Effect of salinity on Cd and Zn availability

Transcription

1 Symposium no. 33 Paper no. 08 Presentation: poster Effect of salinity on Cd and Zn availability KHOSHGOFTARMENSH A.H., JAAFARI B. and SHARIATMADARI H. Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Abstract A greenhouse experiment with two amounts of Zn (0 and mg Zn kg -1 as ZnSO 4 fertilizer), five salinity levels and three replications was conducted. Five irrigation water with salinity levels of 0, 60, 1 and 180 mm NaCl, and 1 mm NaNO 3 were applied. These salts levels were typical of those found in irrigation waters in wheat fields of central Iran. Wheat was seeded in pots and after 45 days, shoot was harvested. Trace metal concentrations were determined in the shoot by AAS. After plant harvest, soil solution was extracted form the soil. Electrical conductivity, ph, and concentration of anions and cations were determined. Activities of metal species in solution were predicted using the computer program MINTEQA2. Salinizing with NaCl resulted in higher concentrations of Cd in the saturation extract. By increasing of NaCl levels in irrigation water, shoot dry matter decreased, especially in no ZnSO 4 treatment. Application of Zn-fertilizer had a positive effect on salt-tolerance of plant and increased dry matter of shoot. Treating the soil with NaCl altered the distribution of Cd species (Cd 2+, CdCl +, etc.) in the soil solution and increased total concentration of Cd T. Shoot Cd concentrations were positively related to total Cd T concentration and waterextractable Cl. By application of Zn fertilizer, shoot Cd concentrations decreased significantly. AsCl is known to mobilize soil Cd, elevated Cd concentrations in the wheat appear to be largely a result of the use of saline irrigation waters and high application of P-fertilizers. Application of Zn-fertilizer had a positive effect on plant growth. Keywords: cadmium, salinity, zinc, wheat Introduction There is growing evidence that salt and heavy metals pollution of soils are seriously increasing in many parts of the dry areas (Helal et al., 1995). Although adverse effects of salt accumulation on soils and crops have been researched intensively (Richards, 1954; Helal, 1983), little attention has been paid to the role of soil and water salinity in heavy metal transfer to the food chain (Helal, 1996). Cadmium uptake by food is a concern because Cd is a heavy metal that is potentially toxic when consumed (Norvell et al., 00). Soil is the principal source of Cd accumulated by plants. Generally, soil characteristics such as concentration and form of metal in the soil, ph, organic matter content, clay content, interactions with other elements, fertilizer practices, and Zn availability are considered the major factors determining the bioavailability of Cd in soil (Smolders et al., 1998; Norvell et al., 00). In addition, there appears enhancement of Cd uptake in some crops by elevated salinity or chloride (Bingham et al., 1984; Li et 08-1

2 al., 1994; McLaughlin et al., 1994, 1997; Smolder and McLaughlin, 1996; Weggler- Beaton et al., 00). The results of these studies suggest that the enhancement of Cd uptake in the presence of salinity or Cl - may be a general phenomenon that occurs in many crops, including the major cereal crops (Norvell et al., 00). High amounts of P-fertilizers have been used in salt-affected soils of central Iran, during the last years. Most of the fertilizers used contained elevated quantity of Cd that might be plant available and adversely affected food quality. It considers enhanced Cd uptake by salinity. In addition, zinc deficiency is a major problem in salt-affected soils of central Iran; therefore, the purpose of this study was to investigate the influence of salinity on Cd and Zn availability in the soil and their accumulation by wheat. Material and Methods The soil material was collected from the upper layer of a Cd-polluted soil at the Qom province. This aridisol was taken into production since Cadmium accumulation in this area is attributed to the high application of P-fertilizers contained Cd as impurity. Some characteristics of this soil have been shown in Table 1. Field soil sieved through a 5-mm diameter screen and brought to the greenhouse. A greenhouse experiment with two amounts of Zn (0 and mg Zn kg -1 as ZnSO 4 fertilizer), five salinity levels and three replications was conducted. Five irrigation water with salinity levels of 0, 60, 1 and 180 mm NaCl, and 1 mm NaNO 3 were applied. These salts levels were typical of those found in irrigation waters in wheat fields of Qom, Iran. The soils (<5-mm diameter) were put in pots. At planting, a uniform application of N-P-K fertilizers [100 mg N, P, and K each kg -1 soil as urea, (NH 4 ) 2 SO 4 and K 2 SO 4 ] was made to each soil. Wheat seeds were sown, thinned to five plants pot -1 after 10 days, and grown for 45 days. Table 1 Some soil characteristics. Characteristic Unit Amount ph CaCO 3 22 Clay 54 Silt 36 Sand 10 Available-P 18 Available-K 210 EDTA-extractable Zn 0.9 EDTA-extractable Cd 0.5 At harvest, shoots were cut at the soil surface and washed, dried at 70 0 C and digested. Trace metal concentrations in digest solutions were determined by AAS. After harvesting the wheat, a sub sample of soil was collected from each pot, air dried and sieved through a 2-mm diameter screen, and prepared for chemical analysis. Each soil sample was saturated with deionized water, mixed to have a paste of uniform consistency, and after standing overnight, was transferred to a suction flask for extraction of the soil solution (US salinity laboratory staff, 1954). The ph value and electrical conductivity (EC) of the extract and chemical analysis for Ca, Mg, Na, K, 08-2

