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Environmental Pollution 159 (2011) 3018e3027

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Environmental Pollution

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Phytostabilization of semiarid soils residually contaminated with traceelements using by-products: Sustainability and risks

Alfredo Pérez-de-Mora a,*, Paula Madejón a, Pilar Burgos a, Francisco Cabrera a,Nicholas W. Lepp b, Engracia Madejón a

a Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS), CSIC, PO Box 1052, 41080 Sevilla, Spainb 35, Victoria Road, Formby, Liverpool L37 7DH, UK

a r t i c l e i n f o

Article history:Received 5 December 2010Received in revised form3 April 2011Accepted 7 April 2011

Keywords:AmendmentsAssisted natural remediationHeavy metalsResidual contaminationPioneer vegetation

* Corresponding author.E-mail address: perezdemora@gmail.com (A. Pére

0269-7491/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.envpol.2011.04.015

a b s t r a c t

We investigated the efficiency of various by-products (sugarbeet lime, biosolid compost and leo-nardite), based on single or repeated applications to field plots, on the establishment of a vegetationcover compatible with a stabilization strategy on a multi-element (As, Cd, Cu, Pb and Zn) contaminatedsoil 4e6 years after initial amendment applications. Results indicate that the need for re-treatment isamendment- and element-dependent; in some cases, a single application may reduce trace elementconcentrations in above-ground biomass and enhance the establishment of a healthy vegetation cover.Amendment performance as evaluated by % cover, biomass and number of colonizing taxa differs;however, changes in plant community composition are not necessarily amendment-specific. Althoughthe translocation of trace elements to the plant biotic compartment is greater in re-vegetated areas,overall loss of trace elements due to soil erosion and plant uptake is usually smaller compared to thatin bare soil.

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1. Introduction

Residual trace element contamination is a major concernfollowing clean-up of highly-contaminated sites, where the mostaffected layer is generally excavated and removed for further pro-cessing or disposal. The term “residual” accounts for thosecontaminants associated with spills or other contamination eventswhich, despite clean-up operations, are still present at concentra-tions significantly above local geochemical background levels. Thelong-term effects of such residual contaminants are poorly under-stood and significant changes in contaminant bioavailability andmobility may be observed as a result of local shifts in soil pH, redoxconditions, or organic matter transformations (Basta et al., 2005;Hartley et al., 2009).

Trace elements are amongst the most widespread soilcontaminants and are difficult and expensive to remove (Dickinson,2000). The high costs and disruption of the natural soil character-istics associated with off-site physical and chemical treatmentssuch as heat treatment, soil washing or solidification make

z-de-Mora).

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these techniques unsuitable for treating extensive areas whereresidual contamination is present. Alternatively, more sustainablestrategies based on the utilization of plants and/or soil additives insitu have been used to reduce wider dispersal of residual contam-inants (Adriano et al., 2004; Mench et al., 2007; Kumpiene et al.,2008).

In semiarid areas pioneer plant communities in trace element-affected soils are well-adapted to stress due to the presence ofcontaminants, and poor nutrient and water availability (Freitaset al., 2004; Madejón et al., 2009). Natural vegetation alsoprovides physical protection against soil erosion by wind andwater (Norland and Veith, 1995). In addition, evapotranspirationlimits losses to groundwater by evapotranspiration of soil water byvegetation (Tordoff et al., 2000). Pioneer herbaceous vegetationmay play a chief role in the cycling of trace elements during theearly stages of secondary succession in contaminated areas,particularly in semiarid climates. Trace elements removed by suchvegetation may enter different biogeochemical cycles dependingon i) their translocation to above-ground parts, ii) the presenceand access of grazing livestock to plants, and iii) whether traceelements re-enter the soil following plant decay as litter or asso-ciated with burnt matter following a fire event. Reduced biogeo-chemical cycles in which transfer from soil to other compartments

Table 1Mean values (�SD) of some characteristics of the non-amended soil (n¼ 9) in 2007and the amendments (n¼ 3).

Amendment

Soil SL BC LE

pH 3.45� 0.22 9.04� 0.08 6.93� 0.03 6.08� 0.07TOC (g 100 g�1) 0.99� 0.31 6.70� 1.55 19.5� 1.22 28.9� 0.39% moisture content ND 40e50 25e30 25N-kjeldahl (g kg�1) 0.94� 0.15 9.80� 0.40 13.1� 0.60 11.7� 0.20P (g kg�1) 0.46� 0.55 0.40� 0.03 12.4� 1.80 5.10� 0.60K (g kg�1) 5.18� 1.10 39.7� 0.80 9.30� 2.30 5.30� 0.50Fe (mg kg�1) 42442� 5225 2037� 552 15615� 870 41010� 721Mn (mg kg�1) 450� 120 297� 10.3 257� 24.8 66.2� 1.41As (mg kg�1) 145� 71.0 1.63� 0.34 5.63� 1.48 34.9� 3.46Cd (mg kg�1) 2.57� 0.27 0.43� 0.15 0.73� 0.40 0.83� 0.11Cu (mg kg�1) 112� 26.3 51.0� 8.20 121� 5.66 28.2� 2.40Pb (mg kg�1) 286� 162 39.2� 6.70 137� 26.2 22.0� 2.33Zn (mg kg�1) 205� 41.7 138� 31.0 258� 18.4 64.5� 1.06

