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Dissolved organic matter dynamic in the Amazon basin: Sorption by mineral surfaces

Marcela A.P. Pérez a,⁎, Patricia Moreira-Turcq a,b, Hervé Gallard c, Thierry Allard e, Marc F. Benedetti d,⁎a Departamento de Geoquímica, Instituto de Química, Universidade Federal Fluminense, Outeiro de São João Batista, s/n, 24020-007 Niterói, RJ, Brazilb IRD-Institut de Recherche pour le Développement, GET, HYBAM, 93143 Bondy, Francec Laboratoire Chimie et Microbiologie de l'Eau (LCME), UMR CNRS 6008, Univ. de Poitiers, 86022 Poitiers, Franced Univ. Paris Diderot, Sorbonne Paris Cité, IPGP, UMR 7154 CNRS, 75205 Paris Cedex 13, Francee Institut de Minéralogie et de Physique des Milieux Condensés, Univ. Pierre et Marie Curie, Univ. Paris Diderot, IPGP, UMR CNRS 7590, 75015 Paris, France

a b s t r a c ta r t i c l e i n f o

Article history:Received 14 July 2010Received in revised form 1 May 2011Accepted 2 May 2011Available online 18 May 2011

Editor: J. Fein

Keyword:RiversOrganic matterReactivityAdsorptionDOCSediment

In aquatic systems, soprtion of organic matter (OM) on environmental surfaces or its preference to remaindissolved is highly important for determining its potential transport and/or susceptibility to degradation. In theAmazon and other major rivers of the world, transported OM is either adsorbed to fine minerals or remainsdissolved. The fate of autochthonous OM in Amazon floodplains and allochthonous OM from the river can beaddressed by characterizing the nature of OM interactions with the sediment material. This goal was pursued inthis study using Rio Negro and Amazon River samples as well as those collected from floodplain lakes duringphytoplanktonic blooms, small black rivers, soil solution issued from podzolic areas in the Rio Negro watershedand “terra firme” stream flowing into the floodplain. The amount of carbon surface loading expressed as theadsorbedorganic carbonperunit of surface areaof the substratewasobtainedbybatchexperimentswithmineralphases representative of soils, suspended matter found in the Amazon basin and extracted OM fractions (thehydrophobic, HPO and the transphilic, TPH fractions of OM). The bulk dissolved organic matter samples werecharacterized by carbon and nitrogen isotopic measurements as well as elementary composition, specific UVabsorbance (SUVA),molecularweight (Mw) and FTIR spectroscopy. To our knowledge, these are the first C andNisotopic composition data reported for extracted OM fractions in the Amazon basin. The Rio Negro basin watersamples had high [DOC] correlated with conductivity. The SUVA values ranged from 4.0 to 7.3 m−1l mg−1C andwerewithin the rangeofmeasurements reported for theAmazonRiver. The δ13C valuesof all HPO fractions variedfrom −27.7 to −30.2‰. The δ13C values for the TPH fractions were systematically 1‰ higher than theirrespective HPO fraction. An increasing trend between the weight average Mw of the DOC as function of the C/Nratio (the higher C/N the higherMw) is reported. This trend is inferred to result frombothmixing between 2 end-members andOMfractionation. Carbon surface loadings calculated forHPO and TPH fractions rangedbetween 23to 309 μgC m−2 and 38 to 145 μgC m-2, respectively. Carbon surface loadings with goethite and the Rio Negroorganicmatter fractions cluster around the average value of 24.5±4 μgC m-2.With the help of the pHdependantdissociation measurements and NICA-Donnan model parameters, it was possible to estimate the amount ofdeprotonated sites on the OM interacting with the sedimentmaterial. The amount of carbon surface loadingwascorrelated with the calculated residual negative charge of the OM fractions (i.e. HPO or TPH). These analysesshowed that higher residual negative charges were associatedwith lower carbon surface loadings on the treatedsediment. The decrease of the negative charge was suggested to reduce the net electrostatic repulsion betweenthe OM fractions and the clay surfaces and promote ligand exchange reactions governing sorption to clays. Thepreferential uptake of the high molecular weight fraction demonstrates that the chemical nature of the organicmatter remaining in solution differs from that of the adsorbed fraction.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Riverine transport of organic matter (OM) from land to searepresents a major link in the global cycles of bioactive elements,

which modulates the biosphere over geological time. These terrestrialOM losses support significant heterotrophic activity within rivers,estuaries, and marine systems (Kaplan and Newbold, 1993; Mayeret al., 1998). Three dominant processes of OM cycling are advectivetransport, degradation, and adsorption. Of these, the important role ofadsorption was only recently appreciated. The intimate association ofOM with mineral surface significantly decreases its bioavailability(Keil et al., 1994; Nelson et al., 1994). In contrast, Henrichs (1995)demonstrated that easily reversible sorption would not preservelabile OM.

Chemical Geology 286 (2011) 158–168

⁎ Corresponding authors at: Departamento de Geociências, Universidade Federal doAmazonas, CEP: 69077-000 Manaus, AM, Brazil.

E-mail addresses: marcelap.perez@yahoo.fr (M.A.P. Pérez), benedetti@ipgp.fr(M.F. Benedetti).

0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2011.05.004

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Dissolved organic matter (DOM, i.e. OMb0.7 μm) is chemicallyheterogeneous and composed of classes of organic substances that canbe differentiated based on acid, base, or neutral properties, hydro-phobicity, molecular weight or some combinations of these charac-teristics (Brown et al., 2004). Variation in reactivity among humicsubstances, a major fraction of the DOM derived from differentsources of organic material, has been previously reported (Benedettiet al., 1995; McKnight et al., 2002). These variations in reactivity ofDOM molecules are related to variations in molecular size and thepresence of specific functional groups (Chin et al., 1994; Cabanisset al., 2000; McKnight et al., 2002). Evidence for preferentialadsorption of higher molecular weight (Mw), more aromatic andcarboxyl rich, and more hydrophobic fractions of the DOM pointtoward such fractionation (Davis and Gloor, 1981; Jardine et al., 1989;McKnight et al., 1992; Day et al., 1994; Chorover and Amistadi, 2001).More recent data confirmed that adsorptive fractionation of DOMoccurred as a result of competition for mineral binding sites (Gu et al.,1995; Kaiser and Zech, 1997). Most mineral surfaces in the biospheremaintain strong physicochemical associations with organic mole-cules, gels, and micro aggregates (Oades, 1989; Mayer, 1994;Christensen, 1996) at relatively consistent organic carbon (OC) tosurface area (SA) ratios of 0.5–1.1 mg OC m−2 SA (Mayer, 1994;Hedges and Keil, 1995; Keil et al., 1997). For mineral surfaces — OMinteractions six binding processes are forecast: ligand exchange,cation bridges, anion exchange, cation exchange, van der Waalsinteractions and hydrophobic bonding (Arnarson and Keil, 2000;Tombácz et al., 2004; Feng et al., 2005). The influence of solutionchemistry (e.g. ionic strength, pH and solution cation) on OMadsorption to clay mineral surfaces was extensively studied and OMadsorption determined to increase with increasing ionic strength, anddecreasing pH (Baham and Sposito, 1994; Arnarson and Keil, 2000;Satterberg et al., 2003). For soils, Kaiser and Guggenberger (2003)concluded that mineralogy was one of the primary controls of therelation between surface area and sorption of OM within a soilhorizon.

