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Speciation distribution of Cd, Pb, Cu, and Zn in contaminated Phaeozem in north-east China using single and sequential extraction procedures.

Introduction

Contamination by heavy metals in the soil environment is of great concern because of their toxicity and threat to human health (Zhou and Song 2004). Unlike organic contaminants, most metals in the soil environment do not undergo microbial or chemical degradation, and therefore concentrations of metals persist in soils for a long time after their input (Guo et al. 2006). Heavy metals remaining in the soil environment seep into groundwater or even channel into the food chain in crops growing on contaminated soils (Lin et al. 1998). Much research has been conducted on heavy metal contamination in soils from various anthropogenic sources such as industrial waste, automobile emission, mining or processing activities, and agricultural practice (Zhou and Huang 2001). Originally, most analytical measurements dealt with the total content of particulate metals in analysed samples. Fewer attempts have been made to evaluate the speciation of metals in particulate forms (i.e. the partitioning among various forms in which they might exist) (Tessier et al. 1979; Li et al. 1995; McGrath et al. 1999; Kot and Namiesnik 2000). The use of total concentration as a criterion to assess potential effects of soil contamination is not sufficient, because fate and toxicity of heavy metals in a contaminated soil is greatly controlled by speciation in the soil (Ernst 1996; Lo and Yang 1998; Peakall and Burger 2003). Biochemical and toxicological investigations have also shown that, for living organisms, chemical forms of a specific element, or the oxidation state in which that an element is introduced into the environment, is crucial, as well as the quantities (Lock and Janssen 2001). It is widely recognised that to assess environmental impact of heavy metal contamination in soils, the determination of the metal speciation will give more information about the potential for release of contaminants and further derived processes of migration and toxicity (Navas and Lindhorfer 2003). Therefore, in ecological risk assessment, it is necessary to determine chemical partitioning among various soil-chemical phases (Qian et al. 1996; Kot and Namiesnik 2000; Navas and Lindhorfer 2003).

The occurrence of heavy metals in soils affects their solubility, which directly influences their bioavailability (Ma and Rao 1997; Zhou and Sun 2002). Metal bioavailability is often correlated with the free-metal concentration, because the free ion is often the most bioavailable form of a dissolved metal. A method widely used to assess the lability and bioavailability of heavy metals in soils is the leaching of soils by chemical extractants. According to this, single and sequential extraction procedures have been applied using various extractants. Single extraction is a short procedure that evaluates the most labile, mobile, and mobilisable forms in soils, and allows a substantial time saving (Maiz et al. 1997). Single extraction with less aggressive solutions, called 'soft' or 'mild' extractants, such as neutral salts, provides a relative empirical method for evaluating the mobility of heavy metals, while extraction by chelating agents such as DTPA and EDTA and dilute acids could provide the correlation between extractability and plant uptake (Maiz et al. 2000; Fangueiro et al. 2002). Sequential extraction provides detailed information on distribution in different association forms, which are the potential source of metal available to biota (Qian et al. 1996; Zhou 2003). Usually, the fractions considered are: (1) exchangeable, (2) bound to carbonates, (3) bound to Fe and Mn oxides, (4) bound to organic matter, (5) residues (Salomons and Forstner 1980; Maiz et al. 2000; Zhou and Sun 2002).

Phaeozem (http://www.fao.org/ag/agl/agll/key2soil.stm) is a typical soil type in north-east China, which is one of the national food provision areas because of its high organic matter contents and intensive farming. The quality and quantity of agricultural products in this area were influenced by interruptions during various agricultural and industrial processes in recent decades. However, most researchers consider this a 'clean' area (Guo and Zhou 2004). Very little information on contamination status of this area exists at present. The first survey conducted at a regional scale analysed the distribution of contamination sources of the cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) in a wide variety of land uses and indicated a close relation between metal contents and source types. Several sites with possible multiple contamination were also identified after analysing the total contents of contaminants (Guo and Zhou 2006). Based on this previous research, the aim of this work was to further study the distribution of metal speciation in the contaminated Phaeozem in the Harbin industrial district, in order to determine plant availability and contaminant mobility and to provide information for risk assessment on Phaeozem contamination.

