Soil amendments reduce trace element solubility in a contaminated soil and allow regrowth of natural vegetation

Environmental Pollution 139 (2006) 40e52 www.elsevier.com/locate/envpol

Soil amendments reduce trace element solubility in a contaminated soil and allow regrowth of natural vegetation Engracia Madejo´n*, Alfredo Pe´rez de Mora, Efraı´ n Felipe, Pilar Burgos, Francisco Cabrera Instituto de Recursos Naturales y Agrobiologı´a de Sevilla. IRNAS-CSIC, Avenida Reina Mercedes 10, P.O. Box 1052, 41080 Seville, Spain Received 15 December 2004; accepted 23 April 2005

Soil amendments affect soil chemistry and allow revegetation of soils contaminated by trace elements. Abstract We tested the effects of three amendments (a biosolid compost, a sugar beet lime, and a combination of leonardite plus sugar beet lime) on trace element stabilisation and spontaneous revegetation of a trace element contaminated soil. Soil properties were analysed before and after amendment application. Spontaneous vegetation growing on the experimental plot was studied by three surveys in terms of number of taxa colonising, percentage vegetation cover and plant biomass. Macronutrients and trace element concentrations of the five most frequent species were analysed. The results showed a positive effect of the amendments both on soil chemical properties and vegetation. All amendments increased soil pH and TOC content and reduced CaCl2-soluble-trace element concentrations. Colonisation by wild plants was enhanced in all amended treatments. The nutritional status of the five species studied was improved in some cases, while a general reduction in trace element concentrations of the aboveground parts was observed in all treated plots. The results obtained show that natural assisted remediation has potential for success on a field scale reducing trace element entry in the food chain. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Amendments; Assisted natural remediation; Plant development; Soil quality; Trace elements

1. Introduction Remediation of soils contaminated with trace elements is mainly based either on the extraction or the stabilisation of the contaminants. While physico-chemical extraction techniques generally imply degradation of soil structure and high costs, stabilisation techniques can improve soil physico-chemical and biological properties, do not generate by-products, are less expensive and therefore are more suitable for remediation of extensive

* Corresponding author. Tel.: C34 95 462 4711; fax: C34 95 462 4002. E-mail address: [email protected] (E. Madejo´n). 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2005.04.034

areas of low-value land (Mench et al., 2003). The potential of assisted natural remediation for remediation of trace element polluted soils has been recognised (Adriano et al., 2004). The technique is based on the use of amendments to accelerate those processes (sorption, precipitation and complexation reactions) that take place naturally in soils to reduce mobility and bioavailability of trace elements (Bolan and Duraisamy, 2003; Hartley et al., 2004). Due to their restricted nature, natural attenuation processes alone may not be sufficient in mitigating risks from trace elements (Adriano et al., 2004). Moreover, assisted natural remediation may enhance microbial activity, plant colonisation and development and thus augment a restart nutrient cycling in affected soils.

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

Vegetation plays a crucial role in the restoration of such degraded areas, because it prevents wind-blow of contaminated particles and reduces water pollution (Tordoff et al., 2000). Nevertheless, trace element uptake by plants implies several hazards such as introduction into the food chain (McLaughin, 2001). Thus, a study of soil properties and vegetation development is necessary to evaluate the effectiveness of remediation measures on ecosystem ecology and risk posed by the trace element content of a soil. The failure of the tailings pond dam at the Aznalco´llar pyrite mine, 45 km west of Seville, Spain, in April 1998 released about 6 million cubic meters of slurry composed of acidic water containing heavy metals and other toxic elements that affected ca. 4500 ha of land along the Agrio and Guadiamar river valleys. A long strip, approximately 300 m wide and 40 km long, was covered by a layer (2e30 cm thick) of black sludge (Cabrera et al., 1999; Grimalt et al., 1999). Severe As, Bi, Cd, Cu, Pb, Sb, Tl and Zn pollution was observed in most of the sludge-affected soils (Cabrera et al., 1999). Remediation works started soon after the accident. Toxic sludge and a variable layer of topsoil (10e30 cm) were mechanically removed; a second phase of the remediation consisted of addition of liming materials whilst a third phase encompassed the addition of organic matter and iron-rich clayey materials (Aguilar et al., 2004). Despite these clean-up and amending operations, soils in the affected zone still exhibit consistent trace element contamination. In many cases, total soil trace element concentrations increased after the remediation. This increase can be related to remains of sludge left on the surface of the soil and buried during sludge removal, liming, manuring and harrowing (Cabrera, 2000; Ayora et al., 2001). After these short-term measures, the Regional Government, ‘Junta de Andalucı´ a’ carried out a programme in the affected area to assess trace element stabilisation, reduce phytotoxicity and establish a vegetation cover. This programme has been called ‘Green corridor of the Guadiamar River’. The corridor should connect the lowland protected areas (Don˜ana National Park) with the forested mountains (Sierra Norte Natural Park) (CMA, 2003). In order to re-establish vegetation cover and make a ‘green corridor’, typical both Mediterranean forest and riparian forest species have been established in the area. Nevertheless, most soils have been spontaneously colonised by wild plants (mainly annual plants) well adapted to the local climatic conditions and little affected by contamination. The effect of the amendments employed on the colonisation, nutritional status and trace element concentrations of these plants has not been previously investigated although these plants play an essential role in soil stabilisation, nutrient cycling and organic matter turnover.

41

In this study we investigated the effects of three different amendments (sugar beet lime, biosolid compost and leonarditeCsugar beet lime) on soil chemical properties, trace element availability and wild plants growing in an experimental plot established on a soil contaminated by the Aznalco´llar mine accident. Plant species were identified, biomass production measured and vegetation cover estimated. Nutrient and trace element concentrations of the five most abundant plant species were also compared with phytotoxic and zootoxic concentrations given in the literature to evaluate potential risk of these elements in the food chain.