In aquatic systems, whether an organic molecule adsorbs orremains dissolved determines in large part its potential transport andsusceptibility to degradation (Aufdenkampe et al., 2001). In theAmazon basin, 85% of transported organic matter is either colloidal orremains dissolved (Benedetti et al., 2003a, 2003b; Moreira-Turcq etal., 2003). The floodplains of large rivers are built by the formation ofbars and the accumulation of sediment carried in diffuse overbankflows and channelized flows (Dunne et al., 1998). Floodplains affecterosion, transport and sedimentation flux budgets in the watershedsystem and are of special importance for the carbon cycle due to theirhigh productivity. Amazon floodplains or várzeas constitute a largesource of atmospheric carbon, as methane (Bartlett and Harris, 1993),and as CO2 (Richey et al., 2002). Floodplains are also an importantsource of OM for the Amazon main stem (Junk, 1997; Moreira-Turcqet al., 2005). Quay et al. (1992) demonstrated that about 40% of theOM supportingmicrobial respiration in the Amazon River was derivedfrom várzea grasses, although these grasses supply only 10% of theparticulate organic carbon (POC) in the Amazon. The fate ofautochthonous OM in Amazon várzeas and allochthonous OM fromthe river must be addressed by characterizing the nature of OMinteractions with the sediment material either suspended or depos-ited at the bottom of the várzea lakes during burial. In addition, thedevelopment of podzols in lateritic landscapes of the upper Amazonbasin contributes to the major pathway of OM export by the blackwaters of the Rio Negro watershed. The mechanisms that control thefate of this large pool and its transfer to the rivers must be addressedsince the contributions of processes that control OM concentrationsand reactivity along the Amazon basin are still issues of debate.Hedges et al. (2000) presented a regional “chromatographic”model toaccount for the evolution of OM from soils to the Amazon's main stemand concluded that selective sorption of OM onto minerals was the

key process that affects the properties of different fractions of organiccarbon of the rivers. Prior studies concerned with the evolution of OMalso concluded that further exploration of its composition andreactivity towards mineral phases are necessary to understand OMpartitioning processes in soils and riparian zones (Amon and Benner,1996; Hedges et al., 2000).

This work focuses on the dynamics of OM in one of the largesttropical watershed in the world (i.e. the Amazon River and its blackriver tributary the Rio Negro). Our objectives were to identify themain geochemical processes responsible for changes in the dissolvedorganic matter characteristics with emphasis on those controlling thedistribution of the organic matter (i.e. dilution, degradation or/andsorption) along the main stem. To address these issues, we studiedthe sorption behavior of different extracted DOM fractions collected inthe most characteristic parts of the Amazon watershed system. Thefractionation approach should help to fill the knowledge gaps asresults of similar experiments made with bulk samples are often moredifficult to interpret in terms of processes. The samples were collectedfrom the main stema of the Rio Negro and Amazon River as well asfrom floodplain lakes during phytoplanktonic blooms and small blackrivers. This set was enhanced by samples of soil solution issued frompodzolic areas in the Rio Negro watershed and “terra firme” streamsflowing into the floodplain. The amount of carbon surface loadingexpressed as the OC/SA ratio was obtained by batch experiments withextracted fractions of the DOM and mineral phases representative ofsoils or suspended matter found in the Amazon basin. In addition, thepH dependent dissociation of the OM fractions was obtained by pHtitration data. The residual negative charge (i.e. amount of un-protonated sites) was calculated with the NICA-Donnan model at pHvalues measured during the adsorption experiments. These fractionsand the bulk dissolved organic matter samples were characterized byC and N isotopic measurements. Their elementary composition,specific UV absorbance (SUVA), molecular weight (Mw) and FTIRspectroscopy were also determined.

2. Field sites and sampling

The Amazon basin is divided into several units with distinctmorphostructural characteristics. The variable climatic conditions isthese unites also support the development of organic hydromorphicand podzolic soils. The later soilds to be located in the northern part ofthe Amazon basin. The Rio Negro sub-basin (700000 km2) occupies12% of the Amazon basin (Fig. 1). It extends from 73.25° to 59.35°longitude West and from 5.4° North to 3.35° latitude South (Frappartet al., 2007). The flood period in the Rio Negro generally lasts fromMay to August, whereas lowwater period takes place from Septemberto February (Frappart et al., 2007). The Rio Negro is one of the majortributaries of the Amazon River, as it accounts for 30% of the totalAmazon River discharge. It is recognized that OM of the Rio Negrooriginates from podzols (Sioli, 1984) and the black waters of the RioNegro carry a high load of labile organic molecules issued from soils(mainly podzols and laterites) of the central Amazonbasin (Nascimentoet al., 2004 and references therein). The concentrations of major ions inthe acidic Rio Negro waters are low (Benedetti et al., 2003a, 2003b).Themajor tributaries of the Rio Negro aswell as the numerous brooksthat drain the uplands of the plateaus may exhibit distinct colorscategories defined by Sioli (1984) (i.e. clear, black or white). The RioJaú is a right riverbank tributary of the Rio Negro with a watershedarea (i.e. 10000 km2) entirely covered by “terra firme forest”.Downstream the Rio Negro, the podzols are scarce and only represent5% of the surface area of the Rio Jaú sub-catchment. They occupysmall depressions in the central part of elongated plateaus and areweakly incised by a diffuse network of brooks (Nascimento et al.,2004). Chemical erosion predominantly acts at the margin of thesedepressions and is assigned to the lateral development of podzols

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(associated to black waters) into clay-depleted laterites or Acrisols(associated to clear waters).

The “Lago Grande de Curuai” floodplain (Fig. 1) is located nearÓbidos at 850 km upstream of the Atlantic Ocean (Moreira-Turcqet al., 2004) on the right riverbank of the Amazon River's main stem(01°50′S–02°15′S and 55°00′W–56°05′W). The Curuai floodplain isthe largest in the lower part of the basin and its area corresponds to13% of the total floodplains area in the Amazon basin betweenManacapuru and Óbidos (Maurice-Bourgoin et al., 2007).Water levelsin the floodplain measured at the Curuai gaging station were withintens of centimeters of the Amazon River's water levels at the Óbidosstation, and the annual water-level fluctuations are approximatelysynchronous with the river's fluctuations. Maximum and minimumlevels occur duringMay–June and November–December, respectively.Generally, water flows from the Amazon River into the floodplain inJanuary, flows in or out from February to late April, and flows out tothe Amazon River from late April to late October. The mean annualresidence time of water on the floodplain is approximately 3 months(Bonnet et al., 2008). The floodplain system comprises several whitewater lakes and black water lakes, some of them linked to the AmazonRiver system. A “terra firme” area defines the south border of its basin.The northern border of the floodplain is the river's bank that ischaracterized by young forests. The soils are mainly composed ofrecent alluvial deposits and laterites issued from the Alter do Chãoformation (Lucas, 1989). Extensive banks of semi-aquatic herbaceous(i.e. Paspalum repens, Paspalum fasciculatum and Echinochloapolystachya) colonize the major channels (Engle et al., 2008).Extensive phytoplankton blooms mainly in the falling periods arefound into some lakes.

