Zinc speciation and its relationship to plant uptake, sewage sludge disposal and drainage conditions of agricultural land

Trace metal accumulation in plants resulting from application of sewage sludge to agricultural soils has received considerable attention due to potentially adverse effects of heavy metals on human health. The solubility and speciation of zinc (Zn) were determined by sequential extractions as a function of pH and sewage sludge addition in A and B horizon samples of three soil series of a topohydrosequence from the Coastal Plain of Maryland; a Galestown loamy sand (Psammentic Hapludults), a Bertie silt loam (Typic Hapludults), and an Othello silt loam (Typic Endoaquults). Water soluble Zn for the Galestown, Bertie, and Othello A and B horizons in all sludge treatments decreased with increasing pH and cation exchange capacity (CEC). The exchangeable Zn levels in all treatments were not significantly correlated with the CEC. The Zn sorption in sludge-amended soils was energized to overcome exchangeable retention by electrostatic attraction at about pH 5.8. The mechanism to account for an abrupt transition to predominantly sesquioxide-bound forms of retention above pH 5.8 may be based on the specific adsorption associated with higher bonding energies. Plant-soil interaction studies were conducted to determine the effect of oxidation-reduction status (redox) on the solubility of Zn by growing rice (Oryza sativa L., Calrose-76), wheat (Triticum aestivum L.), and barley (Hordeum vulgare L.) in sludge-amended A horizon samples of Galestown, Bertie, and Othello soils in a growth chamber. Major differences between plants were that rice had higher levels of Zn and a more positive growth response to fluctuating redox condition than did wheat and barley. A larger fraction of oxide forms of Zn was found in all flooded and sludge-amended soils, while the treatment of no flooding showed higher exchangeable Zn. Increase in pH in flooded soils was the principal determinant of the speciation of Zn, whereas the role of sludge amendment was less important than pH. These results are relevant to concerns about heavy metal contamination of food and mobility in soils of different drainage classes when sewage sludge is used as a nutrient source and waste product on agricultural land.