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Published online 27 February 2006
Published in Soil Sci Soc Am J 70:582-589 (2006)
DOI: 10.2136/sssaj2005.0136
© 2006 Soil Science Society of America
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Soil Fertility & Plant Nutrition

Cadmium and Zinc in Saline Soil Solutions and their Concentrations in Wheat

Amir H. Khoshgoftarmanesha,*, H. Shariatmadaria, N. Karimianb, M. Kalbasia and S. E. A. T. M. van der Zeec

a Dep. of Soil Science, College of Agriculture, Univ. Tech., 84154 Isfahan, Iran
b Dep. Soil Science, College of Agriculture, University of Shiraz, Iran
c Wageningen Univ., Nieuwe Kanaal, 6709 Wageningen, The Netherlands

* Corresponding author (amirhkhosh{at}cc.iut.ac.ir)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil salinity and plant genotype may affect bioavailability of Cd and Zn to wheat. The aim of this study was to investigate the relationship between concentrations of Zn and Cd in saline soil solutions and in different wheat genotypes. A greenhouse experiment with four bread wheat genotypes (Triticum aestivum L. cv. Rushan, Kavir, Cross, and Falat), and a durum wheat (Triticum durum L. cv. Durum), at four salinity levels of irrigation water (0, 60, 120, and 180 mM NaCl) in triplicate was conducted. After 45 d of growth, the shoots were harvested, and Zn and Cd concentrations were determined in the shoots. Speciation of Cd and Zn in saturation paste extract was modeled using MINTEQA2. A significant (P < 0.05) correlation was observed between model results and Cd and Zn species measured using Amberlite resin. Cadmium and Zn speciation in MINTEQA2-calculated soil solution was affected by salinity but not by wheat genotype. The major Cd species present in MINTEQA2-calculated soil solution were free Cd2+, CdCl+, and CdSO40, that increased with increasing salinity. Free Zn2+ was the dominant Zn-species for all salinities and decreased with increasing salinity. Increasing salinity resulted in significant increases and decreases in shoot Cd and Zn concentrations, respectively, of the Zn-inefficient genotypes. Cadmium concentrations in shoots of Durum and Kavir genotypes were best predicted by CdCl+ concentrations in solution. In contrast, free Zn2+ ion concentrations in MINTEQA2 calculated soil solution were best related to shoot Zn concentrations in Zn-efficient genotypes. Under saline conditions, Zn and Cd speciation effects on bioavailability thus depend on both plant genotype and the metal of interest.

Abbreviations: AAS, atomic absorption spectrometry • DOC, dissolved organic carbon • EC, electrical conductivity • GFAAS, Graphite Furnace Atomic Absorption Spectrometry • SWRI, Soil and Water Research Institute


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALTHOUGH SALINE SOILS represent over 30% of arable lands in Iran (Khoshgoftar et al., 2004), limited research has been conducted on the effect of the soil aqueous phase chemistry of trace metal ions on their bioavailability in these soils. In saline soils, the ionic strength (I) of soil solutions and the concentrations of ligand anions are high (Naidu et al., 1995). The environmental implications of salinization in terms of uptake of metals by crops have yet to be fully assessed (McLaughlin et al., 1997: Khoshgoftar et al., 2004) despite the importance of metal uptake by crops. Cadmium uptake by crops is of concern because Cd is potentially toxic (Norvell et al., 2000) when consumed at low concentrations (0.2 mg kg–1 as a proposed maximum Cd level in cereal grains) (Codex Alimentarius Commission, 1999) whereas Zn is an essential micronutrient, which is deficient in many regions worldwide, such as in the salt-affected soils of central Iran (Khoshgoftarmanesh et al., 2004).

Soil solution contains ions, which may exist as free, hydrated ions and/or as dissolved species that are complexed with organic or inorganic ligands (Helmke, 1999). Determination of soil solution trace metal free ion activities is of great importance in the study of plant metal uptake because it is thought that metal speciation and ionic activity, rather than the total concentration of a dissolved metal, determine plant uptake (Kasawneh, 1971; Bingham et al., 1984).

