POTENTIALLY HARMFUL ELEMENTS IN SOIL-PLANT INTERACTIONS

The transfer of heavy metals to barley plants from soilsamended with sewage sludge with different heavymetal burdens

José Martín Soriano-Disla & Ignacio Gómez &

José Navarro-Pedreño & Manuel M. Jordán

Received: 23 May 2013 /Accepted: 14 August 2013 /Published online: 27 August 2013# Springer-Verlag Berlin Heidelberg 2013

AbstractPurpose Our main aim objective was to evaluate the transferof Cd, Cr, Cu, Ni, Pb and Zn to barley (Hordeum vulgare )grown in various soils previously amended with two sewagesludges containing different concentrations of heavy metals.This allowed us to examine the transfer of heavv metals tobarley roots and shoots and the occurrence of restrictionmechanisms as function of soil type and for different heavymetal concentration scenarios.Material and methods A greenhouse experiment wasperformed to evaluate the transfer of heavy metals to barleygrown in 36 agricultural soils from different parts of Spainpreviously amended with a single dose (equivalent to 50 t dryweight ha−1) of two sewage sludges with contrasting levels ofheavy metals (common and spiked sludge: CS and SS).Results and discussion In soils amended with CS, heavymetals were transferred to roots in the order (mean values ofthe bio-concentration ratio in roots, BCFRoots, in brackets): Cu(2.4)~Ni (2.3)>Cd (2.1)>Zn (1.8)>Cr (0.7)~Pb (0.6); simi-lar values were found for the soils amended with SS. Themean values of the soil-to-shoot ratio were: Cd (0.44)~Zn(0.39)~Cu (0.39)>Cr (0.20)>Ni (0.09)>Pb (0.01) for CS-amended soils; Zn (0.24)>Cu (0.15)~Cd (0.14)>Ni (0.05)~Cr (0.03)>Pb (0.006) for SS-amended soils. Heavy metalswere transferred from roots to shoots in the following order(mean values of the ratio concentration of heavy metals in

shoots to roots in brackets): Cr (0.33)>Zn (0.24)~Cd (0.22)>Cu (0.19)>Ni (0.04)>Pb (0.02) for CS-amended soils; Zn(0.14)>Cd (0.09)~Cu (0.08)>Cr (0.05)>Ni (0.02)~Pb(0.010) for SS-amended soils.Conclusions Soils weakly restricted the mobility of heavymetals to roots, plant physiology restricted the transfer ofheavy metals from roots to shoots, observing further restric-tion at high heavy metal loadings, and the transfer of Cd, Cuand Zn from soils to shoots was greater than for Cr, Ni and Pb.Stepwise multiple linear regressions revealed that soils withhigh sand content allowed greater soil-plant transfer of Cr, Cu,Pb and Zn. For Cd and Ni, soils with low pH and soil organicC, respectively, posed the highest risk.

Keywords Bio-concentration factor . Heavymetals . Roots .

Sewage sludge . Shoots . Soil properties

1 Introduction

The production of sewage sludge in Europe has increaseddramatically in the last decades. Because of economic re-straints and environmental concerns about land-filling andincineration, interest in land application of sewage sludgecontinues to grow (O’Connor et al. 2005). The land applica-tion of sewage sludge, which are produced locally and areavailable at a relative low cost, enhances the recycling ofnutrients and organic matter to soil and can contribute to thereduction of the atmospheric concentration of CO2, increasingthe levels of soil organic carbon (SOC) and associated fertility(Ajwa and Tabatabai 1994; Bernal et al. 1998). This practice isof particular relevance in soils from arid and semi-arid re-gions, where soil erosion and decline of organic matter are

Responsible editor: Jaume Bech

J. M. Soriano-Disla : I. Gómez : J. Navarro-Pedreño :M. M. Jordán (*)Department of Agrochemistry and Environment, University MiguelHernández, Avenida de la Universidad S/N, 03202 Elche, Spaine-mail: manuel.jordan@umh.es

J Soils Sediments (2014) 14:687–696DOI 10.1007/s11368-013-0773-4

major threats (Albaladejo et al. 2013). However, concernshave been raised regarding the presence of heavy metals fromdomestic, light industrial, commercial and urban runoffsources which can result in soil andwater contamination, plantphytotoxicity and enter the food chain via accumulation inplants (Barceló and Poschenrieder 1990; Qian et al. 1996;McBride 2003). The transfer of essential and nonessentialheavy metals to plants, including Cd, Cr, Cu, Ni, Pb and Zn,poses health risks to human and animals (McLaughlin et al.1999; Peralta-Videa et al. 2009; Ali et al. 2013).

Plant uptake is one of the major pathways bywhich sludge-borne potentially toxic heavy metals enter the food chain(Chaney 1990). The evaluation of the amount of heavy metalstransferred to plants is of crucial importance for risk assess-ment and environmental regulation. This transfer greatly dif-fers as function of the metal (Navarro-Pedreño et al. 1997;McLaughlin et al. 2000; McBride 2003). Roots are in directcontact with the soil solution, and the concentration of heavymetals in roots is generally used as indicative of soil metalbioavailability (Chaignon et al. 2003). The transfer of heavymetals from soils to roots is a complex process influenced by amyriad of factors including soil properties, environmentalconditions, plant physiology and rhizosphere biochemistry(Basta et al. 2005). Among soil properties, pH, organic matter,amorphous Fe and Mn oxides, calcium carbonate content,mineralogy and texture have been found to control heavymetal uptake (McBride 1994; Kabata-Pendias 2004). Thetransfer of heavy metals from roots to shoots is controlled byplant physiology (Kalis et al. 2008) and greatly determineswhich fraction might potentially enter the food chain.

