.4±14.3
FeIIIa Cl− NO− NO− SO2− pHb DOC
2 3 4
(µeql−1) (µeql−1) (µeql−1) (µeql−1) (µeql−1) (mgl−1)
Spring 4 0.6±0.2 11.5±4.1 0.0±0.0 3.6±6.2 11.1±1.2 4.0±0.08 37.4±1.9
W1-20 1 1.5 8.2 4.1 8.6 8.6 4.1 34.4
W2-150 1 2.1 19.7 32.3 8.9 35.1 4.2 40.2
S1-240 4 3.9±2.6 16.6±9.9 0.0±0.0 74.6±10.8 24.2±9.4 4.0±0.09 15.2±0.7
S1-510 4 1.2±0.5 11.2±2.8 2.0±2.0 41.8±7.2 17.4±6.2 4.6±0.08 2.3±0.4
S2-490 4 1.5±0.6 13.3±9.7 0.0±0.0 11.2±5.1 22.8±7.1 4.6±0.13 4.0±1.6
S4-400 4 3.9±1.8 32.5±18.1 3.9±3.9 23.4±10.4 34.7±14.1 4.8±0.07 4.9±2.0
S6-300 4 3.3±0.9 12.5±1.1 0.0±0.0 6.5±4.6 15.0±4.3 5.1±0.17 5.4±0.9
aCalculatedforthesumofAlorFeIIIspecies,respectively.
bValuesaftercorrection;seetext.
Pathway n°1
550
1.2
0.7 500
3+-1Al, Si (mg.L) 000000......123456 1234500000 DOC (mg.L), pH*1-1 2+3+-1Fe, Fe(mg.L) 000001......024680 223344050505000000 E(mV)H
0
0.0 0,0 150
Spring W1- W2- S1- S1- S2- S3- S4- S5- S6- Spring W1- W2- S1- S1- S2- S3- S4- S5- S6-
20 150 240 510 490 380 400 380 300 20 150 240 510 490 380 400 380 300
1.86±0.46 Pathway n°2 1.58±0.72
550
3+-1Al, Si (mg.L) 000000......234567 23450000 DOC (mg.L), pH-1 2+3+-1Fe, Fe(mg.L) 000011......246802 233445505050000000(mV)EH
0.1 10 *10 0.0 200
0.0 0,0 150
S1- S2- S3- SCB- S1- S2- S3- SCB-
240 160 160 15 240 160 160 15
Al3+ Si DOC pH Fe2+ Fe3+ EH
Fig.5.AveragecompositionofthesampledwatersalongthepathwaysNo.1andNo.2.Verticalbarsgivethecalculatedstandarddeviation
foreachsetofdata.
www.biogeosciences.net/9/3705/2012/ Biogeosciences,9,3705–3720,2012
3714 Y.Lucasetal.: Amazonianpodzol-ferralsolsoilsystemwithwhitekaolin
Fe2+,Fe3+ concentrationsandEH –theFe2+ concentra- Table4.Resultofmodelingthemicrotitrationcurvesby4discrete
tionsincreasedwhenpassingfrompointS1-240(0.10mgl−1 acidicsites–pKaandsitedensityLT.
on average) to point S2-160 (0.21mgl−1 on average), then
decreased on point S3-160 (0.05mgl−1 on average). The Spring
Fe2+ concentrations on point S2-160 exhibited, however, a pKa 3.5±0.6 4.7±0.4 6.9±2.6 9.3±1.5
highvariability,withastandarddeviation(0.24)higherthan LT(meqgC−1) 17.2±0.4 9.5±0.4 8.1±2.3 5.4±5.6
theaveragevalue.ThehighFe2+concentrationswerealways W2-150
relatedtolowerE values,whichreflectontheaverageval- pKa 4.1±0.3 5.2±0.1 8.7±0.1 10.6±0.1
uesofE :501,39H9and482mVforpointsS1-240,S2-160 LT(meqgC−1) 10.9±0.3 7.1±0.5 2.6±0.3 6.6±3.5
H
and S3-160, respectively. There is no, however, correlation
between the bulk of Fe2+ and EH values (R2=0.23). The 20 11 20 11
Fe3+concentrationsremainedalwayslowerthan0.08mgl−1 15 15
for both S2-160 and S3-160 points. The waters sampled at %)10 9 %)10 9
pavoeinratgSeC1B.5-815anhdad0.h7i9gmhgFle−21+,raensdpeFceti3v+elcyo)nwchenentractoiomnpsa(roend pH error ( -055 7pH pH error (-505 7pH
to any other sampling point, and relatively low E values -10 5 -10 5
H -15 -15
(372mVonaverage). SPRING W2-150
-20 3 -20 3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Base added Base added
3.3 DOMcharacterization
Measurement point µQ water titration result of modelling pH error on modelling
Microtitrations were performed on samples collected at Fig.6.MicrotitrationcurvesofDOM-richgroundwatersandresult
Spring and W2-150 points. An example of microtitration ofmodelingwiththePROSECEsoftware.