Experiments involving solution hydroponic culture of plants have usually but not always suggested that free ionic Cd and Zn are absorbed by plants. In these experiments, the speciation of metals in soil solution, either using computer models or experimental data, consistently showed that only free metal ion is absorbed by plants (Checkai et al., 1987; Cabrera et al., 1988; Lorenz et al., 1997). In contrast to these results, McLaughlin et al. (1997) indicated tuber Cd concentrations of potato (Solanun tuberosum L.) were not related to Cd2+ activities in saline/sodic soil solutions, but were related to activities of the chloro-complexes.

We recently demonstrated that soil salinity is a key factor in controlling Cd uptake by wheat in central Iran (Khoshgoftar et al., 2004). It has been postulated that under saline conditions, high chloride (Cl–) concentrations in soil solution could increase the degree to which Cd is chloro-complexed (McLaughlin et al., 1994). Chloro-complexation raises total Cd concentrations in solution (Garcia-Miragaya and Page, 1976) and could lead to enhanced Cd uptake by crops through either faster Cd diffusion to roots or greater Cd uptake if chloro-complexes are transported across the root membrane (McLaughlin et al., 1997).

In normal soils, Zn is mainly present as Zn2+ and as metal-organic matter complexes (Sposito, 1981; McBride, 1989; Holm et al., 1995), but limited information is available to the speciation of Zn in saline soil solutions. Salinizing with NaCl has been shown to decrease free Zn2+ concentrations in soil solution (Khoshgoftar et al., 2004).

There are few studies where links have been established between concentrations of Cd and Zn species in saline soils solutions and in different wheat genotypes. The objective of this study was to investigate the impact of salinity on Cd and Zn speciation in soil solution and concentrations in wheat tissue.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A Cd-polluted surface (0–30 cm) soil was collected from Qom province in central Iran. The soil is classified as Typic Calcigypsids (Soil Survey Staff, 1999). Cadmium accumulation in this area is attributed to many years of large P-fertilizer application with Cd associated as an impurity. Selected properties of this soil are shown in Table 1.


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  		Table 1. Selected chemical properties of soil at the beginning of the experiment.

 
Soil pH was measured using a digital pH-meter (Model 691, Metrohm AG Herisau Switzerland) (Thomas, 1996a) and electrical conductivity (EC) using an EC-meter (Model Ohm-644, Metrohm AG Herisau Switzerland) (Rhoades, 1996). Organic matter content was determined by the Walkley and Black method (Nelson and Sommers, 1982). Percentages of clay, silt and sand were measured using the Hydrometer method (Gee and Bauder, 1986). The CaCO3 equivalent was determined by neutralizing with HCl and back titration with NaOH (Black et al., 1965). Available-P content in the soil was extracted from the soil with 0.5 M NaHCO3 (Olsen and Sommers, 1982) and was determined by a colorometric method (Black et al., 1965). Available-K was extracted with ammonium acetate and determined on a flame-photometer (Thomas, 1996b). One hundred mg of air-dried soil subsamples were digested in a mixture of HNO3–HClO4–HF on a hot plate until the digest turned into a light yellowish sticky mass. A 10% HNO3 solution was added to the digest until a volume of 10 mL was obtained for analysis of total Cd and Zn (Black et al., 1965) and then determined using atomic absorption spectrometry (AAS) (Black et al., 1965).

A bulk soil sample (about 240 kg) was dried, thoroughly mixed, and sieved to remove particles > 5 mm. Polyethylene pots (37 cm height, 17 cm diameter) were first filled with a 5-cm layer of well-washed sand to improve drainage. Then, homogenized soil weighing 2.5 kg was put into pots. At planting, uniform rates of N and K fertilizers [100 mg kg–1N and K as (NH4)2SO4 and K2SO4, respectively] were applied to each pot. These fertilizer rates were determined based on the SWRI fertilizer recommendation method (Milani et al., 1998) and mixed thoroughly with soil before addition to the pots. This greenhouse experiment with five wheat genotypes (Cross, Rushan, Kavir, Falat, Durum), and four salinity levels was conducted under natural daylight conditions. Durum and Kavir are Zn-inefficient genotypes that produce much higher dry matter yields when Zn is added compared with Zn deficient conditions (Khoshgoftarmanesh et al., 2004). In contrast, Falat, Rushan, and Cross are Zn-efficient genotypes with a low response to Zn addition to Zn-deficient soils. The four salinity levels were created by adding NaCl to irrigation water to achieve 0, 60, 120, and 180 mM concentrations, respectively. The salinity levels and ionic composition were typical of irrigation water used in wheat fields of the Qom province of Iran. Wheat was seeded in pots, thinned to five plants per pot after 10 d, and grown for 45 d. For the first 10 d after sowing, soil moisture was maintained near 75% of field capacity using deionized water. Thereafter, the pots were maintained near field capacity with frequent watering to adjust weight by using the respective salinized irrigation waters.