TOC¼ total organic carbon; BC¼ biosolid compost; LE¼ leonardite; SL¼ sugarbeetlime; ND¼ not determined.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e3027 3019

is minimized are possible if pioneer species show metal avoidancetraits. Minimizing accumulation of trace elements in above-ground biomass is a major challenge for any large-scale phyto-management programme (Domínguez et al., 2008). Usingby-products such as lime or alkaline organic materials (e.g.composts) as soil amendments can ameliorate both soil acidityand nutrient availability whilst simultaneously reducing phytoa-vailability of trace elements, and enhancing plant growth and re-valuing these materials (Madejón et al., 2006). In addition, someby-products improve soil aeration and soil water retention,enhancing microbial turnover and reducing the risk of drought.Such processes can significantly accelerate redevelopment ofderelict land under semiarid conditions (Caravaca et al., 2003;Bastida et al., 2009). However, trace element stability in soil maybe reversed as time progresses due to various mechanisms (e.g.oxidation of metal sulfides, reduction of iron and manganeseoxides, and mineralization of organo-trace element complexes andformation of soluble organo-trace element complexes). The pres-ence of additional trace elements in some soil amendments addsfurther complexity to the remediation equation (Clemente et al.,2010).

Repeated amendment incorporations are commonly employedto prevent changes in soil characteristics and maintain traceelement immobilization in soil. However, the longevity of suchamendments in the field, particularly in semiarid areas, remainsuncertain. The experiments described below assess if there isa need for re-treatment with various amendments to achievea sustainable, metal-avoiding vegetation cover in a soil residuallycontaminated with trace elements (As, Cd, Cu, Pb and Zn). Thethree-year study (2006, 2007 and 2008) was conducted on a siteaffected by the Aznalcóllar mine spill (SW Spain, 1998), wherea remediation experiment had been initiated in 2002 followingclean-up operations in the area (Grimalt and Macpherson, 1999).Previously, Madejón et al. (2010) showed that total concentrationsin soil generally did not change after repeated amendment appli-cations (except for Zn) and also that 0.01 M-CaCl2-extractableconcentrations were significantly reduced in amended soil. Organictreatments (BC and LE) required repeated incorporations (4 appli-cations) to achieve similar results to the inorganic treatment (2applications).

The specific aims of this work are to investigate the effects ofvarious amendments and repeated additions on: i) plant coloni-zation and growth, ii) the composition of the pioneer plantcommunity growing on the affected soil, and iii) the accumulationof trace elements in above-ground biomass and, in turn, the risksassociated with this uptake.

2. Materials and methods

2.1. Study area

The study site is an experimental field (“El Vicario”) that was affected by theAznalcóllar mine spill. This is located on the right bank of the Guadiamar river(latitude N 37� 260 2100, longitude W 06� 120 5900), 10 km downstream from theAznalcóllar mine. The only remediation work carried out in this field was theinitial removal of the sludge together with a layer of underlying topsoil. The soilfrom this area is a clayey loam (21.1% clay, 29.1% silt and 49.8% sand) classified asTypic Xerofluvent (USDA Soil Survey Staff, 1996). The main characteristics of thesoil are shown in Table 1. The annual rainfall for each period of the study was asfollows: 732 mm for 2006, 411 mm for 2007 and 547 mm for 2008. Averageannual temperature is 19 �C (min. 9 �C in January, max. 27 �C in July) and annualaverage rainfall is 484 mm. Details about the mine spill and the remediationmeasures undertaken to establish a Green corridor in the affected area can befound elsewhere (Grimalt and Macpherson, 1999; CMA, 2003). Despite clean-upoperations and remediation works, there is substantial residual trace elementcontamination in the area. Controlling transfer of trace elements within thesoileplanteanimal system is one of the main challenges for the success of theGreen Corridor.

2.2. Experimental design

Three amendments e two organic and one inorganic e were tested. The twoorganic amendments were biosolid compost (BC) from the SUFISA wastewatertreatment plant (Jerez de la Frontera, Southern Spain) and leonardite (LE), a lowgrade coal rich in humic acids (DAYMSA, Zaragoza, Northern Spain). The inorganicamendment was sugarbeet lime (SL), a residual material from the sugarbeetmanufacturing process with 70e80% (dry basis) CaCO3 (EBRO-AGRÍCOLAS, San Joséde la Rinconada, Southern Spain). The most relevant characteristics of the amend-ments are shown in Table 1. For more details on the characterization of theamendments see Text S1 (supplementary material).

