Single and Sequential Extraction of Cadmium in Some Highly Calcareous Soils of Southwestern Iran

Journal of Soil Science and Plant Nutrition, 2013, 13(1), 153-164 Single and Sequential Extraction of Cadmium in Some Highly Calcareous Soils of Southwestern Iran A. Khanmirzaei*1, K. Bazargan2, A. Amir Moezzi1, B. K. Richards3, K. Shahbazi2 Department of Soil Science, Science, college of Agriculture, Shahid Chamran University, Ahwaz, Iran. 2Soil and Water Research Institute of Iran, Karaj, 31785-311 Iran 3Department of Biological and Environmental Engineering, Riley-Robb Hall, Cornell University, Ithaca, NY 14853-5701, USA. *Corresponding author. email: Tel.: +98-261-3203575 1 Abstract: Thirty-two surface soil samples with different physical and chemical properties were collected from calcareous soils of arid regions of Khuzestan Province, Southwestern Iran. Actual and potential cadmium (Cd) bioavailability was assessed by means of a greenhouse study, two different single extractions: 0.005 DTPA and 0.05 EDTA and two sequential extraction procedures: modified five-step Tessier method and the three-step BCR method. Extraction results were correlated with grain Cd uptake by Durum wheat (Triticum durum L. ) grown under greenhouse conditions. Cadmium in the durum grain ranged from 0.064 to 0.273 mg kg-1. The results showed better suitability of DTPA extraction in these highly carbonated soils than EDTA extraction for predicting Cd phytoavailability. DTPA –extractable Cd was positively related to soil electrical conductivity (p < 0.01, r = 0.65). Cadmium was predominantly associated with carbonate fraction in these soils (Tessier procedure 40% and BCR 44%).1 M NH4OAc which extracted the soluble + exchangeable fraction in the Tessier scheme may also provide useful information on Cd availability in studied soils. Keyword: contamination, cadmium, sequential fractionation, wheat 1. Introduction The chemical form of the metal determines whether it is bioavailable which are bound or associated with different soil constituents (Shuman, 1991). The degree of metal association with soil constituents depends on soil physical and chemical properties primarily soil pH, soil solution ionic composition, chloride concentration in soil solution, CEC, clay mineral content and types, organic matter content, calcium carbonate equivalent, oxides of Fe, Mn, and Al, and redox conditions (Naidu et al., 1994). Single and Sequential extraction techniques are widely used to characterize the metal fractions in soils and sediments and predict their mobility and bioavailability (Ahnstrom and Parker, 1999; Tessier, 1979). A single extraction method is designed to dissolve a phase whose elemental content is easily mobilizable and ideally is correlated with the 153 154 Khanmirzaei et al. plant bioavailability of the element. In sequential extraction techniques, a sample is treated with a series of progressively harsher reagents to dissolve increasingly recalcitrant forms. In the case of heavy metals, these methods operationally estimate the amounts of metals, in various solid pools, which commonly termed exchangeable, carbonate bound, Fe– Mn oxide bound (Reducible), organic (Oxidisable) and residual. Ideally, the reagents are chosen to selectively attack a specific soil compartment with minimal dissolution of non-targeted phases (Ahnstrom and Parker, 1999). Although these techniques have been in use for four past decades (Gibbs, 1973; Tessier et al., 1979), there are no universally accepted procedures in terms of reagents and their order in the sequence (Sutherland et al., 2000). This variability limits both method validation as well as comparability of results obtained under multiple extraction regimes. In order to establish a uniform extraction procedure the European Standards, Measurements and Testing Programme (formerly BCR) introduced the standardized sequential extraction procedure (Ure et al., 1993). This procedure has been standardized and applied to a variety of matrices including sediments, soils; sewage sludge, mining wastes etc., and some modifications have been suggested during the evaluation of the scheme (Sahuquillo et al., 1999; Sulkowski et al., 2006). Of particular concern here is the fact that the BCR scheme considered the soluble, exchangeable and carbonates fractions as a single fraction with uniform potential bioavailability. However, in highly calcareous soils the low-solubility carbonate fraction is predominant (Rajaei et al., 2006).This suggests that the BCR scheme may be inadequate for predicting potential Cd solubility and plant availability. As such, the potentially bioavailable fraction which correlates with plant uptake should be considered and separately distinguished in the sequential extraction schemes. The soils of Khuzestan province, a primary crop production area in Southwestern Iran, are extremely carbonated, with more than 40 percent carbonate Journal of Soil Science and Plant Nutrition, 2013,13(1), 153-164 content in most parts. Agricultural activities are extensive in the area and application of inorganic fertilizers, especially phosphorous, has caused elevated Cd levels in soils and grain crops produced in the area (Jafarnejadi et al., 2011). Nevertheless, there has been very limited information on cadmium Cd status and, chemical forms, which is needed to evaluate the actual and potential Cd mobility and bioavailability in the region’s soils. To evaluate the adoption of the BCR scheme modified for carbonate rich soils (Sulkowski et al., 2006) in calcareous soils of the region, we compared the results of this scheme with those of the extraction protocol of Tessier et al. (1979) which is, typically used for analysis of the calcareous soils of Iran (Abbaspour et al., 2007; Jalali and Khanlari, 2008). These were also compared with standard single step extraction protocols (DTPA and EDTA). The objectives of this study, therefore, were (i) to evaluate the Cd phytoavailability by growing a wheat crop in a greenhouse condition, (ii) to the determine the major soil fractions that contain bioavailable Cd in a wide range of soils of Khuzestan province using two single and two sequential extraction schemes. 2. Methods and Materials 2.1. The Study area and soil sampling This investigation was conducted using 32 surface soil (0-20 cm) samples collected at different randomly selected farms in an area of approximately 4000 km2 , representing different soil types of calcareous soils in arid region of Khuzestan Province, Southwestern Iran,(47° 40’ to 50 °33’ N and 29° 57’ to 57° 33’ E). The climate vary from semiarid to arid with a mean annual precipitation and class A pan evaporation of 240 and 3000 mm, respectively (Farshi et al., 1997). The physiography of the region ranged from piedmont plains to lowlands in the south. The soils are highly calcareous and are mostly classified as Entisols, Single and sequencial extraction of cadmium in some highly calcareaus soils Aridisols and Inceptisols17. The region’s primary crops are wheat (Tritcum aestivum L.), barley (Hor (...truncated)