Published in Soil Sci. Soc. Am. J. 68:1885-1889 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Division S-4—Soil Fertility & Plant Nutrition
Salinity and Zinc Application Effects on Phytoavailability of Cadmium and Zinc
A. H. Khoshgoftara,*,
H. Shariatmadaria,
N. Karimianb,
M. Kalbasia,
S. E. A. T. M. van der Zeec and
D. R. Parkerd
a Dep. of Soil Sciences, College of Agriculture, Univ. of Technology, Isfahan, Iran, 84154
b Dep. of Soil Sciences, College of Agriculture, Univ. of Shiraz, Iran
c Dep. of Environmental Sciences, Section of Soil Quality, Wageningen Univ., P.O. Box 8005, 6700 EC Wageningen, the Netherlands
d Dep. of Environmental Sciences, Univ. of California, Riverside, 92521, CA, USA
* Corresponding author (ahkhoshgoftar{at}yahoo.com)
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ABSTRACT
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Salinity and Zn deficiency in soils are two factors that may change the phytoavailability of Zn and Cd. The aim of this study was to determine the effects of salinity and Zn application on soil Cd and Zn solubility and their concentration in wheat shoots. A greenhouse experiment with wheat (Triticum aestivum L. cv. Rushan) consisting of two levels of Zn (0 and 15 mg Zn kg–1, in the form of ZnSO4), and five salinity levels of irrigation water (0, 60, 120, and 180 mM NaCl, and 120 mM NaNO3) in triplicate was conducted. Wheat was seeded in pots. After 45 d of growth, the shoots were harvested, and Zn and Cd concentrations were determined. After harvesting, electrical conductivity (EC), pH, and concentrations of anions and cations were determined in soil saturation extracts. Concentrations of Cd and Zn species in soil solution were predicted using the speciation program MINTEQA2. Increasing salinity increased total Cd (CdT), Cd2+, CdCl+, CdHCO3+, and CdCl20 concentrations in the soil solution, whereas no such effect was found for the NaNO3 treatment. Higher salinity decreased the total Zn (ZnT) and free Zn2+ concentrations in the soil solution and decreased Zn concentrations in the wheat shoots. With application of Zn fertilizer, shoot Cd concentrations decreased by 11 to 90%, whereas Zn concentration increased by 75 to 103%. Increasing salinity of irrigation water decreased shoot dry matter, especially if no ZnSO4 was applied. Application of Zn had a positive effect on salt tolerance of plant and increased dry matter of shoot.
Abbreviations: EC, electrical conductivity • GFAAS, Graphite Furnace Atomic Absorption Spectrophotometer • ISWRI, Iranian Soil and Water Research Institute
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INTRODUCTION
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ALTHOUGH ADVERSE EFFECTS of salt accumulation in soil and crops have been investigated intensively (Helal et al., 1995; Richards 1954), relatively little attention has been paid to the influence of either soil or irrigation water salinity on heavy metal uptake by crops (Helal et al., 1996). Cadmium uptake by crops is a serious concern because of its potential toxicity to humans (Smolders and McLaughlin, 1996; Weggler-Beaton et al., 2000).
Soil is the principal source of Cd accumulated by plants. Many characteristics such as concentration and form of metal in the soil, pH, organic matter content, clay content, concentration of Zn, other cations, complexing ligands, and fertilization practices, have been recognized as major factors that determine the bioavailability of Cd in soil (Boekhold and van der Zee, 1994; Norvell et al., 2000; Smolders et al., 1998). In addition, there is evidence of enhanced Cd uptake by some crops due to elevated salinity or Cl– concentrations (Bingham et al., 1984; Li et al., 1994; McLaughlin et al., 1994, 1997; Smolders and McLaughlin, 1996; Weggler-Beaton et al., 2000). These studies suggest that enhanced Cd uptake due to high salinity or Cl– concentrations may be expected for many crops, including major cereal crops.
During the last 30 yr, large amounts of P fertilizers have been used in salt-affected soils of central Iran. Most of the inorganic fertilizers contained relatively high concentrations of Cd. The Cd that has been applied with P fertilizer may well be plant available and adversely affect food quality.
