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Environmental and Experimental Botany 67 (2009) 403–414

Contents lists available at ScienceDirect

Environmental and Experimental Botany

journa l homepage: www.e lsev ier .com/ locate /envexpbot

istribution, movement and availability of Cd and Zn in a dredged sedimentultivated with Salix alba

ean-Philippe Bedell a,∗, Xavier Capillaa, Claire Girya, Christophe Schwartzb,ean-Louis Morelb, Yves Perrodina

Université de Lyon, Laboratoire des Sciences de l’Environnement, Ecole Nationale des Travaux Publics de l’Etat, Rue Maurice Audin, 69518 Vaulx-en-Velin Cedex, FranceLaboratoire Sols et Environnement, INPL (ENSAIA)/INRA UMR 1120, 2 avenue de la forêt de Haye, BP 172, 54505 Vandoeuvre-les-Nancy, France

r t i c l e i n f o

rticle history:eceived 20 March 2009eceived in revised form 27 July 2009ccepted 13 August 2009

eywords:drowthetal mobility

ercolatesalix albaedimentn

a b s t r a c t

Willows occur as volunteer vegetation on sediment-derived soils, such as dredged sediments, landfillcover or stockpile deposits, and are used as phytoremediators on such soils. The present study evaluatesgrowth and metal uptake by Salix alba grown on a contaminated dredge sediment for 209 days undergreenhouse conditions. At the end of the study, the aerial parts of the S. alba plants had grown to heightsof between 80 and 117 cm. Biomass and Cd and Zn concentration in the roots, stems and leaves, at 70,112 and 209 days, showed that Cd and Zn had been bioaccumulated, especially in the leaves.

At the three sampling dates, Cd and Zn extractability and pH measurements were also carriedout on samples of two soil layers (0–15 and 15–30 cm) from both the planted and the control pots.Cd and Zn extractability were assessed using single extraction procedures (0.01 M CaCl2; 1 M HNO3;CaCl2–TEA–DTPA). The two metals showed similar variations in CaCl2 and HNO3 extractabilities, but thiswas not the case for DTPA extractability. The greatest variations were observed in the upper soil layers ofthe control pots. In the planted pots, the CaCl2 extractability of Zn decreased in the upper layer, and theHNO3 extractability of Zn increased in the lower layer. The pH of the upper soil layer was always higherthan the pH of the bottom layer. In addition, we monitored several parameters of the percolates fromboth the planted and the control sediments, including pH, Eh, conductivity, dissolved organic carbon, Znand Cd concentrations, and presence of certain cations/anions. Dissolved organic carbon, and Cd and Znconcentrations increased steadily over time. There were no significant differences between the planted

and the control pots. After 209 days, the characteristics of the control sediment reflected the effects ofageing in that the CaCl2-extractable Cd and Zn concentrations had decreased compared with the initialconcentrations. Conversely, the concentrations of HNO3-extractable Cd and Zn had increased. A frac-tion of the metal initially extracted by CaCl2 (considered as exchangeable) became less available withtime. After 112 days, the plants had extracted approximately 2.8 mg of Zn. At the same time, the CaCl2

uppe-extr

extractability of Zn in theZn from the pool of CaCl2

. Introduction

Dredging is often carried out when sediment accumulationncreases the risk of flooding or decreases the draught of navigable

aterways, or when the sediment represents a proven ecolog-cal or health risk. The dredged sediment can be left in waterstockpiled, deposited in borrow pits), used for landscaping areas

uch as waste disposal sites (Mohan et al., 1997) or for restor-ng marshes (Ford et al., 1999), or it can be stockpiled on landCapilla et al., 2006). In practice, the 6 million m3 of sedimentsredged annually from French continental waters is disposed of

∗ Corresponding author. Tel.: +33 4 7204 7081; fax: +33 4 7204 7743.E-mail address: bedell@entpe.fr (J.-P. Bedell).

098-8472/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.envexpbot.2009.08.001

r, rooted layer decreased by 2.6 mg. We can assume that S. alba extractedactable Zn.

© 2009 Elsevier B.V. All rights reserved.

as follows: 70% is re-deposited in water or on land, 10% is usedfor public works, 7% is spread on farmland and 3% is stock-piled or used in miscellaneous fills or processes (Carpentier et al.,2002).

In the case of a deposit located near a river or a canal, pol-lutants can be transferred to the surrounding soil, surface waterand groundwater (Bedell et al., 2003), affecting microorganismsand the terrestrial and aquatic flora and fauna. Plants can growon such deposits (Capilla et al., 2006) and, via their root systems,they can modify the fate of pollutants within contaminated matri-

ces (Adriano et al., 2004), especially since the chemical conditionsof the rhizosphere can be very different from those in the bulk soil(Morel et al., 1986; Wang et al., 2001). A range of processes maybe involved, including the fixation and adsorption of pollutants or,conversely, their mobilization (e.g., due to changes in pH at the rhi-

404 J.-P. Bedell et al. / Environmental and Experimental Botany 67 (2009) 403–414

Table 1Physico-chemical characteristics of the sediment.

pH (water) Total calcium(g kg−1)

POlsen

(g kg−1)Organic C(g kg−1)

N (g kg−1) C/N Total conductivity(mS cm−1)

Particle sizedistribution (in %)

Total metal content (mg kg−1 DW)

Clay Silt Sand Cd Cr Cu Ni Zn Fe Al

zc

sb2absViV2Wsectmbe

gastmamWaa2mcta(i72o2o2addctTagtir

8.2 514 0.090 93.4 2.74 34 0.17

ospheric level). These processes can cause physical or chemicalhanges in the sediment.

Willows occur as volunteer vegetation on sediment-derivedoils, such as dredged sediments, landfills, stockpile deposits, over-ank sedimentation zones and freshwater tidal marshes (Bal et al.,001; Marseille et al., 2000a; Vervaeke et al., 2001; Vandecasteele etl., 2002a; Capilla et al., 2006). As a result, willows provide potentialiomonitors for contaminated, sediment-derived soils, with foliaramples being used to assess metal bioavailability, as described byandecasteele et al. (2002a, 2005). However, metal concentrations

n willows are highly dependent on species (Lunácková et al., 2003;andecasteele et al., 2004, 2005), clone (Landberg and Greger,002; Vandecasteele et al., 2005), growth performance (Klang-estin and Perttu, 2002), root density and distribution within the

oil profile (Keller et al., 2003), and sampling period (Vandecasteelet al., 2004, 2005). In addition, as willows are easy to grow fromuttings and have high growth rates, they are well suited both foresting metal bioavailability and accumulation, and for phytore-

ediation, which can be combined with energy production fromiomass (Dimitriou et al., 2006; Lewandowski et al., 2006; Vervaeket al., 2006; Meers et al., 2007).