Materials and methods

Sampling

The study area is situated in Harbin, Heilongjiang Province, China, which is located in the continental temperate monsoon zone, with a dry and cold winter and a warm and wet summer. The annual mean temperature is 5[degrees]C. The annual average precipitation ranges from 500 to 600 mm, of which 50-70% occurs during May-September, and the frostless duration is around 120 days per year. The study site (Fig. 1) was near the Chemo-Industrial Road, with different industries such as plastic, paint, pesticide, and cement factories. Other major anthropogenic effects in this area are traffic exhaust emissions and agricultural activities. The soil is loose and black in colour. High contents of organic matter and cation exchange capacity are the main points of this kind of soil.

[FIGURE 1 OMITTED]

The studied area is about 10 000 [m.sup.2]. Fifteen surface (0-0.20m) and 15 subsurface (0.20-0.40 m) soil samples were collected following a tessellation pattern on August 2004 from a contaminated site in the studied area. Soil samples were air dried, ground to pass through a 2-mm plastic sieve, and milled in an agate pot to a fine powder (<170 um), then stored at room temperature in polyethylene flasks. Representative plants of Zea mays (L.) (Longdan 23), Glycine max (L.) Merr, Artemisia annua (L.), Artemisia sieversiana (Ehrh.) Willd., and Erigeron canadensis (L.) mainly growing in the contaminated site were harvested at the reproductive flowering phase. The aboveground material of the plant samples was rinsed with distilled water, dried at 105[degrees]C for I h and at 80[degrees]C for 24 h, ground, and stored in glass containers until analysis.

Soil properties and total metal contents

Soil pH was determined in a soil : water (1:2.5) suspension by using a pH meter. Organic matter and cation exchange capacity (CEC) determinations were carried out following the standard methods of chemical analysis (Zhou et al. 2003). Some main soil properties for the surface and subsurface samples are listed in Table 1. The pH values ranged from acidic to basic (6.2-6.9). The organic matter contents were in a broad range and the average at the surface was <32 mg/kg. CEC ranged from 15 to 32 cmol/kg. These values were mostly below the usual ranges of agricultural Phaeozem (Li and Zheng 1989; Shen 1998).

In order to assess the extent of metal contamination that was accumulated in the soil and plant samples and to provide an assessment of the long-term potential effects, total metal contents were determined after strong acid digestion. The acids used for soil and plant samples were HN[O.sub.3]-HCl[O.sub.4] (HN[O.sub.3]:HCl[O.sub.4] 3:1) (Guo and Zhou 2006).

Single extraction procedure

Aliquots of the dry soil samples were accurately weighed into 100-mL acid-cleaned polystyrene centrifuge tubes. Different extraction solutions were added and the tubes were shaken at ambient temperature on a variable speed reciprocal shaker at 220 strokes/min for 2 h (Maiz et al. 2000). Experimental details are listed in Table 2. After the extraction, the sample solutions were centrifuged at 5000 r.p.m, for 15 min and then filtered. The supernatant was used for analysis. All experiments were carried out at a room temperature of 25 [+ or -] 1[degrees]C.

Sequential extraction

The sequential extraction was developed from that of Tessier et al. (1979) using the same terminology. This procedure was selected because it is well documented, widely used, and has been adapted to the study of soils and dusts (Clevenger 1990; Navas and Lindhorfer 2003). The extraction was carried out progressively on an initial weight of 2.0 g of test material, which was contained in a centrifuge tube (polypropylene, 100 mL) and shaking with variable speed reciprocal shaker. The shaking speed was kept at 220 strokes/min. The extractants and operationally defined chemical fractions were as follows:

Fraction 1, exchangeable phase: the samples were extracted at room temperature for 1 h with 16 mL of 1 mol Mg[Cl.sub.2]/L (pH 7) with continuous stirring;

Fraction 2, carbonate phase: the wash residue of step 1 was extracted at room temperature with 16 mL of 1 mol NaOAc/L (adjusted to pH 5 with HOAc) for 5 h with continuous shaking;