The experimental area (20� 50 m) was divided into 12 plots (7� 8 m each),with a margin of 1 m (long) and 2 m (wide) between plots. The experiment was setup in October 2002. Treatments were an unamended control (NA), SL applied at rateof 30 Mg ha�1 y�1, BC at a rate of 30 Mg ha�1 y�1, and LE at a rate of 25 Mg ha�1 y�1

plus 10 Mg ha�1 y�1 of SL. The application rates (fresh wt basis) were of the samemagnitude as those applied to other areas affected by the spill within the GuadiamarGreen Corridor (Antón Pacheco et al., 2001) and were below the maximumpermitted limits for annual trace element loading established by the EuropeanUnion (Directive 86/278/EEC) (CEC, 1986). Amendments were applied for twoconsecutive years (October 2002 and October 2003) and incorporated into the top0.15 m of soil using a rotary tiller (RL328 Honda). Non-amended subplots were tilledin an identical manner. In October 2005, each subplot was divided into equal halves.One half remained unamended in the following years (Doses 2, D2) (SL2, BC2 andLE2), whereas the other half received the same amendment at the same rate foranother two years (October 2005 and October 2006; Doses 4, D4) (SL4, BC4 and LE4).The experiment was carried out in a completely randomized block designwith threereplicates per treatment.

2.3. Soil sampling and plant survey and sampling

Details regarding the soil analysis and the soil sampling can be found in Text S1and Text S2 (supplementary material), respectively; for information on general soilproperties and trace elements in soil under the different treatments see Madejónet al. (2010). For the detailed plant survey, each plot was originally divided intofour parts (4� 3.5 m), establishing 48 different vegetation sampling units. Multipleamendment additions therefore resulted in 18 sampling units receiving 4 amend-ment doses (6 sampling units per treatment), 18 sampling units receiving 2amendment doses (6 sampling units per treatment) and 12 non-amended samplingunits. Plant surveys and samplings were carried out in the spring of 2006, 2007 and2008. For each survey, a 0.03� 0.03 m quadrant was used (Cox, 1990). The quadrantwas randomly placed three times within each sampling unit. For biomass estima-tions plants were clipped from the soil surface, dried in the lab at 70 �C and weighedusing a digital scale. Plant species were identified and listed and the vegetation coverestimated in the field prior to clipping. Plant species were determined and namedfollowing the keys and nomenclature for vascular plants proposed by Valdés et al.(1987).

Three different plant species: Raphanus raphanistrum L. (2006), Poa annua L.(2007), and Lamarckia aurea (L.) Moench (2006, 2007 and 2008), generally abundantin all treatments, were collected for shoot chemical analysis. It should be noted thatnon-washed plants might be more representative of the real environmental risk forgrazers (Madejón et al., 2002). In this study, we wanted to focus on above-groundaccumulation of trace elements solely due to plant uptake to study whether accu-mulation is reduced or not in amended soil. Plant samples were washed for at least15 s with a 0.1 M HCl solution and for 1 s with deionized water. Plant material was

Fig. 1. Mean values and standard errors of a) percentage of plant cover, b) above-ground biomass and c) number of vascular plant species in each treatment. Signifi-cant differences between treatments for each year are indicated by the respective F andp values.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e30273020

then dried at 70 �C, ground and passed through a 500 mm stainless-steel sieve. Traceelements (As, Cd, Cu, Pb and Zn) in plant material (1 g) were extracted by wetoxidation with 4 mL of concentrated HNO3 (suprapur; density 1.39 g cm�3) underpressure in a microwave digester. The program consisted of three consecutive steps(5 min each) of power (250, 450 and 600W). Trace elements in the extracts weredetermined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using an IRIS ADVANTAGE spectrometer (Thermo Jarrel Ash Corporation, MA,USA) and expressed on a dry wt basis. The accuracy of the analytical methods wasassessed through BCR analysis (Community Bureau of Reference) of a plant sample(CRM 279, Sea lettuce). The certified concentrations and % of recovery were asfollows: As 3.09 mg kg�1,103%; Cd 0.27 mg kg�1,114%; Cu 13.14 mg kg�1, 96.6%; Pb13.48 mg kg�1, 113% and Zn 51.3 mg kg�1, 108%. Additional information regardingmacronutrient concentrations in plants can be found as supplementary material(Text S3; Table S3).

2.4. Estimation of annual loss of trace elements from soil

Details regarding the annual loss of trace elements from soil can be found in TextS4 (supplementary material).

2.5. Statistical analysis

Significant differences between treatments in terms of biomass, plant cover,number of species and trace element concentrations in above-ground biomass weretested by ANOVA, considering the treatments as the independent variable. Analyzedvariables were checked for normality and transformed when necessary prior toANOVA. Post hoc analysis was based on Tukey’s test when variances were equal,whereas GameseHowell’s test was used in case of unequal variances. The signifi-cance level awas set at 0.05. A correlation analysis was performed to determine therelationship between soil- and plant-related properties. The P levels reported(P� 0.01 and P� 0.05) were based on Pearson’s coefficients.

A multivariate analysis using CANOCO 4.5 was performed to investigate theplant community composition in the various treatments and assess the importanceof the environmental variables measured (direct gradient analysis). Redundancyanalysis (RDA) was selected over canonical correspondence analysis (CCA), becauseof better performance of RDA with our data set (more details in Text S5;supplementary material).