2. Materials and methods 2.1. Site description and experimental procedure The study site is an experimental field named ‘El Vicario’, affected by the toxic Aznalco´llar mine spill (Fig. 1), located on the right margin of the Guadiamar river (latitude 37  26#21$ N, longitude 06  12#59$ W), 10 km downstream from the Aznalco´llar mine. The only remediation work carried out in this field was the removal of the sludge from the surface of the soil together with a layer of soil of around 10e15 cm. The soil was a clay loam (21.1% clay, 29.1% silt and 49.8% sand) classified as Typic Xerofluvent (Soil Survey Staff, 1996). Before the spill, the area was mainly occupied by cropland (sunflower, wheat, sorghum) and orchards. Cultivation of food crops in the spill-affected area was prohibited by law after the accident. Within the field an experimental plot (20!50 m) was divided into 12 subplots of 7!8 m each, with a margin of 1 m (long) and 2 m (wide) between plots (Fig. 2a). Three amendments d two organic and one inorganic d from different sources were used. The two organic amendments were a biosolid compost (BC) from the wastewater treatment plant of SUFISA (Jerez de la Frontera, southern Spain) and a leonardite (LE), a low rank coal rich in humic acids (DAYMSA, Zaragoza, northern Spain). The inorganic amendment was sugar beet lime (SL), a residual material from the sugar beet manufacturing process with 70e80% (dry basis) of CaCO3 (EBRO-AGRI´COLAS, San Jose´ de la Rinconada, southern Spain). The most relevant characteristics of the amendments are listed in Table 1. The following rates (fresh basis) were applied twice (October 2002 and October 2003): (i) 30 Mg haÿ1 of BC (treatment BC), (ii) 25 Mg haÿ1 of LE mixed with 10 mg haÿ1 of SL (treatment LESL), and (iii) 30 Mg haÿ1 of SL (treatment SL). The doses were within the common rage of amendment application in land treating. These doses were below the maximum permitted limits for trace element annual load established by the European

42

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

Fig. 1. Location of the experimental plot of Vicario (rectangle).

Union (Directive 86/278/EEC) (CEC, 1986) for sewage sludge application. A treatment without amendment addition (NA) was also established. The amendments were mixed with the top soil (0e15 cm) of each subplot using a motor hoes (RL328 Honda). This tillage was also performed in the non-amended subplots. The experiment was carried out in a completely randomised block design with three replicates per treatment (Fig. 2b).

2.2. Soil sampling Soil samples (0 to 15 cm depth) were collected from 48 sites (four sites per subplot) on a 14!45 m (Fig. 2b) grid over the experimental plot. At each location three soil cores were taken to make a composite sample representative of each location. Soils were sampled twice: September 2002 (before the application of amendments) and September 2003 (one year after the

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

43

b)

11

12

9

10

7

8

5

6

3

4

1

2

50 m

a)

Sampling points

3m

1m

NA BC

2m

LEOSL

4m

SL

20 m Fig. 2. (a) Soil sampling locations in the experimental plot. (b) Distribution of amendments in the experimental plot.

application of amendments and before the second amendment application). All soil samples were air-dried, crushed and sieved through a 2 mm sieve, then ground with an agate mortar grinder to !60 mm prior to determination of S and trace elements.

Table 1 Mean values and standard deviation of some characteristics of the amendments Amendment

pH TOC (%) N (%) P (%) K (%) As (mg kgÿ1) Cd (mg kgÿ1) Cu (mg kgÿ1) Mn (mg kgÿ1) Pb (mg kgÿ1) Zn (mg kgÿ1)

BC

LE

SL

6.93G0.03 19.5G1.22 1.31G0.06 1.24G0.02 0.93G0.02 5.63G1.48 0.73G0.40 121G5.66 257G24.8 137G26.2 258G18.4

6.08G0.07 28.9G0.39 1.17G0.02 0.04G0.001 3.97G0.08 34.9G3.46 0.83G0.11 28.2G2.40 66.2G1.41 22.0G2.33 64.5G1.06

9.04G0.08 6.70G1.55 0.98G0.04 0.51G0.06 0.53G0.05 1.63G0.34 0.43G0.15 51.0G8.20 297G10.3 39.2G6.70 138G31.0

TOC, total organic carbon; BC, biosolid compost; LE, leonardite; SL, sugar beet lime (nZ3).

2.3. Plant survey and sampling Due to the heterogeneity of the chemical characteristics of the soil, observed in the first sampling, each subplot was divided equally into four parts (4 x 3.5 m), establishing 48 different vegetation sampling sites. Due to the different succession of wild plants in the experimental plot three surveys were conducted (December 2003, March 2004 and June 2004). For each survey, a 30!30 cm quadrat was used (Cox, 1990). The quadrat was randomly placed three times within each sampling site. Plant biomass was clipped and weighed, plant species were listed and vegetation cover estimated. Plant species were determined and named following the keys and nomenclature proposed by Valde´s et al. (1987). The most frequent species Oxalis pes-caprae L., Poa annua L., Lamarckia aurea (L.) Moench, Raphanus raphanistrum L., Medicago polymorpha L. were collected for shoot chemical analysis. In addition, control plants of the same species were collected from adjacent unaffected soils for comparison. The most relevant characteristics of these unaffected soils were: (i) pH 7.46; As 7.50 mg kgÿ1, Cd 0.30 mg kgÿ1, Cu 65.7 mg kgÿ1, Pb 48.5 mg kgÿ1, Zn 65.6 mg kgÿ1 for soil supporting

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

44

Oxalis pes-caprae L., Poa annua L. and Medicago polymorpha L. controls; (ii) pH 7.66; As 5.19 mg kgÿ1, Cd 0.16 mg kgÿ1, Cu 19.2 mg kgÿ1, Pb 32.4 mg kgÿ1, Zn 43.7 mg kgÿ1 for soil supporting Raphanus raphanistrum L. control; (iii) pH 4.77; As 11.6 mg kgÿ1, Cd 0.63 mg kgÿ1, Cu 18.3 mg kgÿ1, Pb 33.4 mg kgÿ1, Zn 61.5 mg kgÿ1 for soil supporting Lamarckia aurea (L.) Moench. Plant samples were washed for at least 15 s with a 0.1 N HCl solution and for 10 s with distilled water. Plant material was then dried at 70  C, ground and passed through a 500-mm stainless-steel sieve.