3 SO 4, HCO 3, P, NO 3 and the trace metals Cd, Fe, and Zn were carried out. The chemical composition data were used with the MINTEQA2 computer program (Lindsey and Ajwa, 1992) to speciate Cd and Zn in the saturation extracts (the free ion Cd 2+, Zn 2+, and complexes with Cl -, SO 2-4, NO - 3, PO 3-4 and OH - ). Results Increasing rates of NaCl decreased plant growth significantly (Figure 1). A significant decrease of plant growth was found with 1 mm NaNO 3 treatment. However, no significant yield difference (at p<0.05) was found between plants grown in soils amended with equivalent rates of NaCl and NaNO 3. ) -1 shoot weight (g plant NaNO NaNO Salinity treatments without Zn with Zn Figure 1 Shoot weight of wheat in relation to water salinity and Zn treatmant. In both soils with and without ZnSO 4, the addition of NaCl resulted in a significant (p<0.05) increase in Cd concentrations in shoots (Table 2). The results presented in Table 2 show furthermore, that Cd concentration in shoots almost was doubled between the 0 and 1 mm NaCl treatment, whereas, salinizing with NaNO 3 had no significant effect on Cd concentrations in shoots. In both soils with and without ZnSO 4, the addition of NaCl resulted in a significant (p<0.05) decrease in Zn concentration in shoots but this effect was more pronounced in soils without ZnSO 4 treatment. No effects of NaNO 3 treatment were found on Zn concentration in shoots. Increasing salt rates increased Na concentration in the plant but it had no effect on K, Ca, and Mg concentrations (data not shown). These salt effects were independent of the type of Na salt used. Treatment with ZnSO 4, increased Zn concentration in shoots, significantly (p<0.05) (Table 2). There was a general decline in Cd accumulation in shoots under ZnSO 4 treatment. There was no effect of salt treatments on soil ph and it was the same in different salinity treatments. Solution concentrations of Mg 2+, K +, Ca 2+, SO 2-4, and HCO - 3 were mostly unchanged by salt treatments, while Na + concentration increased in line with both NaCl and NaNO 3 treatments (Table 3). Chloride concentration increased in line as salt rates increased in NaCl-treated soils (Table 3). Cadmium concentrations in soil solution increased significantly (p<0.01) with increasing NaCl concentration for both with and without ZnSO 4 treatments (Table 4). 08-3

4 Table 2 Cadmium and Zn accumulation in wheat. Salt rate Accumulation ( ) Zn Cd Without Zn With Zn Table 3 The measured cations and anions in soil solutions. Salt rate Concentration (cmol L -1 ) [Cl - ] [SO 4 2- ] [Ca 2+ ] [Mg 2+ ] [HCO 3 - ] [Na + ] [K + ] Without Zn 6 12 With Zn Although salinizing with NaNO 3 resulted higher concentrations of Cd in the saturation extract than control treatment, the effect of salinization was more pronounced in the NaCl treatments. Table 4 shows a summary of the calculated chemical speciation of Cd in the saturation extracts of soil with and without ZnSO 4. The calculated Cd 2+ concentrations were unaffected by the NaCl treatment for both soils with and without ZnSO 4, whereas CdCl + complexes increased significantly by increase of NaCl rates. In contrast, NaNO 3 treatment had no effect on Cd 2+ and CdCl + concentrations in soil solution. Treatment with ZnSO 4 increased total concentration of Zn and Cd in the soil solution, but it had no effect on distribution of Cd 2+ and CdCl + species

