The Transfer and Fate of Cadmium and Zinc from Sewage Sludge Amended Agricultural Soil in an Arthropod Food Chain.

Abstract

The re-cycling of sewage sludgeis the most significant entry point for trace metalsin to the agroecosystem. However, the investigation of the transfer and fate of trace metals in soil-plant-arthropod systems in an agricultural context has received little attention, despite the potential threat that secondary toxicity to predatory arthropodsposes to the biological control of crop pests.In this study, an agricultural soil was amended with sewage sludgeat rates up to an equivalent of 100 t (dry solids) ha-1.The subsequenttransfer of Zn and Cd through an ecologically relevant soil-crop-aphid-arthropod predator systemwasinvestigatedin a series of pot trials. Results show that Zn was transferred to a greater extent than Cd between all components of the system, except between the roots and shoots. Cadmium was only biomagnified in roots and was biominimised in shoots, aphids and ladybirds. Zinc was biomagnified in roots, shoots, andaphids compared to the soil, but concentrationsin ladybirds were similar to those in the aphids they consumed.Differences between winter and spring wheat were found to have a larger influence on the transfer of Cd and Zn in the systemthan differences between winter wheat and spring barley. It was also shown that the rose grain aphid (Metopolophium dirhodum) accumulateshigher concentrationsof Cd than the grain aphid (Sitobion avenae).Whilst concentrationsof Zn did not differ between the two species of aphid, concentrations in M. dirhodum appearedto be more closely regulated than in S.avenae. Consumptionof S.avenaeby the fourth instar larvae of the seven-spottedladybird (Coccinella septempunctata)did not result in significant differences between treatmentsin newly emergedadults.This was partly the result of the sequestrationof the two metalsin the pupal exuviae. This mechanism had a greater effect on the Cd concentrations in newly emerged adult ladybirds than on Zn concentrations.In a further experiment,there appeared to be no pathway for the transfer of Cd from aphids to adult ladybirds, but a pathway was indicatedfor Zn. The implications of the results are discussed in relation to the agroecosystem andthe wider environment.