Different willow species have adopted one of two basic strate-ies for developing metal tolerance: metal exclusion and metalccumulation (Baker, 1981; Baker and Walker, 1990). Metal exclu-ion, which involves avoiding metal uptake and restricting metalransport to leaves (De Vos et al., 1991), is usually used by pseudo-

etallophytes, a group of plants that grow on both contaminatednd non-contaminated soils. True metallophytes only grow onetal-contaminated and naturally metal-rich soils (Baker, 1987).illows are leaf accumulators for zinc (Zn) and cadmium (Cd),

nd root accumulators for copper (Cu), chromium (Cr), nickel (Ni)nd lead (Pb) (Punshon and Dickinson, 1997; Stolz and Greger,002; Rosselli et al., 2003). The accumulation and distribution ofetals was also confirmed by a greenhouse study of two S. alba

lones (Vandecasteele et al., 2005), by the potential of short rota-ion of five willow species for phytoextraction (Meers et al., 2007),nd using on-site measurements of foliar metal concentrationsVervaeke et al., 2003). Foliar Cd concentrations have been shown toncrease during the growing season, with concentrations exceeding0 mg kg−1 dry weight (DW) just before leaf shedding (Vervaeke,004). Recent studies have macrolocalized Cd in the tips and edgesf young leaves on a S. alba viminalis L. tolerant clone (Cosio et al.,006), and microlocalized the main Cd sink in the pectin-rich layersf the collenchyma cell walls of the leaf veins (Vollenweider et al.,006). Nevertheless, Vollenweider et al. (2006) concluded that cellnd tissue responses to Cd stress depend on Cd exposure and leafevelopment, and on whether storage or intoxication reactions pre-ominate. Thus S. alba trees growing on dredged-sediment depositsan mobilize Cd from the sublayer of the sediment and recycle ito the surface of the stand through leaf fall (Beyer et al., 1990).his raises concerns of increased Cd mobility in natural ecosystems

nd the risk of food chain contamination. However, it also sug-ests that Cd-contaminated sediments and soils can be depollutedhrough the repeated harvesting of the wood and leaf biomass thats produced; that is to say, carefully managed and monitored shortotation forestry systems can be used as phytoextractors (Östman,

25.8 29.7 44.5 2.9 48.3 178.2 40.0 640.0 24.8 37.2

1994; Greger and Landberg, 1999). Meers et al. (2007) studied thepotential of five willow species for stabilizing and removing con-taminants from contaminated soils. They estimated 5–27 kg ha−1

for Zn and 0.25–0.65 kg ha−1 for Cd that could potentially be annu-ally extracted with two willow clones studied (Meers et al., 2007).Recently, results in a hydroponic system of Mleczek et al. (2009)showed also metal accumulation by willow and concluded as anindicator of the remediation capacity of willow.

In this article we present the results of a greenhouse study ofS. alba plants grown for 209 days on a dredged sediment. Severalparameters of the plants, sediments and percolates were analyzedafter 70, 112 and 209 days. The aims of our study were to: (i)evaluate the impact of tree growth on Cd and Zn mobility in adredged sediment, (ii) describe the distribution of metals in theplant-sediment-percolates system and thus (iii) evaluate and dis-cuss the evolution of Cd and Zn bioavailability during the growthof S. alba on a contaminated sediment.

2. Materials and methods

The characteristics of the sediment selected for the trial wererepresentative of the average characteristics of ten sediments stud-ied in an investigation of different deposits from around France(Capilla, 2005; Capilla et al., 2006). The identification of this sedi-ment as an appropriate “candidate” for studying the mobility andbioavailability of Cd and Zn in the presence of S. alba was based onits metal concentrations, agronomic properties (such as POlsen) andmaximal rooting depth, as well as on the presence of S. alba at thesediment sampling site (Capilla, 2005; Capilla et al., 2006).

2.1. Sediment characteristics

Samples of the upper, rooted horizon (0–20 cm) of a deposit atFlers-en-Escrebieux (Nord, France) were collected in April 2004.The sediment samples were mixed and homogenized before beinganalyzed (Table 1). Sediment grain size, pH in water (NF ISO 10390),organic C ratio (NF ISO 10694, combustion method), total N (NFISO 13878, combustion method), total calcium (NF ISO 10693),CEC (NF X 31-130), exchangeable cations (concentrations of Ca,Mg, K and Na exchangeable with cobaltihexamine), available phos-phorous (Olsen method), and total conductivity were determinedafter air drying and sieving at 2 mm. Total Cd, Zn, Cu, Pb, Cr andNi concentrations were determined after acid mineralisation byHF in a microwave digestion system (CEM, MARS 5). The miner-alized extracts were assayed using ICP-AES and/or Flame AtomicAbsorption (Zeeman effect) (Zhao et al., 1994). Certified ReferenceMaterial (CRM 281 from BCR) and internal samples were used tocheck that complete dissolution occurred during the mineralisa-tion step. The total Cd and Zn contents of the samples (Table 1)were also determined, as these are two of the most commonlyobserved metal pollutants in sediment deposits (Vandecasteele et

al., 2002b; Capilla et al., 2006). Samples of the sediment were sub-ject to three selective chemical extractions at the beginning of theexperiment, and then after 70, 112 and 209 days. Of the variousextractants described in the literature, we chose CaCl2, HNO3 anddiethylenetriamine-pentaacetic acid (DTPA). CaCl2 extractability

Expe

oaacu(ago

wge(is0pwt2fCb8fia

ts

J.-P. Bedell et al. / Environmental and

ften appears to provide an efficient tool for evaluating bioavail-bility (Aten and Gupta, 1996; Mc Bride et al., 2004; Sterckeman etl., 2004). It is also a way of evaluating exchangeable pools throughationic exchange reactions (Lebourg, 1996). Other authors havesed HNO3 for evaluating residual metals in soils and sedimentsKabata-Pendias, 2004; Mertens et al., 2001), or for measuring met-ls bounds to oxides (Planquart et al., 1999). DTPA extractabilityives access to exchangeable fractions and fractions chelated withrganic matter and hydroxides (Lebourg, 1996).

