Capacity of soil amendments in lowering the phytoavailability of sludge-borne zinc

One way to reduce the phytoavailability of an excess of metals such as zinc in soil is through the addition of amendments. The effectiveness of inorga nic materials such as basic slags, magnetite, maghemite, hematite, birnessi te, hydrous manganese oxide, steel shots, and beringite, was evaluated in a pot experiment with a coarse sandy soil contaminated by sludge-borne zinc. Zinc extractability and phytoavailability were investigated using single s oil extraction by 0.1 M calcium nitrate and vegetation experiments with dwa rf bean (Phaseolus vulgaris L.) and ryegrass (Lolium multiflorum). A phytot oxicity test based on the activities of enzymes involved in the stress meta bolism caused by toxic Zn concentrations in the primary leaves of dwarf bea ns was also performed. Based on the addition rate, birnessite (10 g.kg(-1) soil, dry weight DW) resulted in the maximum decrease in extractable Zn fro m the contaminated soil. Beringite gave similar results but at 50 g.kg(-1) soil DW. Birnessite and beringite treatments were the most effective to red uce Zn assimilation by dwarf bean, and in consequence phytotoxicity. Subseq uent harvests of ryegrass confirmed the beringite effect on Zn uptake 5 mon ths following the soil treatment. For birnessite, Zn availability to ryegra ss shoots increased however at the third harvest and reached the level of t he untreated Zn-contaminated soil.