Research on the transfer of heavy metals from soils to plantsfollowing sewage sludge amendment has been focused onevaluating the ability of surrogates methods (mainly chemicalextractants) to assess heavy metal bioavailability in soils(McBride et al. 2003; Soriano-Disla et al. 2010), the responseof different plants (Sauerbeck 1991; Natal-Da-Luz et al. 2009),different sewage sludge rates and availability of heavy metals(Bose and Bhattacharyya 2008; Antoniadis et al. 2010; Li et al.2012) and phytostabilisation mechanisms (Santibáñez et al.2008). However, a particular metal can behave entirely differ-ently in different soils (McBride 2003). The evaluation of thetransfer of several heavy metals occurring together (e.g. insewage sludge) to roots, and from roots to shoots, from varioussoils amended with sewage sludge has received, comparably,little attention. These studies have commonly involved a re-duced set of sludged soils, under a range of conditions oftenrestricted to pH values in the neutral to acidic range (Sauerbeck1991; Hooda et al. 1997). As discussed by Hamon et al. (1999)and Basta et al. (2005), the study of the factors (soil or plant)that are primarily responsible for limiting metal uptake at highsoil heavy metal concentrations still needs further research.

Our main aim objective was to evaluate the transfer of Cd,Cr, Cu, Ni, Pb and Zn to barley (Hordeum vulgare ) grown in

various soils previously amended with two sewage sludgescontaining different concentrations of heavy metals. The useof contrasting soils and sewage sludge with common and highconcentrations of heavy metals allowed us to examine thetransfer of heavy metals to barley roots and shoots and theoccurrence of restriction mechanisms as function of soil typeand for different heavy metal concentration scenarios.

2 Material and methods

2.1 Soil samples

Thirty-six agricultural soils across Spain were chosen to covera wide range of properties. The soils, sampled from theploughed layer (0–30 cm), were air-dried, homogenised andsieved (<2 mm). Some soil characteristics are shown inTable 1: pH (1:2.5w /v, distilled water), equivalent calciumcarbonate (CO3

2−) determined by using the Bernard calcimeter(Hulseman 1966), SOC determined by potassium dichromateoxidation (Nelson and Sommers 1982), texture determined bythe Bouyoucos method (Gee and Bauder 1986), amorphous FeandMn extracted by an ammonium oxalate/oxalic acid extrac-tion (Houba et al. 1989) and determined by atomic absorptionspectrophotometry (Unicam 969, UK) and total heavy metalsdetermined after microwave acid digestion using HNO3/H2O2

at a ratio of 4:1 (v /v) (Moral et al. 1996).

2.2 Sewage sludge

The sludges used in the experiment (Table 2) were obtainedfrom a wastewater treatment plant in Alicante (southeasternSpain; common sludge, CS) and after spiking the CS withheavy metal salts (spiked sludge, PS). The spiking experimentwas performed following the protocol proposed by Kandpalet al. (2004), and it is fully described in Soriano-Disla et al.(2011). Sewage sludge characterisation was performed fol-lowing the same methods than the previously described forsoils. In addition, Kjeldahl Nitrogen (Bremner and Mulvaney1982) and total P, after microwave acid extraction according toMoral et al. (1996), were determined.

2.3 Greenhouse experiment

The experiment was carried out in a greenhouse [mean (±stan-dard deviation) temperature and relative humidity of 22±5 °Cand 70±10%, respectively]. Two replicates of 675 g of eachsoil, previously air-dried and passed through a 2-mm sieve,weremixedwith the sewage sludges (CS and SS) at a single doseof 15.71 g dry weight kg−1 (equivalent to 50 t dry weight ha−1,assuming 30 cm of ploughed layer). Plastic pots (11 cm length,11.5 cm internal diameter) were filled with the mixtures and keptat approximately 60%water holding capacity (Foster 1995) with

688 J Soils Sediments (2014) 14:687–696

distilledwater. Thesemixtureswere incubated for 4weeks beforeseeding to promote the decrease of phytotoxic substances such asNH4

+ (Pascual et al. 2004). Barley seeds (H. vulgare) werethoroughly rinsed with deionised water and germinated in thedark on moist filter paper for 24 h at 20 °C. Thirty-five barleyseeds were sown per pot and thinned to 20 plants after 2 weeks.The plants were harvested 8 weeks after sowing and wereseparated into shoots and roots. Roots were thoroughly rinsedwith tap and deionisedwater, then the roots were dried in an oven

at 65 °C for 2 days to constant weight for the determination ofmetal concentration (after microwave acid digestion). After fil-tering through a 90-mm quantitative filter paper (Filter-Lab,Spain), the metal concentration in the digests was determinedby inductively coupled plasma-optical emission spectroscopy(Perkin Elmer 4300 DV, USA). Standard reference soil andsewage sludge (Community Bureau of Reference, BCR, 141Rand 145R) were used for quality control in the quantification ofheavy metals.