Fraction 3, Fe-Mn oxide phase: the residue of step 2 was further extracted with 16 mL of 0.04 mol N[H.sub.2]OH.HCl/L in 25% HOAc (v/v) heated in a water bath for 6 h at 96[degrees]C with occasionally agitation;

Fraction 4, organic phase: the residue from the Fe-Mn oxide fraction was extracted with 4 mL of 0.02 tool HN[O.sub.3]/L and 6 mL of [H.sub.2][O.sub.2] 30% (adjusted to pH 2 with HN[O.sub.3]). The mixture was heated to 85[degrees]C for 2 h, with occasional agitation. A second 6 mL of [H.sub.2][O.sub.2] 30% (pH 2 with HN[O.sub.3]) was added and the mixture heated again to 85[degrees]C for another 3 h with intermittent agitation. After cooling, 6 mL of 3.2 M N[H.sub.4]OAc in 20%(v/v) HN[O.sub.3] was added and the samples diluted to 30 mL and agitated continuously for 30 min;

Fraction 5, residue: the residue of step 4 was digested with 18 mL HN[O.sub.3] (65%) and 6 mL HCl[O.sub.4] (40%) in 180 [+ or -] 5[degrees]C electro-thermal tank. When the sample was closely dried and became opalescent, it was diluted to 25 mL in a test tube with 5% HN[O.sub.3].

After each successive extraction, separation was done after centrifuging for 30min. The supernatant was filtered and placed in a tube for measuring. The residue was washed with 20 mL deionised water (DIW) followed by vigorous shaking, and then followed by centrifuging for the separation before the next extraction. All experiments were carried out at a room temperature of 25 [+ or -] 1[degrees]C.

Analytical methods

Guaranteed reagents and DIW were used throughout the study. All pipettes, tubes, and containers were washed with a detergent, then soaked for 24 h in 2% nitric acid (HN[O.sub.3]) solution and rinsed repeatedly with DIW. Total metal concentrations of the supernatants from each step were analysed by the graphite furnace atomic absorption spectrophotometer. Blanks were used for background correction and other sources of error. At least l duplicate and 1 spike sample were run for 5 samples in the acid digestion to verify precision of the analytical methods. The spike recovery and precision were found to be within 100 [+ or -] 10%. Samples were in triplicate in both single and sequential extraction procedures (LSR test, P < 0.05).

Results

Results of single extraction

Eight reagents classified in the 3 types are listed in Table 2 and were used to evaluate the potential mobility and bioavailability of metals in the contaminated Phaeozem. The metal extraction efficiency obtained with the single extraction procedures studied can be compared from the results presented in Table 3.

In the surface soil samples, the extractabilities of the 4 metals obtained with chelating reagents were generally higher than with dilute acid and neutral salts. The 2 chelating reagents could extract out >80%, 40%, 30%, and 20% of the total Cd, Pb, Cu, and Zn, respectively. EDTA hads a higher chelating capacity for the 4 metals in Phaeozem than DTPA. HOAc had higher extracting efficiency than HCl. The neutral salts had different extraction efficiencies for the 4 metals. The lowest extraction efficiency of Cd was obtained with the Mg[(N[O.sub.3]).sub.2], but it had a higher efficiency for Zn and Cu. Among the neutral salts, high Cd and Pb extractability was obtained with N[H.sub.4]Cl.

In the subsurface soil, the extraction procedure with HOAc had the highest efficiency for Cd and Pb. Among the neutral salts, Mg[(N[O.sub.3]).sub.2] had the most stable extraction efficiency for the 4 metals. The lowest extraction efficiency was obtained with Ca[Cl.sub.2], but differences among extractants with neutral salts or chelating reagents were not as clear as that for the surface soil samples.

Results of sequential extraction

As shown in Fig. 2, the fractions of Pb, Cu, and Zn were mainly bound to the residual phase with 40-72% recovery for the surface and 68-84% for the subsurface soil samples. Compared with the residual phase, much lower proportions of Pb, Cu, and Zn were extracted in the exchangeable and carbonate phases, which on average account, respectively, for 1.2% and 3.3% of total Pb concentration, 4.6% and 2.1% of total Cu, and 0.7% and 7.3% of total Zn in the surface soil samples. In the subsurface soil samples, insignificant amounts of Pb and Cu were retained in the carbonate and exchangeable phases and, similarly, negligible amounts of Zn were retained in these phases.