2.4. Chemical analysis Soil pH was measured in a 1/2.5 sample/1 M KCl extract after shaking for 1 h. Total organic carbon (TOC) was analysed by dichromate oxidation and titration with ferrous ammonium sulphate (Walkley and Black, 1934). Pseudo-total trace element contents and S concentration in soil samples (!60 mm) were determined after aqua regia digestion in a microwave oven (Microwave Laboratory Satation Mileston ETHOS 900, Milestone s.r.l., Sorisole, Italy). The term ‘pseudo-total’ accounts for the aqua regia digestion, because it does not completely destroy silicates. Available trace elements were determined by extracting samples with 0.05 M EDTA solution pH 7 (Quevauviller et al., 1998). Soil CaCl2-soluble trace element concentrations were determined in 1/10 soil sample (2 mm)/0.01 M CaCl2 extracts (Ure et al., 1993). Plant material was analysed for N by Kjeldahl digestion. Mineral nutrients (P, K, Ca, Mg and S) and trace elements (As, Cd, Cu, Pb and Zn) were extracted by wet oxidation with concentrated HNO3 (suprapur; density 1.39 g cmÿ3) under pressure in a microwave digester. Analysis of mineral nutrients and trace elements in the extracts of soil and plants were determined by ICP-OES in an IRIS ADVANTAGE, Thermo Jarrel Ash Corporation spectrometer and expressed on a dry basis. The accuracy of the analytical methods was assessed through BCR analysis (Community Bureau of Reference) of a plant sample (CRM 279, Sea lettuce) and soil sample (CRM 277, estuarine sediment) (Table 2). The Table 2 Analysis of BCR references samples. CRM 277 (estuarine sediment) and CRM 279 (Sea lettuce)

As Cd Cu Pb Zn

CRM 277 (Estuarine sediment)

CRM 279 (Sea lettuce)

Certified

Experimental

Certified

Experimental

43.7 11.9 102 146 547

42.8G0.83 12.4G0.20 119G1.52 147G0.01 507G16.0

3.09 0.28 13.1 13.5 51.3

2.03G0.11 0.43G0.10 13.5G0.50 11.4G0.79 48.2G1.70

Experimental values calculated from (nZ6).

detection limits of the method used were: As!0.01 mg Lÿ1, Cd!0.001 mg Lÿ1, Cu!0.005 mg Lÿ1, Pb!0.01 mg Lÿ1, Zn!0.001 mg Lÿ1. 2.5. Statistical analysis Mean and standard deviation (SD), were determined for all data. Data was tested for normality with the KolmogoroveSmirnov (K-S) test. Correlation analysis was performed to determine the relationship between the values of the different parameters analysed in soil and plants. The significance level reported ( p!0.01 and p!0.05) is based on Pearson’s coefficients. To study differences between treatments, the data were analysed by ANOVA, considering the treatments as the independent variable. The means were separated by the Tukey’s test, using a significance level of p!0.05. All statistical analyses were carried out using SPSS 11.5 for Windows.

3. Results and discussion 3.1. Soil pollution Initial measurements of soil parameters showed that the field plot was very heterogeneous (Table 3). This could be attributed to remains of sludge left buried in the soil after the clean-up operations (Cabrera, 2000). In addition, acidic water accumulated in surface puddles

Table 3 Mean, standard deviation (SD), variation coefficient (CV), minimum and maximum values of some soil chemical properties before remediation (nZ48) Variable

Mean

SD

pH TOC (%) S (mg kgÿ1)

3.86 0.92 8693

1.32 0.16 5740

34.2 17.6 66.0

2.45 0.62 529

7.28 1.33 25 663

211 4.44 119 471 381

103 1.16 26.6 216 136

48.8 26.1 22.3 45.9 35.6

58.9 1.79 84.2 159 134

421 8.26 193 1100 812

Available (EDTA) (mg kgÿ1) As 3.58 4.88 Cd 0.57 0.29 Cu 34.1 9.55 Pb 5.21 4.24 Zn 96.3 35.0

136 51.0 28.0 81.3 36.4

0.26 0.07 17.0 0.55 26.9

24.2 1.50 60.3 15.9 187

Total (mg kgÿ1) As Cd Cu Pb Zn

Soluble (CaCl2) (mg kgÿ1) Cd 0.037 Cu 1.50 Zn 9.86

CV (%) Min.

0.026 1.22 6.46

70.3 81.3 65.5

0.000 0.092 0.023

Max.