curveisgiveninFig.6.Threerepetitionsweremadeforeach
sample.Theacid-basepropertiesoftheDOCwereevaluated
by fitting these results with the PROSECE software (Gar- 4 Discussionsandconclusion
nier et al., 2004a, b) which consider a discrete distribution
4.1 Evolutionofthepercolatingwatersalongthecatena
of acidic sites whose acidic constants pKa and site densi-
i
ties L are determined by fitting the experimental curves.
T,i 4.1.1 Watersinthepodzolicarea
Theoptimalnumberofacidicsitesisdefinedasthesimplest
combination that conduces to the minimal fitting bias value
Thespringwaterandthewatercirculatingintheupperpart
(sum of the absolute differences between experimental and
of the white sands had the highest DOC and the lower pH
calculatedpHvalues).Accordingly,adistributionof4acidic
and Si values compared to all other samples and very low
siteswassufficienttoallowacorrectfittingofexperimental Al,Fe2+andFe3+values,intherangeofvaluesobservedby
curves.TheresultsaregiveninTable4.Thetotalcalculated
Cornu et al. (1997, 1998) in a similar podzolic area – simi-
sitedensitieswere40.2±6.1and27.2±3.6meqgC−1forthe
larsoilandvegetation–situatednearManaus.DissolvedSi,
SpringandtheW2-150samples,respectively.
Al and Fe found here can come from canopy leaching, lit-
Smallcarboxylicacididentificationandquantificationfor
ter mineralization and dissolution of quartz and trace min-
samplescollectedatpointsSpring,W2-150,S1-510,S3-380
erals. This water is aggressive with regard to clay or iron
and S4-400 aregiven inTable 5. Withregard tothe totalof
minerals, due to its acidity and complexing capacity of the
theidentifiedsmallcarboxylicacids,aceticacidrepresented
DOC which greatly enhances the weathering rates (Robert
between38and61%,formic,succinic,oxalicandcitricacids
andBerthelin,1986).Whenitreachesthetransitionbetween
between4and27%,andlacticacidremainedlowerthan4%.
white sands and kaolin horizon, in the Bh-Bhs horizons, it
Thesumofsmallcarboxylicacidswashigherthan2mgCl−1
dissolveskaoliniteandFe-orAl-oxidessothattheconcen-
in the DOC-rich samples and lower than 1mglCl−1 in the
trations of Si, Al and Fe in the percolating water increase.
DOC-poorsamples.Theproportionofsmallcarboxylicacids
The dissolved Si/Al molar ratio in the Bhs horizons was
intheDOC,however,waslowerthan10%intheDOC-rich
verycloseto1,whichisinaccordancewithacongruentdis-
samples (Spring and W2-150) and higher than 47% for the
solution of kaolinite. The up and down movements of the
other samples. The SUVA index (Table 5) values are in
254 watertableperchedovertheBh-Bhsfavortheupwarddisper-
accordance with these results. The two DOC-rich samples
sionofsolutes,whichexplainsthatconcentrationsatmedium
exhibitedaSUVA indexmorethantwotimeshigherthan
254 depth in the white sands are intermediate between concen-
thoseoftheDOC-poorsamples,whichindicatethatthefor-
trations at the topsoil and within the Bhs. The decrease of
mercontainmanymorehumicsubstancesthanthelatter.
DOC concentration when water passes through the Bh-Bhs
is due to the DOC adsorption on kaolinite or Fe-oxides and
gibbsitesurfaces(Davis,1982;KaiserandZech,2000).Such
adsorptionwasobservedinAmazonianpodzolseverywhere
Biogeosciences,9,3705–3720,2012 www.biogeosciences.net/9/3705/2012/