The role of roots in the phytoextraction of bioavailable zinc

Phytoextraction is a non-invasive remediation method using plants. Phytotech Foundation and AGB in Berne are performing phytoextraction on an old landfill of a former hot dip galvanization plant, using sunflowers and tobaccos as extractors. Every year since 2007, total and soluble zinc concentrations in soil as well as zinc concentrations in shoot of the plants have been measured. To study the role of roots in phytoextraction, 14 root samples were harvested in autumn 2010 and their zinc concentration was measured. The results show that sunflowers were more efficient in fast transfer of zinc from root to shoot. They were also compared with the 4-year measurements and a mass balance was performed. This has shown that phytoextraction was so efficient that even the decrease in soluble zinc did not explain itself the concentrations in the plant. The roots may mobilize total zinc, which might re-load the soluble zinc pool in soil. In parallel, a pot experiment was performed at laboratory scale with young tobacco plants growing on polluted soils, monitoring root, stem, leaves, leachates and soil zinc concentrations. A mass balance was also performed, which confirmed the previous statement, even at young stage and for detailed values. Finally thanks to both experiments, it was suggested that there should be a potential concentration for each part of plants. Once this concentration reached, transfer of zinc could occur from the root to the stem, and then from the stem to the leaves where it is accumulated.