Trophic barriers to fertilizer Cd bioaccumulation through the food chain: a case study using a plant--insect predator pathway.

The objective of this study was to assess the uptake and subsequent transfer of Cd and Zn from a soil amended with a single application (150 kg P ha(-1)) of triple super phosphate fertilizer to wheat plants, aphids, and a predator and biocontrol agent of aphids, lacewings. The fertilizer amended soil and wheat plants grown on this soil had elevated concentrations of Cd compared to the controls, but similar concentrations of Zn. Aphids feeding on wheat plants on the fertilized soil had between three and seven times the concentrations of Cd and Zn observed in aphids feeding on the control plants. However, the lacewings showed no significant accumulation of Cd or Zn, and no differences in larval performance were recorded. Changes in the availability of Cd and Zn in the soils and the transfer through the plant-insect pathway were monitored using isotope dilution, by labeling the soils with carrier-free (109)Cd and (65)Zn. Decreases in the specific activities for Cd in the plants and aphids were observed for the fertilized soils compared to the controls, suggesting an increase in bioavailable Cd. On the fertilized soils the Cd:Zn ratio of the phloem-feeding aphids (0.008) was significantly less than the host plants (0.025), indicating a reduced relative uptake of Cd and a possible barrier for Cd along the soil--plant--herbivorous insect pathway--reducing uptake by phloem feeders and subsequently their predators.