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

Abstract

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 inorganic materials such as basic slags, magnetite, maghemite, hematite, birnessite, 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 soil extraction by 0.1 M calcium nitrate and vegetation experiments with dwarf bean (Phaseolus vulgaris L.) and ryegrass (Lolium multiflorum). A phytotoxicity test based on the activities of enzymes involved in the stress metabolism caused by toxic Zn concentrations in the primary leaves of dwarf beans 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 from the contaminated soil. Beringite gave similar results but at 50 g.kg(-1) soil DW. Birnessite and beringite treatments were the most effective to reduce Zn assimilation by dwarf bean, and in consequence phytotoxicity. Subsequent harvests of ryegrass confirmed the beringite effect on Zn uptake 5 months following the soil treatment. For birnessite, Zn availability to ryegrass shoots increased however at the third harvest and reached the level of the untreated Zn-contaminated soil.