Table 1 Selected variables of the agricultural soils

Soil pH CO32−

(%)SOC(g kg−1)

Clay(%)

Sand(%)

AmFe(mg kg−1)

AmMn(mg kg−1)

Cda Cr Pb Cu Zn Ni

1 8.6 24.1 5.3 36 49 277 164 0.12 28.0 7.1 13.7 26.5 16.6

2 8.4 36.2 13.0 40 23 855 131 0.26 40.3 18.9 24.2 44.4 18.4

3 8.6 6.0 3.2 16 77 222 37 0.10 18.7 9.2 6.7 23.1 9.5

4 8.9 11.5 2.7 7 91 111 11 0.10 5.0 2.3 1.4 4.3 4.6

5 8.5 35.5 9.1 16 69 244 53 0.14 20.0 8.5 4.2 15.1 8.5

6 8.6 44.2 7.6 21 51 174 30 0.20 33.1 9.0 11.0 27.0 14.0

7 8.3 19.2 5.7 20 41 248 203 0.13 24.5 9.7 7.7 32.5 10.8

8 5.9 <0.1 4.0 18 72 203 40 0.09 12.6 5.4 5.2 18.3 5.9

9 5.6 <0.1 3.8 18 54 276 46 0.05 14.5 7.0 4.2 17.5 6.3

10 5.5 <0.1 2.9 10 75 289 204 0.07 7.7 15.9 3.8 17.4 4.6

11 5.7 <0.1 2.2 6 85 200 125 0.06 6.6 5.0 1.9 11.1 4.1

12 5.8 <0.1 6.5 17 61 610 335 0.06 29.9 12.8 3.4 12.5 10.7

13 6.0 <0.1 7.0 13 59 612 331 0.06 26.4 13.2 2.9 15.3 11.6

14 8.8 19.1 3.7 14 79 180 24 0.10 14.6 6.5 4.3 12.3 8.1

15 8.6 16.5 5.6 10 81 223 20 0.10 13.6 6.8 5.1 9.2 6.2

16 7.7 9.3 9.1 4 41 1024 77 0.03 10.7 6.0 15.5 20.7 23.0

17 8.5 51.8 14.9 26 25 278 33 0.23 10.8 90.9 9.5 32.9 20.0

18 8.7 20.1 6.8 16 73 172 17 0.10 7.6 7.4 6.1 13.0 14.3

19 9.0 17.1 2.7 13 80 102 14 0.06 2.6 2.4 3.6 8.5 12.5

20 8.7 17.0 6.3 10 77 159 19 0.07 4.1 3.2 3.7 12.0 15.6

21 9.1 18.0 4.4 12 82 152 13 0.11 3.2 3.0 9.1 9.6 13.1

22 9.1 13.4 3.9 9 83 153 16 0.08 6.0 4.1 5.5 24.3 12.0

23 8.5 35.8 7.4 17 60 288 34 0.12 8.6 8.2 5.5 22.6 16.1

24 8.7 40.4 9.0 20 55 302 38 0.11 17.3 9.2 7.8 22.9 18.5

25 8.7 78.7 9.9 18 59 71 7 0.10 9.4 2.4 4.1 12.2 20.6

26 8.4 59.7 18.6 13 65 105 15 0.12 8.0 6.7 45.0 14.7 14.4

27 8.5 24.9 20.0 18 47 292 85 0.12 16.5 13.4 8.2 36.4 22.0

28 8.6 29.0 17.1 16 56 369 63 0.14 13.8 13.8 8.4 30.3 23.8

29 8.3 22.9 18.0 11 73 308 44 0.14 9.2 8.3 6.8 25.2 19.9

30 8.7 47.7 8.5 13 71 260 36 0.11 6.9 6.4 7.1 18.7 19.4

31 8.8 30.5 6.4 17 64 185 36 0.10 7.9 7.8 9.4 22.1 21.0

32 8.8 54.1 8.7 35 28 145 23 0.13 10.8 8.6 12.7 32.7 25.3

33 8.9 52.4 11.8 22 30 142 16 0.20 11.4 8.9 14.6 39.0 27.7

34 8.8 30.5 13.6 14 66 195 22 0.13 10.1 9.4 6.1 21.0 20.9

35 8.1 16.0 5.2 <1 40 181 44 0.03 14.7 3.4 9.5 24.3 22.5

36 8.6 21.3 8.5 24 52 369 134 0.08 19.0 16.0 12.2 43.8 26.6

CO32− equivalent calcium carbonate, SOC soil organic carbon, AmFe and Mn amorphous Fe and Mn

aMetal concentrations as determined after acid digestion (milligrams per kilogram)

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2.4 Statistical analysis

Three different ratios are calculated in this study (Table 4); thebio-concentration factor in roots and shoots (BCFRoots andBCFShoots; Sauerbeck 1991): the concentration of heavy metalsin roots or shoot to the total concentration of heavymetals in thesludged soils, estimated from the total concentration of heavymetals in soils and sewage sludge mixed in the experimentalpots; the concentration of heavy metals in roots and shoots ofsoils amended with SS to soils amended with CS (SS/CS); the

transfer factor (TF): the concentration of heavy metal in shootsto the concentration in roots for CS- and PS-amended soils(Marchiol et al. 2004).

Pearson correlations, expressed in terms of the coefficient ofcorrelation (r), and stepwise multiple linear regression (SMLR),were performed to relate theBCFRoots andBCFShoots in both soilsamendedwith CS and SS to soil properties (Table 5). The SMLRequations, accompanied by the coefficient of determination (R2),followed the general model y=a+∑bnxn where y is the value ofBCF, a and bn are regression intercept and coefficients, respec-tively, and xn represents the soil properties used in the model.Prior to the development of the bivariate and multivariate anal-ysis, the distributional values of soil properties and BCFRoots andBCFShoots were examined in order to detect asymmetry anddepartures from normality. Parameter values were root or logtransformed when required to fulfil the conditions required bythe statistical analyses. Statistical analyses were performed usingthe softwares SPSS v.21.0. and Microsoft Excel 2007.

