Table 281

Mean Concentrations of Tha in Wheat Seedlings and in Soil where Plants Were Grown and Ratio of Th Content in Plants Grown in Th-Enriched Soil to That in Control

Control Experiment (+Th) Ratio experiment/control

b Differences between control and experiment are significant at P < 0.01. c Differences between control and experiment are significant at P < 0.001.

FIGURE 28.1 Variations in Th concentration in clean (control) and artificially contaminated with Th soil within 2, 4, and 7 days after adding Th to the experimental soil.

Application of maize in phytoremediation studies is described in numerous reports in the literature [39-41]. Maize can grow quickly, has high biomass, and has been found to accumulate different metals in its shoots and roots [42]. For example, Kalisova-Spirochova et al. [43] compared maize and sunflower grown in highly contaminated soil (Zn: 75,000 mg kg-1; Pb: 16,000 mg kg-1). The highest values of accumulation of Zn and Pb were found in roots and in leaves of corn (whole plant: 1158 mg kg-1 for Zn and 500 mg kg-1 for Pb). Sunflower showed considerably lower accumulation ability (whole plant: 47 mg kg-1 for Zn and 9 mg kg-1 for Pb).

Keltjens and van Beusichem [44] compared an uptake of Cu and Cd by wheat (Triticum aestevum) and maize (Zea mays) grown in nutrient solution. They found that, at identical external Cd levels, maize accumulated significantly more Cd in the shoots than wheat did, making maize a more pronounced "shoot Cd accumulator" than wheat. Unfortunately, similar metal uptake by these plants when they grow in soil cannot be predicted. In this case, quite opposite effects may be observed; these may be explained by differences that can arise when comparing experiments on metal uptake by plants grown in soil and culture solutions.

Much less information is available on uptake of metals by other cereal crops. Madrid and Kirkham [45] studied an uptake of Cd, Fe, Mn, Ni, and Pb by barley (Hordeum vulgare L.) grown in animal-waste lagoon soil. They found that amendment of the soil with EDTA resulted in an increase of uptake of Fe, Mn, Ni, and Pb. Luo and Rimmer [46] studied multielement toxicity at the early stages of barley growth. It was shown that growth of barley was controlled by the amount of plant-available Zn, which depended on the amounts of added Zn and added Cu. The effect of the added Cu was to increase the toxicity of the added Zn. Adams et al. [47] compared metal uptake by wheat and barley and found that, when it was grown under comparable soil conditions, barley could take up much lower amounts of Cd than wheat could.

Cobb et al. [48] compared three plant species (oats, radish, and lettuce) grown in soil contaminated with Cd, Cr, Ni, and Pb. They found that oats were the most tolerant compared to other plant species and were able to accumulate rather high amounts of Cd and Ni.

Knox et al. [49] studied an uptake of Cd by rye, maize, and oats grown in soils with different levels of Cd contamination. It was found that an increase of Cd content in soil up to 20 mg kg-1

resulted in a significant increase of Cd level in all the plant species: oats accumulated 112 mg kg-1 Cd in their leaves, and concentrations of Cd in leaves of rye and maize were 43 and 41 mg kg-1, respectively. When concentration of Cd was increased up to 40 mg kg-1, Cd contents in leaves were 80, 98, and 198 mg kg-1 in rye, maize, and oats, respectively. Rye was found to be able to accumulate more Cu and Zn in its shoots compared to wheat and oats grown in the same soils [50].

Schumann and Sumner [51] performed experiments with sorghum grown in silty loam soil amended and non-amended with by-products. It was shown that As concentrations in sorghum leaves were linearly correlated (R2 = 0.895) to total As in soil. An application of coal combustion by-products (fly ash) for crop fertilization led to a significant (four times) increase of As content in leaves that finally reached the level of 120 mg kg-1. Estevez Alvarez et al. [19] studied the uptake of Cr, Cd, Ni, Pb, and Zn by sorghum and toxicity of the metals for this species. They observed a remarkable effect of Zn and Cr on the growth of the plant. Even insignificant increases of Zn and Cr concentrations in soil resulted in a death of sorghum. On the other hand, Cu, Ni, and Pb did not affect the plant yield.

The author's experimental data on oats and barley indicate that these plant species may be used for metal phytoextraction in slightly and moderately contaminated soils [15,16]. Vegetation tests showed that growth of oats and barley in different soils contaminated with various metals and metalloids resulted in a decrease of metal concentrations in soil solutions. As an example, Figure 28.2 illustrates variations in Cu and Zn contents in soil samples treated with 1 M ammonium acetate, pH 4.8 (this soil extraction can characterize sorbed and loosely bound metals) [52]. It is important to note that, after soil cultivation, total amounts of several elements in the soils also decreased (Table 28.2).

Behavior of As in the urban soils was the most interesting. During a 1-month vegetation test, As content in slightly contaminated soils cultivated with oats decreased up to the level of As in the clean soil (Table 28.2). Concentration of As in moderately contaminated soil also decreased significantly. In this case, such an effect was observed as a result of cultivation of soil with both oats and barley.

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