Samples (O1, O3, O4, O20) were collected at the Rio Negro and Jaúrivers (Fig. 1) during a lowwater level cruise (October 2004). Sampleswere also taken in the forest, consisting in one stream (O15), oneigapó (a small black water stream in the flooded forest, recharged bythe river (O6), and three podzols perched aquifers (O16, O18, O19).

At the Curuai floodplain (or várzea), samples were collectedduring a rising water episode in March 2006. In order to understandthe fate of the different types of DOM, water samples were taken atseven stations in different parts of the várzea lakes. They consisted ofthree white water samples (W1, W2 and W3) and one black watersample (B1). Two samples were collected during a phytoplanktonicbloom (PB1 and PB2). One sample was also collected in the AmazonRiver near Óbidos (AR) (Fig. 1).

3. Methodology

3.1. Field operations

The pH and conductivity were measured in situ with a multi-parameter probe (®YSI 600XLM). Surface water samples from theCuruai floodplain were processed onboard using tangential-flowultrafiltration (TFU, Millipore Pelicon device and Durapore mem-branes with a nominal cutoffs of 0.2 μm) to remove the particulatematerial before the concentration of the DOM by reverse osmosis(RO). Detailed information on TFU procedures can be found elsewhere(Benedetti et al., 2003a, 2003b). TFU membranes were cleaned with10 to 50 liters of clean water produced in the field by a RO devicebetween each new sample. For each sample, a known volume of waterwas filtered with the clean ultra filtration membranes, and then the

Fig. 1. Location of the “Lago Grande de Curuai” Floodplain on the right bank of the Amazon River's mainstream (01°50′S - 02°15′S e 55°00′W - 56°05′W) and the Negro riverWatershed, it extends from 73.25° to 59.35° West and from 5.4° North to 3.35° South. Sampling points are shown as defined in the text.

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TFU filtrates were concentrated into 1 to 2.6 L by RO. The black riverwater samples were treated directly by RO. The RO procedure was thesame for all samples, 100 to 200 L of water were processed, the first5 L were discarded to prevent sample cross contamination during theRO treatment. Each sample was concentrated into 1 to 2.6 L. All watersamples were also treated by classical filtration with glass fiber filters(GF/Fb0.7 μm) to measure dissolved organic carbon concentration([DOC] in mg L−1) as well as bulk DOM properties such as SUVA,molecular weight and C/N ratios. All samples, including RO concen-trates were stored in pre-combusted glassware in the dark andacidified in the field with H3PO4 (up to pH 2). The aliquots taken forMw determinations were not acidified.

3.2. Dissolved organic matter fractionation and chemical analysis

RO concentrated fractions were extracted, in the laboratoryfollowing the procedure described by Leenheer (1981) and Thurmanand Malcolm (1981), on XAD-8 and XAD-4 resins, the elutedsubstances corresponding to the hydrophobic (HPO) and thetransphilic (TPH) fraction, respectively. After the extraction, therecovered organic material was freezed-dried and stored as such inglass bottles.

Organic carbon and total nitrogen concentrations were measuredwith a Shimadzu TOC-VCSH analyzer in the NPOC mode and with aTNM-1 coupled unit, respectively. The detection limits for the organiccarbon and the total nitrogen were 100 μg l−1 and 50 μg l−1,respectively.

Specific UV absorbance (SUVA) is a measure of the aromaticity ofthe dissolved organic matter and is defined as the absorbance of awater sample at 254 nm divided by the DOC concentration ([DOC]).The absorbance was measured with a UV–Vis Hitachi U2000-Spectrophotometer.

High-pressure size exclusion chromatography (HPSEC) DOMmolecular weight determination (i.e. filteredb0.7 μm)was performedwith a chromatographic system coupled with an UV absorbancedetector at the wavelength of 260 nm, using a REPROSIL 200 SECcolumn. The carrier solution was a 10 mM sodium acetate solution(pH=7.0) and the flow rate was 1 ml min−1. Polystyrene sulfonate(PSS) standards of molar mass 1400, 4300, 6800, 13000 and 17000Daltons were used as standards. Number — (Mn) and weight-averaged (Mw) molecular weights for the samples were determinedusing equations described by Yau et al. (1979). Water samples DOCconcentration for HPSEC determination ranged from 4 to 7 mg L−1

and were diluted twice if necessary.Additional characterization of the different organic moieties of

HPO and TPH fractions was performed by FTIR (Fourier TransformInfrared) spectroscopy using an aliquot of freeze-dried samplesmixedwith KBr.

3.3. Potentiometric acid–base titration

The pH dependent dissociation of HPO and TPH fractions wasdetermined using a fully automated titration system as described inBenedetti et al. (2002). The titrations were performed on re-suspended freeze-dried OM diluted in a 0.01 N NaNO3 solution toreach a final concentration of 1.2 gC L−1. The data analysis of thetitration was performed as previously described by Benedetti et al.(1996).

3.4. Isotopic measurements

Total organic carbon (TOC), total nitrogen (TN, i.e., organic andinorganic nitrogen) of HPO and TPH fractions, δ13C and δ15N isotoperatios were measured on the same sample aliquots by EA-IRMS(Carlo-Erba NA-1500 NC Elemental Analyser on line with a FisonsOptima Isotope Ratio Mass Spectrometer). The δ13C and δ15N values

are reported in per mil (‰) relative to Pee Dee Belemnite (PDB)standard and relative to air N2, respectively. TOC and TN concen-trations are reported in mg g−1 of dry sample. Analytical preci-sions (±1σ) were ±0.1‰ and±0.2‰ for δ13C and δ15N, respectively.The average precisions for concentration measurements were ±0.1 mgC g−1 for TOC and ±0.05 mgN g−1 for TN. Data reproducibilitywas checked by triplicate analysis of selected samples and of atyrosine laboratory standard (δ13C=23.2±0.1‰ and δ15N=10.05±0.30‰).

3.5. Adsorption protocols

In order to decipher between the complementary processesresponsible of the DOM fate in the Amazon basin, adsorptionexperiments withmineral phases representative of soils or suspendedmatter found in the basin were made. The batch method was usedwith the purified organic matter fractions (i.e. HPO, TPH) taken in theRio Negro and várzea lakes systems. The organic matter concen-trations in the two experiments were chosen to ensure detectableorganic carbon concentrations in the supernatant solutions once theadsorption was completed.

The adsorption experiments with the Rio Negro organic matterwere conducted with a synthetic goethite (specific BET area of94 m2g−1) that is representative of a major reactive mineral found insoils in the Rio Negro basin as demonstrated by Fritsch et al. (2009).The experiments were carried out following an adaptation of Korshinet al. (1996). Aliquots of the HPO and the TPH fractions were dilutedto obtain solutions with a final concentration of 20±1 mgC L−1. Tenml of the later were put in contact with 60 mg of goethite for eachadsorption test. The final average pH values ranged from 3.9 to 4.4.Blanks experiments were conducted in a similar way. The solutionswere stirred for half an hour in the absence of light at roomtemperature (23±2 °C). Afterwards the solutions were first centri-fuged at 7 000 rpm for 30 min and then filtered with GF/F.