All measurements were carried out in four replicates (excepthere otherwise indicated). The reagents used were of analytical

rade or of ultrapure type, and the glassware and plastic contain-rs were cleaned prior to the tests by soaking them in 5% HNO3v/v) for 12 h, and then rinsing them several times with deion-zed water. For CaCl2 extraction and HNO3 extraction, 10 g of freshediment were shaken for 2 h in 100 mL of either 0.01 M CaCl2 or.1 M HNO3. After centrifugation (10 min at 3500 rpm), the sus-ension was filtered (0.22 �m cellulose acetate filter) and acidifiedith HNO3 (ultrapure grade reagent) to prevent metal precipita-

ion and to inhibit microbial growth before analysis (Capilla et al.,007). For DTPA extraction, 10 g of fresh sediment were shakenor 2 h in 20 mL of solution (0.1 M triéthanolamin TEA, 0.01 MaCl2 and 0.005 M DTPA buffered at pH 7.3) as recommendedy ISO 14870 (AFNOR, 2004). After centrifugation (10 min at000 rpm), the suspension was filtered (0.45 �m cellulose acetate

lter) and acidified with HNO3 (ultrapure grade reagent) beforenalysis.

Depending on detection limits, the Cd contents of the solu-ions were measured using graphite furnace atomic absorptionpectrometry (GF-AAS) or by inductively coupled plasma atomic

Fig. 1. Experimental design and measurements carried o

rimental Botany 67 (2009) 403–414 405

emission spectrometry (ICP-AES). Zn was only measured usingICP-AES. Certified samples, such as Surface Water Level 2 (Spec-trapure standards), were used to check the calibration of the metalstudied.

2.2. Greenhouse experiment

Twenty-four plastic pots (height 40 cm; diameter 40 cm) werefilled with approximately 18 kg DW of sediment and placed in agreenhouse on April 24th 2004. For the 209 days of the study,the night/daylight period of the greenhouse was 10/14 h and thetemperature varied between 17.4 and 31.2 ◦C (Fig. 1). Such temper-atures were of the same order as the optimal growth temperaturesfor a number of plant species (Marschner, 1995). The first set of potsdid not contain plants and are referred to as “control”. All the pots inthe second set contained two willow cuttings and are referred to as“planted” (Fig. 1). Each S. alba cutting was 40-cm long, and plantedwith 20-cm below the sediment. The S. alba cuttings were takenfrom a single tree at the nursery of the “Compagnie Nationale duRhône” (CNR). The 24 pots were placed randomly in the greenhouseand watered three times a week with demineralized water in orderto keep the sediment’s moisture content constant (approximately110% of field capacity). During the 209 days of our study, each potwas given either 400 mm (control) or 620 mm (planted) of water.This is comparable with the average rainfall in France (618.8 mm)

for the periods 1978–1989 and 1997–1998 (Celle et al., 2002). Sed-iment percolates were sampled using vacuum-dialyze bags oncea week. After 70, 112 and 209 days of culture, three “total sam-plings” were carried out by harvesting six pots (three control andthree planted) in order to collect biomass (roots, stems and leaves)

ut for the Salix alba pot experiment over 209 days.

406 J.-P. Bedell et al. / Environmental and Experimental Botany 67 (2009) 403–414

aerial

aa

2

mmstdad0H(t[(1p1twac

iplflwluUA

2

no

Fig. 2. Changes in length of S. alba

nd sediment samples, which were taken from two layers (0–15nd 15–30 cm) within the pots.

.3. Measurements

The maximum length of the aerial part of each plant waseasured once a week (Fig. 1) and the length, biomass andetal content of the different parts of the plants were mea-

ured at each total sampling time. The metal concentrations inhe aerial parts (stems and leaves) and roots of the plants wereetermined as follows. After washing three times with tap waternd rinsing three times with deionised water, the plants wereried for 48 h at 70 ◦C, and then crushed in an agate mortar. A.5–0.1-g sample of dry matter was then mineralized in 8 mLNO3 and 2 mL H2O2 and heated in a CEM type microwave oven

Mars 5). After filtration (average filtration) through ash-free fil-er paper (Prolabo) and adjustment in a 25 mL phial with HNO30.1 M], the metals were analyzed using inductive coupled plasmaICP) and flame atomic absorption (Zeeman effect) (Zhao et al.,994). Sediment samples were also analyzed at each total sam-ling time. The sediments were sampled in two layers: 0–15 and5–30 cm. Samples of each layer were collected from the cen-er of the pots in order to avoid border effects and each sampleas homogenized before sub-sampling. Cd and Zn contents were

ssayed as total concentrations and as chemical selective extractiononcentrations.

Once a week, the vacuum-dialyze bags were collected andmmediately sub-sampled to allow pH, conductivity, and redoxotential (Eh) measurements to be carried out on the perco-

ates. The percolates were also analyzed for Cd and Zn usingame atomic absorption. Dissolved organic carbon (DOC) contentsere measured after oxidation of the organic carbon using alka-

ine persulfate under ultraviolet radiation, and CO2 was measuredsing an infrared gas analyzer (model LABTOC®, PPM, Sevenoaks,K) according to the French standard methodology (NF EN 1484,FNOR, 1997).

.4. Statistical analysis

Statistical analyses were carried out using STATISTICA©. The sig-ificance of our hypotheses concerning the impact of plant growthn Cd and Zn mobility was assessed at 5%. Non-parametric tests

parts at various periods of growth.

(Mann–Whitney) were used when the hypotheses for the paramet-ric test (ANOVA) were not confirmed.

3. Results

3.1. Plant parameters

3.1.1. Plant growth and biomassTwo weeks after the cuttings were planted, buds appeared.

Growth of aerial parts was monitored weekly and maximumlengths (in centimeters) are shown in Fig. 2. Growth rates wereweak but variable during the first 60 days: most plants grewbetween 30 and 50 cm (Fig. 2), although others, such as sampleNo. 2a in Fig. 2, died. Differences in growth rates persisted forthe first 118 days of the study, after which all the plants grew atapproximately the same rate, reaching lengths ranging from 80 to117 cm at harvest (Fig. 2). The willows’ survival rate at 209 dayswas around 50%, which is comparable to the 54% survival rate after3 years’ growth reported by Ledin (1998) for a willow plantationin Sweden. However, the mortality rate in our study was muchhigher than the 7–31% plant mortality obtained by Vandecasteeleet al. (2007) in a 111-day experiment with S. alba cinereaon a submerged sediment-derived soil that underwent gradualterrestrialisation.