[FIGURE 2 OMITTED]

The percentage distribution of Cd was different from the rest of the metals. The residual of Cd was <10% in the surface and 30% in the subsurface samples. However, 32% and 47% of Cd from surface and subsurface samples, respectively, was associated with the exchangeable phases. In the surface samples, the lower proportion of total Cd was extracted in the organic phase (9.4%), and 22% and 33% of Cd was associated with the carbonate and oxide phase, respectively. In the subsurface samples, Cd in association with the carbonate phase was below detection limit (2 [micro]g/kg), and 4.4% was associated with oxide phase.

Assessment of bioavailability and mobility of the heavy metals

As can be seen in Table 4, the plant species on the contaminated Phaeozem had different contents of the 4 metals. Erigeron canadensis had the highest contents of the 4 metals. The 4 metals presented different relationships between extractability and plant uptake (Fig. 3). The Cd concentration in surface soil samples extracted by chelating reagents was close to the concentrations in Erigeron canadensis and Artemisia annua, and the concentrations of Cd in Glycine max and Artemisia sieversiana were close to those from the dilute acid. Concentration of Cd in Zea mays was much lower than that in the surface soil sample extracted by different reagents. Pb concentrations in the 5 plant species were close to the available Pb extracted by neutral salts. The concentration of Cu in Zea mays was similar to the Cu extracted by dilute acids, whereas concentrations in the other 4 plant species were close to that extracted by chelators in the surface soil samples. Zn concentration in the plants was higher than the potentially available Zn extracted by the 3 types of reagents.

[FIGURE 3 OMITTED]

Discussion

A large number of single extraction procedures were reviewed by Ginepro et al. (1996) and Pueyo et al. (2004). They stated that wide ranges of extracting efficiencies are possible depending on the different chemical properties of the selected extractants. In this study, the part of the 4 metals extractable by chelating reagents was greater than that extractable by dilute acids and by neutral salts in surface and subsurface soil samples. The maximum percentage extractable by chelating reagents was closer to the plant concentrations of Cd, Cu, and Zn in the contaminated Phaeozem. The acid-base properties of EDTA and DTPA could result in competition on the binding sites of metal ions and production of the new compound in the soil. They tend to release metals out of the non-silicate bound phase and, therefore, correlate well with plants contents and with the plant-available fraction for the 3 metals. The percentage extractable by neutral salts generally demonstrated the potentially mobile portion of the metals in Phaeozem. This part is far lower than that extracted by dilute acid and chelating reagent, but the contents of Pb in the 5 plant species were close to the portion extractable by neutral salts. Conductimetric experiments showed that the binding of Cd, Cu, and Zn has a significant electrostatic contribution, whereas for Pb there is a higher level of covalence in the binding of humic acids, which could result in the higher release and movement of Cd but lower mobility of Pb in the soil-plant system (Pinheiro et al. 1994). Results indicated N[H.sub.4]Cl had the most stable extractability among the 4 neutral salts. This could be due to the possible reaction of these elements with N[H.sub.3] and to the higher salt concentration of the N[H.sub.4]Cl solution (Pueyo et al. 2004). Based on the comparison of the available parts extracted by chelating reagents with those by neutral salts in the surface soil samples, Cd and Zn could be more easily accumulated in the plants, whereas Pb and Cu were less mobile and fixed in soils. Pb and Cu have lower solubility and are more stable in compound form than Cd and Zn in the studied area, with a high content of organic matter, which had been noted in previous work (Pinheiro et al. 1994; Zhang et al. 2001; Weng et al. 2002).