The adsorption of the Amazon–Curuai floodplain HPO and TPHfractions was conducted with the organic-free b63 μm fraction of thesediment from the Curuai Várzea Lake. This material is representativeof the suspended sediments transported and deposited by theAmazon River (Amorim et al., 2009). The organic carbonwas removedfrom this surface sediment by treatment with NaOCl 10% until organiccarbon concentrations in the extraction solutions were below thedetection limit of Shimadzu TOC-VCSH analyzer. The sedimentmineralogical composition was: illite (20%), smectite-montmorillon-ite (47%) and kaolinite (32%) (Amorim, pers. comm.). The BET surfacearea of the sediment after treatment was 11±0.5 m2 g−1. Aliquots ofHPO and TPH fractions were diluted to obtain solutions with a finalconcentration of 4±0.2 mgC L−1. An aliquot of 50 ml of the later wasput in contact with 10 mg of the treated sediment for each adsorptiontest. The final pH of the experiments ranged from 5 to 6. Preliminarytests showed that after 30 min the adsorption process was notcompleted (data not shown here) therefore a longer equilibrationtime was used for the experiments made with the treated sediment.The solutions were stirred for 24 h, in the absence of light at roomtemperature (23±2 °C), then centrifuged at 7 000 rpm for 30 minand filtered over GF/F. Blanks were also conducted to correct sampleresults.

Total organic carbon and total nitrogen concentrations as well asSUVA values were measured for all filtered solutions collected aftercompleted sorption tests.

4. Results

4.1. Bulk water sample chemistry

The Rio Negro basin water samples had [DOC] strongly correlatedwith conductivity values (r2=0.96) (Table 2 and Fig. 2A). Samples

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conductivity ranged from 8 to 114 μS cm−1. The highest valuescorresponding to the highest [DOC] measured in the podzols perchedaquifers samples. The same trend was not observed for the Curuaifloodplain water samples (r2=0.62). The conductivity remainedconstant (46±19 μS cm−1). However, the conductivity measured inthe black streamwas low (B1: 7 μS cm−1) and close to the value foundfor the Rio Negro water sample (O1). The evolution of the [DOC] isreported as function of the pH, in Fig. 2B. Two end-members areobserved (1) water samples with low pH (3.35±0.3) and high [DOC]and (2)water sampleswith circum neutral pH and lower [DOC]. It wasnoteworthy that in Fig. 2B, like for conductivity data (Fig. 2A) sampleB1 of the Curuai floodplain is close to the Rio Negro water samplevalue. The other samples of the Curuai floodplain had constant [DOC]and pH values characteristic of the Amazon River.

The SUVA values (Table 2) ranged from 4 to 7.3 m−1L mg−1C forsample B1, the Rio Negro water samples, the Amazon River (AR) andsome várzea lake waters (W1-W3). These values were close to theSUVA measurement reported for the Amazon River colloids (4.2) byBenedetti et al. (2002). The high SUVA waters are generally enrichedin hydrophobic OM, such as humic substances (Leenheer and Croué,2003). The weight average molecular weight (Mw) and the numberaverage molecular weight (Mn, not shown here) values (Table 2)were within the range of values reported by Benedetti et al. (2002,2003) and Alasonati et al. (2010) for similar samples. The Rio Negrowater samples (O1, O3, O4 and O20) had significantly lower valuescompared to the other samples. Polydispersity values (i.e. 1.52 to2.04) were in the range of values reported by Cabaniss et al. (2000) forvarious aquatic fulvic acids and hydrophobic fractions of DOM.

4.2. Isolated hydrophobic and transphilic substances

Mass balance calculations made for the extraction showed thatHPO+TPH fractions always represented more than 75% of the OMcollected by RO (i.e. OMb0.2 μm) underlining the relevance of thesefractions. The following general trendswere observed for the bulk C andN contents of HPO and TPH (Table 3): C/N ratios of the HPO fractionswere always higher than the C/N ratios of the TPH fractions. Theenrichment in N was measured in the TPH fractions at both sampling

Table 1Experimental conditions for the adsorption experiments on different solid phases relevant for Amazonian aquatic systems.

Source of OM pH range Type of OM Type of solid phase BET (m2g−1) Reaction time

Rio Negro watershed water 3.78–4.38 Transphilic (TPH) and hydrophobic (HPO) fractions Synthetic Goethite 94±0.5 30 minCuruai floodplain water 5–6 Transphilic (TPH) and hydrophobic (HPO) fractions Treated sediment 11±0.5 24 h

Table 2Curuai floodplain (Rising stage) and Negro River (low stage) water sample characteristic and properties of the dissolved organic matter (DOMb0.7 μm). n.d. not determined.Cond.: conductivity in μS cm−1; DOC: dissolved organic carbon in mg l−1; C/N: organic carbon to organic nitrogen ratio in moles, the organic nitrogen concentration was belowdetection limits for the Curuai water samples; SUVA: specific UV absorbance in m−1l mg−1C; Mw:Weight Average Molecular Weight; Mn: Number Average Molecular Weight bothmeasured by SEC in Daltons.

Station Site pH Cond. DOC C/N SUVA Mw Mn Mw/Mn

Curuai floodplain (CF) AR Amazon river 6.92 55 4.9 n.d. 4.69 2694 1438 1.9W1 Salé lake 7.22 52 4.7 n.d. 4.80 2654 1460 1.8W2 Poção lake 7.28 53 4.9 n.d. 5.11 2493 1407 1.8W3 Santa Ninha lake 7.04 n.d. 5.7 n.d. 7.32 3063 1669 1.8B1 Piraquara stream 5.69 7 3.7 n.d. 4.10 2208 1280 1.7PB1 Grande do Poção lake 6.82 55 4.1 n.d. 3.28 2169 1264 1.7PB2 Grande lake 7.06 55 4.8 n.d. 3.83 1933 1132 1.7

Rio Negro watershed (RNW) O1 Negro River 5.78 11 7.1 24 4.62 1415 978 1.3O3 Negro downstream Jaú 4.35 12 10.1 28 4.65 1684 1108 1.6O4 Negro upstream Jaú 4.41 12 10.7 26 4.67 1703 1120 1.6O20 Jaú River 5.05 11 7.6 17 4.00 1076 820 1.6O6 Igapó 4.27 22 11.6 19 4.00 2240 1422 1.6O15 Bonito stream 3.62 65 57.7 41 4.97 2259 1438 1.6O16 Podzol pit III 3.54 86 75.6 48 4.76 2350 1513 1.6O18 Podzol pit V 3.69 79 65.8 51 5.18 2795 1788 1.6O19 Podzol pit IV 3.55 114 89.0 51 4.93 2657 1653 1.6

Fig. 2. Relation between conductivity (A), dissolved organic carbon (B) and pH into theNegro River watershed (RNW) at low-flow for the bulk samplesb0.7 μm, and in theCuruai floodplain (CF) at rising-flow for the bulk samples, 0.22 μm.

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sites. The C/N ratios of the Rio Negro HPO water samples (43±5) werehigher than the ratios of the Curuai HPO fractions (33±2) while theratios were the same for all TPH fractions (16±2). These results werein agreement with previously published data for similar extractedfractions (Abbt-Braun and Jahnel, 2001; Croué et al., 2003).