At harvest, shoots were cut at the soil surface, washed with deionized water, dried at 70°C, ground for 48 h, ashed at 550°C for 8 h, and the ash dissolved in 2 M HCl (Chapman and Pratt, 1961). Concentrations of Zn and Cd in the digest solutions were determined by graphite furnace atomic absorption spectrometry (GFAAS) (PerkinElmer 3400, PerkinElmer, Wellesley, MA).

A thermodynamic model and an experimental test were used to evaluate chemical species of trace metals in soil solutions. To estimate free Cd2+ and Zn2+ species experimentally, we used the method developed by Holm et al. (1995) based on the calculation of the partition coefficient of free ions between a Ca-saturated ion-exchange resin (Amberlite) and solutions. In addition, we used the geochemical speciation program, MINTEQA2 (Allison et al., 1991) to estimate activity of chemical species in solution.

After harvesting the wheat, a sample of 700 g of each soil was collected from each pot, air-dried, and sieved to pass a 2-mm sieve. Each soil sample was saturated with deionized water, mixed to a paste of uniform consistency and, after standing overnight, was transferred to a suction flask for extraction of the soil solution (Rhoades, 1996). Soil pH, EC of the saturated extract and concentrations of the trace metals Cd, and Zn were measured using the methods described earlier. Calcium and Mg were analyzed using AAS and Na and K using an atomic emission spectrophotometer (AA/AE spectrophotometer 157, Washington, MA). Concentration of phosphate (PO43–) in extracts was determined by the method of Murphy and Riley (1962). The extract was analyzed for nitrate (NO3–) by steam distillation (Keeney and Nelson, 1996) and SO4 by standard BaSO4 gravimetry and turbidimetry (Greenberg et al., 1985). Dissolved organic carbon (DOC) in soil extracts was measured using a DOC analyzer (Dohrmann D-80). Chemical composition data obtained was used as MINTEQA2 input (Allison et al., 1991) and the Gaussian DOC was assumed to calculate saturated extract Cd and Zn concentrations (the free ion Cd2+, Zn2+ and their complexes with Cl–, sulfate [SO42–], NO3–, PO43–, and hydroxide [OH–]).

The accuracies of Cd and Zn analyses were controlled by analyzing certified standard materials and including blanks in digestion batches. Analysis of NIST soil standard (San Joaquin #2709; certified Cd and Zn concentration, 0.38 ± 0.01 and 106 ± 3 µg g–1, respectively) gave Cd and Zn concentrations of 0.35 ± 0.04 and 103 ± 4 µg g–1, respectively. Recovery of Cd and Zn for apple leaf standard (#1573A) was 92 and 94%.

The experiment was set up in a completely randomized factorial design with three replicates. Results were analyzed using ANOVA procedures and means were separated using protected LSD at the 0.05 probability level (SAS Institute, 1988). In addition, correlation analyses between Cd and Zn concentrations in wheat shoots and soil solution were performed by using this package.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Solution Na+, Cl–, SO42–, Ca2+, and HCO3– concentrations increased proportionally with increased salinity levels, while Mg2+ and K+ were mostly unchanged (Table 2).


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  		Table 2. Measured total cations and anions in the saturated paste soil solutions.