Metal concentrations in the 5 plants varied over a wide range. Owing to the variety of soil chemical conditions in metal-contaminated soil and the evolutionary potential of plants to adapt to extreme environments, a number of plant species are able to colonise naturally metal-enriched soils and to maintain a functional ecosystem. Metal concentrations of Cd, Cu, and Zn in Zea mays were low, and it can grow naturally in the contaminated site owing to its metal-resistant characteristics with the help of mycorrhizal fungi, by binding heavy metals into the roots and restricting their ability to migrate into the plant. The metal-binding capacity of mycorrhizal fungi is so striking that both the uptake of metals from the soil and their subsequent translocation to Zea mays may be effectively restricted when exposed to pollution, whereas it can enhance the plant access to nutrients such as P (Ernst 1996; Christie et al. 2004; Gaur and Adholeya 2004; Chiu et al. 2005). Metal concentrations of Cd, Cu, and Zn in Erigeron canadensis and Artemisia annua are higher and close to those in the soils. The major processes involved in the accumulation of heavy metals from soil to the plants possibly include bioactivation of metals in the rhizosphere through root-microbe interaction; and detoxification of metals by distributing to the apoplasts, e.g. binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins, and sequestration of metals into the vacuole by tonoplast-located transporters (Wei and Zhou 2004; Yang et al. 2005; Qureshi et al. 2005). The results have shown high availability of Cd and Zn in Glycine max. This important crop in the study site could be at risk with increasing input of the 2 metals.

In sequential extraction, Cu, Pb, and Zn were strongly retained in the residual phase in all the samples. These heavy metals are contained in the crystal lattices of minerals with strong bonds and consequently they will not be released into the environment. According to Navas and Lindhorfer (2003), processes of metal mobilization-immobilisation are affected by a variety of soil properties. The free ion activity in the soil is largely determined by metal bonding to the solid phase. Solid organic matter, dissolved organic matter, clay, and ion hydroxides could have a significant impact on the metal activity. High contents of organic matter in Phaeozem would contribute to the different bonding capacities of heavy metals. For example, Pb has a much greater and more stable affinity than Cd with soil sorbents after introducing into the soil environment. In this study, a higher proportion (33%) of Pb was associated with the oxide phase, and only in the case of a change in the redox conditions towards reductive would it be released from oxides. The high percentage of Cd in the exchangeable phase (32-47%) represents the mobile and bioavailable fractions. In this phase, Cd has the more labile bonds and can be more easily released into the environment. The Cd bound to the carbonate fraction would be released if conditions become acidic. This could be the supplement for the available Cd equilibrium in soil solution. Insignificant amounts of Cu in the exchangeable phase appeared in the sequential extraction. This could be closely connected with its lower background concentration in the tested samples (Guo and Zhou 2004).

The distribution of heavy metal speciation in the surface soil samples was different from that in the subsurface soil samples. The total and extractable metal concentrations in surface soil samples are generally higher than in the subsurface samples and clean soils (Table 1). Total concentrations of Cd, Pb, and Zn greatly increased compared with the background values at the study site (Guo and Zhou 2006). The increasing contents of the 3 metals were mostly concentrated in the surface soil. External sources were the main contributors to the increasing Cd, Pb, and Zn. The percentage of the residual fraction of the metals apparently increased in the subsurface soil samples, whereas the percentage of exchangeable phase Cd was relatively similar in the 2 soil layers. The source of Cd could result in the high bioavailability and mobility. According to its total content in the study area, Cd is the biggest danger in the contaminated site corresponding with its source and speciation distribution. The most mobile fraction of Cd in the contaminated area could transfer to the plants or the groundwater and endanger public safety.

Acknowledgments

This work was financially supported by the Ministry of Science and Technology, People's Republic of China as a 973 project (approval No. 2004CB418503) and by the National Natural Science Foundation of China as a project for 'Distinguished Young Scholars' (approval No. 20225722), and the China-Russia Joint Centre of Natural Resources, Ecology and Environmental Sciences, Chinese Academy of Sciences. The authors would like to thank all the helpers in the Key Laboratory of Ecological Process, Chinese Academy of Sciences.

Manuscript received 12 July 2005, accepted 11 January 2006

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G. L. Guo (A,B), Q. X. Zhou (A,C,E), P. V. Koval (D), and G. A. Belogolova (D)

(A) Key Laboratory of Terrestrial Ecological Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, P.R. China.

(B) Graduate School of the Chinese Academy of Sciences, Beijing 100039, P.R. China.

(C) College of Environmental Sciences and Engineering, Nankai University, Tianjin 300071, P.R. China.

