BIOAVAILABILITY OF COPPER AND ZINC IN SOILS TREATED WITH ALKALINE STABILIZED SEWAGE SLUDGES

A glasshouse experiment was conducted to determine the bioavailability of Cu and Zn in a sandy loam and a clay loam, Spring barley (Hordeum vulgare L. cv. Forrester) was grown for 40 d in the two soils followin g application of alkaline-stabilized sewage sludge. The soils were mir ed with sludge product to give mixtures equivalent to fresh sludge cak e application rates of 30, 90, and 120 Mg/ha, Unamended controls and a ll sludge-treated soils except those receiving 90 Mg/ha were sequentia lly extracted to fractionate Cu and Zn into five operationally defined chemical forms. About 74% of total Cu and 80% of total Zn remained in the RESD fraction of the sludge-amended soils, The largest extractabl e Cu fraction was OM-Cu while the largest extractable Zn fraction was FeMnOX-Zn in both types of sludge product and in both soils. The EXCH and CARB fractions together comprised <2% of total Cu and Zn in sludge -amended soils, The sludge product raised soil pH and, except for EXCH -Zn, increased the concentrations of both metals in the four extractab le soil fractions, The sludge product increased barley shoot uptake of Cu by 112% and of Zn by 67% on average. However, sludge? application decreased the shoot Zn concentration by 39% in the clay loam, This may be due to a decrease in EXCH Zn in the soil together with a dilution effect in the shoots resulting from a positive yield response to the s ludge cake. Plant metal uptake was correlated (P < 0.05) with the vari ous soil fractions, particularly EXCH-Cu and CARB-Zn.