Further recent advances in knowledge also help to evaluate the likelihood of using trees for phytoremediation.
Tree foliage can contain high concentrations of heavy metals, especially Cd and Zn. On highly contaminated sites, these may be at their peak values just before leaf senescence, appearing not to be translocated and redistributed within the plant prior to leaf fall. If these metals return to the rhizosphere, they represent a significant pool of potentially bioavailable metal. Under such circumstances, phytoremediation will fail in its aim of cleaning up the soil. It does, however, raise the question of improving offtake of metal by harvesting the trees prior to leaf fall. This is not normal practice in SRC systems in which the stems without leaves are harvested over the winter period. With long-term tree covers that are not regularly harvested (and in the years intervening in the usual 3-year SRC harvest), this recycling process of metals to surface soil may be significant.
Roots usually contain the highest concentrations of all metals in trees; depending on the size of the root biomass, this could be a significant amount. Death and decomposition of roots during normal growth processes or following harvest of above-ground biomass probably release heavy metals back into the rhizosphere. Because the roots can contain a significant pool of metals, it may be that the root bole should be harvested at the end of the SRC cycle (after 25 to 30 years) or when a mature tree is cut down.
Root density and the depth of rooting are particularly significant in the context of phytoreme-diation . Studies on 246 coppice stools of five Salix and five Populus clones in four different soil types at seven sites in the U.K., with a stool age of 3 to 9 years, found rooting to a depth of more than 1.3 m; however, 75 to 95% of roots were in the top 36 cm. Wetter soils had shallower root systems . It has also been found that Salix cinerea has a much reduced uptake of Cd and Zn into leaves and bark when grown in wetland compared to that in drier soils .
Although the two are often reported together simply as "stem" tissue, tree bark usually contains a higher concentration of metals than wood does [14,86-88]. The higher biomass of wood means that it contains a greater pool of metal than the bark does. Wood and bark (stem) tissue contains a significant pool of heavy metals (concentration x biomass yield), one that increases with the age of the tree. This pool of metals is much less bioavailable than those in the roots and leaves. Regular cutting of trees in the SRC system is designed to increase stem biomass, but also increases the ratio of bark to wood and thus may increase this pool of immobilized metal.
Uptake data are frequently converted to BCFs that allows a better comparison to be made of the metal uptake abilities of trees grown under different conditions.
concentration in plant tissue (mg kg-1)
concentration in soil (mg kg- )
Care must be taken when collating such data because of the different strengths and types of acids and extraction processes that are routinely used to measure soil concentrations of heavy metals (see Section 20.4.1). Table 20.2 shows recently published BCFs for uptake of Cd, Cu, Ni, Pb, and Zn by trees. BCF values > 1 indicate that metal is accumulated in the tree relative to the soil.
This approach suggests potential for enhanced uptake of Cd by Salix, especially when only slightly elevated concentration of Cd is in the substrate. Zinc uptake may also be significant but, in view of limited zootoxicity of this metal, is of less concern on brownfield land. In general, transfer of Cu, Ni, and Pb from soil to plant is very poor.
Although BCF values can give a guide to the uptake of metals from soil, the concentration of a metal in plant tissue does not alone give any information about the absolute amount of metal removed from the soil. Knowledge of the biomass yield is also required in order to calculate metal offtake (concentration x yield). Some estimates of metal offtake obtained from field trials using Salix are given in Table 20.3. Some of the trials measured offtake over a number of years; these have been corrected to an annual figure to allow comparison between trials of differing duration. As pointed out earlier, yearly values may not be equal and, with coppiced systems especially, the effects of tree growth and harvest may be important.
Such figures do, however, show some consistent features. Cadmium, Cu, Ni, and Pb are removed at the rate of the order of tens of grams per hectare per year, whereas the value for Zn is about 100 times higher. When compared with estimates of the metal content of the soil to rooting depth, such offtake figures generally give estimates of hundreds to tens of thousands of years to reduce the soil metal contents to acceptable values. Only the case of Cd, in which the amount taken out by the trees can be a significant percentage of the soil metal, offers hope of cleanup within a realistic time scale (Table 20.4).
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