The tri-trophic transfer of Zn from the agricultural use of sewage sludge

An agricultural soil in Southern England was amended with two annual applications of sewage sludge at rates of 0, 10 and 15 t ha−1 dry solids. Soil concentrations of Zn reached 31.1 μg g−1 and the subsequent uptake was measured in wheat plants (Triticum aestivum), grain aphids (Sitobion avenae) and a predatory carabid beetle (Bembidion lampros) in order to test the hypothesis that Zn was transferred through this tri-trophic food chain. Concentrations of Zn in the wheat plants, aphids and predatory beetles reached 31.7, 116.0 and 242.2 μg g−1 dry wt., respectively. A potential exposure pathway from soil→plant→herbivore→predator was identified, the concentration of Zn in the predatory beetle being related to the amount of aphids eaten and sewage sludge application rate. The concentrations of Zn in the sludge-amended soils were all well within current UK regulation limits and the relevance of exposure pathways at sub-critical concentrations when considering metal loading rates for sewage sludge amended soils is discussed.