(D) Vinogradov Institute of Geochemistry, PO Box 4019, Irtutsk-33, 664033, Russia.

(E) Corresponding author. Email: Zhouqixing2003@yahoo.com
Table 1. Properties of the studied Phaeozem samples

Soil pH Organic
 matter
 (g/kg)

Surface Range 6.2-6.9 24.3136
 Mean 6.47 31.6

Subsurface Range 6.3-6.7 13.2-31.8
 Mean 6.35 15.8

Clean soil Range 6.35-6.67 18.6-58.4
 Mean 6.51 37.8

Soil CEC Cd
 (cmol/kg)

Surface Range 15.8-31.4 0.68-2.29
 Mean 20.1 1.93

Subsurface Range 15.1-23.6 0.13-0.59
 Mean 17.2 0.45

Clean soil Range 15.0-41.9 0.06-0.19
 Mean 35.5 0.099

Soil Total metal concentration (mg/kg)

 Zn Pb Cu

Surface Range 103.2-214.3 48.6-07.9 18.3-25.4
 Mean 145.23 57.5 19.8

Subsurface Range 41.3-68.2 14.9-23.5 11.4-18.3
 Mean 53.60 17.2 13.4

Clean soil Range 44.42-61.29 3.44-29.62 10.34-20.26
 Mean 52.0 11.43 16.91

Table 2. Summary of the experimental procedures
for the single-step extractions

Extractant Abbreviation

Magnesium chloride Mg[Cl.sub.2]
Magnesium nitrate Mg[(N[O.sub.3]).sub.2]
Calcium chloride Ca[Cl.sub.2]
Ammonium chloride N[H.sub.4]Cl
Hydrochloric acid HCl
Acetic acid HOAc
Ethylenediaminetetraacetic acid EDTA
Diethylenetriaminepentaacetic acid DTPA

Extractant Condition

Magnesium chloride 1 mol/L, pH 7
Magnesium nitrate 1 mol/L, pH 7
Calcium chloride 0.1 mol/L
Ammonium chloride 1 mol/L, pH 7
Hydrochloric acid 0.1 mol/L
Acetic acid 0.43 mol/L
Ethylenediaminetetraacetic acid 0.05 mol/L, pH 7
Diethylenetriaminepentaacetic acid 0.01 mol/L, pH 7

Extractant Reference

Magnesium chloride Navas and Lindhorfer (2003)
Magnesium nitrate Shao et al. (1994)
Calcium chloride Houba et al. (1996)
Ammonium chloride Xia and Liu (1994)
Hydrochloric acid Lu et al. (2003)
Acetic acid Ure et al. (1993)
Ethylenediaminetetraacetic acid Fangueiro et al. (2002)
Diethylenetriaminepentaacetic acid Prokop et al. (2003)

Table 3. Metal contents (mean concentration [+ or -] s.d., mg/kg)
from the single extraction on surface and subsurface contaminated
Phaeozem by different reagents

Within columns, values followed by the same letter are not
significantly different at P = 0.05 (LSR test)

 Surface soil (mg/kg)

Extractant Cd Pb

Mg[Cl.sub.2] 0.87 [+ or -] 0.04d 0.79 [+ or -] 0.106f
Mg[(N[O.sub.3])
 .sub.2] 0.25 [+ or -] 0.032g 0.82 [+ or -] 0.24f
Ca[Cl.sub.] 0.45 [+ or -] 0.003f 1.09 [+ or -] 0.123e
N[H.sub.4]Cl 0.86 [+ or -] 0.096d 1.3 [+ or -] 0.121e
HCl 0.64 [+ or -] 0.061e 2.1 [+ or -] 0.112d
HOAc 1.05 [+ or -] 0.045c 9.72 [+ or -] 0.22c
EDTA 1.68 [+ or -] 0.016a 26.65 [+ or -] 0.2a
DTPA 1.53 [+ or -] 0.041b 23.06 [+ or -] 0.325b

 Surface soil (mg/kg)