0.105 4.70 23.3

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

could have contributed to the heterogeneity of pH, total S and trace element distribution (Clemente et al., 2003). One of the main objectives of the remediation treatment was to assess the effect of different amendments (BC, LESL and SL) on several soil chemical properties. Table 4 shows the mean values of the pH, TOC and total S and the total, soluble (CaCl2) and extractable (EDTA) trace element concentrations in 2003, one year after application of the amendments. Soil pH was less acidic in BC and SL treatments (Table 4), mainly due to the buffering effect of the biosolid compost and the CaCO3 content of the sugar beet lime applied in these two treatments. In spite of the application of leonardite together with sugar beet lime, this mixture (LESL) did not cause a significant increase in the pH. This was probably due to the more acidic nature of the leonardite which may counteract the alkalinity of the sugar beet lime (Table 1). The organic treatments showed increases in soil TOC content in comparison with the SL and NA treatments (Table 4). This is especially interesting in this particular case, where the soil was highly degraded, totally or partially devoid of its surface litter and upper soil horizon. Therefore organic amendments increased the organic matter content of the soil, potentially enhancing fertility, structure and water retention. Mean values of total concentrations of As, Cd, Cu, Pb and Zn before and after the application of the Table 4 Mean values of pH, TOC, S content and total, available and soluble trace element concentrations in soils treated with the different amendments after one year of the first amendment application Variable

Treatments NA

BC

LESL

SL

pH TOC (%) S (mg kgÿ1)

3.19 (a) 0.81 (a) 6463 (a)

4.67 (bc) 1.38 (b) 5506 (a)

4.17 (ab) 1.50 (b) 8059 (a)

6.01 (c) 1.18 (ab) 5395 (a)

Total (mg kgÿ1) As Cd Cu Pb Zn

201 (a) 4.32 (a) 112 (a) 484 (a) 293 (a)

168 (a) 4.80 (a) 120 (a) 381 (a) 415 (a)

201 (a) 4.75 (a) 123 (a) 453 (a) 344 (a)

162 (a) 4.44 (a) 109 (a) 386 (a) 351 (a)

Available (mg kgÿ1) As Cd Cu Pb Zn

3.61 0.86 34.6 3.10 96.4

Soluble (mg kgÿ1) Cd Cu Zn

0.062 (c) 2.73 (b) 15.2 (b)

(a) (a) (a) (a) (a)

2.80 (a) 0.84 (a) 31.7 (a) 5.07 (a) 100 (a)

3.14 (a) 0.58 (a) 31.7 (a) 2.44 (a) 105 (a)

4.13 (a) 0.86 (a) 36.9 (a) 5.27 (a) 107 (a)

0.031 (ab) 0.61 (a) 7.62 (a)

0.047 (bc) 2.17 (b) 9.39 (a)

0.019 (a) 1.17 (a) 6.59 (a)

Values followed by the same letter in the same row do not differ significantly ( p!0.05).

45

amendments (Tables 3 and 4) were much higher than soil background values: As 18.9 mg kgÿ1; Cd 0.33 mg kgÿ1; Cu 30.9 mg kgÿ1; Pb 38.2 mg kgÿ1; and Zn 109 mg kgÿ1 (Cabrera et al., 1999). Thus, the concentration of trace elements in the experimental plot was still high despite the clean-up operations. Differences of pseudototal trace element concentrations between treatments were not expected since trace elements cannot be degraded in contrast with organic contaminants. However, solubility and bioavailability of trace elements can be more important in remediation studies than total or pseudo-total concentrations of these elements in the contaminated soil, because they represent the most labile fractions subject to leaching, and uptake by plant and microorganisms (Adriano, 2001). CaCl2-soluble Cd, Cu and Zn concentrations were similar to those found by Clemente et al. (2003) and Nagel et al. (2003) in other sites of the Guadiamar valley. The SL and BC treatments were the most effective in reducing CaCl2soluble Cd, Cu and Zn concentrations, whilst the LESL treatment was less effective (Table 4). This behaviour seems to be related to pH changes. It is well known that increasing the pH value of soil leads to a decrease in mobility of cationic trace elements, while adsorption capacity increases (Brallier et al., 1996). It has been proved that each unit of increase in pH results in approximately a 2-fold decrease in heavy metal concentrations (Zn, Ni and Cd) in the soil solution (Christensen, 1984; Sanders et al., 1986). Amendments did not have a clear effect on soil EDTA-extractable trace element concentrations and no differences were found between treatments. The values obtained were similar to those reported by Madejo´n et al. (2004) in soils affected by the mine spill for As, Cd and Zn while they were lower for Cu and Pb. Although EDTA extractable concentration is considered to be related to the fraction of trace element available to plants, results may differ when compared with other soil extractants and with plant uptake (Kabata-Pendias, 2001).

3.2. Effect on spontaneous vegetation 3.2.1. Plant development In the surveys of the experimental plot, 43 vascular plant species, representing 42 genera and 16 families were found. Gramineae (11) and Compositae (10) were the most represented families followed by Leguminosae, Cruciferae, Caryophyllaceae (3, respectively) and Umbelliferae (2). Most of these plants were annuals (34), while biannuals (5) and perennials (4) were less represented. Table 5 shows the species that were present in more than 55% of the sampling sites under the different treatments. Plant colonisation resulted from plant propagules from the surrounding area and the soil seed

46

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

Table 5 Most abundant species (O55%) in the affected plot for the different treatments December 2003

March 2004

June 2004

Treatment (NA) Poa annua L. Lamarckia aurea (L.) Moench

Lamarckia aurea (L.) Moench

Lamarckia aurea (L.) Moench Cynodon dactylon (L.) Pers

Treatment (BC) Poa annua L. Lamarckia aurea (L.) Moench Calendula arvensis L. Medicago polymorpha L. Raphanus raphanistrum L. Oxalis pes-caprae L. Sonchus oleraceus L. Malva sylvestris L. Treatment (LESL) Poa annua L. Lamarckia aurea (L.) Moench Juncus bufonius L. Raphanus raphanistrum L. Calendula arvensis L.