 
The free Cd2+ and Zn2+ ions concentration of soil solutions calculated using the MINTEQA2 model was compared with those determined experimentally using Amberlite resin. Significant (P < 0.05) correlations were observed between model results and experimental data (Fig. 1a and b). Several computer programs (e.g., GEOCHEM) have been used to calculate free metal ion concentration in soil solution. The applicability of computer speciation models needs experimental verification. McGrath et al. (1986) demonstrated that experimentally determined (ion-selective electrode and ion exchange equilibrium methods) and computed (GEOCHEM) free ion concentrations in pure solutions with known concentrations of various ligands were identical. Holm et al. (1995) compared the proportions of free Zn2+ and Cd2+, determined by calculation and the resin technique in Cl+ and SO42+ solutions, and observed a good relationship between the methods. Accordingly, comparisons of the ion exchange resin method and MINTEQA2 calculations demonstrated excellent agreement for free hydrated Zn2+ and Cu2+ in the studied soils (Fotovat and Naidu, 1997). The strong relationship between the measured and MINTEQA2 calculated free Zn2+ and Cd2+ in the current study suggests that the MINTEQA2 model can be used for the determination of concentrations of Zn2+ and Cd2+ in saline soil solutions.


Figure 1
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  		Fig. 1. Relationship between free (a) Cd and (b) Zn concentrations in soil solution determined with speciation model and experimentally with a cation exchange resin.

 
Total cadmium concentration in soil extracts, as calculated by MINTEQA2, significantly increased (P < 0.05) with increasing NaCl concentration (Table 3). The increase of total Cd was slightly larger for the Zn-efficient genotypes. This might be due to pH effects, that is, that Zn-efficient and inefficient genotypes have differences in the acid exudation. Accordingly, the solution pH in the soils in which Zn-efficient genotypes were grown were significantly (P < 0.05) lower than the soils under growing of Zn-inefficient genotypes (Table 4). The increase of total Cd with salinity could be attributed to Cd replacement by Na+ on exchange sites and subsequent complexation with Cl– in soil solution (Bingham et al., 1984). Cadmium speciation in MINTEQA2-calculated soil solution changed with salinity treatments but it was unchanged between genotypes. The major Cd species present in MINTEQA2-calculated soil solution were free Cd2+ ion, CdCl+, CdSO40, and CdHCO3+species (Table 3). Other species including DOC-complexes were negligible. By considering the very low content of DOC and high Ca concentration of solution in our soil, the inclusion of DOC would have had little effect on speciation of metals in solution (Khoshgoftar et al., 2004). At higher salinity the ratio of Cd2+/total-Cd was smaller, although free Cd2+ concentrations also increased with increasing salinity levels.


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  		Table 3. Main chemical species of Cd and Zn in soil extracts from different wheat genotypes with different irrigation water salinity levels calculated with MINTEQA2 program.{dagger}

 

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  		Table 4. Soil solution pH from different wheat genotypes with different irrigation water salinity levels.

 
Increasing NaCl salinity significantly (P < 0.05) increased concentration of CdCl+ complex in MINTEQA2-calculated soil extracts proportionally with increased Cl– ion concentration. In 120 and 180 mM NaCl treatments, more than half of the total Cd was found to be in the form of chloride complexes (Table 3). Chloride is known to form stable complexes with Cd2+ (Hahne and Kroontje, 1973), which leads to desorption of Cd from exchange sites in soil (Garcia-Miragaya and Page, 1976). This explains the reasonably good relationship between NaCl salinity level and Cd concentration in MINTEQA2-calculated soil solution. Concentration of CdSO40 complex was relatively high in soil solution. This was because the stability constant for CdSO40 (log K = 2.5) is very similar to that for CdCl+ (log K = 2.0) (Lindsay, 1979; Smith and Martell, 1981). But SO42– complexed less Cd than Cl–. The concentrations of CdHCO3+ and CdSO40 complexes in soil extracts increased with salinity. The increase in free Cd2+, CdHCO3+, and CdSO40 concentrations by salinity is probably due to the ionic strength and uncommon ion effects (Fotovat and Naidu, 1998). Accordingly, NaCl salinity increased concentration of SO42– in the soil extracts (Table 2).

Soil salinity decreased the total concentration of Zn in MINTEQA2-calculated soil extracts (Table 3). This might be due to Cd desorption and Zn adsorption on the free surface site.