Leaf growth was comparable for all the pots (Fig. 2). After 100days, the length of the aerial parts varied from 28 to 66 cm. This issimilar to the 46.2–63.3 cm leaf growth reported by Vandecasteeleet al. (2005) for greenhouse cultures of the S. alba viminalis clone“Aage” after 100 days, but lower than the leaf growth these sameauthors obtained for the S. alba fragilis clone “Belgisch Rood”(52.3–91.3 cm). These differences may be due to the differentbehaviors of the species and clones, or to differences in experimen-tal conditions. They may also have been caused by the phytotoxiceffect of Cu (Mant et al., 2003): substrate Cu concentrations of100 mg kg−1 have been shown to reduce the growth rate of S. albaviminalis by a factor of 2 (Mant et al., 2003). However, Mant et al.(2003) grew their plants under hydroponic conditions, whereas

our experiment used a solid substrate. In addition, although thepseudo-total Cu concentration of our sediment was 178.2 mg kg−1,the Cu content of the percolates collected at 196 days was only23.7 mg kg−1 (data not shown), which is too low to induce copperphytotoxicity. All elements make them thinking that such dif-

J.-P. Bedell et al. / Environmental and Expe

Table 2Biomass production by Salix alba.

Date (in days) Parts of the plant Biomass (in g FW)

70Roots 1.0*Stems 1.6*Leaves 5.3*

112

Roots 2.3a

±1.4Stems 2.7b

±1.7Leaves 5.5c

±4.8

209

Roots 19d

±10.1Stems 29.4e

±172Leaves 44.7f

±21.4

Values followed by the same superscript letter are not significantly different atpSs

fi

talai(fioaeiid

oo(fr

the first (70 days) and last (209 days) total samples. However, the

TE

VS

< 0.05.tandard errors of the means (n = 4) are in italics, except * at 70 days where only oneample could be obtained from the different pots.

erences observed in comparison with other authors cannot benduced by a copper phytotoxicity.

Biomass measurements were made at the three total samplingimes, with separate values being measured for the roots, stemsnd leaves (Table 2). The high standard deviations for stem andeaf production show that these two parameters were very vari-ble (Table 2); nevertheless, we observed significant differencesn biomass production between the 112-day and 209-day samplesTable 2). By the first sampling time (Fig. 2) there had been insuf-cient growth to allow us to collect enough biomass to carry outur tests in triplicate. After 112 days, stem, root and leaf growthppeared to accelerate, with biomass production for these threelements showing, respectively, 11-, 8- and 8-fold increases dur-ng the period between 112 and 209 days (Table 2). These increasesn biomass correlate with the increases in growth length measureduring the same period (Fig. 2).

Our biomass production results were in the same range as thosebtained by Rosselli et al. (2003) for two clones (local and Swedish)

f pot-grown S. alba viminalis measured after 3 and 8 monthswhereas Rosselli et al. measured single plants, our results are totalsor three samples. Therefore, for the comparison, we divided ouresults by three). Rosselli et al. (2003) reported biomass produc-

able 3volution of Zn and Cd concentrations in the different parts of S. alba during plant growth

Date (in days) Elements

(=initial cuttings)

Zn

Cd

70

Zn

Cd

112

Zn

Cd

209

Zn

Cd

alues followed by the same letter-number are not significantly different at p < 0.05.tandard errors of the means (n = 3) are in italics, except * where only a single sample cou

rimental Botany 67 (2009) 403–414 407

tion (per pot) of between 1 mg (minimum, measured in roots) and3 mg (maximum, measured in stems or leaves) at 3 months, and of4 mg (minimum, measured in leaves), 8 mg (measured in roots) and22.5 mg (maximum, measured in stems) at 8 months. Furthermore,Vandecasteele et al. (2005) in a greenhouse study of S. alba viminalisand S. alba fragilis gave root biomass results at 100 days of between0.34 ± 0.2 g DW (minimum) and 1.24 ± 0.27 g DW (maximum). Thisis comparable to the root biomass of 0.32 ± 0.2 g DW we obtained at112 days. Vandecasteele et al. (2007) also obtained similar resultsafter 111 days’ growth (0.1–1.3 g DW) in a greenhouse study of S.alba cinerea.

In the present study, the root/leaf ratio – calculated asroot/leaves + stem using fresh weights (FW) – increased betweenthe first total samples at 70 days (0.14) and the second total samplesat 112 days (0.28) but decreased slightly (0.25) for the 209-day totalsamples. This decrease indicates a nutrient shortage or imbalance,or nutrient toxicity. In response to decreased nutrient availability,plants tend to increase the allocation of assimilates to aerial parts;therefore, they need to exploit a larger volume of soil. A similarstrategy has been reported for phosphorus but not for potassium(Ericsson et al., 1992). A greater root/leaf ratio would probably beadvantageous for applications such as wastewater treatment andphytoextraction, as enhanced root production will increase the sur-face area available for interactions, especially for the formationand/or attachment of biofilms. During a 3-year study of S. albaviminalis cultivated on a sandy loam, Martin and Stephens (2006)reported a root/leaf ratio of 0.6 for plants grown without waterstress and 0.8 for plants grown with water stress. These results arecomparable with the root/leaf ratios obtained in our study (0.62,0.85 and 0.65 after 70, 112 and 209 days, respectively). However,plant root/leaf ratios are affected by a number of factors, includ-ing shortages of nitrogen and phosphorous, which increase theroot/leaf ratio, and nutrition (Ericsson et al., 1996).

3.1.2. Extraction of metals by plantsGreger and Lanbderg’s (1999) study of Cd concentrations in the

stems of four different S. alba clones from three locations in Swedengave values ranging from 0.2 to 1.44 g kg−1 DW. We obtained simi-lar values and noted a small and non-significant increase between

Cd concentrations measured after 70 days were three times greaterthan the concentrations measured in the initial cuttings (Table 3).

There were large variations in Cd and Zn uptake among the 104clones of S. alba analyzed by Greger and Landberg (1999), with

(in mg kg−1 DW).

Roots Stems Leaves

28.6a2

±5.30.36b2

±0.06

579* 133c2 538a3

±19 ±543.55* 1.0d2 0.76b3

±0.14 ±0.07

445a1 208e2 883c3

±21.2 ±58 ±599.8b1 0.86d2 1.21d3,d2

±11 ±0.4 ±0.26

455c1 347f2 1437e3

±57 ±37 ±2493.2d1 1.65g2 2.12f3,d1,g2

±0.6 ±0.2 ±0.4

ld be obtained for all the roots from the different pots.

4 Experimental Botany 67 (2009) 403–414

CvraDaa“ci4atwD(ccac(

odoaZ

odtC7bi1

ttalolgpowCrictf

et

TEg

Table 5Evolution of the sediment pH during plant growth.