The δ13C values of all HPO fractions (Table 3, with a precision at ±1σ equal to±0.1‰ and±0.2‰ for δ13C and δ15N, respectively) variedfrom −27.7 to −30.2‰ among which the Rio Negro samples had thelowest values. The δ13C values for the TPH fractions were consistentlyhigher by 1‰ than their respective HPO fraction. At Curuai várzealakes, δ15N varied from+1.8 to +3.2‰ and from+2.7 to +4.1‰ forthe hydrophobic and transphilic fractions, respectively. The highestvalues were founded for samples collected during the phytoplank-tonic bloom (PB1, PB2) and the TPH δ15N values were higher thanHPO δ15N ones (Table 3). Conversely, δ15N values of HPO (1.7±0.9)and TPH (1.9±1.0) samples collected in the Rio Negro watershedwere not statistically different. The δ13C values were within the rangeof the values reported byMayorga et al. (2005) for DOC and POC in theAmazon watershed. The δ15N values were in agreement with the dataobtained by Aufdenkampe et al. (2007) for ultrafiltrated DOM andPOC in river samples taken from the Andes to the lowland Amazonmain stem, with uniform δ15N values regardless of the elevation or thedistance downstream the main stem.

Results of the pH dependent dissociation (Fig. S1) of HPO and TPHfractions are summarized in Table 4 and expressed as the amount ofreactive groups and proton binding parameters derived within theNICA-Donnan model framework (Kinniburgh et al., 1999). Theparameter values were within the range of generic parameters forhumic and fulvic acids obtained with the same modeling approach by

Milne et al. (2001). No major difference is observed between the twoRio Negro water samples fractions parameters values. For the Curuailakewater samples, the only observed difference is a higher amount oflow affinity sites or carboxylic type groups (Qmax,1) compared to theamount of high affinity sites or phenolic type groups (Qmax,2).

The FTIR spectra are used to identify chemical moieties andcharacterize the organic compounds found in the Curuai floodplainsystem (Fig. S2). For the Curuai floodplain, the FTIR spectra of bothfractions at all stations were similar. Moreira-Turcq et al. (2003)analyzed particulate organic matter (POM) after adsorption experi-ments with DOM onto alumina (α-Al2O3) particles. The adsorbedmaterial presented the same aliphatic CH2 and CH3 stretching at 2935and 2850 cm−1, C=O stretching of COOH at 1 730 cm−1. At 1640 and1510 cm−1 were observed C=O stretching from amide groups(amide I band) and N–H deformation and C=N stretching (amide IIband) (Nadja et al., 1995). The phenolic C–O stretching at 1460–1450 cm−1 and the two bands at 1090 and 1030 cm−1 were alwayspresent. These bands were also observed in the absorption spectra ofcolloidal organic matter (5 kilo daltonsbOMb0.2 μm). The compar-ison of the spectroscopic data obtained for HPO and TPH and naturalorganic matter collected by different means revealed that they allcontained the same reactive organic moieties and that the results ofadsorption test can be generalized to the bulk organic matter.

4.3. Adsorption experiments

Natural organic matter is a heterogeneous mixture of differentorganic molecules that may have different adsorption affinities for the

Table 3Elementary and isotopic composition of the hydrophobic (HPO) and transphilic (TPH) fractions isolated from the dissolved organic matter collected in the Curuai floodplain andNegro River systems.

System Station HPO TPH

OC(%)

N(%)

C/N δ13C(‰ vs PDB)

δ15N(‰ vs Air)

OC(%)

N(%)

C/N δ13C(‰ vs PDB)

δ15N(‰ vs Air)

Curuai floodplain(CF)

AR 52.8 1.5 35 −29.6 1.8 42.2 2.2 19 −28.6 2.7W1 51.8 1.5 35 −29.3 2.3 40.8 2.8 15 −28.3 3.3W2 52.4 1.6 32 −28.5 2.3 46.2 3.2 19 −27.7 3.3W3 51.7 1.4 36 −29.3 2.3 – – – – –

B1 51.9 1.7 31 −27.7 2.4 – – –- – –

PB1 51.0 1.7 30 −28.9 3.0 – – – – –

Rio Negrowatershed(RNW)

PB2 52.4 1.8 30 −28.8 3.2 43.6 3.0 14 −27.8 4.1O4 49.5 1.2 42 −29.7 1.3 46.9 3.1 15 −28.9 1.5O20 52.1 1.2 45 −30.2 2.3 46.4 2.6 18 −29.2 2.4O6 50.3 1.3 38 −30.1 2.2 45.3 3.2 14 −29.1 2.2O15 50.7 1.3 39 −30.1 2.5 46.7 3.3 14 −29.0 3.1O16 48.9 9.7 50 −29.3 0.3 45.3 2.3 20 −28.7 0.4

Table 4Reactive group Proton binding parameters derived with the NICA-Donnan model (Kinniburgh et al., 1999). Parameters definitions: Qmax,1 and Qmax,2 (meq g−1 C) corresponding tocarboxylic type-group and phenolic type-group site density, respectively. KH,1 and KH,2 are the respective median proton affinity constant for each type groups. mH,1 and mH,2 are thechemical heterogeneity parameter for each group. Only residuals of the fitting are shown since r2 values were all above 0.995.

System Fraction Station Qmax,1 KH,1 mH,1 Qmax,2 KH,2 mH,2 Qmax ,1+Qmax,2 Residuals

Curuai floodplain(CF)

AR 5.26±0.26 4.47±0.01 0.76±0.03 3.17±0.40 7.98±0.12 0.33±0.04 8.43±0.66 0.66W1 7.39±0.13 4.33±0.01 0.61±0.01 2.85±0.18 8.95±0.06 0.37±0.02 10.24±0.31 1.54W2 6.07±0.21 4.47±0.01 0.70±0.02 3.97±0.49 8.87±0.11 0.27±0.03 10.04±0.70 0.84

HPO W3 5.73±0.13 4.39±0.01 0.63±0.01 2.93±0.24 9.29±0.09 0.34±0.03 8.66±0.37 0.66B1 5.51±0.32 4.46±0.01 0.79±0.03 3.94±0.56 8.02±0.13 0.29±0.04 9.45±0.88 0.7PB1 5.67±0.13 4.26±0.03 0.70±0.04 3.54±0.16 9.78±0.15 0.26±0.01 9.21±0.29 1.07PB2 4.20±0.33 4.45±0.01 0.98±0.06 2.41±0.47 7.44±0.23 0.36±0.07 6.61±0.80 0.7W1 5.24±0.24 4.37±0.01 0.73±0.02 2.97±0.27 7.49±0.13 0.36±0.03 8.21±0.51 0.71

TPH W2 9.75±0.32 4.13±0.1 0.60±0.01 3.03±0.23 7.73±0.09 0.38±0.02 12.78±0.55 2.72PB1 6.15±0.36 4.29±0.11 0.69±0.02 3.63±0.72 8.10±0.19 0.23±0.05 9.78±1.08 1.34

Rio Negro watershed(RNW)

HPO O4 4.56±0.13 4.38±0.03 0.74±0.04 4.72±0.26 8.28±0.28 0.25±0.02 9.28±0.39 0.56O15 4.90±0.70 4.43±0.03 0.75±0.14 3.92±1.1 8.10±0.66 0.28±0.17 8.82±1.8 0.67

TPH O4 4.46±0.30 4.38±0.04 0.72±0.07 4.08±0.50 8.28±0.28 0.29±0.08 8.54±0.80 0.46O15 5.26±0.72 4.43±0.04 0.81±0.14 3.60±1.00 8.10±0.66 0.26±0.16 8.86±1.72 0.69

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mineral surfaces, therefore competitive or preferential adsorption ofspecific fractions are expected.

