Metals whose densities exceed 6 g cm are usually referred to as heavy metals. Some (Co, Cr, Cu, Fe, Mn, Mo, Sn, V, W, Zn) of these are essential for life processes whereas others (Cd, Pb, Hg) have no known useful biological role; all of them are injurious to living organisms at higher concentrations.
Davies et al. (1987) have studied the relationships between leaf and soil concentrations of Cd, Cu, Pb and Zn, using radish (Raphanus sativus) as the experimental plant which was grown on 46 garden soils. These soils were characterized by varying degrees of metal contamination. In some pots of contaminated soil, they added lime to modify soil pH.
Liming was found to greatly reduce foliar Zn concentrations. The pH effect appears to be exercised through a modification of the release of Zn from other solid compartments to the exchange sites (Davies et al., 1987). Liming was less effective in reducing the foliar Cd levels.
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However, Cd levels could be greatly reduced by flooding the pots with tap water; such flooding lowered the redox potential. Davies et al. found that neither changes in pH nor redox potential are effective in modifying tissue Pb concentrations. Lead occurs naturally in soils at concentrations typically within the range 10-200 mg kg-1 in the absence of local mineralization.
Harrison and Johnson (1987) have made recent estimates of the separate air and soil-derived contribution of lead to a range of crop plants. According to them, values of the soil lead concentration factor (lead in plant in mg kg-1 dry weight, divided by total lead in soil in mg kg-1 dry weight, in the absence of an air lead concentration) fall within the range of I-30 x 10-3.
They found that in most cases, both air and soil contribute a significant proportion of the lead content of plant tissues. In peas and beans, they found that the plant parts which are not exposed to the atmosphere, do not accumulate atmospheric lead. In contrast, lead appeared to be translocated to unexposed tissues in certain other plants e.g., lettuce and radish.
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Some studies of heavy metal cycling in forests of varying levels of pollution have been made during the last few decades (Hughes, 1981; Mayer, 1981; Davies, 1983). Most workers agree that the soil and organic litter layer components of woodland ecosystems are the ultimate sinks for heavy metals in aerially contaminated areas. Once soils have become contaminated, they are unlikely to be depleted of lead and other metals by leaching (Davies, 1983).
It is now known that the major part of the total ecosystem burden of heavy metals is found in the soil and organic litter components. The distribution of heavy metal concentrations in soil profiles generally shows high levels of contamination in the surface horizons only (Mayer, 1981; Davies, 1983).
Mobility differs between metals and according to the type of soil and type and degree of contamination involved. Cadmium and zinc appear to be relatively more mobile in soils than such metals as lead and copper (Martin and Coughtrey, 1987). Soil properties which influence metal mobility include pH, caution exchange capacity, organic matter content, free lime content, clay content, manganese, aluminium and iron oxide content, soil texture, redox potential and leaching rate (Martin and Coughtrey, 1987).
Martin and Coughtrey determined the metal concentrations in litter and soil profiles of aerially-contaminated woodland over a period of 100 years. Data for lead, zinc and cadmium at various depths in the soil profile were used to determine total contents per unit area and to compare distributions within the profile with time.
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Their work demonstrates that the mobility of lead, zinc and cadmium is considerably greater than that which would be inferred from existing literatures. Predictions of the Martin and Coughtrey’s model suggest that the total loss of metals from the 0-20 cm layer of the profile could amount to 2-3 gnr2 for Cd, 150-200 gnr2 for Zn, and 10-50 gnr2 for lead over the next 10-100 years.
In the case of Cd, partial equilibrium in that layer may be reached within a period of a decade and this would be associated with the fe- establishment of a classic concentration-depth profile generally interpreted as representing immobility in soil at aerially-contaminated sites (Martin and Coughtrey, 1987).
Martin and Coughtrey emphasize the rate of loss of metals from organic layers as being an important factor determining the movements of metals in soil. They conclude that a major factor in the apparent recent increase in mobility is the considerable decline in the pH of litter and soil over the past decade.