Treatment 70 Days 112 Days 209 Days

Control

0–15 cm7.17a1 7.19a2 7.19a1,a3

±0.02 ±0.08 ±0.12

15–30 cm7.01b1 7.08 b2 7.05b1,b2,b3

±0.02 ±0.03 ±0.17

With plants

0–15 cm7.13c1 7.20c2,a2 7.17c1,c2,c3,a3

±0.02 ±0.02 ±0.14

15–30 cm7.05d1 7.12d2 7.06b3,c3,d1,d2

±0.02 ±0.03 ±0.17

08 J.-P. Bedell et al. / Environmental and

d concentrations in the leaves and roots of 104 clones of S. albaiminalis ranging from 0.2 to 8.5 and from 4.1 to 291 mg kg−1 DW,espectively. In the same study, Zn concentrations were between 14nd 1813 g kg−1 DW in the leaves, and between 72 and 2140 g kg−1

W in the roots (Greger and Landberg, 1999). Field studies of S.lba cinerea L. have given leaf Cd concentrations of between 2.5nd 25.9 mg kg−1 DW (Vandecasteele et al., 2005). S. alba viminalisOrm” growing on a dredged-sediment disposal site showed (i) Znoncentrations of 275–674 mg kg−1 in leaves and 173–250 mg kg−1

n roots, and (ii) Cd concentrations of 5.0–8.8 mg kg−1 in leaves and.4–12.2 mg kg−1 in roots (Meers et al., 2005). Samples (the entireerial part of each plant) collected from willow plants growing onhe deposit from which the sediment used in the present studyas taken gave Cd concentrations of between 1.5 and 34.5 mg kg−1

W, and Zn concentrations of between 347 and 1100 mg kg−1 DWCapilla, 2005; Capilla et al., 2006). Our results for Cd and Znoncentrations covered the same range of values but with loweroncentrations in the roots than in the leaves. Both Cd and Znccumulated in the leaves. In S. alba viminalis, above-ground con-entrations of all metals were higher in the leaves than in the barkMeers et al., 2005; Vervaeke et al., 2003).

In the present study, Zn concentrations in the roots were stablever time, but Zn concentrations in the stems increased 4.5-folduring the first 70 days, and 1.5-fold and 1.6-fold during the peri-ds 70–112 days and 112–209 days, respectively (Table 3). Zn alsoccumulated in the leaves, with similar increases (1.6-fold) in then concentrations of the leaves and the stems (Table 3).

The trends in Cd concentrations were very different to the trendsbserved for Zn. Cd concentrations in the roots increased betweenay 70 and day 112 of the experiment (Table 3); however, theyhen fell 3-fold between day 112 and the end of the study (Table 3).d concentrations in the stems increased 2.8-fold during the first0 days, were stable up to day 112, and then increased 1.9-foldetween 112 and 209 days (Table 3). Cd concentrations in the leaves

ncreased 1.6-fold between 70 and 112 days and 1.75-fold between12 and 209 days (Table 3).

Bioconcentration factors (Table 4) were calculated by dividinghe heavy metal concentration in the above-ground plant tissue byhe total heavy metal concentrations in the sediment (Rosselli etl., 2003). At harvest, the resulting bioconcentration factors for theeaves were 2.25 for Zn and 0.73 for Cd (Table 4). During a studyf a local clone of S. alba viminalis Rosselli et al. (2003) reportedeaf bioconcentration factors of 0.95 for Zn and 1.42 for Cd in pot-rown samples but only 0.37 for Zn and 0.83 for Cd in field grownlants. S. alba leaf samples collected from willow plants growingn the deposit from which the sediment used in the present studyas taken gave bioconcentration factors of 0.6 for Zn and 2.3 ford (Capilla, 2005; Capilla et al., 2006). The Cd and Zn contentsecorded in the present study concurred with the results of the fieldnventory carried out by Capilla et al. (2006). However, the biocon-entration factor for Zn was higher in our greenhouse experiment

han it was under field conditions, and the bioconcentration factoror Cd was lower than under field conditions.

The Zn content of plants varies considerably, reflecting differentxternal factors related to the ecosystem in which a plant grows ando its genotype. Plants grown in Zn-contaminated soils accumulate

able 4volution of the bioconcentration factor for the different parts of S. alba during plantrowth.

Date (in days) Roots Stems Leaves

Zn Cd Zn Cd Zn Cd

70 0.90 1.22 0.21 0.34 0.84 0.26112 0.70 3.38 0.33 0.30 1.38 0.42209 0.71 1.10 0.54 0.57 2.25 0.73

Values followed by the same superscript letter-number are not significantly differ-ent at p < 0.05.Standard errors of the means (n = 4) are in italics. Different layers were not comparedat different dates.

a large proportion of the metal in their roots (Kabata-Pendias andPendias, 1992).

Although Cd is considered a nonessential element for metabolicprocesses, it is absorbed by roots and leaves. Nevertheless, sev-eral characteristics of the plant and of the soil, such as soil pH,affect Cd uptake (Kabata-Pendias and Pendias, 1992). Cd in plantsis relatively mobile; however, the translocation of Cd through planttissues may be restricted because Cd is easily stored at active-compound exchange sites, which are mostly located in the cell wallsat pectin sites (Vollenweider et al., 2006).

There is debate about how Zn and Cd ions interact, as bothantagonism and synergism between the two elements in uptake-transport processes have been reported. In addition, interactionswith other metals can occur (with Cu, for example), as can reactionswith other elements, such as Ca and P, which are known to corre-late negatively with Zn in soils (Kabata-Pendias and Pendias, 1992).Nevertheless, our study showed that both metals, but particularlyZn, accumulated over time, and not just in the roots.

The parameters recorded in the present study show that S. albawas able to grow on a contaminated sediment under our experi-mental conditions. Moreover, bioaccumulation of both Zn and Cdtook place, especially in aerial parts, such as the leaves. Theseresults confirm that S. alba has the potential to be used for thephytoextraction of Zn and Cd from a polluted, dredged sediment.

3.2. Sediment parameters

3.2.1. pHThe pH measurements taken during the study are shown in

Table 5. All the samples collected during growth had a lower pHthan the initial sediment (pH 8.2). The pH values obtained after112 days (for the both layers) were different to those measuredafter 70 days but not significantly different to those measured after209 days (Table 5).

The pH of the upper sediment layer (0–15 cm) was always higherthan the pH of the lower layer (15–30 cm). For the control potsthe pH of the upper sediment layer did not change significantlyduring the experimental period. The pH of the lower layer increasedslightly between 70 and 112 days. There was no further changebetween 112 and 209 days. For the planted pots the pH of bothlayers increased significantly between 70 and 112 days but therewas no significant change in pH between 112 and 209 days.