3 Results and discussion

3.1 Heavy metals in barley roots and shoots and as functionof the initial heavy metal input

The mean concentrations, median, ranges and first and thirdquartile of heavy metals analysed in roots and shoots, and forboth soils amended with CS and SS, are found in Table 3. The

Table 2 Selected properties of the sewage sludges

Properties Common Spiked

pH 7.8 7.6

EC, ms cm−1 4.9 7.3

Organic C, g kg−1 234 250

Kjeldahl N, g kg−1 58 57

Total P, g kg−1 22 23

Cda, mg kg−1 0.8 39.9

Cr, mg kg−1 36 1,990

Cu, mg kg−1 207.0 1,710

Ni, mg kg−1 14 370

Pb, mg kg−1 39 1,086

Zn, mg kg−1 646 4,020

EC electrical conductivityaMetal concentration as determined after acid digestion

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Table 3 Descriptive statistics of the concentration of heavy metals in roots and shoots of plants grown in soils amended with common (CS) and spikedsludge (SS)

Cd Cr Cu Ni Pb Zn

Roots

CS Mean±SD 0.23±0.07 7.2±2.6 25.4±11.1 26.5±4.3 5.50±5.08 57.4±15.8

1st–3rd quartile 0.19–0.27 5.2–8.7 17.6–30.9 24.0–26.9 3.60–6.10 48.2–63.2

Median 0.24 6.8 20.8 25.4 4.81 51.8

Range 0.13–0.41 3.1–13.8 11.8–56.4 21.9–44.6 1.37–32.34 39.7–123.9

SS Mean±SD 1.78±0.74 40.9±26.0 112.2±72.4 51.9±22.1 27.44±17.37 196.3±81.9

1st–3rd quartile 1.27–2.08 24.6–52.2 61.9–142.5 38.3–53.0 18.57–33.91 139.7–247.2

Median 1.56 33.8 90.1 45.1 22.61 167.4

Range 0.93–4.64 11.7–144.5 27.0–363.6 32.9–149.7 4.50–84.82 79.0–490.6

Shoots

CS Mean±SD 0.05±0.02 2.1±0.7 4.2±0.6 1.1±0.3 0.09±0.09 13.4±5.0

1st–3rd quartile 0.03–0.06 1.6–2.5 3.8–4.6 0.8–1.3 0.05–0.09 10.2–14.4

Median 0.05 2.1 4.2 1.1 0.07 12.5

Range 0.02–0.09 1.0–4.2 2.7–5.6 0.5–1.9 b.d.l.–0.42 6.2–28.3

SS Mean±SD 0.14±0.04 1.6±0.5 6.8±1.3 1.1±0.4 0.21±0.14 23.1±7.0

1st–3rd quartile 0.11–0.16 1.2–1.9 6.0–7.5 0.8–1.3 0.12–0.25 18.3–27.0

Median 0.14 1.5 6.6 1 0.17 23.4

Range 0.08–0.27 0.7–3.1 4.1–10.4 0.5–2.2 0.05–0.73 10.9–40.7

SD standard deviation, b.d.l. below the detection limit of the instrument (0.001 mg l−1 )

concentrations of heavy metals in plants varied widely amongmetal, soil, sewage sludge used and part of the plant. Theconcentrations of heavy metals in roots for both soils amendedwith CS and SS followed the order Zn>Cu>Ni>Cr>Pb>Cd.Compared to roots, the concentrations of heavy metals inshoots were lower for both soils amended with SS and CS(Table 3). Metal accumulation in barley shoots followed theorder Zn>Cu>Cr>Ni>Pb>Cd in both set of soils amendedwith CS and SS. The order in which the concentrations ofheavy metals were found in roots and shoots was influencedby the concentrations of heavy metals in the initial mixtures ofsewage sludge and soil.

The SS was designed deliberately to create a “worst-case”scenario (McLaren et al. 2004; Speir et al. 2007) in whichheavy metals in the sludge were, at, or near the concentrationsthat the Directive 86/278/EEC (1986) has set for sewagesludge applications on soils. The spiking of wastes with heavymetals allows high concentrations to be achieved in soils withjust one sewage sludge application. The higher concentrationsof heavy metals observed in roots and shoots in the soilsamended with SS compared to those amended with CS(Table 3) were consistent with the higher concentrations ofheavy metals in the SS. However, the magnitude of the dif-ferences varied across the heavy metals and part of the plant.These differences are better expressed by the ratio of theconcentrations of heavy metals in SS to CS for roots and

shoots (SS/CS, Table 4). According to this ratio, heavy metalsin roots and shoots were sorted as follows:

1. Roots: Cd>Cr>Pb>Cu>Zn>Ni2. Shoots: Cd>Pb>Zn~Cu>Ni>Cr

Differences in heavy metal concentrations found in theroots of plants grown in soils amended with SS comparingto plants grown in soils amended with CS ranged from two-fold, for Ni, up to eightfold for Cd. These results were inagreement with the different amount of heavy metals initiallyavailable in soils amended with SS and CS. This fact indicatesthat soils were not very effective in restricting the mobility ofheavy metals to roots in soils amended with SS.