Treatment (SL) Poa annua L. Lamarckia aurea (L.) Moench Medicago polymorpha L. Oxalis pes-caprae L. Stellaria media (L.) Vill. Raphanus raphanistrum L. Malva sylvestris L. Sonchus oleraceus L. Diplotaxis virgata (Cav.) DC. Leontodon longirrostris (Finch & P.D. Sell) Fumaria officinalis L. Calendula arvensis L.

Oxalis pes-caprae L. Medicago polymorpha L. Raphanus raphanistrum L. Anagallis arvensis L. Sonchus oleraceus L. Calendula arvensis L. Malva sylvestris L.

Raphanus raphanistrum L. Bromus rubens L.

Lamarckia aurea (L.) Moench Medicago polymorpha L. Raphanus raphanistrum L. Poa annua L. Sonchus oleraceus L. Pulicaria paludosa Link Oxalis pes-caprae L. Calendula arvensis L.

bank, as all the identified species occurred naturally in proximity to the test plot. Fig. 3a shows the number of species found in each subplot in the three surveys. Species richness was greater in the amended subplots compared to the NA subplots in each survey. Although a similar number of species were found in some of each particular subplot in the three surveys (Fig. 3a), species were different on each occasion due to their different life cycles. Vegetation cover increased at each subplot with time (Fig. 3b) and it was higher in the amended subplots. A similar pattern was observed for plant biomass (Fig. 3c). This is due to the progressive colonisation and increasing plant development with time. Moreover, under a Mediterranean condition, greatest development of annuals occurs in spring, coinciding with the March and June surveys. In general, species richness, vegetation cover and biomass production were significantly higher in the amended subplots (Table 6) in each survey, especially in the SL and BC treatments. This pointed to a positive

Lamarckia aurea (L.) Moench Vulpia myuros (L.) C.C. Gmelin Vulpia ciliata Dumort Lolium multiflorum Lam.

Lamarckia aurea (L.) Moench Raphanus raphanistrum L. Bromus rubens L. Pulicaria paludosa Link Vulpia myurus (L.) C.C. Gmelin Lolium multiflorum Lam.

Lamarckia aurea (L.) Moench Pulicaria paludosa Link Vulpia myuros (L.) C.C. Gmelin Vulpia ciliata Dumort

effect of the amendments in enhancing plant colonisation and plant development in spite of the chemical heterogeneity revealed by soil analyses. The results obtained in the NA subplots for these three variables clearly indicate the adverse conditions (poor soil fertility and residual contamination) of the non-amended soil. The positive effects of the amendment might be related to an increase in soil pH (Table 4). There were strong positive correlations ( p!0.01) between soil pH and the number of species (rZ0.712), vegetation cover (rZ0.794) and biomass production (rZ0.642). The increase in soil pH reduced trace element solubility (Table 4) and thus potential toxicity to plants and microorganisms. Moreover the availability of macronutrients (Ca, Mg, K, P, N, and S) as well as molybdenum and boron is curtailed in strongly acid soils (Brady and Weil, 2003). Therefore the increase in soil pH seems to be the most important amendment effect in reducing trace element solubility. Moreover, the nutrients added through the amendments could also contribute to improve soil fertility in this soil.

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

14

Dec-03 Mar-04 Jun-04

number of species

12

47

a)

10 8 6 4 2 0

1-NA

2-SL

3-BC 4-LESL 5-SL

6-NA 7-LESL 8-BC

9-NA 10-SL 11-BC 12-LESL

Subplot-treatment 100

b)

Dec-03 Mar-04 Jun-04

90

Vegetation cover (

)

80 70 60 50 40 30 20 10 00

1-NA

2-SL

3-BC 4-LESL 5-SL

6-NA 7-LESL 8-BC

9-NA 10-SL 11-BC 12-LESL

Subplot-treatment 8000 7500

c)

Dec-03 Mar-04 Jun-04

7000 6500

biomass (kg ha-1 dw)

6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 1-NA

2-SL

3-BC 4-LESL 5-SL

6-NA 7-LESL 8-BC

9-NA 10-SL 11-BC 12-LESL

Subplot-treatment Fig. 3. Evolution of the number of species, vegetation cover and biomass production values in the different subplots. Bars are standard deviation values.

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

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Table 6 Mean values of number of species, vegetation cover and biomass production in the plots treated with the different amendments in the three surveys performed Treatment NA

BC

LESL

SL

No. of species Dec 03 Mar 04 Jun 04

1.83 (a) 2.42 (a) 3.00 (a)

8.75 (b) 9.50 (bc) 7.92 (b)

7.00 (b) 6.00 (ab) 8.92 (b)

10.4 (b) 11.2 (c) 8.25 (b)

Plant cover (%) Dec 03 Mar 04 Jun 04

16.3 (a) 17.3 (a) 32.1 (a)

57.5 (b) 65.5 (b) 81.7 (b)

43.5 (ab) 51.7 (ab) 65.7 (ab)

71.9 (b) 75.4 (b) 79.9 (b)

Biomass (kg haÿ1) Dec 03 Mar 04 Jun 04

118 (a) 225 (a) 809 (a)

1884 (b) 2863 (b) 4066 (b)

1150 (ab) 2943 (b) 4557 (b)

1955 (b) 2999 (b) 4668 (b)

Values followed by the same letter in the same row do not differ significantly ( p!0.05).