3. Results

3.1. Plant cover, biomass and richness

In general, plant cover was between 20 and 40% higher inamended plots compared to non-amended plots in all years. Sug-arbeet lime (SL) and biosolid compost (BC) performed better thanthe leonarditeelimemixture (LE) (Fig.1a). Overall, vegetation coverimproved from 2006 to 2007, regardless of the treatment. Resultsfrom 2008 were similar to 2007. Re-application of amendments didnot significantly increase vegetation cover.

Above-ground biomass yields were also significantly greaterfrom amended plots compared to the non-amended controls, being20e50 fold greater in 2006 and 3e7 fold greater in subsequentyears (Fig. 1b). In treated plots, above-ground biomass in 2006 wassubstantially higher than that in subsequent years. This is probablythe result of the high level of precipitation received that year anddifferences in plant community composition (supplementarymaterial, Table S1). Re-treatment had a positive effect on biomassin the LE subplots in 2006 and 2008; no further differencesbetween D2 and D4 treatments were observed.

Plant diversity was consistently higher in treated soils comparedto non-amended soil (Fig. 1c). As a rule, amended soils supportedbetween 10 and 17 plant species, whereas control plots only sus-tained 4e7 different species. Re-treatment generally resulted inincreased richness, especially in the case of LE, although differenceswere not always significant.

3.2. Plant community composition

A total of 53 vascular plant species representing 48 genera and19 families was recorded during the three surveys (2006, 2007 and2008). Most species were annual (37), although biennials (12)

and perennials (4) were also present. Poaceae and Asteraceae werethe best represented families, each contributing 22e30% of thetotal species recorded. These were followed by Brassicacea, Car-yophyllaceae and Fabaceae. The most frequent species, defined asthose present in more than 55% of the sampling units are shown inTable 2. Presence of occasional species, defined as those found inone or two sampling units was rare (just 6e8 species each year).

Table 2Most frequent herbaceous plant species in the experimental plot (above 55%).

2006 2007 2008

Anagalis arvensis L. Andryala integrifolia L. Anagalis arvensis L.Andryala

integrifolia L.Bromus rubens L. Andryala integrifolia L.

Bromus rubens L. Chrysanthemumcoronarium L.

Avena sterilis L.

Lamarckia aurea(L.) Moench

Lamarckia aurea(L.) Moench

Anacyclus radiatus Loisel

Medicagopolymorpha L.

Poa annua L. Bromus diandrus Roth.

Raphanusraphanistrum L.

Lamarckia aurea (L.)Moench

Sonchus oleraceus L.Vulpia myuros

(L.) C.C. Gmel

33 species 42 species 40 species9 Poaceae 9 Poaceae 12 Poaceae8 Asteraceae 12 Asteraceae 10 Asteraceae3 Brassicaceae 4 Brassicaceae 2 Brassicaceae2 Caryophyllaceae 3 Caryophyllaceae 3 Caryophyllaceae2 Fabaceae 3 Fabaceae 4 Fabaceae1 Apiaceae 1 Apiaceae 1 Apiaceae1 Chenopodiaceae 1 Chenopodiaceae 1 Boraginaceae1 Fumariaceae 1 Fumariaceae 1 Fumariaceae1 Primulaceae 1 Primulaceae 1 Primulaceae1 Malvaceae 1 Malvaceae 1 Malvaceae1 Papaveraceae 1 Papaveraceae 1 Papaveraceae1 Lamiaceae 1 Lamiaceae 1 Cyperaceae1 Oxadlidaceae 1 Oxalidaceae 1 Plantaginaceae1 Polygonaceae 1 Convolvulaceae 1 Convolvulaceae

1 Polygonaceae1 Cyperaceae

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e3027 3021

These mainly occurred in treated plots (2006 and 2007), but in2008 three out of six occasional species were recorded in non-treated plots (NA).

Differences between treatments in plant community composi-tion were investigated using RDA. Results suggest differences bothin 2006 and 2007 between the structures of the plant communityin treated plots compared to that in control plots (Fig. 2 andFigure S1, supplementary material). There were, however, no clearpatterns suggesting shifts in the composition of the vegetation dueto a specific amendment, nor did re-treatment have a consistenteffect on the structure of the plant community in treated plots. Atotal of four canonical axes was interpreted. For 2006, these

20a

Axis 1

NA SL2

SL4

BC2

BC4

LE2

LE4

0.1-

8.08.0-

Axis

2

Fig. 2. Redundancy analysis of sampling units based on absence and presence of plant smultivariate space and b) scores (arrows) of the plant species and the environmental vandryala¼ A. integrifolia; bromus.¼ B. rubens; Cd-sol¼ 0.01 M-CaCl2-extractable Cd in soil; Cmedicago¼M. polymorpha; nr of sps¼ number of species; sonchus o.¼ S. oleraceus; TOC¼ t

explained 40% of the total variance observed for the species data(31% for the first two axis), whereas in 2007 it was only 30% (20% forthe first two axis). Fig. 2b (year 2006) and Figure S1b (year 2007,supplementary material) show the environmental variables andthe plants species that most importantly contributed to the ordi-nation of samples in the multivariate space.