In order to get an assessment of potential exposure of heavymetals to humans, differences observed in shoots represent abetter estimation than those observed in roots. The SS/CS ratiofor Cr, Cu, Ni and Zn was below 1.8 (Table 4) in accordancewith the similar concentrations observed for these heavy metalsin the shoots of plants grown in SS and CS (Table 3).Conversely, higher SS/CS ratios were observed in shoots forCd and Pb (SS/CS>3, Table 4), highly conditioned by the weakconcentrations of these heavy metals found in the shoots ofplants grown in CS. These results, as compared to thoseobtained in roots, suggest the existence of a defencemechanismin the plant which limits the transmission of heavy metalsthrough the food chain by their accumulation in roots, the so-

Table 4 Mean±standard deviation (and range) for the ratios of theconcentration of heavy metals in roots and shoots of soils amended withspiked sludge (SS) to soils amended with common sludge (CS), ratios ofthe concentration of heavy metals in soils to their concentration in roots

and shoots (Bio-concentration Factor, BCF) for both soils amended withCS and SS, and ratios of the concentration of heavy metals in shoots toroots (Transfer factor, TF) for soils amended with CS and SS

Cd Cr Cu Ni Pb Zn

SS/CS

Root 8.0±3.4(3.9–21.7)

6.3±5.1(1.7–27.4)

4.5±2.3(1.2–11.2)

2.0±1.0(1.2–6.3)

5.7±2.4(1.2–11.6)

3.5±1.5(1.6–7.2)

Shoot 3.4±1.5(1.5–6.8)

0.8±0.4(0.4–2.3)

1.6±0.3(1.1–2.3)

1.1±0.5(0.5–2.6)

3.1±2.9(0.2–12.4)

1.8±0.5(0.9–3.0)

BCFRootsCS 2.1±1.0

(0.9–5.6)0.7±0.4(0.1–1.8)

2.4±1.5(0.8–6.0)

2.3±1.5(0.8–6.4)

0.6±0.4(0.2–1.6)

1.8±0.7(0.7–3.7)

SS 1.8±0.8(0.8–4.8)

0.7±0.5(0.2–2.8)

2.5±1.8(0.6–9.0)

2.6±2.0(1.0–12.0)

0.8±0.5(0.1–2.9)

1.8±0.8(0.6–4.8)

BCFShootsCS 0.44±.0.32

(0.14–1.63)0.20±0.14(0.02–0.63)

0.39±0.13(0.15–0.71)

0.09±0.06(0.03–0.29)

0.01±0.01(n.a.–0.06)

0.39±0.14(0.15–0.85)

SS 0.14±0.04(0.08–0.26)

0.03±0.01(0.01–0.06)

0.15±0.03(0.09–0.25)

0.05±0.03(0.02–0.17)

0.006±0.005(0.002–0.022)

0.21±0.07(0.10–0.35)

Shoots/roots (TF)

CS 0.22±0.11(0.07–0.59)

0.33±0.17(0.12–0.74)

0.19±0.07(0.09–0.37)

0.04±0.01(0.01–0.08)

0.02±0.02(n.a.–0.07)

0.24±0.09(0.10–0.56)

SS 0.09±0.04(0.02–0.21)

0.05±0.03(0.01–0.16)

0.08±0.04(0.02–0.20)

0.02±0.01(0.01–0.04)

0.010±0.008(0.002–0.036)

0.14±0.07(0.02–0.38)

n.a. data not available due to concentration of Pb below the detection limit of the instrument

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called root barrier (Adriano 2001; Basta et al. 2005). The degreeof protection for the different heavy metals and the mechanismsresponsible are further investigated in Sections 3.2 and 3.3through the study of the BCF and TF, respectively.

3.2 Transfer of heavy metals from soils to barley rootsand shoots

More important than the total amount of heavy metals observedin plants is the relative mobility of these metals assessed by theBCF (Table 4). The evaluation of the BCF represents a simplemethod to characterise quantitatively the transfer of availableheavy metals from the soil to the plant (Branzini et al. 2012).

The order in which heavy metals were transferred from soilsto roots was: Cu~Ni>Cd>Zn>Cr~Pb (BCFRoots, Table 4).The order and values of BCFRoots in soils amended with CSwere similar to those observed in soils amended with SS. Thisconfirms our previous results indicating the weak efficacy ofsoils controlling the transfer of heavy metals from soils to roots.Our results also imply that the high concentrations of heavymetals in SS did not influence the ratio in which these metalswere transferred from soils to roots. The weak control of soilproperties on the transfer of heavy metals from soil to rootmight be related to the important role that dissolved organicmatter (DOM) contained in the sludge has in the mobility ofheavy metals in soils (McBride et al. 1997; Ashworth andAlloway 2008). Dissolved organic matter forms organo-metalcomplexes with heavy metals which may lead to an enhancedpotential for their migration and bioavailability (Ashworth andAlloway 2008). Our results might be also influenced by plantphysiology since the mechanisms developed by roots for theexclusion of heavy metals at high external bioavailable con-centrations are weaker than those developed in the absorptionof deficient elements (Kabata-Pendias 2004). The transfer ofheavy metals from soils to plants might be also affected by ourtime scale since soil retention mechanisms are capable ofreducing trace elements mobility over a longer time period(Antoniadis et al. 2010).