In addition, low Zn availability is common in wheat-growing areas of the world (Takkar and Walker, 1993), such as central Iran. Although large quantities of Zn have been removed from the soils via crop removal, limited attention has been paid to fertilizing with Zn (Takkar and Walker, 1993). Soil salinity is frequently associated with alkaline soils deficient in available Zn (Tinker and Lauchli, 1984). Zinc is not available because in alkaline soils (high pH) Zn sorbs or precipitates in unavailable forms. Furthermore, alkaline soils are often also saline. Soil salinity may reduce Zn uptake due to stronger competition by salt cations at the root surface (Tinker and Lauchli, 1984).
In addition, salinity may enhance Cd uptake and thus decrease Zn accumulation in wheat due to the well-known antagonistic relationship that exists between Zn and Cd (Jalil et al., 1994). Cadmium is chemically similar to Zn and thus Cd may substitutes for Zn in chemical and physiological processes in the soil and plant (Jalil et al., 1994).
The purpose of this study was to investigate the influence of salinity on the solubility of Cd and Zn in soil and their uptake by wheat. Furthermore, Zn fertilization was tested as a means of reducing salinity stress and decreasing Cd uptake by wheat.
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MATERIALS AND METHODS
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Soil Sample
A Cd-polluted surface (0–30 cm) soil was collected from a field in Qom province, central Iran. The soil is classified as a Typic Calcigypsids (Soil Survey Staff, 1999). Cadmium accumulation in this area is mainly attributed to many years' high applications of P fertilizers that contain Cd as an impurity (Baghoori, 1998). Selected properties of this soil are shown in Table 1.
Soil pH (1:2 soil/water) was measured using a pH-meter (Model 691, Metrohm AG Herisau, Switzerland) and EC in soil saturation extracts using an EC-meter (Model Ohm-644; Metrohm AG Herisau, Switzerland). The CaCO3 equivalent was determined by neutralizing with HCl and back titration with NaOH (Black et al., 1965). Percentages of clay, silt, and sand were measured using the Hydrometer method (Chapman and Pratt, 1961). Available-P content in the soil was extracted from the soil with 0.5 M NaHCO3 (Olsen and Sommers, 1990) and was determined by a colorometric method (Black et al., 1965). Available K was extracted with ammonium acetate and determined on a flame-photometer (Chapman and Pratt, 1961). The total Cd and Zn content were determined by microwave digestion (USEPA, 1995; Method 3051). About 0.500 g of air-dried soil subsample (<150 µm) was digested in XP1500plus Teflon-PFA microwave vessel (CEM Corp, Mattews, NC) using 9 mL of HNO3 and 3 mL of HCl, then filtered through Whatman no. 42 filters, transferred to 50-mL volumetric flasks and diluted with deionized, distilled water (DDW). Chelate-extractable Zn and Cd were extracted using 0.005 M DTPA-TEA (Lindsay and Norvell, 1978) and then determined on a graphite furnace atomic absorption spectrophotometer (GFAAS) (PerkinElmer 3400, PerkinElmer, Wellesley, MA) with deuterium background correction and 0.1% (v/v) H3PO4 acid used as a modifier (Black et al., 1965).
The accuracy of Cd and Zn analysis was 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 mg kg–1, respectively) gave Cd and Zn concentrations of 0.35 ± 0.04 and 103 ± 4 mg kg–1, respectively. Recovery of Cd and Zn for apple leaf standard (#1515) was 91 and 96%, respectively.
Experiment
A bulk soil sample (about 240 kg) was dried, thoroughly mixed, and sieved to remove particles >5 mm and brought to the greenhouse. Homogenized soil weighing 2.5 kg was put into polyethylene pots (27 cm height, 17 cm diameter). Pots were first filled with a 5-cm layer of well-washed sand to improve drainage. A greenhouse experiment with two levels of Zn application (0 and 15 mg Zn kg–1 dry soil, in the form of ZnSO4), and five salinity levels was conducted under natural daylight conditions. The five salinity levels were achieved by using irrigation water with salinities of 0, 60, 120, and 180 mM NaCl, and 120 mM NaNO3, respectively. The salinity levels and ionic composition were typical of irrigation water used in wheat fields of the Qom province of Iran. A NaNO3 treatment was also included to enable distinction between the effects of osmotic stress and of chloro–Cd complexation in solution on plant uptake. At planting, uniform rates of N and K fertilizers [100 mg kg–1 N and K each as (NH4)2SO4 and K2SO4, respectively] were applied to each pot. These fertilizer rates were determined on the basis of the soil and water research institute (SWRI) fertilizer recommendation method (Milani et al., 1998) and mixed thoroughly with soil before planting. Wheat seeds were sown, 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 field capacity using deionized water. Thereafter, the pots were maintained near water-holding capacity with frequent watering to weight.