3.3. Micronutrients and trace element concentrations in plants

Significantly lower concentrations ofMn and Fewere recorded inRaphanus, Poa and Lamarckia in 2006 growing in amended plotscompared to those of plants growing in non-amended plots (NA)(Figs. 3e5). Re-treatment of soil showed no consistent trendsregardingmicronutrient concentrations.Meanvaluesof Fe andMninRaphanuswere found to be lower in plots re-amendedwith compost(BC4) and leonardite mixture (LE4), but not with sugarbeet lime(SL4). For Poa lower concentrations of Mn were observed in all re-amended plots, whereas a similar effect for Fe was only found inSL4 (Fig. 4a and b). In the case of Lamarckia, lower concentrations ofboth Fe and Mn were observed in plants growing on plots re-amended with leonardite mixture (LE4) in 2007. Lower concentra-tions of Mn in plants from BC4 were found in 2008 (Fig. 5b), but nofurther evidence of re-treatment effects was observed.

Arsenic concentrations in all plants and for all years were below2 mg kg�1 in plants growing on both amended and non-amendedplots. No differences between treatments were found. Concentra-tions of Cd, Cu, Pb and Zn followed a similar trend as that describedfor Mn and Fe in 2006; mean concentrations of these elements inRaphanus and Lamarckia (2006) were, in general, significantlylower in plants from amended plots compared to those of plantsgrowing in non-amended plots (Figs. 3 and 5). Something similarwas observed for Poa in 2007 (Fig. 4), although differences were notalways significant. Re-treatment significantly reduced concentra-tions of Cd and Pb in SL4 (Fig. 4c and e). For Lamarckia, differencesbetween amended and non-amended soils in 2007 were lesssalient compared to Poa. Significant differences were found forplants growing on SL2 and LE4. In 2008, no significant differencesbetween treatments were observed.

3.4. Soileplant transfer

Table S2 (supplementary information) shows [Element]Plant/[Element]Soil ratios based on average concentrations (mg kg�1) in

06

0.20.2-

0.20.2-

anagalis

andryala

bromus

diplotaxis

fumaria

malva

medicago

sonchus o.

no plant

TOCCd sol

Cu solcover

nr sps

Axis 1

Axis

2

b 2006

5.1

pecies and environmental variables in 2006: a) ordination of sampling units in theariables that contributed the most to the variance observed. anagalis¼ A. arvensis;u-sol¼ 0.01 M-CaCl2-extractable Cu in soil; fumaria¼ F. officinalis; malva¼M. sylvestris;otal organic C.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e30273022

Raphanus and Lamarckia for 2006 and Poa and Lamarckia for 2007and average total trace element concentrations in soil (mg kg�1).Results showed that the relative translocation of As and Pb (0.01),two of the most important contaminants of the spill in terms ofgeochemical background levels, was much lower than that of otherelements such as Cd, Mn or Zn (0.2e1). For the latter threeelements, ratios were also higher in non-amended control plotscompared to amended plots.

3.5. Cycling of trace elements: comparison of bare soil vsphytostabilized soils

Losses of micronutrients and trace elements from soil erosionprocesses were generally greater than those from plant uptake,except for Mn and Zn (Table 3). In the case of Fe, As and Pb plantuptake only represented 1e8% of the total mass of trace element

Fig. 3. Mean concentrations and standard errors of micronutrients and trace elements in Rindicated by the respective F and p values.

lost by erosion processes, whilst in the case of Mn (25e100%), Cd(14e64%), Cu (6e24%) and Zn (22e120%) the proportion washigher. In general, the presence of a vegetation cover reduced theannual loss of trace elements from amended soil between 20 and85%, with the exception of Mn and Zn in 2006 (Table 3). In 2007 asspontaneous vegetation started to colonize the non-amended soil,the average loss of micronutrients and trace elements in controlsdecreased by approx. 36%.

4. Discussion

4.1. Vegetation cover, biomass and species richness

Our results showed that incorporation of by-products asamendments into the soil had a durable and positive effect onplant growth as shown by data on vegetation cover, biomass and

aphanus plants in 2006. Significant differences between treatments for each year are

Fig. 4. Mean concentrations and standard errors of micronutrients and trace elements in Poa plants in 2007. Significant differences between treatments for each year are indicatedby the respective F and p values.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e3027 3023

number of colonizing taxa; all amended soils showed significantlyhigher values of these variables compared to non-amended soils(Fig. 1). Significant and positive correlations between these aspectsof vegetation dynamics were found with both soil pH and totalorganic C concentrations (Table 4); negative correlations with0.01 M-CaCl2-extractable Cd, Cu and Zn from soil were alsoobserved (Table 4). The alkalinizing effect of both organicand inorganic amendments can ameliorate soil acidity and reducetrace element bioavailable concentrations in soil at the sametime without significantly increasing the total trace elementcontent in soil (Madejón et al., 2010). Our data showed that re-treating the soil with the leonarditeelime mixture significantlyimproved biomass yields in 2006 and 2008. In addition, repeated

incorporations of all amendments generally resulted in largernumber of colonizing taxa, although plots were generally domi-nated by a reduced number of species as indicated by plantabundance data (Table 2). Repeated additions of amendments maythus improve soil properties for colonization and growth of traceelement-tolerant plants; for instance, nutrients can exerta protective effect against metal and metalloid toxicity in plants(Mengel and Kirkby, 1987).