According to the BCFShoots (Table 4), the order in whichheavy metals were transferred from soils to the shoots of theplants grown in soils amended with CS was Cd~Zn~Cu>Cr>Ni>Pb. The order was essentially the same for plants grown insoils amended with SS (Zn>Cu~Cd>Ni~Cr>Pb), in agree-ment with that found by Pascual et al. (2004) for ryegrass(Lolium multiflorum) grown in a soil amended with a highdose of sewage sludge. The order of mobility as expressed bythe BCFShoots was different to that found in roots. But, moreimportantly, the BCFShoots values found for all the heavymetals were considerably lower than the BCFRoots, suggestingthe accumulation of metal in roots, a phenomenon that waspreviously defined as “root barrier”. Metal retention in rootplays an important role by preventing an excessive andtoxic accumulation in edible parts of the plant. Plants using

this strategy retain most of the heavy metals taken up fromsoils in root cells, detoxifying them by chelation in thecytoplasm or storing them into vacuoles (Barceló andPoschenrieder 1990; Doncheva et al. 2009). We also observedthat, contrary to roots, the BCFShoots in the soils amended withSS for all the heavy metals studied were lower than thoseobserved in CS. Hooda et al. (1997) found an overall decreasein the BCF of wheat grains for Cd, Cu, Ni, Pb and Zn in plantsgrown in sludge contaminated soils (n =13) compared touncontaminated soils (n =9). Our results are in agreement withthose found by Hamon et al. (1999) and McBride (2003) andconfirm that the restriction of the transfer of heavy metals tothe edible parts was dependent on the plant physiology, andspecifically on the transfer of heavy metals from roots toshoots. Our results proved that this mechanism was relativelyindependent on the soil and more efficient at high concentra-tions of heavy metals.

Orientative BCF values for heavy metals have been pro-vided by Alloway and Jackson (1991) and Sauerbeck (1991).However, comparisons are difficult since, apart from the typeof metal, the values of BCF in plants grown in soils amendedwith sewage sludge have been reported to be dependent on thesoil properties, plant specie and part of the plant studied(Sauerbeck 1991; Hooda et al. 1997; Pascual et al. 2004;McLaughlin et al. 2006).

3.3 Transfer from barley roots to shoots: risk posedby the heavy metals

The transfer of heavy metals from roots to shoots varieddepending on the heavy metal and type of sludge applied(CS and SS; Table 4). According to the TF, heavy metals weresorted as follows:

1. Soils amended with CS: Cr>Zn~Cd>Cu>Ni>Pb2. Soils amended with SS: Zn>Cd~Cu>Cr>Ni~Pb

We observed lower TF values in soils amended with SScompared to soils amended with CS (Table 4), thus indicating,and as previously discussed, that plant restricted the transfer ofheavy metals from roots to shoots, and this restriction was moreimportant at high concentrations available in roots. These factsare illustrated in Fig. 1, which shows the relationships betweenthe concentrations of heavy metals in roots and shoots in bothsoils amended with CS and SS. As can be seen in Fig. 1, and inaccordance with the root restriction observed, the concentra-tions of heavy metals in shoots were not lineally related to theconcentrations of heavy metals in roots, and in some cases (Cr,Ni and Pb) not related at all. The highest reduction in the TF forplants grown in soils amended with SS compared to thosegrown in CS was observed for Cr, with the rest of elementsexperiencing similar reductions (approximately half). Thus, theroot restriction was more important for Cr than for the rest ofelements in soils amended with SS (see Cr plot in Fig. 1).

692 J Soils Sediments (2014) 14:687–696

Nickel and Pb were the heavy metals that appeared to be highlyaccumulated in roots, and this occurred independently on theconcentrations of heavy metals available in roots (Fig. 1 andTable 4). Conversely, Cd, Cu and Zn were relatively lessretained by roots (Table 4). For these metals, there was arelationship between the concentrations found in roots andshoots (see Fig. 1).

In light of the results found for the TF ratio, which confirmthose previously found for BCFShoots, Cd, Cu and Zn were theheavy metals that posed the greatest hazard regarding theirhigher mobility to shoots in both set of soils amended with CSand SS. Conversely, Cr, Ni and Pb posed relatively lower risk.Our data set includes various soils covering a wide range ofsoil properties and amended with sewage sludge differing inthe heavy metal concentrations; hence, comparison with sim-ilar data is difficult. Our results confirm previous concernsrelated to the transfer of Cd, Cu and Zn through the food chainand the relatively less risk posed by Cr and Pb (Sauerbeck1991; Hooda et al. 1997; McLaughlin et al. 1999, 2000;Kabata-Pendias 2004) using a wide range of soil properties.However, contrasting results have been found in the literatureregarding the mobility of Ni (McLaughlin et al. 2000). Asdiscussed by Scott-Fordsmand (1997), the majority of re-search on Ni transfer to plants is based on experiments in-volving the direct addition of soluble Ni to soils, making Nimore available than would be expected and thus influencingthe high mobility of Ni in other studies.

Among these elements, Cd is especially worrying due tothe high toxicity and long body retention time (Alloway andJackson 1991; McLaughlin et al. 1996). On the other hand,lower risk is posed by Cr that is strongly retained by rootsthrough the accumulation in the vacuoles of the root cells(Shanker et al. 2005). According to Peralta-Videa et al.(2009), Cr is poorly translocated to aereal plant tissues dueto its demonstrated toxicity. The reported studies have shownthat Cr in plants is mainly Cr3+, which at low concentrationshas been identified as required for animals.

