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FIGURE 3.2 Effect of soil pH on percentage of Cd sorbed onto soils. Data concern five New Zealand soils that received 2 mg Cd kg-1 as Cd(NO3)2. (Data obtained from Gray, C.W. et al., Austr. J. Soil Res., 36, 199-216, 1998.)

3.3.1.3 Redox Potential

Redox effects are particularly important in soils of aquic moisture regime (low lying soils in relief depressions, poorly drained, with water table normally into the root zone) and paddy soils intentionally flooded for cultivation reasons (rise fields). Generally, low redox values (reducing conditions) result in low metal availability, due to the formation of HS- and the subsequent precipitation of sulfide-metal solids out of the solution [42]. However, in soils in which Fe and Mn hydrous oxides are predominant, this trend may be altered. Prolonged reducing conditions may cause total depletion of hydrous oxides. Even if oxic conditions prevail again, the contribution of these phases to the sorptive capacity of the soils is not recovered. It is found that, when cultivated, such soils (developing prolonged anoxic conditions) exhibit an impressive increase in heavy metal uptake by test plants compared to other soils with no history in waterlogged regimes [43].

3.3.1.4 Competition Effects among Metals

The competition or synergism between heavy metals and/or other ions present in the soil system has a very significant effect on metal mobility in the solution and in the root zone. An overview of antagonistic and synergistic behavior of the elements is given in Figure 3.3. A lot of research effort has been put into the investigation of the interactions between Cd and Zn and the findings appear to be contradictory. Works supporting synergism suggest a mechanism by which the two metals compete for the same adsorption sites, and thus the soil retention capacity of the soil is reduced for any given metal when the other increases in concentration in the solution. Thus, when both elements are present in the soil solution, they exhibit enhanced bioavailability and reduced retention [44].

On the other hand, when plant uptake is concerned, antagonism is the usual effect because it is found that plant roots favor the uptake of the metal found in higher relevant concentrations over the other (e.g., McLaughlin et al. [45]). A well recognized competition in plant uptake occurs between micronutrients (such as Ca, P, and S) and heavy metals. This is due to favorable plant selectivity for the nutrients over heavy metals and to precipitation of insoluble Cd(PO4)2, especially

Adjacent to roots

Adjacent to roots

Plants

FIGURE 3.3 Interactions of heavy metals within plants and at the root surface. 'a: antagonism, P : synergism, Y antagonism and/or synergism, 8: possible antagonism.' (Redrawn from Kabata-Pendias, A. and Pendias, H., Trace Elements in Soils and Plants, 2nd ed., CRC Press, Boca Raton, FL, 1992, 1-87; 131-141. With permission.)

Plants

FIGURE 3.3 Interactions of heavy metals within plants and at the root surface. 'a: antagonism, P : synergism, Y antagonism and/or synergism, 8: possible antagonism.' (Redrawn from Kabata-Pendias, A. and Pendias, H., Trace Elements in Soils and Plants, 2nd ed., CRC Press, Boca Raton, FL, 1992, 1-87; 131-141. With permission.)

at neutral to alkaline pH, that depletes Cd of the soil solution [46]. Also, Pb mimics the physiological behavior of Ca, and thus it may inhibit several enzymes in plants when Ca is not readily available and the plant takes up Pb rather than Ca [47].

3.3.1.5 The Effect of Carbonates

At high pH values, calcite (CaCO3) sorbs Cd as CdCO3 and reduces its availability. The same effect is evident for other metals too. This provides a further explanation as to why liming of acidic soils is effective for reducing metal availability to crop plants. Above pH 7.3, the order of carbonate solids precipitating is [17]:

Papadopoulos and Powell [48] suggested for this sequence that the affinity of calcite for metals is related to their ionic radii. This is more evident for Cd2+, which, at high pH, substitutes for Ca2+ by chemisorption because the Cd radius fits better for this process.

3.3.1.6 Chloride Ions

A factor especially important in arid soils is the effect of chloride ions, which are found to enhance heavy metal availability, particularly that of Cd [49]. Chloride complexes with Cd (CdCl-, CdCl20) may be very stable and inhibit Cd from being adsorbed onto soil solids [50]. Chloride ions can be found in soils because of environmental reasons (soil salinity due to high evapotranspiration), poor irrigation practices, or sewage sludge application to soils prone to salinity.

3.3.2 Heavy Metal Accumulation in Crop Plants

All the preceding factors governing heavy metal availability may affect plant uptake of heavy metals. However, it should be stressed that other parameters related to plant physiology (such as plant genotype) are not studied here, but are of great importance as well. One of the greatest

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