Alkaline sewage sludge solids affect the chemical speciation and bioavailability of Cu and Zn in the rhizosphere soil solution

Perennial ryegrass (Lolium perenne cv. Magella) was grown in a glasshouse experiment in which nylon mesh bags separated the soil into ‘rhizosphere’ and ‘non-rhizosphere’ zones. The soil was mixed with four application rates of an organic fertiliser made by mixing sewage sludge cake with cement kiln dust and composting. Soil solution was obtained by low speed centrifugation and filtration after addition of water (1:1 soil:solution ratio). A resin exchange method was used to distinguish between labile and non-labile forms of dissolved copper (Cu) and zinc (Zn). Soil solution pH, electrical conductivity (EC), absorbance at 360 nm (A360) and Cu and Zn were determined, together with plant (shoot + root) Cu and Zn concentration and uptake.Sludge cake application increased total soluble Cu but decreased total soluble Zn in the soil solution compared to unamended controls and these effects were associated with increased soil solution pH, EC and AMI. Soluble Cu accumulated while soluble Zn was depleted in the rhizosphere compared to the bulk soil. Cu occurred mainly in non-labile form, probably as stable organic complexes, while Zn was mostly in labile form, possibly as free ions and weak complexes. Sludge cake decreased the labile form of each metal and increased the non-labile form in the rhizosphere. It increased plant yield and uptake of Cu and Zn but did not change the plant concentrations of the two metals. The strong influence of the organic fertiliser on plant Cu and Zn uptake by promoting plant growth and by changing the solubility and distribution of the metals in the rhizosphere soil solution is discussed.