Experiments to measure adsorption of the Rio Negro DOM fractionson goethite resulted in low residual [DOC] since 60 to 80% of the initialorganic carbon was adsorbed (Table 5). Carbon surface loadings for thefour tested samples were very similar and cluster around the averagevalue of 24.5±4 μgC m−2. This carbon loading is smaller than the valuecorresponding to a mineral coated by a monolayer of organic matter(Keil et al., 1994) as well as values reported for organic matter loadingon goethite that ranged from180 μgC m−2 to 310 μgC m−2 (Meier et al.,1999; Chorover andAmistadi, 2001). The lowpHvalue, the short time ofinteraction (30 min vs 17 h for Chorover and Amistadi, 2001) and thehigh specific surface area (94 m2 g−1 vs 50 m2 g−1 for Chorover andAmistadi, 2001) are factors that could explain the lower OC loading.

After HPO adsorption experiments, SUVA values of the residual DOMwere similar to the initial values (i.e. SUVA=3±0.5 m−1l mg−1C). Thesorption experiments made with TPH showed however a significantdecrease of the SUVA value from 2±0.2 to 1±0.5 m−1l mg−1Csuggesting the preferential sorption of the more aromatic fractions.

For the Curuai organic matter fractions, the residual [DOC] (3.83±0.11 mg L−1) were higher as samples adsorbed only between 5 and 20%of the initial [DOC] (4.16±0.19 mg L−1). The hydrophobic substances(HPO) showed a higher carbon adsorption than transphilic ones (TPH).Carbon surface loadings calculated for HPO and TPH fractions rangedbetween 23 to 309 μgOC m−2 and 38 to 145 μgOC m−2, respectively.Much higher carbon surface loadings (i.e. 500 to 1000 μgOCm−2) wereexpected if the surface of the treated sediment was coated by amonolayer of organic matter (Keil et al., 1994). The highest surfaceloadingwas obtainedwith the HPO and TPH fractions from the AmazonRiver (AR) (Table 5). The HPO and TPH fractions of phytoplanktonicbloom(PB2) had the lowest surface loading, associated to the lowest pHvalue. For some unknown reasons, sorption did not occur with the HPOfraction of sample W1, although all other chemical characteristics werevery close to those reported for W2 or W3 (see Tables 2, 3 and 4). Aftersorption, because of the high residual [DOC], SUVA values of theremaining DOM were similar to the values measured before sorption(SUVA=5±0.5 and 3±0.5 m-1l mg-1C for HPO and TPH fractions,respectively).

5. Discussion

The dynamics of organic matter (OM) is a key topic that coversvarious fields of biogeochemistry ecology and climate change. Quanti-tave knowledge of fluxes and sources of OM is important to validate itsuse as a tracer of ongoing processes in soils or rivers. The understandingof processes that modify the OM in pristine riverine systems, such asthose found in the Amazon basin, are highly important as it allowsascertaining the fate of OM in estuarine or oceanic environments aswell

as establishing a model of climate changes that often include thisreactive compartment.

In the discussion that follows, wewill first try to explain the trendsobserved between the C isotopic and elementary compositions of theDOM fractions, these trends would then account for the DOMbehavior in the Amazon basin systems. In a second part, the changesof the DOM concentration in the black river and the floodplainsystems are addressed and complementary processes (i.e. dilution,degradation and adsorption onto mineral phases) are discussed withthe help of the data obtained during laboratory sorption experiments.For the floodplain system, the impact of the selective sorption on thefate of organic nitrogen will also be addressed.

5.1. Sources and relationships between isolated fraction characteristics

Three important features are observed between δ13C values and C/N ratio in Fig. 3: (1) a general negative trend is observed between δ13Cvalues and C/N ratios, (2) δ13C enrichment for DOM fractions fromfloodplain samples, and (3) the HPO fractions of both ecosystems hadhigh C/N ratios, typical of a greater influence of terrestrial organicmatter (i.e. Terra Firme forest). The C/N ratioswere also higher than theratios of the TPH fractions (richer in N). The extracted fractionsrepresented at least 75% of the [DOC], so these trends would accountfor most of the DOM behavior in Amazon basin system.

The high C/N values reinforce the notion of pedogenic origin of theRio Negro DOM since high C/N ratios are measured in the podzols(Fig. 3). Hedges et al. (1994) also found, for ultrafiltrated dissolvedorganic matter (UDOM, ~1 nm), a high C/N ratio for the Rio Negro

Table 5Sorption of dissolved organic matter fractions (HPO & TPH). The adsorbed organic carbon (OC) to specific surface Area ratio (SA) was calculated for the different type of organicmatter collected in rivers and floodplain of the Amazon basin with the BET SA given in Table 1. pH corresponding to the final values after equilibration, same final pH was reached forboth HPO and TPH fractions.

Fraction HPO TPH

Station pHfinal

Initial DOC(mg l−1)

Final DOC(mg l−1)

OC/SA(μgC m−2)

Initial DOC(mg l−1)

Final DOC(mg l−1)

OC/SA(μgC m−2)

Curuai floodplain(CF)

AR 5.7±0.1 4.47±0.04 3.79±0.04 309±20 3.72±0.04 3.41±0.04 145±20W1 6.2±0.1 4.05±0.04 4.02±0.04 23±20 – – –

W2 6.3±0.1 4.14±0.04 3.82±0.04 145±20 4.39±0.04 4.15±0.04 109±20W3 6.4±0.1 4.17±0.04 3.74±0.04 195±20 3.43±0.04 3.15±0.04 127±20B1 5.4±0.1 – – – 3.07±0.04 2.85±0.04 100±20PB1 5.9±0.1 4.24±0.04 3.90±0.04 155±20 – – –

PB2 5.3±0.1 3.91±0.04 3.71±0.04 91±20 3.40±0.04 3.32±0.04 38±20Rio Negro watershed(RNW)

O4 3.9±0.1 16.80±0.1 3.10±0.04 24±2 19.20±0.1 4.70±0.04 25±2O15 4.4±0.1 19.20±0.1 7.10±0.07 21±2 20.20±0.1 4.40±0.04 28±2

Fig. 3. Isotopic composition of the hydrophobic (HPO) and transphilic (TPH) fractions ofthe dissolved organic matter in the Negro River watershed (RNW) and the Curuaifloodplain (CF) as function of the C/N ratio. Different potential major sources of carbonare added for comparison. Date for forest soils and tree and tree leaves are taken fromBernardes et al., 2004.