3.2.2. Chemical selective extractions

Initial sediment pseudo-total trace element concentrations

(Table 1) were similar to the concentrations measured by Capillaet al. (2006) at a number of French dredged-sediment disposalsites. Meers et al. (2005) also reported comparable concentrationsfor a moderately contaminated dredged-sediment disposal site, at

J.-P. Bedell et al. / Environmental and Experimental Botany 67 (2009) 403–414 409

F asured 0.05.

wt

3eaec1t7idtCtvaWeeuft

is(CvAofl

ig. 3. CaCl2-extractable and HNO3-extractable zinc and cadmium concentration meates. Histograms with the same letter-number are not significantly different at p <

hich they found intra-site variations in Cd, Zn and Cr concentra-ions of 1.5–3.1, 503–945 and 39–71 mg kg−1, respectively.

.2.2.1. CaCl2 extraction—control pots. After 209 days, CaCl2-xtractable Zn concentrations in the control pots were two-and--half times lower than in the initial sediment (Fig. 3A). Thextractable Zn concentrations measured in the upper layer of theontrol sediment decreased significantly between the 70-day and12-day samples, but the CaCl2-extractable Zn concentration inhe 209-day sample was similar to the concentration measured at0 days (Fig. 3A). A similar pattern was found for the lower sed-

ment layer, with the CaCl2-extractable Zn concentration at 209ays being significantly lower than the concentrations at the otherwo sampling dates. However, the decrease in the concentration ofaCl2-extractable Zn was higher in the upper sediment layer than inhe lower layer (Fig. 3A). Meers et al. (2005) reported an intra-siteariation in CaCl2-extractable Zn concentrations of between 424nd 660 mg kg−1 for pseudo-total Zn contents of 503–945 mg kg−1.e found lower concentrations of CaCl2-extractable Zn than Meers

t al. (2005) but these differences may be due to differences inxperimental method. For example, Meers et al. (2005) used a liq-id/solid ratio of 5/1, whereas we used a ratio of 10/1. As a result, theraction of exchangeable Zn may have been lower in our experimenthan in that of Meers et al. (2005).

During the first 70 days of our experiment a significant decreasen CaCl2-extractable Cd concentrations was recorded for the upperediment layer (to 18 �g kg−1) but not for the lower sediment layerFig. 3B). In the period from 70 days to 112 days, CaCl2-extractabled concentrations in both layers decreased to around half the initial

alue, after which there was no further significant change (Fig. 3B).part from the decrease in the CaCl2-extractable Cd concentrationf the upper layer at 70 days, there were no other significant dif-erences in CaCl2-extractable Cd concentrations between the twoayers. The values of CaCl2-extractable Cd were very low (as were

ments for the two sediment layers of the control and planted pots at three samplingStandard errors of the means (n = 4) are represented by bars on histograms.

the values for Zn), which is in accordance with the below-detectionlimit results obtained by Meers et al. (2005).

3.2.2.2. CaCl2 extraction—planted pots. As in the control pots, theplanted sediment samples showed a two-fold decrease in CaCl2-extractable Zn concentrations at 70 days (Fig. 3A). Although theCaCl2-extractable Zn concentrations in the 112- and 209-day sam-ples of the upper sediment layer were slightly higher than theconcentrations in the control sediments, the values were of thesame order (Fig. 3A). For the lower sediment layer, there were nodifferences between the CaCl2-extractable Zn concentrations of theplanted and control sediments.

Similar results were obtained for both layers of the planted sedi-ment, with the concentrations of CaCl2-extractable Cd after 70 daysbeing significantly lower than the concentrations in the fresh sed-iment. Further decreases were recorded between 70 and 112 days,but concentrations increased between 112 and 209 days. However,the standard deviations of the measurements made at 209 dayswere very large, suggesting that these results should be interpretedwith caution (Fig. 3B).

3.2.2.3. HNO3 extraction—control pots. For both sediment lay-ers, HNO3-extractable Zn concentrations were stable (around115 mg kg−1) between the initial sample and the 70-day sample(Fig. 3C). Significant increases in HNO3-extractable Zn concentra-tions were noted at 112 days, especially in the lower layer, forwhich a concentration of 138 mg kg−1 was measured (Fig. 3C).HNO3-extractable Zn concentrations were significantly lower(70–80 mg kg−1) in the 209-day samples.

We observed a similar evolution in HNO3-extractable Cd con-centrations, with no significant change in concentrations betweeninitial sediment and 70 days samples, followed by an increasebetween 70 and 112 days, especially for the lower sediment layer,and a large decrease between 112 and 209 days (Fig. 3D).

410 J.-P. Bedell et al. / Environmental and Expe

Table 6Evolution of DTPA extraction for the sediment during plant growth.

Samples Zn (mg kg−1 DW) Cd (�g kg−1 DW)

Initial208a1 233a2

±16.30 ±19.77

70 DaysControl

0–15 cm179b1,c1,d1 521.7b2,d2,g2

±14.99 ±30.82

15–30 cm179b1,e1,f1 508.9c2,d2,e2

±6.90 ±29.10

With plants

0–15 cm187c1,g1 519.1f2,g2,h2

±1.84 ±55.69

15–30 cm188.5a1,e1,g1,h1 529.3e2,f2,i2

±11.71 ±79.14

112 DaysControl

0–15 cm161.3d1 538.7j2,b2,k2

±8.8 ±42.14

15–30 cm174.4f1,i1,j1 552.9j2,l2,m2

±4.02 ±14.13

With plants

0–15 cm169.9k1 559.6 h2, k2, n2

±6.62 ±26.71

15–30 cm177h1,i1,k1,l1 544i2,l2,n2

±3.93 ±19.33

209 DaysControl

0–15 cm165.8d1,m1,n1 584.2b2,o2,p2

±8.68 ±70.01

15–30 cm165.8j1,m1,o1 540.2c2,m2,o2,q2

±5.59 ±65.36

With plants

0–15 cm170.7k1,n1,p1 545.3h2,p2

±6.66 ±49.69172h1,l1,o1,p1 537.8i2,q2

Ve

3aviiaw(

ts

3oobwscai

lite

15–30 cm ±13.45 ±52.16

alues followed by the same superscript letter-number are not significantly differ-nt at p < 0.05. Standard errors of the means (n = 4) are in italics.

.2.2.4. HNO3 extraction—planted pots. In the planted pots, the vari-tions in HNO3-extractable Zn concentrations were similar to theariations recorded for the control pots, with no significant changen Zn values during the first 70 days, followed by a significantncrease in HNO3-extractable Zn concentrations at 112 days andlarge decrease in Zn concentration at 209 days. The same patternas observed for both the upper and the lower sediment layers

Fig. 3C).This pattern was repeated for HNO3-extractable Cd concentra-

ions, with the concentrations measured in the planted pots beingimilar to those measured in the control pots (Fig. 3D).