Extractant Cu Zn

Mg[Cl.sub.2] 0.64 [+ or -] 0.021f 0.83 [+ or -] 0.233f
Mg[(N[O.sub.3])
 .sub.2] 0.79 [+ or -] 0.086e 2.41 [+ or -] 0.006e
Ca[Cl.sub.] 0.25 [+ or -] 0.055g 0.30 [+ or -] 0.006f
N[H.sub.4]Cl 0.34 [+ or -] 0.027g 2.07 [+ or -] 0.398e
HCl 1.12 [+ or -] 0.012d 7.80 [+ or -] 0.246d
HOAc 1.52 [+ or -] 0.02c 13.43 [+ or -] 1.048c
EDTA 8.46 [+ or -] 0.153a 34.43 [+ or -] 0.099a
DTPA 6.37 [+ or -] 0.097b 29.44 [+ or -] 0.3376

 Subsurface soil (mg/kg)

Extractant Cd Pb

Mg[Cl.sub.2] 0.25 [+ or -] 0.043bc 0.69 [+ or -] 0.06c
Mg[(N[O.sub.3])
 .sub.2] 0.10 [+ or -] 0.011d 0.93 [+ or -] 0.01c
Ca[Cl.sub.] 0.10 [+ or -] 0.006d 0.26 [+ or -] 0.025c
N[H.sub.4]Cl 0.12 [+ or -] 0.007cd 0.23 [+ or -] 0.046c
HCl 0.23 [+ or -] 0.003bcd 0.88 [+ or -] 0.084c
HOAc 0.59 [+ or -] 0.003a 4.54 [+ or -] 0.006a
EDTA 0.37 [+ or -] 0.149b 3.44 [+ or -] 1.04ab
DTPA 0.30 [+ or -] 0.148b 2.81 [+ or -] 0.993b

 Subsurface soil (mg/kg)

Extractant Cu Zn

Mg[Cl.sub.2] 0.38 [+ or -] 0.006e 0.77 [+ or -] 0.195de
Mg[(N[O.sub.3])
 .sub.2] 0.56 [+ or -] 0.133d 1.22 [+ or -] 0.163cd
Ca[Cl.sub.] 0.13 [+ or -] 0.01f 0.02 [+ or -] 0.006f
N[H.sub.4]Cl 0.17 [+ or -] 0.065f 0.41 [+ or -] 0.08ef
HCl 1.10 [+ or -] 0.006c 1.42 [+ or -] 0.031c
HOAc 1.70 [+ or -] 0b 1.51 [+ or -] 0.006c
EDTA 3.67 [+ or -] 0.145a 4.50 [+ or -] 0.338a
DTPA 3.66 [+ or -] 0.051a 3.21 [+ or -] 0.58lb

Table 4. Metal concentrations (mean [+ or -] s.d., mg/kg) in the
representative plants growing in the contaminated Phaeozem

Within columns, values followed by the same letter are not
significantly different at P= 0.05 (LSR test)

Plants Cd Pb

Erigeron canadensis 2.33 [+ or -] 0.48a 3.31 [+ or -] 0.66a
Artemisia annua 1.62 [+ or -] 0.45b 1.56 [+ or -] 0.43b
Artemisia sieversiana 0.93 [+ or -] 0.12c 1.25 [+ or -] 0.20b
Glycine max 0.62 [+ or -] 0.13cd 0.75 [+ or -] 0.21b
Zea mays 0.14 [+ or -] 0.04d 0.83 [+ or -] 0.41b

Plants Cu Zn

Erigeron canadensis 6.89 [+ or -] 0.85a 52.83 [+ or -] 10.53a
Artemisia annua 5.28 [+ or -] 1.09ab 56.42 [+ or -] 10.12a
Artemisia sieversiana 3.24 [+ or -] 0.86cd 31.78 [+ or -] 7.11a
Glycine max 3.77 [+ or -] 0.86bc 51.57 [+ or -] 5.28a
Zea mays 1.82 [+ or -] 0.43d 22.73 [+ or -] 5.46b
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Article Details
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Author:Guo, G.L.; Zhou, Q.X.; Koval, P.V.; Belogolova, G.A.
Publication:Australian Journal of Soil Research
Geographic Code:9CHIN
Date:Mar 15, 2006
Words:6148
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