At harvest, shoots were cut at the soil surface, washed, dried at 70°C, ground, ashed at 550°C for 8 h, and the ash was dissolved in HCl (Chapman and Pratt, 1961). Concentrations of Zn and Cd in the digest solutions were determined by GFAAS.
After harvesting the wheat, a sample of 700 g soil was collected from each pot, air-dried, and sieved with a 2-mm diameter sieve. Each soil sample 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 (Richards, 1954). Soil pH, electrical conductivity (EC) of the saturated extract, and concentrations of Ca, Mg, Na, K, SO4, HCO3, P, NO3, as well as the trace metals Cd, Fe, and Zn were measured using the methods described earlier. Chemical composition data obtained was used as input data to MINTEQA2 computer program (Allison et al., 1991) to calculate concentrations of the soluble inorganic species of Cd and Zn in the saturation extracts (including the free cations, Cd2+ and Zn2+, and their complexes with Cl–, SO42–, NO3–, PO43–, and OH–). The MINTEQA2 database was used for stability constants (log K).
Statistical Analysis
The experiment was set up in a completely randomized design with factorially arranged treatments and three replications. Results were analyzed by analysis of variance using the SAS computer program (SAS Institute, 1988).
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RESULTS
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Effect of Salinity on pH, and Concentration of Soluble Anions and Cations
Soil pH was not significantly affected by salinity of irrigation water (data not shown). Concentrations of Mg2+, K+, Ca2+, SO42–, and HCO–3 in saturated extracts were mostly unchanged by salt treatments, while Na+ concentration increased more or less proportionally to the applied Na+ in NaCl and NaNO3 (Table 2). Chloride concentration increased likewise proportionally with increased NaCl levels for those treatments (Table 2). Concentration of PO43– and NO3– ions was neglectable.
Effects of Salinity and Zinc Application on Total Concentration and Metal Species in Soil Solution
Cadmium concentrations in soil solution increased significantly (p < 0.05) with increasing NaCl concentration both for the experiments where ZnSO4 was applied and for those where it was not (Table 2). Application of NaNO3 resulted in little or no increase in Cd concentration of saturation extract. The effect of NaCl salinization, on the other hand, was significant and proportional to the NaCl rates for both with and without Zn treatments. Zinc concentrations remained independent of added NaCl and NaNO3 concentrations. Treatment with ZnSO4 had little or no effect on total dissolved Zn.
Free calculated Cd2+ ion concentration was less affected by increasing the NaCl applied to the soils than were the CdHCO3+ and CdCl+ complex concentrations, and this was the case both for soils with and without ZnSO4 additions (Table 3). In contrast, NaNO3 treatment had little or no effect on Cd2+ and CdCl+ ion concentrations in soil solution, as it did not lead to Cd-complexation. Treatment with ZnSO4 increased total concentration of Cd in soil solution, but it had no effect on distribution of Cd2+, CdHCO3+, and CdCl+ species (Table 3). Addition of Zn did not affect the Zn2+ and ZnHCO3+ concentrations (Table 3), whereas there was a two-fold increase in solution ZnSO4 as a result of Zn addition to the soil (Tables 2 and 3).
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Table 3. The concentrations of Cd and Zn species in soil solutions calculated using MINTEQA2 with the average measured Zn and Cd and ligand concentrations of the saturated paste.
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Effects of Salinity and Zinc Application on Concentrations of Metal in Shoots
Both with and without Zn addition, increasing the NaCl concentration of irrigation water was associated with a significant (p < 0.05) increase in Cd concentrations in shoots, whereas, application of NaNO3 had no significant effect on Cd concentrations in shoots (Table 4). Cadmium concentration in shoot tissue was lower in the treatments that included Zn addition than in the treatments that did not include added Zn. Zinc concentration of shoots was higher in plants received the ZnSO4 treatment. Sodium chloride treatments, however, decreased Zn accumulation in shoot. On average, concentration of Zn in the shoots decreased about 36% as the result of NaCl treatment of irrigation water. Sodium nitrate treatment of irrigation water had no effect on Zn concentration in the shoots (Table 4).