.2.2.5. DTPA. DTPA-extracted Zn concentrations (noted Zn-DTPA)f the 70-, 112- and 209-day samples were lower than the Zn-DTPAf the initial sediment for both the planted and control pots and foroth the upper and lower sediment layers. The only sample forhich this decrease was not statistically significant was the lower

ediment layer of the planted pots (Table 6). The DTPA-extracted Cdoncentrations (Cd-DTPA) of all the samples from both the plantednd control pots were significantly higher than the Cd-DTPA of thenitial sediment (Table 6).

Apart from a decrease in the Zn-DTPA of the upper sedimentayer in the planted pots between 70 and 112 days, and an increasen Cd-DTPA in the lower sediment layer of the control pots duringhe same period (Table 6), we did not find any significant differ-nces in Zn-DTPA or Cd-DTPA for either of the two sediment layers

rimental Botany 67 (2009) 403–414

at any time during the experiment. In their study of a moderatelycontaminated, dredged-sediment disposal site, Meers et al. (2005)recorded an intra-site variation of 62–131 mg kg−1 for Zn extractedby DTPA. This range is lower than our Zn-DTPA measurements(between 161 and 208 mg kg−1). In addition, the Cd-DTPA values(1–1.7 mg kg−1) reported by Meers et al. (2005) were higher thanthe values obtained in our experiment (around 0.5 mg kg−1).

The present experiment did not show any significant differencein Zn-DTPA or Cd-DTPA between the planted and control pots,except in the 112-day sample of the upper sediment layer of theplanted pots, which showed an increase in Zn (Table 6). Theseresults are consistent with those obtained by Hammer and Keller(2002), who recorded a decrease in Cd-DTPA and an increase in Zn-DTPA in the rhizosphere of S. alba viminalis grown for 90 days inan acid soil, but no variation in plants grown in a calcareous soil(compared with control pots). The sediment used in our experi-ment was more similar to the calcareous soil used by Hammer andKeller (2002) than to the acid soil.

After 112 days, Zn-DTPA in the control pots was higher in thelower sediment layer than in the upper layer. We did not detectany other differences in Zn-DTPA between the two sediment layersat any of the other sampling times (Table 6). Such an increase withdepth was not observed for Cd-DTPA, but a decrease in Cd-DTPAwas detected in the 209-day sample of the lower sediment layerfrom the planted pots (Table 6).

To summarize, the sediment parameters and the pool of CaCl2-and HNO3-extractable metals seemed to change over time inthe same way both for Zn and Cd. This was not the case forthe DTPA-extractable Zn and Cd fractions. It should be notedthat the pattern of variations in the CaCl2-extractable Zn andCd fractions was the mirror image of the pattern for the HNO3-extractable Zn and Cd fractions. In the control pots the biggestchanges were recorded for the upper sediment layer, whereas inthe planted pots there were differences in both layers, most notablya decrease in CaCl2-extractable Zn in the upper sediment layer,and an increase in HNO3-extractable Zn in the lower sedimentlayer.

3.3. Percolates

3.3.1. Changes in physico-chemical characteristicsOur measurements did not reveal any significant changes in con-

ductivity, pH or Eh over time for either the planted or the controlpots (Fig. 4).

During the first 15 days, conductivity rose from around1.95–2.5 mS cm−1, a value that was maintained until day 105. Afterday 105, the conductivity gradually decreased to reach a value of1.7 mS cm−1 at the end of the experiment (Fig. 4A).

The initial pH of the percolates was 6.9. During the first monththis value increased to 7.4, a value that was maintained until day105. The pH then decreased to 7.1, although the measurementstaken between days 105 and 209 showed much greater variabil-ity than the measurements taken during the first 105 days. Theone significant difference in the pH values for the control andplanted pots occurred between 126 and 131 days, when the pHof the planted pots rose to 7.63, whereas the pH of the controlpots remained around 7.15. High pH values were also measuredfor planted pots between 56 and 63 days; however, the dif-ferences between these values were not statistically significantbecause of the large standard deviations of the measurements(Fig. 4B).

Initial Eh was 380 mV for both the planted and control pots(Fig. 4C). This parameter then decreased to 280 mV at 98 days,before increasing to 340 mV at 147 days. Eh decreased once morebetween 147 and 201 days, reaching a value of 310 mV. Duringthe last 8 days of the experiment the Eh values for the planted

J.-P. Bedell et al. / Environmental and Experimental Botany 67 (2009) 403–414 411

F e percp

apTtdEmavar

pspapcfiddasteciiatttf

ig. 4. Changes in conductivity (A), pH (B), Eh (C) and K+ concentration (D) of thercolates’ pots available at the date) are represented by bars on graphs.

nd control pots started to diverge, with the Eh of the plantedots increasing and the Eh values for the control pots decreasing.here were also significant differences between the Eh values forhe planted and control pots for the samples taken at 98 and 101ays, between 112 and 138 days and on the day of harvest (Fig. 4C).h values between 450 and 550 mV indicate an oxidizing environ-ent in which denitrification can occur; Eh values of 330 mV or less

re typical of environments without free O2 (Marschner, 1995). Thealues obtained during our experiment were all around 330 mVnd therefore typical of saturation conditions without oxidationeactions.

Nitrites, ammonium and phosphates were not detected in theercolates (data not shown). Any significant differences in theodium, chlorate, sulfate, nitrate and calcium contents of thelanted and control pots were masked by the high standard devi-tions of the data, which were due to the heterogeneity of theercolates (data not shown). The potassium content of the per-olates at the beginning of the experiment was 18–20 mg. Thisgure increased slightly to around 22–25 mg at 70 days and thenecreased rapidly between 70 and 105 days. Between 105 and 119ays, the potassium content of the percolates from both the plantednd the control pots increased rapidly. After 119 days, the potas-ium content for the planted pots decreased much more quicklyhan the potassium content for the control pots: at the end of thexperiment the potassium content for the planted pots was 10 mg,ompared with 19 mg for the control pots (Fig. 4D). The decreasesn potassium content correlate with the reduction in conductiv-ty recorded for the same period and with the decreases in the

nion concentrations (e.g., chlorate ions) of the percolates duringhe experiment (data not shown). The changes in the K+ content ofhe percolates from the planted pots can be related to the adsorp-ion of this nutrient by the plants. Such variations were also foundor nitrates, especially for the planted sediments (data not shown).

olates from the control and planted pots. Standard errors of the measures (of all

3.3.2. Changes in DOC, Zn and Cd concentrations of percolatesThe cumulative DOC, Zn and Cd concentrations of the perco-

lates increased regularly. The variability between individual potswas high for both the planted and the control sediment (Fig. 5).These huge standard deviations render significant differences (ifany) as statistically invalid. Nevertheless, we noted that mean DOCconcentrations were higher in the percolates from the planted potsthan in those from the control pots. This can mostly be attributedto the release of exudates by the roots. With the same limits due tothe high standard deviation, we noted that the lower mean Zn andCd concentrations were in the percolates from the planted pots.May be these lower concentrations of Cd and Zn could be due tothe absorption of metals by the plants (Fig. 5).