5 Table 4 Concentrations of Cd species in soil solutions calculated using MINTEQA2 with the average measured metal and ligand concentrations. Salt rate Cd complexation, of Cd T Concentration (cmol L -1 ) [Cd 2+ ] [CdCl + ] [CdCl 0 2 ] [CdSO 0 4 ] [Cd T ] [Zn T ] Without Zn 6 12 With Zn Discussion In agreement with salt effect on the solubility of Cd in soil, salt-water irrigation increased the concentration of this metal in wheat, significantly (Table 2). The water salinity decreased the growth of wheat, significantly (Figure 1). The decline of plant growth was proportional to the water salinity levels. The results presented in the Table 2 show furthermore, that enhancement of Cd uptake by NaCl salinity was counteracted partly by ZnSO 4 treatment. The speciation calculations for the soil solutions predict that the activity of Cd 2+ in soil solution is well buffered by the solid phase, despite the mobilization of Cd by elevated Cd levels (Smolders et al., 1998). This observation confirms earlier findings that CdCl n 2-n complexes do not quantitatively adsorb on soil or clay (Tomminghof et al., 1995). Increasing the rate of NaCl addition to soil increased Cd concentration in the plant, whereas no such effect was found for NaNO 3 treatment. Thus, it can be concluded that the effect of NaCl on Cd availability and Cd accumulation was partly due to formation of CdCl n 2-n complexes and not to ion exchange of Na for Cd on soil surfaces (Smolders et al., 1998). The hypothesis that Cl salinity increases Cd concentrations in plant through osmotic stress affecting root function can also be discounted since 1 mm NaNO 3 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 was also demonstrated recently in different experiments (Smolders and McLaughlin, 1996; Smolders et al., 1998). We concluded that the effect of Cl on Cd uptake could be explained by CdCl n 2-n complexes in soil solution are also available for plant uptake. Treatment with ZnSO 4, decreased Cd accumulation in the wheat, in all of the salinity treatments. Treating the soil with ZnSO 4 had no significant effect on the distribution of Cd species in the soil solution, whereas increased total concentrations of soluble-zn (Table 4). 08-5

6 As Cl is known to mobilize soil Cd and increase its phytoavailability, elevated Cd concentrations in the wheat appear to be largely a result of the use of saline irrigation waters and high application of P-fertilizers. Application of Zn-fertilizer has a positive effect on both plant growth and grain quality. Therefore, zinc has an important role to improve of qualitative and quantitative yield of wheat in saline conditions. Acknowledgments We thank Mr. Bolandnazar, Mr. Naini and Mr. Mirzapour for their cooperation. References Bingham, F.T., G. Sposito and J.E. Strong The effect of chloride on the availability of cadmium. J. Environ. Qual. 13: Helal, H.M Effects of sodium chloride on ionic relations, energy status and dry matter synthesis by various crop plant species. D. Sc. (Habil) Thesis, Justus-Liebig University, Giessen, Germany. Helal, H.M., M. Abdel Monem and F. Azam Heavy metal uptake by L. italicum as affected by salt water irrigation. In R. Prost (ed.). Third international Conference on the Biogeochemistry of Trace Elements. Proceedings. Paris, France. Helal, H.M., S.A. Haque, A.B. Ramadan and E. Schung Salinity-heavy metal interactions as evaluated by soil extraction and plant analysis. Commun. Soil Sci. Plant Anal. 27(5-8): Lie, Y., R.L. Chaney and A.A. Schneiter Effect of soil chloride level on cadmium concentration in sunflower kernels. Plant Soil 167: McLaughlin, M.J., L.T. Palmer, K.G. Tiller, T.A. Beech and M.K. Smart Increased soil salinity causes elevated cadmium concentrations in field grown potato tubers. J. Environ. Qual. 23: McLaughlin, M.J., K.G. Tiller and M.K. Smart Speciation of cadmium in soil solutions of saline/sodic soils and relationships with cadmium in potato tubers (Solanum tuberosum L.). Aust. J. Soil Res. 35: Norvell, W.A., J. Wu, D.G. Hopkins and R.M. Welch. 00. Association of cadmium in durum wheat grain with soil chloride and chelate-extractable soil cadmium. Soil Sci. Soc. J. 64: Richards, L.A Diagnosis and Improvement of Saline and Alkali Soils. US. Department of Agriculture, Washington, DC. Smolders, E., R.M. Lambergts, M.J. McLaughlin and K.G. Tiller Effect of soil solution chloride on cadmium availability to Swiss chard. J. Environ. Qual. 27: Smolders, E. and M.J. McLaughlin Chloride increases cadmium uptake in Swiss chard in a resin-buffered nutrient solution. Soil Sci. Soc. Am. J. 60: Temminghof, E.J.M., S.E.A.T.M. Van DerZee, and F.A.M. De Haan Speciation and calcium competition effects on cadmium sorption by sandy soil at various phs. Eur. J. Soil Sci. 46: U.S. Salinity Laboratory Staff Diagnosis and Improvement of Saline and Alkali Soils. Agric. Handbook No. 60, USDA. US. Government Printing Office, Washington, DC. 08-6

7 Weggler-Beaton, K., M.J. McLaughlin and R.D. Graham. 00. Salinity increases cadmium uptake by wheat and Swiss chard from soil amended with biosolids. Aust. J. Soil Res. 38: