THE BIOAVAILABILITY OF CD AND ZN FROM SOILS AMENDED WITH SEWAGE-SLUDGE TO WINTER-WHEAT AND SUBSEQUENTLY TO THE GRAIN APHID SITOBION-AVENAE

An agricultural soil in southern England was amended with a single app lication 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 3 4.3 mg kg(-1), respectively, dependent on sewage sludge application ra te. Concentrations of Cd and Zn in the shoots of winter wheat grown we re dependent on the rate of sewage sludge application to the soil. Con centrations of Cd in the ears of wheat were not dependent on sewage sl udge application rate, whilst Zn concentrations were dependent. Grain aphids (Sitobion avenae) collected from the host plants accumulated co ncentrations 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 o n the ears on which they were feeding). Regression analysis demonstrat ed that the concentrations of Cd and Zn in S. avenae could be predicte d from the concentrations of these metals in the soil. The retention c haracteristics of the sludge amended soils and control soils for Cd an d Zn were assessed by batch adsorption experiments. From the sorption data it was observed that the sewage sludge amended soils showed no si gnificant change in Cd affinity when compared to the controls soils. H owever, for Zn, the reduction in the Freundlich distribution coefficie nt of the isotherms of the sludge treated soils were significant when compared to the control soils - indicating a reduced affinity for Zn a t these relatively low sewage sludge application rates. (C) 1997 Elsev ier Science B.V.