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(52), and lower uniform C/N ratios (32–36) for other main rivers,including the Amazon River at Óbidos. Slightly higher C/N values weremeasured for Rio Negro watershed samples (38–50) than for Curuaifloodplain samples (30–35), mainly for HPO fractions (Fig. 3). Thesignificant difference between HPO (30–50) and TPH (14–20) C/Nvalues (Fig. 3) is explained by higher amounts of humic substances inHPO fractions than in TPH fractions. Ertel et al. (1986) isolated humicand fulvic acids by XAD-8 resin and explained the higher C/N ratios forhumic acids in the Amazon River (33±5) by the influence of RioNegro waters (C/N=58±0.3). In the same study, C/N ratios of 85, forfulvic acids, were measured in black water tributaries in the Rio Negrowatershed while in the other Amazonian tributaries the values wererelatively uniform (59±7). This is in agreement with the titrationdata (Table 4) that were compared with the generic values obtainedby Milne et al. (2001) for fulvic and humic acids confirms the fact inthe Rio Negro organic matter is more humic like.

Organic matter found in floodplain lakes results from a mixing ofseveral sources (soils, in situ production of grasses and phytoplank-ton). Martinelli et al. (2003) estimated that 36% of the organic carbonpresent in floodplain sediments (along the Solimões/Amazon River)originates from in situ production and that riverine POC provides theremaining 64%. Junk (1997) and Melack and Engle (2009) studiedother Amazonian floodplains and demonstrated that aquatic macro-phytes and phytoplankton were the primary sources of organiccarbon. The δ13C enrichment in Curuai floodplain (−27.7 to−29.6‰)is interpreted as a greater contribution of dissolved bioorganicdetritus from C4 grasses. Martinelli et al. (2003) demonstrated, inthe Solimões-Amazon River system, a downstream increase of anisotopically heavier particulate organic matter produced by grasses. Inthe Negro River, organic matter was mainly derived from forest DOM(−28.7 to −30.2‰). However, the lower SUVA values measured forthe samples collected during an algal bloom (PB1 and PB2) werebelow the average value (5.2) of the Curuai floodplain samples andreflect the influence of a fresher pool of less aromatic algal DOM.

5.2. Fate of the dissolved organic matter: processes at work

Variousmechanisms that control the fate of this large pool of carbonand its transfer to the ocean must be addressed since their relativecontribution along the Amazon Basin is still a matter of debate. Threemajor processes are expected to be important in these processes:dilution to account for concentration changes such as the concentrationof iron bound to organicmatter in theRioNegrowatershed (Allard et al.,2011), differential degradation of different molecular size fractions ofthe DOC (Amon and Benner, 1996) and sorption onto mineral phases(Moreira-Turcq et al., 2003; Aufdenkampe et al., 2007).

The changes in DOM concentration, conductivity and the amount ofcarbon adsorbed during the laboratory experiments together withchanges in bulk properties of the DOMwill be used to address the relativeimportance of the three expected processes first for the Rio Negrowatershed system and then for the Amazon River and floodplain lakes.

A negatively charged organic matter results from the dissociationof the carboxylic and phenolic type groups on the carbon skeletonidentified by the proton titration. With the help of the NICA-Donnanmodel and the resulting fitted parameters (Table 4) it is possible tocalculate the amount of carboxylic and phenolic type groups (Qmax,1+Qmax,2) that are still deprotonated at field pH values. The amount ofnegative charge or deprotonated sites corresponds to the differencebetween the total number of sites (i.e. Qmax,1+Qmax,2) and the amountof protonated sites QH,t calculated by the NICA-Donnan equation givenbelow (Benedetti et al., 1996):

QH;t = Q max;1 ×K̃H;1 Hþ� �n om1

1 + K̃H;1 Hþ½ �n om1

+ Q max;2K̃H;2 Hþ� �n om2

1 + K̃H;2 Hþ½ �n om2

where QH,t is the total amount of protons bound to the organic matter,Qmax is type 1 or 2 (i.e. carboxylic or phenolic) total site density. K̃H isthe proton median binding constant, and [H+] is the concentration ofprotons in the Donnan phase. m1 or m2 is an heterogeneity parameterthat reflects the combined effect of the intrinsic OMheterogeneity andthe proton specific heterogeneity.

In the Rio Negro watershed, the calculated amount of deproto-nated sites for the HPO and TPH fractions ranged from −350 to−750 μeqg OM−1. The later values were converted into a residualnegative charge for the bulk DOM that ranged from −24 to−67 μeq L−1. Because the dissolved cations and anions concentra-tions of these rivers are in the micromolar range (Benedetti et al.,2003a, 2003b), the conductivity value at a given sampling point canbe concluded to result from the DOM charge. The decrease ofthe conductivity in the Rio Negro system (Fig. 2A) could resultfrom the dilution with water originating from other parts of thewatershed, notably those dominated by clay-depleted laterites(Nascimento et al., 2004) with low [DOC]. The simultaneous decreaseof [DOC] and increase of pH (Fig. 2B) would also result from the samedilution of the organic rich water and watershed dominated by clay-depleted laterites having higher pH values in addition to lower [DOC].

The relationship between the weight average molecular weight(Mw) of the dissolved organic matter as function of the C/N ratio forthe Amazon River and Rio Negro watershed samples is given in Fig. 4.With the exception of one sample, the data points cluster along onetrend line (the higher is the C/N, the higher is the Mw). This trendcould also be interpreted as the result of mixing between 2 end-members. Indeed Allard et al. (2011) proposed that the ironconcentration bound to organic matter can be considered to followa mixing line between laterite-type water with lower [DOC] andpodzol-type water with high C/N ratios and high molecular weight(Nascimento et al., 2004; Alasonati et al., 2010).

According to the assumption of conservative mixing, an averageRio Negro sample corresponds to about a contribution of 25–50% ofpodzol water. This value is consistent with the 33% of podzol surfacecontribution in the Rio Negro basin (Fritsch et al., 2009). However,Aucour et al. (2003) showed that in the Amazon basin the mixing ofrivers with contrasted chemical composition was not conservative forthe dissolved organic matter since 30% of the DOC was lost forinstance at the confluence between the Rio Negro and the Solimões.Another process is therefore needed to account for the evolutionsreported in Figs. 2 and 4.

Amon and Benner (1996) carried out bacterial growth andrespiration measurements and showed that for some riverine DOM,the high molecular weight OM was utilized to a greater extent thanlow molecular weight OM. The preferential degradation of the

Fig. 4. Relation between molecular weight (Mw) and C/N ratio for the bulk samples.(a) fractionb0.7 μm at Negro River watershed (RNW) andb0.22 μm at Amazon River(b) fractionb0.22 μm at Curuai floodplain (CF).

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heaviest molecules along the hydrological pass would account for thetrend in Fig. 4. However, the Rio Negro DOM utilization rates given byAmon and Benner (1996) were the same for both DOM molecularweight fractions and corresponded to 0.36±0.01 μM DOC h−1 (Amonand Benner, 1996). Thus degradation alone cannot explain Fig. 4.

Preferential sorption of the heavier Mw fractions of the DOM inthe soils or on suspended material would also favor the distributionof the data points along the trend line in Fig. 4. Numerous papersreported differential absorption of organic matter on various min-eral substrates or soils (Chorover and Amistadi, 2001; Kaiser andGuggenberger, 2003; Feng et al., 2005). The differential fate of organicmatter molecules in the Rio Negro waters based on the role of size andpossibly reactivity is in agreement with the recent finding on podzolsformation in the Rio Negro basin. It was demonstrated that theaccumulation of organic matter in the Bh soil horizon correspondedto the trapping of highly aromatic humic acid like substances (Bardyet al., 2008) while the fulvic acid like substances (i.e. smaller and lessreactive on a molecular weight basis) would migrate deeper in thesoil and reach the rivers (Bardy et al., 2008; Fritsch, et al., 2009).Moreover, Alasonati et al. (2010) characterized colloidal organicmatter from the Rio Negro watershed and showed the existence of adecrease of size of the colloidal matter when passing from first orderstreams to higher order rivers. The central position of the data pointcorresponding to the Amazon River taken at Óbidos (Fig. 4) would bethe net result of the combined processes at the Amazon watershedscale.