3.2.2. Plant macronutrient contents Medicago and Raphanus were two frequent highly representative species in the amended soils, however these species did not grow in NA subplots. Soil amendments improved conditions to enable seedlings of these species to grow to maturity. The macronutrient concentrations of plants growing in the contaminated soils (amended and non-amended) showed some differences from those of control plants collected from non-affected adjacent areas (Table 7). Nitrogen concentrations in Oxalis and Poa growing in contaminated soil, were lower than in control plants. This was also true for K in Oxalis, Medicago and Raphanus. There were no clear differences in N and K contents of plants from different treatments. The LESL treatment showed higher consistently K values because of the high K content in LE (Table 1). As a rule, P concentrations in plants from the BC and SL treatments were higher than in plants from the NA and LESL treatments. This might be related to the P content of BC and SL applied in these treatments (Table 1). Therefore these two amendments helped to restore P contents to those found in control plants. In a study of wild grasses in the Guadiamar area, Madejo´n et al. (2002) found that Cynodon plants growing in the polluted soils showed reduced uptake of N, P and K. However, the nutritional status of another wild grass (Sorghum) was less affected by soil pollution. The two grasses analysed in this study (Poa and Lamarckia) also showed differences in nutrient uptake in the affected soils (Table 7). Whereas a slight decrease in nutrient content was observed in Poa, Lamarckia had nutrient levels similar to control plants. The pyritic sludge contained elevated S. Sulphur concentrations were higher in plant samples from the

experimental plot compared to control plants (Table 7). Madejo´n et al. (2002) also found higher values of S in Cynodon and Sorghum plants growing on soils affected by the mine accident. In general, similar values were found in all treatments with the exception of Lamarckia and Medicago samples from the LESL treatment, which showed lower S content. Calcium contents of Lamarckia, Medicago and Raphanus in the amended plots were similar or even higher than control plants (Table 7). Madejo´n et al. (2002) found the same behaviour in grass growing in polluted soils, concluding that the increase in tissue Ca may be related to the protective action of Ca against the toxicity of metal and metalloids (Mengel and Kirkby, 1987). In fact, the concentrations of all micronutrients and trace elements studied were significantly and negatively correlated ( p!0.01) with the Ca concentration in plants (rZÿ0.292 for As, rZÿ0.320 for Cd, rZÿ0.557 for Cu, rZÿ0.282 for Mn, and rZÿ0.424 for Pb). Nevertheless, the application of amendments with significant concentrations of Ca could also increase the uptake of this element in the amended subplots, thereby reducing the absorption of trace elements by the plant. Lower concentrations of Mg were found in Oxalis, Poa and Medicago plants growing in the affected soils. In general, there were no differences in Mg concentrations between treatments.

3.2.3. Plant micronutrients and trace elements contents In general, plants growing in the contaminated soils had higher concentrations of micronutrients and trace elements than control plants (Table 8). Nevertheless, trace element concentrations in plants growing in the amended subplots were lower than those measured in NA plants. It was expected that the decrease in trace element concentrations in soil solution (Table 4) would be reflected in the decrease of trace element contents in plants growing in the amended subplots. Manganese concentrations in plants growing in the experimental plot were higher than concentrations found in control plants of the five species analysed. Oxalis, Poa and Lamarckia from amended treatments showed lower Mn concentrations than plants from the NA treatment. The organic matter added through the amendments could reduce Mn availability in soil and thus plant uptake (Gallardo-Lara and Nogales, 1987). In the case of Poa samples from the NA treatment, Mn values reached phytotoxic values (400e1000 mg kgÿ1) (Kabata-Pendias, 2001). The concentration of Cu in plants did not exceed the maximum level tolerated by sheep (25 mg kgÿ1; Chaney, 1989) and were in general, below the phytotoxic range for plants (20e40 mg kgÿ1; Kabata-Pendias, 2001) (Table 8). Copper concentration in Poa and Lamarckia from the NA subplots approached this

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

49

Table 7 Mean values and standard deviation of the macronutrient (g kgÿ1) analysed in the most frequent species of the plot Oxalis

Treatment (no. of plants sampled) NA (nZ6)

BC (nZ10)

LESL (nZ6)

SL (nZ9)

CT (nZ3)

N P K S Ca Mg

20.1G6.30 2.60G0.70 24.8G3.50 2.60G0.40 7.00G1.70 2.50G0.80

20.2G4.30 3.70G1.60 29.0G6.90 2.60G0.70 6.30G1.50 2.50G0.60

17.8G1.50 2.20G0.10 30.5G3.00 2.00G0.30 4.90G0.50 1.80G0.20

18.2G2.90 3.70G1.00 28.1G5.20 2.70G0.30 7.70G1.50 2.90G0.60

27.0G0.30 2.60G0.10 43.4G1.20 2.20G0.10 9.10G0.10 4.70G0.10

Poa

NA (nZ4)

BC (nZ12)

LESL (nZ9)

SL (nZ9)

CT (nZ3)

N P K S Ca Mg

18.2G9.70 1.70G0.30 28.3G2.30 3.40G0.30 2.30G1.10 0.70G0.10

13.9G3.50 3.00G0.40 28.0G4.00 3.60G0.60 4.30G1.10 1.10G0.20

17.4G5.20 2.00G0.50 36.4G5.20 3.20G0.40 4.00G1.30 1.00G0.20

11.7G2.70 2.50G0.50 26.9G4.90 3.70G0.80 5.20G1.50 1.00G0.40

23.5G0.20 2.50G0.20 35.9G0.50 2.60G0.10 5.90G0.50 2.40G0.10

Lamarckia

NA (nZ10)

BC (nZ12)

LESL (nZ10)

SL (nZ11)

CT (nZ3)

N P K S Ca Mg

20.0G0.70 2.70G0.40 29.9G6.70 4.50G0.24 3.00G0.90 1.20G0.40

22.2G3.50 3.00G0.50 27.8G3.10 4.80G1.30 3.30G0.40 1.50G0.40

23.0G5.30 2.40G0.20 32.5G6.20 3.40G0.80 3.00G0.60 1.40G0.40

16.2G2.40 3.10G0.60 25.6G1.70 3.40G0.70 4.40G1.10 1.10G0.10

19.3G2.50 3.00G0.10 24.7G0.60 3.50G0.10 2.40G0.10 1.10G0.10

Medicago

NA (nZ0)