4.2. Plant community composition

Theplant community compositionwasdominatedby PoaceaeandAsteraceae as seen in uncontaminated pastures along the Guadiamar

c

a b

d

e f

Fig. 5. Mean concentrations and standard errors of micronutrients and trace elements in Lamarckia plants in 2006, 2007 and 2008. Significant differences between treatments foreach year are indicated by the respective F and p values.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e30273024

Valley (Madejón et al., 2009). The species recorded in the experi-mental plot were similar to those observed in surrounding areas,which probably acted as plant propagule donors. Multivariate anal-ysis of vascular plants growing in the experimental plot indicateddifferences in plant community composition between amended andnon-amended plots consistent with a higher number of colonizingtaxa in amended plots. Nonetheless, no further evidence wasobserved to support additional differences between amendmenttreatments, nor did re-treatment seem to affect the structure of theplant community. The redundancy analysis also showed that both in2006and2007 (Fig. 2 andFigure S1, supplementarymaterial), speciesrichness contributedmore importantly than environmental variablesrelated to soil properties or trace element concentrations to the total

variance observed in the species data. Previous work has shown thatthe effect of amendments on vegetation growth is most noticeableduring the early stages of plant colonization (Madejón et al., 2006;Pérez-de-Mora et al., 2006). In this manner, once a suitablesubstrate has been established and pioneer vegetation has colonizedsome areas, positive feedback occurring in such vegetation patchesmay bemuchmore relevant to the structure of the plant communitythan direct or indirect effects associated with the incorporation ofamendments. This positive feedback involves an increasing numberof seeds, reduced evaporation by shading and increased infiltrationrates (Gilad et al., 2007; Salazar et al., 2009). In the mid- and long-term this could positively affect adjacent non-amended areas asfound in thenon-treatedplots. It shouldbenoted that competition for

Table 3Comparison between the theoretical mass loss of trace elements from bare soil by erosion processes (B) and the theoretical mass loss of trace elements by soil erosion (S) and plant uptake (P) in the different treatments.Micronutrients and trace elements mass loss due to soil erosion and plant extraction are expressed in g ha�1 y�1 except for Fe, which is expressed in kg ha�1 y�1. The reduction in trace element loss due to the different treatmentscompared to bare soil is expressed as %.a An average soil loss of 4.4 Mg ha�1 y�1 for Mediterranean areas was considered.b An average trace element loss by plant uptakewas calculated based onmean trace element concentrationsof Raphanus and Lamarckia in 2006 and Poa and Lamarckia in 2007. Average concentrations of micronutrients and trace elements in soil from all treatments reported in Madejón et al. (2010) were used.c

Fe Mn As Cd Cu Pb Zn

Average element concentration (mg kg�1) in the studied soil 40,318 652 131 2.89 118 250 300Theoretical element loss in bare soil (B) 177 2868 585 12.7 585 1108 1263

2006Fe Mn As Cd Cu Pb ZnS P % S P % S P % S P % S P % S P % S P %

C 176 0.10 0.86 2842 59.3 e1.15 580 0.27 0.87 12.6 0.24 e0.99 501 2.60 0.41 1099 0.82 0.85 1251 63.0 e4.07SL2 92.5 1.34 47.1 1496 911 16.1 305 1.65 47.5 6.63 2.25 30.1 264 48.4 38.3 578 16.2 46.5 659 713 e8.63SL4 93.1 1.66 46.6 1506 1020 11.9 307 6.11 46.5 6.68 2.52 27.7 266 48.8 37.8 582 22.5 45.7 663 739 e11.0BC2 112 1.53 36.0 1812 1110 e1.87 370 6.14 35.6 8.03 2.74 15.2 320 51.0 26.8 700 20.1 35.2 798 842 e29.8BC4 120 1.32 31.8 1936 1225 e10.2 395 8.42 31.1 8.58 3.14 7.77 342 65.6 19.5 748 30.0 30.1 852 1044 e50.2LE2 140 0.70 20.8 2261 586 0.76 461 2.18 20.8 10.0 1.44 9.88 399 25.7 16.1 874 7.14 20.6 995 526 e20.5LE4 137 1.62 21.6 2223 1556 e31.7 453 8.71 21.0 9.85 3.74 e6.89 392 71.1 8.44 859 22.6 20.7 979 1077 e62.8

