Comparison between fractionation and bioavailability of trace elements in rhizosphere and bulk soils
Introduction
Speciation is defined as a process leading to the identification and determination of the different chemical and physical forms of an element existing in a sample (Kot and Namiesnik, 2000). In the case of soils the speciation of heavy metals is generally related to their physico-chemical forms existed: simple and complex ions in interstitial solution; exchangeable ions; associated with soil organic fractions; occluded or coprecipitated with metal oxides, carbonates or phosphates and other secondary minerals; ions in the crystal lattices of primary minerals (Zhu and Alva, 1993). Based on the consideration of elements being associated with the different geochemical phases of soils there are numerous experimentally defined single and multiple sequential extraction procedures for the elemental speciation analysis (Shuman, 1985; Levesque and Mathur, 1988; Novozamsky et al., 1993; Ure et al., 1993; Leleyter and Probst, 1999). Among them the most widely applied methods were those recommended by Tessier et al. (1979) and by the Community Bureau of Reference (BCR) (Ure et al., 1993). BCR is now called the Standard Measurement and Testing Program of the European Community (STM). The main drawbacks of the sequential extraction are no information of true speciation of elements in soils available in addition to the nonselectivity of extractants used and redistribution of trace elements among geochemical phases during extraction processes (Shan and Chen, 1993; Howard and Brink, 1999). Although these disadvantages of elemental speciation analysis are recognized, the sequential extraction methods are unfortunately still used before the more effective methods available, especially in the case of study on the relationship between the elemental speciations and their bioavailability (LeClaire et al., 1984; Chlopecka and Adriano, 1996; Davidson et al., 1999).
Principally, the methods to study on the bioavailability of trace elements in soils should be analogous to the uptake of metals by biota and need a detailed knowledge of biological uptake mechanisms, particularly the transport of metals through the biological membrane (Town, 2000). In order to achieve these goals, many efforts should be made in the future to understand what speciation of metals present in a sample (Kaneta et al., 1986; Kot and Namiesnik, 2000), where the different forms of metals are localized in plants (Vazquez et al., 1992; Kramer et al., 1996; Kelley et al., 1999), how the different compounds of metals enter the plants apoplastically or intercellularly (Kramer et al., 1997; Salt and Kramer, 2000). Due to the extremely complicated mechanisms involved between the speciation of metals and their bioavailability, at the present time, various empirical approaches are often used to study the speciations of trace elements in soils and the contents of those elements in plants, including single regression (Randall et al., 1976; LeClaire et al., 1984; Chlopecka and Adriano, 1996) and multiple regression analyses (Davies, 1992; Maiz et al., 2000).
It must be pointed out that plants exude many low molecular organic acids, which can influence the speciation of elements in soil solutions and the uptake of elements by plants. Rhizosphere is a small but important subsystem of pedosphere, and the properties of rhizosphere are different largely from those of bulk soils (Kraffczyk et al., 1984; Curl and Truelove, 1986). It also seems that there are different speciations present in dried and wet soils (Kennedy et al., 1997). The aim of this study is to compare the elemental speciation and bioavailability between the rhizoshphere soils and bulk soils. A comparison is also made between the dry and wet soils.
Section snippets
Soils
Soil samples were collected from 15 rural area around China, representing typical Chinese soil with different physical and chemical properties. All soils were taken from the surface layer (0–20 cm) of cultivated soils. The soils were air-dried, ground and sieved to pass through 1 mm of plastic mesh for further use. Precautions were taken to avoid contamination during sampling, drying, grinding and storage.
Soil properties of pH, organic matter (OM) and cation exchange capacity (CEC) were
Speciation difference between air-dried and wet samples
Routinely, air-dried soil samples were used in soil analysis because it is easy to operate. However, when the soils were air-dried some of their chemical and physical properties may be altered. Such changes may be unimportant to total concentration but can significantly affect the speciation of trace elements in soils (Kennedy et al., 1997). Therefore, the veracity of using dried soil samples for the speciation analysis and bioavailability should be reconsidered.
Both the wet and dried
Conclusion
Based on the above comparison of speciation of trace elements in soils and their bioavailability between the air-dried soils and wet soils, between the nonrhizosphere soils and rhizosphere soils the following conclusions can be drawn:
- 1.
Chemical reaction taking place during the air-drying of the wet soil samples result in the increase in water-soluble, exchangeable and carbonate bound form of elements, which form is considered as the most easily uptake speciation by plants.
- 2.
Rhizosphere soil is a
Acknowledgements
This work was jointly supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences and Research Center for Eco-Environmental Sciences.
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