The ratio of Zn2+/total-Zn distinctly decreased with increasing NaCl salinity level; free Zn2+ was, however, the dominant species at all NaCl levels in soil (Table 3). These results were consistent with findings of Holm et al. (1995) and Khoshgoftar et al. (2004), who found that Zn is mainly present as Zn2+ in their soil solutions. Zinc was found to a lesser degree as ZnSO40 in the soil extracts. Sulfate concentration was also high enough (Table 2) in saline soil solutions to lead to relatively significant Zn complexation (up to 29% of total Zn). Other complexes (e.g., ZnHCO3+ and Zn-DOC) should be less important in the soil extracts.

Increasing soil salinity level resulted in significant increases in shoot Cd concentrations of Durum, Falat, and Kavir genotypes, while it had no effect on Cd concentration in Rushan and Cross genotypes (Fig. 2a ). The main effect of increasing salinity on shoot Cd concentration was significant (Fig. 2b). Differences in concentration (Fig. 2c) and uptake of Cd have been shown for different wheat genotypes (Clarke et al., 2002; Ozturk et al., 2003). Lower Cd concentrations in Zn-efficient genotypes may be related to the increased ability to inhibit Cd uptake by roots (Oliver et al., 1995; Li et al., 1997) or the decrease in Cd root translocations. Reduced Cd root translocation is a consequence of stimulated biosynthesis of phytochelatins that tightly bind heavy metals in root cells (Grill et al., 1985). There were no differences in relative Cd translocation from root to shoot among wheat genotypes (data not shown). Thus, we assumed that the differences in shoot Cd concentrations between Zn-inefficient and efficient genotypes were related to differences in Cd uptake by roots.


Figure 2
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  		Fig. 2. (a) Shoot Cd concentration for different wheat genotypes at different irrigation water salinity levels, (b) Main effect of salinity on shoot Cd concentration, (c) Main effect of wheat genotypes on shoot Cd concentration.

 
Shoot Cd concentrations in Zn-inefficient genotypes were best predicted by solution CdCl+ concentration (Table 5). This suggested that the CdCl+ complex was the main determinant of Cd solubility and uptake for Zn-inefficient genotypes Kavir and Durum. McLaughlin et al. (1997) found that tuber Cd concentrations were related to activities of the chloro-complexes, and it has been suggested that Cl-complexed Cd may also be absorbed by plants (Smolders and McLaughlin, 1996). Results also indicate that free Cd2+ concentration, which is often assumed to be the species taken up by plants (Bingham et al., 1984), is not a better indicator of Cd uptake by Zn-inefficient wheat genotypes. In contrast to Zn-inefficient genotypes, no significant relationships were observed between Cd concentration of ‘Cross’, ‘Rushan’, and ‘Falat’ shoots and total Cd, Cd2+, or CdCl+ concentrations (Table 5). This suggests that the Cd uptake system in Zn-efficient genotypes is different from that in the Zn-inefficient genotypes (Oliver et al., 1995; Hart et al., 1998). Influx of Cd across the root cell plasma membrane has been shown to occur via a concentration-dependent process exhibiting saturable kinetics (Hart et al., 1998). The Cd uptake saturable nature may be different among wheat genotypes.


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  		Table 5. Correlation coefficients between Cd concentrations in wheat shoots and Cd species concentrations in soil solution.

 
Increasing NaCl concentration slightly decreased Zn concentrations of shoots in all wheat genotypes (Fig. 3a ). The main effect of increasing salinity on shoot Zn concentration was significant (Fig. 3b). Shoot Zn concentrations varied between the wheat genotypes, and in particular, Cross and Rushan tended to accumulate more Zn in the shoots (Fig. 3c). Such differences in Zn concentration have been widely recognized among wheat genotypes (Cakmak et al., 1997; Kalayci et al., 1999). It appears Zn uptake may be more closely controlled by Zn-efficient genotypes as compared with Kavir and Durum. Differences in shoot Zn concentration between Zn-efficient and Zn-inefficient genotypes could be related to root-mediated alterations in rhizosphere chemistry that involve pH changes or release of organic ligands (Hacisalihoglu and Kochian, 2003). The pH values measured after plant growth (Table 4) suggest that enhanced Zn concentration in Cross, Falat, and Rushan genotypes might be affected by slight reduction in soil pH. Although Zn concentration in MINTEQA2-calculated soil extracts was similar for Zn-efficient and Zn-inefficient genotypes, this is probably due to higher Zn uptake by roots of Zn-efficient genotypes.