3.4 Vulnerability of soils to the transfer of heavy metalsto plants

As discussed above, heavy metals were generally weaklyretained in soils. However, the transfer of heavymetals to plantsstill varied with soil type, suggesting different vulnerability ofsoils. The influence of soil properties on the BCFRoots andBCFShoots, was tested by the performance of Pearson correla-tions and SMLR (Table 5). The assessment of BCF from soilproperties will help to identify which soils are of greatest riskfrom the point of view of the soil-plant transfer of heavy metals(McLaughlin et al. 2006).

The soil factors controlling BCFRoots were very similar insoils amended with CS and SS (Table 5). This is not surprisingsince the values for BCFRoots in soils amended with CS weresimilar to those found in SS (Table 4). Texture was included in

0

0.1

0.2

0.3

0 1.2 2.4 3.6 4.8

0.5

1.5

2.5

3.5

4.5

0 40 80 120 160

2

4.5

7

9.5

12

0 100 200 300 400

0.3

0.8

1.3

1.8

2.3

20 55 90 125 160

0

0.2

0.4

0.6

0.8

0 25 50 75 100

0

12

24

36

48

0 125 250 375 500

Met

als

in s

ho

ots

(m

g k

g-1

)

Metals in roots (mg kg-1)

Cd Cr Cu

ZnNi Pb

Fig. 1 Relationship between the concentrations of heavy metals in roots and shoots in soils amended with common (circles) and spiked sludge (triangles)

J Soils Sediments (2014) 14:687–696 693

all the models for the assessment BCFRoots (Table 5),suggesting that soils with high sand contents are less efficientin the control of the transfer of heavy metals to roots. The sandcontent was the most important soil property in regulating theBCFRoots for Cr, Cu, Pb and Zn. Texture is viewed as one ofthe most important variables driving the availability of heavymetals (Kabata-Pendias 2004). The reasons for the relation-ships with sand are based on the low sorption capacity ofsandy soils. Sandy and sandy loams soils have low clay andusually low organic carbon contents, thus having a lowersorption capacity for heavy metals as compared to loamy orclayey-textured soils (McBride 2003; Basta et al. 2005). Wealso suggest that the promoted mobility of DOM in sandysoils (Soriano-Disla et al. 2012) may be responsible of thepositive relationship between the sand content and the transferof heavy metals.

Just for two heavy metals, Cd and Ni, the sand content wasnot the main soil property in describing their BCFRoots. TheBCFRoots for Cd was mainly controlled by soil pH, indicating

that soils with low pH transfer more Cd to roots. The role of soilpH influencing the solubility and hence mobility of Cd is wellknown (McLaughlin et al. 1996; Bose andBhattacharyya 2008).Both absorption and precipitation reactions are generallyfavoured at increasing pH values (Basta et al. 2005). In calcar-eous soils, the transfer of Cd to plants is restricted by theprecipitation of Cd with carbonates and potential competitionof Ca2+ ions for absorption sites on plant roots (Alloway andJackson 1991; Bose and Bhattacharyya 2008). The BCFRoots forNi was strongly influenced by SOC, hence showing that theroots of the plants grown in soils with low levels SOC are morevulnerable to the transfer of Ni. Soil organic matter is a veryimportant adsorptivemedium for heavymetals in soils (Allowayand Jackson 1991; Kabata-Pendias 2004), and it reveals strongability to sorb Ni (Kabata-Pendias and Pendias 2001).

Despite to the control of plant physiology on the transfer ofheavy metals from roots to shoots, it was still possible to findrelationships between the soil properties and BCFShoots, espe-cially for plants grown in soils amended with CS. The amount

Table 5 Pearson correlation coefficients (r) and stepwise multiple linearregressions (with the coefficient of determination, R2, in brackets) be-tween soil properties and the bio-concentration factor in roots and shoots

(BCFRoots and BCFShoots) in plants grown in soils amended with common(CS) and spiked sludge (SS)

BCFRoots pH CO32− logSOC Clay Sand logAmFe logAmMn Stepwise multiple linear regressions

CS logCd −0.69*** −0.66*** −0.40* −0.55*** 0.36* 0.22ns 0.33* 1.1–0.069pH-0.007Clay (0.66***)

logCr 0.31 ns −0.18 ns −0.33 ns −0.45** 0.63*** −0.38* −0.53** 0.18+0.003Sand-0.09logAmMn (0.53***)

logCu −0.10 ns −0.46** −0.66*** −0.64*** 0.78*** −0.28 ns −0.21 ns 0.53+0.004Sand-0.22logSOC-0.005Clay (0.72***)

logNi −0.54** −0.53** −0.67*** −0.34* 0.57*** −0.15 ns 0.09 ns 0.95–0.22logTOC-0.061pH+0.003Sand (0.65***)

logPb −0.20 ns −0.44** −0.64*** −0.51** 0.76*** −0.40* −0.38* 0.25+0.003Sand-0.14logSOC-0.05logAmMn (0.70***)

logZn −0.03 ns −0.37* −0.58*** −0.60*** 0.82*** −0.35* −0.36* 0.12+0.005Sand (0.68***)

SS logCd −0.56*** −0.55*** −0.48** −0.38* 0.53** 0.05 ns 0.28 ns 0.66–0.050pH+0.003Sand (0.56***)

logCr −0.39* −0.40* −0.52** −0.38* 0.60*** −0.09 ns 0.16 ns 0.29+0.003Sand-0.03pH (0.48***)

logCu −0.14 ns −0.37* −0.61*** −0.53** 0.81*** −0.35* −0.19 ns 0.22+0.007Sand-0.18logSOC (0.70***)

logNi −0.48** −0.57*** −0.77*** −0.45** 0.71*** −0.22 ns 0.02 ns 0.90–0.29logSOC-0.005Sand-0.051pH (0.80***)

logPb −0.43** −0.52** −0.63*** −0.48** 0.71*** −0.13 ns 0.09 ns 0.31+0.004Sand-0.04pH (0.66***)

logZn −0.50** −0.48** −0.62*** −0.43** 0.68*** −0.21 ns 0.07 ns 0.56+0.004Sand-0.05pH (0.68***)

