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 following application of alkaline‐stabilized sewage sludge. The soils were mixed with sludge product to give mixtures equivalent to fresh sludge cake application rates of 30, 90, and 120 Mg/ha. Unamended controls and all sludge‐treated soils except those receiving 90 Mg/ha were sequentially 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 extractable 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 extractable 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 sludge cake. Plant metal uptake was correlated (P < 0.05) with the various soil fractions, particularly EXCH‐Cu and CARB‐Zn.