Figure 3
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  		Fig. 3. (a) Shoot Zn concentration for different wheat genotypes at different irrigation water salinity levels, (b) Main effect of salinity on shoot Zn concentration, (c) Main effect of wheat genotypes on shoot Zn concentration.

 
Free Zn2+ ion concentrations in MINTEQA2-calculated soil solution were significantly correlated with shoot Zn concentrations in the Zn-efficient genotypes ‘Cross’, ‘Rushan’, and ‘Falat’ (Table 6). Increasing soil salinity decreased free Zn2+ concentration, which caused the decrease of shoot Zn concentrations. The hypothesis that plants predominantly absorb the free Zn2+ ion from soil solution has been confirmed in other studies (Cabrera et al., 1988; Sachdev and Deb, 1991; Hamon et al., 1995). For instance, Sachdev and Deb (1991) reported that Zn uptake at different growth stages of rice (Oriza sativa L.) was significantly related to the free soil Zn2+ ion. Our results showed that there was no relationship between free Zn2+ in MINTEQA2-calculated soil solution and shoot Zn concentrations in Durum and Kavir. It appears that at high salinity levels, free Zn2+ ion concentrations is not a good indicator of Zn uptake by Zn-inefficient genotypes ‘Kavir’ and ‘Durum’. These results suggest that enhanced Cd concentrations in solution inhibit root Zn2+ uptake in Zn-inefficient genotypes, which could be attributed to competition for carriers across root membrane. It was clear from the results of present study that Zn2+ and Cd2+ inhibit the uptake of each other in roots of wheat (Hart et al., 2002), although the affinity of the membrane transporter is different for the two ions. Membrane transporter affinity has been shown to be greater for Cd than for Zn (Homma and Hirata, 1984; Hart et al., 2002). It is interesting that, while the affinity of the Zn/Cd carrier is quite high for Cd2+, relatively low Cd2+ activities may be required to inhibit Zn2+ uptake (Hart et al., 2002). In our experiment, greater Zn uptake by Zn-efficient genotypes might be partly due to a high affinity Zn2+ transporter in these genotypes. In support of this contention, the existence of a high affinity transporter for Zn2+ in bread wheat genotypes has been demonstrated (Hacisalihoglu et al., 2001). Further investigation is required to test this hypothesis.


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  		Table 6. Correlation coefficients between Zn concentrations in wheat shoots and Zn species concentrations in soil solution.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In saturated paste extracts of saline soils, Cd existed mainly as chloride complex while the dominant form of Zn was Zn2+, as predicted by MINTEQA2. Increasing soil salinity level significantly increased shoot Cd concentrations of Zn-inefficient wheat genotypes. Predicted concentrations of the CdCl+ complex in soil solution were significantly correlated to shoot Cd concentrations and may be a good indicator of Cd uptake by these wheat genotypes. In contrast to Cd, concentrations of Zn in shoots of all wheat genotypes decreased with increasing NaCl concentration. Predicted free Zn2+ ion concentrations in soil solution were significantly correlated to shoots Zn concentrations for Cross, Rushan, and Falat but not for Kavir and Durum. The results showed that the effect of salinity on Cd and Zn bioavailability differs for different genotypes as well as for different metals. Enhanced Cd concentrations in soil solution could inhibit Zn2+ uptake by roots, which may be attributed to competition for carriers across root membrane.


   ACKNOWLEDGMENTS
 
This work was supported by Isfahan University of Technology and Qom Management and Planning Organization (Project no. 12504).

Received for publication May 2, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Home page
 J. Environ. Qual.Home page
A. H. Khoshgoftarmanesh and R. L. Chaney
Preceding Crop Affects Grain Cadmium and Zinc of Wheat Grown in Saline Soils of Central Iran
J. Environ. Qual., June 27, 2007; 36(4): 1132 - 1136.
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