BC (nZ8)

LESL (nZ7)

SL (nZ10)

CT (nZ3)

N P K S Ca Mg

e e e e e e

30.9G4.60 2.50G0.40 22.0G1.50 5.30G0.80 17.5G1.00 2.30G0.10

29.8G2.90 1.90G0.10 28.0G0.90 3.70G0.50 15.0G2.20 2.00G0.40

29.2G3.60 2.30G0.30 19.3G2.50 4.90G0.90 17.9G3.50 2.50G0.30

36.4G0.10 3.20G0.20 34.3G0.90 3.00G0.20 12.1G0.60 3.20G0.20

Raphanus

NA (nZ0)

BC (nZ12)

LESL (nZ10)

SL (nZ9)

CT (nZ3)

N P K S Ca Mg

e e e e e e

27.9G5.90 2.90G0.60 18.1G4.20 10.9G2.00 39.6G12.9 3.90G1.10

29.1G6.10 2.30G0.40 21.6G4.30 10.8G1.40 36.7G7.60 3.90G0.90

27.3G4.10 2.90G0.60 20.3G3.90 11.2G2.50 39.0G14.2 3.70G0.70

24.1G0.10 3.60G0.10 22.2G0.40 7.20G0.40 22.3G1.40 4.20G0.20

maximum value, while amended plants had Cu concentrations similar to those found in control plants. This could be related to the presence of other cations such as Zn, which could inhibit Cu adsorption at the root level (Kabata-Pendias, 2001), the increase of organic matter content in the soil which would bind Cu (Adriano, 2001) and/or the increase of the soil pH. Concentrations of Zn in plants growing in the contaminated soil were higher than those found in control plants (Table 8). Oxalis, Poa and Lamarckia from subplots NA had high Zn concentrations, in the range considered as excessive or phytotoxic (100e 400 mg kgÿ1) for plants (Kabata-Pendias, 2001). However these concentrations were lower than the toxic value for livestock (Chaney, 1989). Plants can tolerate a relative high content of Cd in soils but if they accumulate large quantities in their

tissues this may cause harmful effects on animals feeding them. Apart from Medicago, all other species exceeded the toxic level (0.5 mg kgÿ1) recommended for livestock (Chaney, 1989). Madejo´n et al. (2002) found similar levels of Cd in wild grasses growing in this affected area. Therefore Cd needs to be carefully monitored. Amendments, especially BC and SL, reduced the Cd concentration in plants when compared to non amended plants, thus natural assisted remediation can be an effective measure in reducing Cd uptake by wild vegetation. Despite the high concentrations of As and Pb in the sludge, plants did not show high levels of these elements in their tissues. Arsenic levels greatly exceeded normal values in soils and has no known function in plants. Nevertheless, As concentrations were only above the detection limit of the method used in Poa and Lamarckia. This can be related to the low availability of As in soils,

E. Madejo´n et al. / Environmental Pollution 139 (2006) 40e52

50

Table 8 Mean values and standard deviation of trace elements (mg kgÿ1) analysed in the most frequent species of the plot Oxalis

Treatment (no. of plants sampled) NA (nZ6)

BC (nZ10)

LESL (nZ6)

SL (nZ9)

CT (nZ3)

Mn Cu Zn Cd As Pb

265G130 12.2G3.37 172G69.0 0.72G0.26 !1.00 3.27G2.89

200G196 9.74G1.12 56.0G33.0 0.30G0.14 !1.00 1.55G1.64

185G65.0 8.44G0.84 65.0G20.0 0.42G0.08 !1.00 1.46G0.67

172G184 10.8G2.68 70.0G71.0 0.49G0.43 !1.00 1.97G1.05

35.6G0.84 12.3G0.60 18.7G2.80 !0.10 !1.00 0.44G0.39

Poa

NA (nZ4)

BC (nZ12)

LESL (nZ9)

SL (nZ9)

CT (nZ3)

Mn Cu Zn Cd As Pb

413G92.0 20.9G2.46 325G89.0 1.39G0.38 5.87G3.14 15.4G9.55

328G114 13.0G2.98 175G56.0 0.87G0.22 2.16G1.62 7.30G3.09

210G84.0 12.9G2.56 150G57.0 0.64G0.18 !1.00 2.91G2.02

118G84.0 10.3G1.81 79.0G35.0 0.68G0.16 !1.00 3.50G2.57

51.5G1.43 12.0G1.00 36.3G1.74 !0.10 !1.00 !1.00

Lamarckia

NA (nZ10)

BC (nZ12)

LESL (nZ10)

SL (nZ11)

CT (nZ3)

Mn Cu Zn Cd As Pb

296G93.0 19.7G10.4 228G118 0.79G0.42 2.50G2.17 5.39G3.40

285G91.0 14.8G4.51 188G39.0 0.53G0.09 1.21G1.07 4.93G3.65

234G34.0 15.0G4.42 146G59.0 0.46G0.14 !1.00 4.22G1.90

157G55.0 11.2G2.26 88.0G26.0 0.43G0.17 !1.00 3.49G1.84

142G5.00 3.67G0.54 17.4G7.05 !0.10 !1.00 !1.00

Medicago

NA (nZ0)

BC (nZ8)

LESL (nZ7)

SL (nZ10)

CT (nZ3)