BCFShoots pH CO32- logTOC Clay Sand logAmFe logAmMn

CS sqrCd −0.43** −0.49** −0.12 ns −0.40* −0.19 ns 0.44** 0.42* 1.64–0.006CO32-0.01Sand-0.02Clay (0.71***)

sqrCr 0.11 ns −0.18 ns −0.37* −0.43** 0.42* −0.43** −0.45** 0.88–0.22logAmMn-0.004CO32- (0.39***)

Cu −0.40* −0.45** −0.64*** −0.50** 0.65*** −0.28 ns −0.07 ns 0.57+0.003Sand-0.03pH-0.15logSOC (0.61***)

sqrNi −0.62*** −0.58*** −0.71*** −0.31 ns 0.44** −0.12 ns 0.19 ns 0.76–0.21logSOC-0.04pH (0.68***)

sqrPb 0.20 ns −0.24 ns −0.46** −0.55** 0.51** −0.25 ns −0.41* 0.22–0.002Clay-0.03logAmMn-0.05logSOC (0.49***)

SqrZn 0.45** 0.16 ns −0.12 ns −0.39* 0.36* −0.59*** −0.70*** 0.95–0.17logAmMn-0.004Clay (0.56***)

SS Cd 0.30 ns 0.04 ns 0.03 ns 0.08 ns −0.28 ns −0.04 ns −0.15 ns ns

sqrCr 0.33 ns 0.03 ns −0.23 ns −0.21 ns 0.29 ns −0.37* −0.42* 0.21–0.03logAmMn (0.18*)

Cu 0.10 ns −0.10 ns −0.54** −0.21 ns 0.53** −0.54** −0.40* 0.34–0.06logSOC-0.06logAmFe (0.48***)

sqrNi −0.38* −0.55** −0.77*** −0.41* 0.64*** −0.24 ns −0.02 ns 0.29–0.15logSOC+0.001Sand (0.66***)

sqrPb 0.18 ns −0.04 ns −0.37* −0.15 ns 0.46** −0.42* −0.33 ns 0.03+0.001Sand (0.22**)

Zn 0.54** 0.33* −0.04 ns −0.13 ns 0.27 ns −0.64*** −0.75*** 0.39–0.12logAmMn (0.56***)

CO32− equivalent calcium carbonate, SOC soil organic carbon, AmFe and Mn amorphous Fe and Mn, ns not significant, sqr square root

*p <0.05, **p<0.01, ***p <0.001

694 J Soils Sediments (2014) 14:687–696

available in roots was conditioned by the transfer from soil toroots, which, as we discussed, was influenced by soil proper-ties. Overall, weaker relationships were found between soilproperties and BCFShoots, in soils amended with SS. This is inagreement with the more important restriction of the transferof heavy metals from roots to shoots in soils amended with SScompared to soils amended with CS. Nickel and Zn were anexception to this general pattern since relatively high relation-ships were found between the soil properties and theirBCFShoots in soils amended with SS. As previously shownfor the Ni BCFRoots, the BCFShoots for this element wasinfluenced by SOC and sand. The BCFShoots for Zn wasnegatively related to the amorphous Mn oxides. Manganeseoxides are well-known adsorbents of heavy metals in soils(Alloway and Jackson 1991). However, the factors regulatingthe BCFShoots for Zn are somewhat inconsistent with thosefound to regulate the BCFRoots of this element.

4 Conclusions

The rationale behind this article is the need to evaluate thetransfer of several heavy metals occurring together (e.g. insewage sludge) to barley plants grown in contrasting soils (notrestricted by e.g. soil pH) amended with sewage sludge con-taining different levels of heavy metals. Our study has clearlyrevealed that barley roots were highly vulnerable againstincreasing concentrations of heavy metals in soils amendedwith sewage sludge, with soil components showing weakefficacy in restricting the mobility of heavy metals from soilto roots. However, metal transfer from roots to shoots wasrestricted and controlled by plant physiology observing fur-ther metal restriction in roots at high levels of heavy metals(SS-amended soils). As for human exposure concerns, Cd, Cuand Zn were the heavy metals which posed the highest risk,being transferred in relative higher proportions to shoots thanCr, Ni and Pb.

Although heavy metals were generally weakly retained insoils, the vulnerability against the transfer to roots varied withsoil type, observing similar patterns in soils amended with CSand SS. Soils with high levels of sand were the most vulner-able to the transfer of Cr, Cu, Pb and Zn from soil to roots.Soils with low pH and SOC promoted the transfer of Cd andNi from soil to roots, respectively. The different vulnerabilityof soils to the transfer of heavy metals from soils to roots stillconditioned their transfer from soil to shoot. However, weakerrelationships were found between soil properties andBCFShoots in soils amended with SS due to further restrictionsobserved in the roots of plants grown in these soils.

Acknowledgments JoseM. Soriano-Disla gratefully acknowledges theGovernment of Valencia for a Post-Doctoral research fellowship(APOSTD/2011/034).

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