Our sorption experiments with the Rio Negro organic matter havelower OC/SA values than the one obtained for the Curuai sediment OC/SA data (Table 5). Preferential sorption of the larger molecules couldhinder further sorption and favor low OC/SA values under ourexperimental conditions. Chorover and Amistadi (2001) reportedthat goethite would effectively remove higher molecular weightmaterial from solution at low equilibrium [DOC] like here. Theseconditions and processes would then explain the higher [DOC]measured in field podzol soil solutions characterized by low pHvalues (Table 2).

Among the Curuai samples, we observed that as the molecularweights increase the δ13C values became more negative and the C/Nratios increase (Fig. 5A and B, respectively). This behavior follows atrend from Amazon River to Curuai lakes. Amon and Benner (1996)reported faster carbon consumption rates for high (1.08 μM DOC h−1)than for low molecular weight DOM (0.24 μM DOC h−1) for RioSolimões. It suggests that the DOM fractionated at Curuai lakes couldbe more degraded than the DOM in the Amazon River. This trendcould result in decreasing average surface affinity of DOM withadvancing degradation regardless of mineral type (Davis and Gloor,1981; McKnight et al., 1992). However, the different physical andchemical characteristics of the samples could also lend to differentsorption interactions of the DOM onto mineral particles.

In the experiments made with the Curuai samples, ionic strengthdid not vary and the pH changeswere less than 0.3 units. The differentOC/SA ratios given in Table 5 must have another origin. The amount ofresidual negative charge or deprotonated sites of the OM interactingwith the treated sediment corresponds to the difference betweenthe total number of sites (i.e. Qmax,1+Qmax,2) and the amount ofprotonated sites QH,t, calculated by the NICA-Donnan equation(Benedetti et al., 1996) and model parameters (Table 4). The amountof carbon surface loading is plotted against the calculated residualnegative charge of the OM fractions (i.e. HPO or TPH) in Fig. 6. Thelater shows that the higher is the amount of un-protonated carboxylicand phenolic type groups, the lower is the carbon surface loading onthe treated sediment. This result agrees with previously publisheddata (Feng et al., 2005 and reference therein). The decrease of thenegative charge reduces the net electrostatic repulsion between theOM fractions and the clay negatively charged surfaces and promotesligand exchange reactions for sorption to clays (Murphy et al., 1994).

Some samples with similar residual negative charge showed howeverdifferent carbon surface loading (Fig. 6). This would follow from theselective sorption of OM fractions by clays surfaces. Montmorillonitehas been reported to react with the larger molecular weight com-ponent of OM due to its higher CEC (Satterberg et al., 2003). It ispossible that the binding sites on montmorillonite surfaces in thetreated sediment were occupied by a more polydisperse OM having agreater potential range of interactions with clay surfaces (Collins et al.,1995; Vermeer and Koopal, 1998; Chorover and Amistadi, 2001).

Fig. 5. (A) Relation between δ13C and Mw (B) Relation between C/N ratio and Mw, forCuruai floodplain (CF) and Amazon River (AR) fractionated samples (Hydrophobicsubstances).

Fig. 6. Adsorbed carbon concentrations related with total amount of deprotonated sitesin the initial adsorption solution for the experiments with the hydrophobic (HPO) andtransphilic (TPH) fractions of Curuai floodplain (CF) samples.

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The above observations indicate that in the Amazon basin riversystem, it is the combination of preferential degradation of DOMfractions and the preferential sorption of the heavier molecularweight fractions (which, as shown in our experiments, is controlledby the amount of protonated carboxylic and phenolic sites of theDOM) thatmodifies the OM characteristics during its “journey”withinthe Amazon basin and will affect its fate in estuarine or oceanicenvironment as well.

5.3. Impact of the selective sorption on organic nitrogen

The preferential uptake of the high molecular weight fraction dueto selective sorption implies that the chemical nature of the organicmatter remaining in solution will differ from the adsorbed fraction.The impact of the selective sorption on a peat-humic acid chemicalcomposition was characterized by Feng et al. (2005) with H-HR-MAS-NMR. They reported the presence of both aromatic and CH3 signals onthe organo-clay complex and CH2 groups were observed to bindpreferentially to montmorillonite while carbohydrates and aminoacids remain in the supernatant. These observations explain thedifference between our results showing the absence of nitrogenenrichment after sorption and the results of Aufdenkampe et al.(2001), which conducted adsorption experiments with dissolvedorganic matter (b0.7 μm GF/F) and reported preferential sorption ofnitrogen rich fractions, evidenced by C/N ratios substantially smallerin the fine particulate organic matter fraction than in the correspond-ing dissolved organic matter. This nitrogen enrichment on suspendedparticles is a dominant characteristic of organic matter in the AmazonRiver (Williams, 1968; Hedges et al., 1994, 2000), and in other riversaround the world (Lewis et al., 1995). However, our results suggestthat the nitrogen enrichment takes place only for a minor part of thetotal pool of the organic matter since HPO and TPH fractions accountfor more than 75% of the [DOC] or that the carbohydrates and aminoacids fractionswere not retained by the two step extraction proceduremaking these fractions less representative of the total dissolvedorganic nitrogen behavior in aquatic systems.

6. Conclusion

In the present study we have shown that in the Amazonwatershed, changes in DOC along the river can be accounted bytaking into consideration several complementary processes, notablythe mixing between carbon rich and carbon poor waters, preferentialdegradation and fractionation of the organic matter during sorptionon sediment or soil material. Lab-scale sorption experiments thatutilized field purified organic matter show that (i) the high molecularweight and aromatic rich compounds will be sorbed preferential byminerals like goethite while (ii) the lower molecular weight and lessaromatic compounds will migrate deeper in the soils and reach thehigher order streams where later they can interact with suspendedclay mineral inducing a second fractionation between dissolved andsorbed organic matter in the lower part of the Amazon basin and infloodplain lakes. The residual negative charge of the dissolved organicmatter is a key property of the organic macromolecules that willdetermine the fate of organic matter issued from pedogenic processesor from in situ production.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.chemgeo.2011.05.004.

Acknowledgments

This study was supported by a joint CNPq (Conselho Nacional deDesenvolvimento e Pesquisa Tecnológica) and IRD (Institut deRecherche pour le Developpement), and HYBAm (Hydrologie etGeochimie du Basin Amazonien). We would like thanking to ANA andCPRM brazilian institutions for their help during cruise works.

Marcela Pérez was sponsored by CAPES and SWE-CNPq PhD fellow-ships in the Geochemistry Department of UFF (Universidade FederalFluminense), Brazil. We would like to thank the two annonimousreviewers for the very helpful and constructive comments. Professor GKorshin (Seattle, USA) is acknowledge for polishing our english :merci Gregory. This paper is IPGP contribution N°: 3132.

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