The bioavailability of Cd and Zn from soils amended with sewage sludge to winter wheat and subsequently to the grain aphid Sitobion avenae

Abstract

An agricultural soil in southern England was amended with a single application of sewage sludge from a municipal source at rates of 0, 5, 7.5, 10, 15 and 20 t ha −1 dry solids. Soil concentrations of Cd and Zn in the field plots ranged from 0.109 to 0.180 mg kg −1 and 26.3 to 34.3 mg kg −1, respectively, dependent on sewage sludge application rate. Concentrations of Cd and Zn in the shoots of winter wheat grown were dependent on the rate of sewage sludge application to the soil. Concentrations of Cd in the ears of wheat were not dependent on sewage sludge application rate, whilst Zn concentrations were dependent. Grain aphids ( Sitobion avenae) collected from the host plants accumulated concentrations of Cd and Zn up to 0.386 mg Cd kg −1 and 319 mg Zn kg −1 (eight and 10 times greater, respectively, than the concentrations on the ears on which they were feeding). Regression analysis demonstrated that the concentrations of Cd and Zn in S. avenae could be predicted from the concentrations of these metals in the soil. The retention characteristics of the sludge amended soils and control soils for Cd and Zn were assessed by batch adsorption experiments. From the sorption data it was observed that the sewage sludge amended soils showed no significant change in Cd affinity when compared to the controls soils. However, for Zn, the reduction in the Freundlich distribution coefficient of the isotherms of the sludge treated soils were significant when compared to the control soils — indicating a reduced affinity for Zn at these relatively low sewage sludge application rates.