4. Discussion and conclusions

4.1. Changes in the sediment with time

A comparison between the characteristics of the initial sedimentand the sediment samples taken from the control pots at differ-ent dates shows the ageing of the sediment. Ageing did not affectthe pH of the sediment. CaCl2-extractable Cd and Zn concentra-tions decreased over time, whereas HNO3-extractable Cd and Znconcentrations increased. Zn-DTPA contents decreased while Cd-DTPA contents increased. Thus, the exchangeable fractions seemedto decrease while the more stable fractions increased. The observeddecrease in the exchangeable fractions could be related to theexport of Zn and Cd via the percolates.

This export is not the only cause for the decrease in the pool ofCaCl2-extractable components, as 2 mg of Zn was exported via thepercolates (the decrease in CaCl2-extractable Zn was 3.5 mg kg−1,which corresponds to around 60 mg per pot). A fraction of the metalinitially extracted by CaCl2 (considered as exchangeable) became

412 J.-P. Bedell et al. / Environmental and Experimental Botany 67 (2009) 403–414

F C) cont n gra

lH

iuo

4i

aeoita

bwlIMooes

cai

ig. 5. Changes in the cumulative total organic carbon (A), zinc (B) and cadmium (he measures (of all percolates’ pots available at the date) are represented by bars o

ess available, which partly explains the increase in the pool ofNO3-extractable components.

The evolution of the different fractions may be due to a decreasen the exchangeable fraction and a subsequent increase in the resid-al fraction but without affecting the fraction of metal linked to therganic matter.

.2. Effect of S. alba on the status of metals in the percolates andn the sediment

The only characteristic of the percolates that was significantlyffected by the presence of S. alba was the K+ content. The pres-nce of S. alba led to an increase in sediment pH. Few effects werebserved on the extractability of Zn and Cd. In the planted pots, anncrease in the concentration of HNO3-extractable Zn was noted forhe lower sediment layer and decreases in the CaCl2-extractable Znnd Zn-DTPA fractions were noted for the upper sediment layer.

After 112 days, approximately 2.8 mg of Zn had been taken upy the plants and the Zn-CaCl2 fraction in the upper sediment layer,hich contained the plants’ roots, decreased by 2.6 mg. It is most

ikely that S. alba takes up Zn from the CaCl2-extractable fractions.n a study of the phytoextraction potential of five willow species,

eers et al. (2007) reported topsoil (25 cm) Zn and Cd reductionsf 1.4–8 mg kg−1 and 0.07–0.20 mg kg−1, respectively, dependingn soil type and level of pollution. The topsoil layer used by Meerst al. (2007) was comparable to the sediment layers in the present

tudy.

Growing corn, colza and ray fatty on sediment under greenhouseonditions causes an acidification of the scrubbing solution, as wells rises in the Zn, Cd, Cu, Pb, Mn and Fe contents and in the DOCsn solution (Marseille et al., 2000b). Such results can be compared

centrations of the percolates from the control and planted pots. Standard errors ofphs.

to the effect of S. alba on metal mobility and/or on the changes ofpercolate parameters.

A plant’s response to heavy metals depends on a large num-ber of factors, including the metal, the plant species and even theplant’s growth stage. Moreover, the solubility of heavy metals insoils, and thus their phytoavailability, is also affected by soil pH,type of mineral colloids and many other factors, such as microbialactivity, redox potential, and aeration (Marschner, 1995). Heavymetals can also affect a plant’s root development (in our study,via growth of S. alba), which can in turn modify the fungi popu-lations (and/or bacterial populations), for example Penicillium orAspergillus spp. in a soil, which are known to affect the bioleachingof Zn or Al (Vachon et al., 1994; White et al., 1997).

S. alba has been shown to accumulate Zn (Kayser et al., 2000;Rosselli et al., 2003) and Cd (Rosselli et al., 2003) and has beenused in phytoextraction experiments (Landberg and Greger, 1996).However, in some of these studies Zn did not reach sufficient con-centrations for the plants to be considered accumulators (Rosselliet al., 2003). This was also the case in our experiment. The mainadvantages of S. alba are its large biomass production, its exten-sive root system, which tends to be deeper and more developedthan the root systems of herbaceous plants, and the fact that itcan easily be grown from cuttings. Current research is focusing onthe possibility of using S. alba in association with chelating agents(Meers et al., 2007; Vandecasteele et al., 2007) or in association withother plants. Other advantages of S. alba include its propagation and

growth characteristics and its rhizosphere effect. This effect, whichis enhanced by microbial or mycorrhizal activities, increases theaccumulation and extraction of metals by plants such as S. alba.

In conclusion, growing S. alba on dredged sediment may affectthe mobility to the plant of Zn and, to a lesser extent, Cd. S. alba can

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J.-P. Bedell et al. / Environmental and

ave a real impact on the available Zn and Cd fractions, withoutecessarily enhancing their accumulation/transfer in the plant. Inarallel with the effect of plant growth on the status of metals,ediment ageing can affect the mobility of Zn and Cd.

Studies are underway to investigate the availability of differentrace elements in dredged sediments in order to more accuratelystimate their bioavailable compartments. This should provide aetter understanding of the role of metal mobility on plant growthnd on the role of plants in modifying metal mobility. In addition, itould be interesting to study the effect of S. alba on the microfloraresent in sediments, in order to evaluate the impact of root devel-pment on the diversity of bacterial populations, and the effects ofhese bacterial populations on metal bioavailability, the transfer of

etals to plants and the biogeochemical cycles of nutrients.

cknowledgements

The authors would like to thank Robert Moretto (EEDEMS) foris help in setting up the greenhouse experiment, Frank PressiatCNR) for providing cuttings of S. alba from the CNR nursery, and

artine Ghidini-Fatus and Marc Danjean (Laboratoire des Sciencese l’Environnement-ENTPE) for their technical assistance.

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