Effects of Salinity and Zinc Application on Shoots Dry Matter
Effect of Zn application and irrigation water salinity on wheat shoot dry weight is shown in Fig. 1
. Plant growth significantly decreased with increasing NaCl rate. The decrease was similar at similar rates of NaCl and NaNO3. Zinc application increased dry weight yield at all salinity levels as compared with control (no Zn application).
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Fig. 1. Shoot weight of wheat in relation to water salinity and Zn treatment. Different letters above bars indicate significant difference among salinity treatments for a particular Zn treatment.
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DISCUSSION
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The effect of applying NaCl to water used for irrigation of soil may have three soil chemical effects, that is, the added Na+ competes with other cations for sorption sites, the added Cl– may complex certain cations, such as Cd, and the ionic strength is increased, which affects activity coefficients as well as the degree of preference of divalent over monovalent cations by the cation exchange complex. Increasing the applied quantity of NaCl to soil increased the total Cd concentration in the soil solution, whereas no such effect was found for the NaNO3 treatment, which does not lead to Cd complexation by Cl–. Thus, we conclude that the effect of NaCl on Cd solubility was mainly due to the formation of CdCln2–n complexes and not to ion exchange of Na for Cd on soil surfaces (Smolders et al., 1998). This observation confirms earlier findings that CdCln2–n complexes do not quantitatively adsorb on soil or clay (Temminghof et al., 1995). Minor cation exchange effects can be seen due to Na application (e.g., for K for the treatment where no Zn was applied). Because solution changes in the cations other than Na are practically absent, the influence of salt on the activity coefficients and on preference of divalent cations may be disregarded for the concentration levels in this experiment because such effects are not visible despite the higher Na competition at higher salt levels. However, it is still possible that precipitation–dissolution reactions control speciation in solution. For the treatment where no Zn was added, such reactions controlled Zn speciation, since addition of ZnSO4 did not increase Zn concentrations in solution. In agreement with effects of salts on the solubility of Cd in soil, salt-water irrigation significantly increased Cd concentration in wheat (Table 4). Increasing NaCl concentration increased Cd concentration in the plant, whereas no such effect was found for the NaNO3 treatment. The hypothesis that Cl salinity increases Cd concentrations in plants through osmotic stress affecting root function can also be discounted since the 120 mM NaNO3 treatment had no effect on plant Cd concentration (Smolders et al., 1998). The lack of any impact of solution ionic strength on Cd uptake by roots of Swiss chard (Beta vulgaris L.) was also demonstrated recently in different experiments (Smolders and McLaughlin, 1996; Smolders et al., 1998). The effect of Cl on Cd uptake could be explained by the fact that CdCln2–n complexes in soil solution are also available for plant uptake (Smolders et al., 1998).
In contrast, the water salinity decreased Zn concentration in wheat probably due to an increase in available Cd in the soil. A negative relationship between Cd and Zn in soil and plant has been reported previously (Jalil et al., 1994; Oliver et al., 1994). The changes in Cd concentrations that resulted from Zn treatment appear to be related in part to increase in shoot dry matter (dilution effect), and suggests that Zn fertilization may lower Cd concentrations in crops in part by ameliorating salinity stress. Application of ZnSO4 decreased Cd accumulation and increased Zn accumulation in wheat in all of the salinity treatments.
The water salinity decreased the growth and yield of wheat significantly (Fig. 1). The decline of plant growth was proportional to the water salinity levels. Application of Zn fertilizer partly counteracted negative effects of salinity on plant growth. Zinc is required for maintenance of integrity of biomembranes (Marschner, 1995). Under Zn deficient conditions there is a typical increase in plasma membrane permeability of root cells (Welch et al., 1982). Increased membrane permeability in Zn-deficient plants might lead to enhanced Cl, Na, and B uptake by Zn-deficient plants, particularly in salt-affected soils. Thus, under these conditions plant growth and yield may decrease (Marschner, 1995). Accordingly, application of Zn increased salt-tolerance of wheat.
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CONCLUSION
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Sodium chloride salinity mobilized soil Cd and increased its phytoavailability. Elevated Cd concentrations in wheat shoots appeared to be largely a result of the use of saline irrigation waters and high Cd content of P fertilizers. Application of Zn fertilizer had a positive effect on plant growth regardless of the degree of irrigation water salinity. Zinc fertilization, furthermore, increased Zn and decreased Cd concentration in wheat shoots.
Received for publication February 17, 2004.
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ABSTRACT
INTRODUCTION