2007Fe Mn As Cd Cu Pb ZnS P % S P % S P % S P % S P % S P % S P %

C 112 0.25 37.0 1805 164 31.3 368 0.77 37.0 8.00 0.30 34.7 318 4.22 36.2 698 2.77 36.9 795 56.4 32.6SL2 44.3 0.74 74.6 717 587 54.5 146 2.09 74.7 3.18 1.26 65.1 127 16.1 71.8 277 7.26 74.4 316 231 56.7SL4 34.0 1.33 80.1 550 375 67.8 112 1.35 80.6 2.44 1.35 70.2 97 21.6 76.6 213 14.5 79.7 242 213 63.9BC2 39.9 1.10 76.9 645 695 53.3 132 1.51 77.2 2.86 1.09 68.9 114 15.8 74.4 249 10.5 76.7 284 213 60.6BC4 26.6 1.06 84.4 430 468 68.7 87.8 1.88 84.7 1.91 1.23 75.4 76 18.4 81.4 166 13.5 83.9 189 251 65.1LE2 79.8 0.73 54.6 1291 312 44.1 263 0.56 54.9 5.72 0.69 49.6 228 9.35 53.1 499 7.25 54.4 568 134 44.4LE4 75.3 1.02 56.9 1219 439 42.2 249 2.26 57.1 5.40 0.79 52.0 215 15.2 54.5 471 12.7 56.5 537 183 43.0

a The percentage of vegetation cover in each treatment was considered for calculations of trace element loss due to soil erosion; f.ex., if the % cover in a plot was 50%, then loss of trace elements solely due to erosion processeswould be half of that in bare soil.

b De la Rosa et al. (2000).c Note that these average values differ from those shown in Table 1 as they include all treatments and not only the non-amended soil.

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Table 4Pearson’s correlations between various environmental parameters in 2006 and2007.

2006 2007

Cover Biomass Nr. sps Cover Biomass Nr. sps

pH 0.692** 0.599** 0.684** 0.638** 0.459** 0.768**TOC 0.301* 0.489** 0.479** 0.336** 0.275** 0.402**sol-Cd e0.503** e0.509** e0.496** e0.540** e0.443** e0.666sol-Cu e0.488** e0.455** e0.765** e0.674** e0.510** e0.764**Sol-Zn e0.489** e0.493** e0.575** e0.622** e0.430** e0.749**

**p� 0.05; *p� 0.01. Nr sps, number of species; TOC, total organic C.

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e30273026

limiting resources such as water also contributes to shape the plantcommunity. These and other environmental factors not assessed inthis study could partly account for the low percentage of varianceexplained by the multivariate analysis.

4.3. Concentrations of trace elements in above-ground biomass

Previous work in the same plots showed that single amendmentincorporations can significantly reduce trace element accumulationin above-ground biomass in the short-term (Madejón et al., 2006;Burgos et al., 2008). Data from this study further supports thedurability and sustainability of this approach in the long-term(Figs. 3 and 4). It is possible that a ‘dilution effect’ resulting fromhigher biomass yields may account for some of the differences intrace element concentrations observed between amended and non-amendment plots, particularly in 2006 (Jarrel and Beverly, 1981);however, our data provides evidence that other mechanisms mayalso be involved; for example, concentrations ofmicronutrients andtrace elements in Lamarckia in 2007 were significantly lower in LE4compared to LE2 despite similar biomass yields (Fig. 5). The samewas observed for Poa in 2007 when SL4 and SL2 were compared(Fig. 4). Depending on the element, amendment and speciesconsidered, re-treatment can be helpful in maintaining lowconcentrations of trace elements. Whether due to a dilution effect,enhanced immobilization or both, foliar concentrations of micro-nutrients and trace elements were as a rule lower in amended plotscompared to non-amended plots in 2006 and 2007 (Figs. 3e5). In2008, concentrations of micronutrients and trace elements weresimilar regardless of the treatment, although only one species(Lamarckia) was represented by all treatments, including no-amendment, and thus could be analyzed. It is known that traceelement uptake by plants exposed to the same soil becomes lessefficient as availability diminisheswith successive extractions everygrowing season (McGrath et al., 2002); thus natural attenuationmechanisms are also important. In general, concentrations of bothmicronutrients and trace elements were higher than those reportedas background for the same plant species (Madejón et al., 2006), butsimilar or lower than those reported in previous studies carried outon the same experimental plot (Madejón et al., 2006; Burgos et al.,2008). However, on occasions phytotoxic concentrations weredetected, particularly in non-amended plots. For instance, Mn andFe reached phytotoxic concentrations in Raphanus and Poa fromnon-amended plots (400e1000 mg kg�1 for both Mn and Fe;Chaney, 1989), whilst concentrations in amended plots were belowthat threshold level (Figs. 3 and 4). Iron concentrations in Lamarckiaplants from both amended and non-amended plots were alsowithin the phytotoxic range in 2007 (Fig. 5). Grasses secrete phy-tosiderophores into the surrounding soil to complex Fe, which thenis taken up as Fe-siderophore complex. High concentrations of Fe ingrasses have been previously observed on other areas of the Gua-diamar Valley (Madejón et al., 2002). The relatively low concen-trations of As and Pb in plants are consistent with other studies

assessing theirmobility and leacheability under controlled and fieldconditions (Pérez-de-Mora et al., 2007; Cabrera et al., 2008).Cadmium, which characteristically shows a high soileplant transfercoefficient, generally reached the maximum level tolerable forcattle (0.5 mg kg�1; Chaney, 1989) in plants growing on non-amended soil, whereas Cd concentrations in plants from amendedplots were generally below that threshold value. Copper concen-trations in plants were also relatively low, possibly due to enhancedretention in soil by non-soluble organic matter (Xiangdong et al.,2001). Zinc, which is known to be readily soluble in soil (Kabata-Pendias, 2004) accumulated at higher concentrations than theother trace elements studied, but still generally below the phyto-toxic range (500e1500 mg kg�1) proposed by Chaney (1989),except for Raphanus from non-amended plots.