Mn Cu Zn Cd As Pb

e e e e e e

64.0G18.0 8.41G1.09 78.0G21.0 0.12G0.13 !1.00 !1.00

97.0G29.0 9.61G0.42 86.0G37.0 0.13G0.10 !1.00 !1.00

59.0G22.0 9.04G0.88 50.0G11.0 !0.10 !1.00 !1.00

36.4G3.10 13.7G0.26 27.0G6.00 !0.10 !1.00 !1.00

Raphanus

NA (nZ0)

BC (nZ12)

LESL (nZ10)

SL (nZ9)

CT (nZ3)

Mn Cu Zn Cd As Pb

e e e e e e

147G87.0 4.54G1.19 194G110 0.58G0.26 !1.00 !1.00

199G164 6.53G1.78 327G123 0.95G0.32 !1.00 !1.00

123G86.0 4.48G1.04 125G99.0 0.52G0.21 !1.00 !1.00

79.0G6.00 4.91G0.49 19.5G6.31 !0.10 !1.00 !1.00

which is even lower at more acid pH values. In these two species, the highest As concentrations were found in samples from the NA and BC treatments, though potentially phytotoxic levels were only reached in Poa from NA (Kabata-Pendias, 2001). Furthermore, As has a comparative low soil-plant transfer coefficient (Kloke et al., 1984) due to its low mobility in the soil. Lead did not reach phytotoxic levels in any of the species studied. Concentrations in Oxalis, Poa and Lamarckia were higher in the experimental plot in comparison with control plants. Lead levels in Medicago and Raphanus were below the detection limit of the method used. The low concentrations of Pb found in most samples could be due to the low solubility of this element at pH levels above 4 (Evans et al., 1995) or to the increased TOC content, which can bind Pb strongly

(Al Chalabi and Hawker, 2000). Moreover, translocation to shoots tends to be greatly limited (about 3% in most cases), although plants can accumulate Pb in their roots. Although trace element concentrations in plants of the amended subplots were in general lower than those of plants from the treatment NA, the total amount of trace elements extracted in the amended plots by the whole vegetation was higher than in the non-amended plots. This fact is unavoidable if the revegetation of the area is the final goal in order to create a ‘green corridor’. It is therefore important to reduce the excessive uptake of trace elements by plants, which can be done successfully by amendments, and to monitor trace element contents of different taxa growing in the affected area.

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Table 9 Pearson’s coefficients between extractable EDTA and soluble CaCl2 trace element concentration in soil and plant trace element concentration As-plant Soil element extractable EDTA Soil element soluble CaCl2 * **

e e

Cd-plant **

ÿ0.262

0.319**

Cu-plant

Mn-plant

Pb-plant *

Zn-plant

e

e

ÿ0.172

0.179*

0.296**

0.345**

e

0.403**

p!0.05. p!0.01.

3.2.4. Trace element bioavailability Two extractants (0.05 M EDTA and 0.01 M CaCl2) were used to estimate trace element bioavailability. The correlation coefficients between soil EDTA-extractable and soil CaCl2-soluble trace elements and concentrations of each element in plant samples of the affected area are shown in Table 9. Apart for Zn, no positive correlations between soil EDTA-extractable and plant concentrations were found for any of the elements studied. EDTA did not seem to be a good extractant to assess trace element bioavailability in our conditions. This extractant alters the chemical environment dissolving greater quantities of elements from the soil-solid phase than are plant available in the short term (McBride et al., 2004) such as some trace elements in poorly crystallised Fe-oxides. Nevertheless, EDTA-extractable trace element contents were positively and significantly correlated with total trace element contents in our soil. On the other hand, positive and significant ( p!0.01) correlation coefficients between soil CaCl2-soluble elements and plant concentrations of each element (Mn, Cd, Cu and Zn) were found (Table 9). CaCl2 has been shown to be at least as effective as other more aggressive tests in representing plant availability (Van Erp et al., 1998; Houba et al., 1990). It is reasonable to expect that the best soil test for predicting plant accumulation of trace elements should be that one in which laboratory extraction occurs under conditions that do not greatly differ from those in the environment of plant roots (Alloway and Jackson, 1991). Therefore CaCl2 seems to be a more suitable extractant to predict trace element biovailability in these soils.

4. Conclusions Assisted natural remediation proved to be a successful and reliable technique for remediation of a trace element contaminated soil at a field scale with minimum maintenance. Based on their total trace element concentrations, soils of the Guadiamar valley still present high contamination. However, this technique reduces trace element availability and thus the risk of entry in the food chain, is cheaper than other methods and is accepted by the Spanish law. In fact, the programme of the ‘green

corridor’ has followed this technique to restore the area. The amendments increased soil pH values and TOC levels and reduced trace element soluble concentrations in comparison with non-amended soil. Moreover, growing of wild vegetation helped by the addition of amendments seems to be a useful phytostabilisation technique. In this case, the application of amendments clearly improved the establishment and colonisation of wild plants. These plants can modify the contaminated habitat and make it more suitable for subsequent plant communities restoring at the same time soil fertility and structure and reducing trace elements mobility. In addition, amendments improved in same instances the nutritional status of the studied species and diminished soil-plant transfer to aboveground parts thereby hampering the risk of trace elements entry in the food chain. Nevertheless, middle- and long-term evolution of the low-solubility compounds formed between organic matter and trace elements is of utmost importance. Solubility of these compounds could either decrease with time, favouring trace metal stabilisation, or increase and act as a ‘chemical time bomb’. For this reason, monitoring in soil and wild vegetation is required to ensure trace element stabilisation and evaluate the need for further applications.

Acknowledgements This study was carried out in the framework of the project REN 200-1519 TECNO supported by the CICYT. Dr. Burgos thanks her I3P program contract financed by the European Social Fund. Mr Perez de Mora thanks to the Spanish MCDE the financial support by the fellowship. The authors thank Dr. P. Madejon for her helpful comments.

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