ZINC EXTRACTABILITY AND PLANT UPTAKE IN FLOODED, ORGANIC WASTE-AMENDED SOILS

Abstract

The study of the distribution of various forms of Zn under fluctuating oxidation-reduction (redox) conditions is necessary to improve predictions of Zn solubility and bioavailability when Zn is added to soils with large quantites of organic C that oxidize over time. We conducted plant-soil interaction studies to determine the effect of redox potential in flooded and unflooded soils on the solubility and uptake of Zn by rice (Oryza sativa L., Calrose-76), wheat (Triticum aestivum L.), and barley (Hordeum vulgare L.) in a growth chamber. Sequential extractions were used to fractionate Zn in biosolids-amended and unamended A horizon samples (25-cm depth) of three soil series of a typical toposequence 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). Addition of organic C (as biosolids) increased the leaf dry matter yield, but not leaf tissue Zn concentration, compared with Zn-amended soil with no organic C addition. Flooding decreased leaf dry matter yield and Zn concentrations in the leaf tissue of all plants, especially in the organic waste-amended soils. Among the crops, rice had higher levels (P < 0.05) of Zn (7.4-34 μmol/g) than did wheat (0.3-4.3 μmol/g) and barley (1.2-6.1 μmol/g). Exchangeable Zn (25 to 48% of added Zn) extracted by Ca(NO 3 ) 2 was correlated with the concentration of leaf Zn of the three crops (r = 0.62, P < 0.05), whereas the fraction of nonexchangeable Zn (48 to 78% of added Zn) extracted by 100 mM NH 2 OH . HCL in 1 M HNO 3 was negatively correlated with leaf Zn (r = -0.49, P < 0.05). Reduced soil conditions depressed exchangeable Zn and enhanced nonexchangeable Zn in all of the ponded soils, apparently the result of the enhancement of surface-induced hydrolysis of Zn caused by increases in pH after flooding. The addition of organic C increased the fraction of Zn in nonexchangeable forms in the flooded soils, whereas the Zn levels in nonexchangeable forms in the unflooded soils were not affected by the addition of organic C. Thus, Zn availability to the crops was more dependent on pH changes induced by flooding than on the addition of organic C.