4.4. Environmental risks: soileplant transfer, soil erosion andphytostabilization

In multi-element contaminated soils different elements showdifferent soileplant transfer coefficients. For the same elementsoileplant transfer coefficients are generally higher in areas of lowbiomass compared to areas of high biomass as a result of a dilutioneffect (Table S2; supplementarymaterial) (Madejón et al., 2009). Thishas important implications for herbivores; a well-established densevegetation cover should be thus less hazardous for these animals interms of trace element intake compared to a less developed plantcoverwhen the total amountof plantmaterial consumed is the same.

Nonetheless, the net translocation of trace elements to the plantbiotic compartment is higher in areas with high biomass yieldscompared to areas with low biomass yields (Table 3); even thoughfoliar concentrations of trace elements are lower (Figs. 3e5). It couldbe argued that if more biomass is available potential consumerscould feed more on these soils. Nonetheless, a well-developedvegetation cover may reduce wind- and water-assisted depositionof contaminated dust on non-contaminated areas and vegetationdecreasing potential intake of trace elements by biota (Bargagli,1998). The structure of the plant community is also important; forexample, grasses will re-grow foliage following grazing, whilstdicots such as radish will not. Thus, a grass-dominated vegetationcover will present greater food chain transfer potential than dicots,even if foliar element concentrations are lower.

The question is whether the establishment of a vegetation cover,despite contaminant-tolerant and low-accumulating, makes senseor not. For this purpose, we calculated a theoretical loss of micro-nutrients and trace elements from bare soil solely due to erosionprocesses and compared it to an average loss of trace elements ineach treatment as a result of both soil erosion and vegetationuptake (Table 3). It should be noted that this is an estimate based onan erosion rate of 4.4 Mg ha�1 y�1 (de la Rosa et al., 2000) andcertain assumptions that may present some limitations (see detailsin Text S4, supplementary material). Our data indicates that thepresence of a vegetation cover is particularly effective reducing theloss of elements with little soileplant transfer such As or Pb, orpresent at very high concentrations in the soil such as Fe both inyears of high (2006) and intermediate (2007) biomass (Table 3). It isalso effective, although to a lesser extent, with elements such as Cd(present at low concentrations in soil but with a high soileplanttransfer) and Cu (intermediate concentrations in soil and soileplanttransfer). Finally, it is only effective for elements present atmoderate to high concentrations and readily translocable (Mn andZn) when biomass yields are not too high (Table 3). Therefore, theestablishment of contaminant-tolerant and low-accumulatingvegetation, although not exempt from some risks, seems ingeneral a reasonable approach for stabilization of trace elements insemiarid soils with long-term potential, particularly in areas with

A. Pérez-de-Mora et al. / Environmental Pollution 159 (2011) 3018e3027 3027

high soil erosion rates. It is noteworthy that despite some trans-location to above-ground parts, a redistribution of trace elementsfrom deeper soil layers to soil surface through litter accumulation isminimized compared to accumulators. Another important aspect ofvegetation covers is that they provide shelter for animal specieswhich may become established in the area (WWF/ADENA, 2004).

5. Conclusions

� By-products used as soil amendments enhance vegetationdynamics and the establishment of a contaminant-tolerant andlow-accumulating vegetation cover in a semiarid soil.

� Successive applications of amendments may be necessary toimprove plant growth and to reduce trace element concen-trations in above-ground biomass, but this depends on the typeof amendments used, the species and the elements considered.

� The structure of the plant community established is indepen-dent of the type of amendment used and re-treatment, providedthat physicalechemical conditions and nutrient concentrationsin soil achieved by different treatments are relatively similar.

� The cycling of trace elements to the biotic compartment is higherin well-re-vegetated areas compared to areas of low vegetation,despite lower foliar concentrations; nonetheless, a low-accumulating and well-established vegetation cover generallyreduces loss of trace elements from soil by erosion processes.

� In most cases phytostabilization entails fewer risks in terms oftrace element mobilization than a non-revegetation strategyfor semiarid soils.

Acknowledgments

Dr. Pérez-de-Mora thanks the Spanish “Ministerio de EducaciónyCiencia” for financial support. This work was funded by the SpanishJoint Commission “CICYT” (project CTM 2004-01985 TECNO).

Appendix. Supplementary material

Supplementary material related to this article can be foundonline at doi:10.1016/j.envpol.2011.04.015.

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