Chemical analysis of the soil reveals the concentration of metals but provides little information on their phytoavailability. Even mobile metals are present in the soil in several forms of different mobility, including hydrated ions, soluble organic and inorganic complexes and constituents of floating colloid particles in the soil solution, as exchangeable ions adsorbed on colloid surfaces and in the silicate crystals of the solid phase. There is a dynamic exchange between all these different forms (Marschner, 1995).
Phytoavailable metal concentrations in the rhizosphere can be estimated by solute transfer models that incorporate the metal concentration in the bulk soil solution, the buffering power of the soil, the diffusion coefficient for the metal, the water movement, the size and morphology of roots and the rate of entry of metals into the roots (Whiting et al., 2003). Other soil properties may also influence the phytoavailable fraction of heavy metals. Such properties are pH, organic matter content, clay particles, moisture content and soil type (Alloway, 1995). For instance, it was shown that spinach (Spinacia oleracea), a species capable of accumulating large amounts of heavy metals, took up lower amounts of metals from a clay than from a sandy soil, even at similar total element contents (Naidu et al., 2003).
There are differences between heavy metals concerning their phyto-availability. Lead and Cu have a high affinity for organic soil constituents, although they may also bind to colloid particles. Zinc has a much lower affinity for soil organic particles than Cu. The phytoavailable fraction of Zn corresponds to its water-soluble and exchangeable forms, which constitute a small portion as compared to the total Zn content of the soil. Cadmium is a constituent of fertilizers and sewage sludge applied to agricultural areas, and is highly mobile and available for plants (Bolan et al., 2003; McBride, 1989; Bell et al., 1991b; Luo and Christie, 1998; Barak and Helmke, 1993; Kabata-Pendias, 2001).
One of the most important properties affecting heavy metal availability is pH. Generally, heavy metals are more soluble at low pH, and thus factors increasing pH may decrease phytoavailability of metals. Availability of Pb is decreased by phosphates by increasing the inorganically bound fraction and pH (Bolan et al., 2003). Lime application to soils also increases pH, counterbalancing Cu release from sewage sludge (Merry et al., 1986), but it often results in lime-chlorosis by decreasing Fe availability (Nikolic and Romheld, 2002). On the contrary, Fe deficiency in most dicots causes an increased activity of root plasmalemma H+-ATPases, decreasing rizosphere pH and increasing the solubility of Fe and other heavy metals (Marschner et al., 1986).
Cieslinski et al. (1997) investigated the low molecular weight organic acids released by five cultivars of a mono- and a dicotyledonous plant (Triticum turgidum and Linum usitatissimum), and detected oxalic, malonic, fumaric, succinic, acetic, malic, citric and tartaric acids in the root exudates. All these compounds decrease the pH of the rhizosphere, thereby increasing the solubility of most heavy metals, but, on the other hand, they may also bind competitively free metal ions. It has been shown that oxalate secreted from the roots of Pb-tolerant rice varieties may reduce the bioavailability of Pb through precipitation (Yang et al., 2000).
Graminaceous plants (following Strategy II in Fe uptake) release phytosiderophores (e.g. mugineic acid and avenic acid), which are especially powerful in complexing Fe3+, but may also mobilize Mn2+, Zn2+ and Cu2+ (Romheld, 1991). Siderophores released by soil microorganisms (e.g.
ferrichrome and ferrioxamine B) may increase the soluble fraction of metals in the soil and may also promote their uptake by plants (Crowley et al., 1987).
Since Fe chlorosis is a serious agricultural problem in certain areas, many procedures have been developed to increase soil Fe availability. One of these is the application to the soil of synthetic chelating agents (García-Marco et al., 2003). As chelating agents bind Fe and other heavy metals, they may be used for increasing the solubility and phytoavailability of toxic metals for decontamination purposes (see below). The application of CDTA or DTPA caused a shift in the distribution of Pb in the soil from more recalcitrant to more soluble forms, greatly increasing the leaching of this metal (Cooper et al., 1999). EDTA, which has a high stability constant with Fe3+, also enhances Pb solubility (Blaylock et al., 1997; Huang et al., 1997). However, EDTA application to soils may result in long-term solubilization of heavy metals. For instance, in a study with two metal contaminated soils, Cu-EDTA and Cd-EDTA complexes were still found in soil pore water 5 months after the application of the chelating agent (McGrath et al., 2002). Metal-EDTA complexes with relatively high stability constants (i.e. Cu-, Fe-, Pb- and Zn-EDTA) are degraded more slowly than those with lower stability constants (i.e. Ca-, Mg- and Mn-EDTA) (Satroutdinov et al., 2000).
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