Earthworm biomass as additional information for risk assessment of heavy metal biomagnification: a case study for dredged sediment-derived soils and polluted floodplain soils

Foliar Cd and Zn concentrations of hybrid poplars commonly planted on sediment-derived soils were assessed in field circumstances. Selected sites covered a range of soil types and plantation characteristics. Reference data for foliar concentrations were established from samples taken in a tree-nursery. Even in the reference situation a large variation in foliar Cd and Zn concentrations was observed, with relative standard deviations in the order of 15%. Foliar concentrations of Cd and Zn in poplars growing on sediment-derived soils increased during the growing season. The accumulation rate was markedly higher on polluted sediment-derived soils than in the reference situation. Poplars grown on polluted sediment derived soils showed elevated and deviating foliar Cd and Zn concentrations (> 7.5 mg Cd kg^-1 DW and 320 mg Zn kg^-1 DW). A thin unpolluted covering layer did not influence foliar concentrations. Regardless of site characteristics, poplar age, species or clone, a significant positive relation was found between soil and foliar concentration for Zn and to a lesser extent for Cd. Bioconcentration factors for Cd and Zn were higher than one in baseline situations, but mostly lower than one on polluted sediment-derived soils. Cd: Zn ratio was on the average twice as high as in the soil. Leaf beetles showed normal body concentrations for Zn, but higher Cd concentrations than in reference situations. BCFs were lower than one on sediment-derived soils. Foliar results indicated a possible threat in long-term habitat development of poplar plantations. This conclusion was confirmed by the significant higher Cd concentrations in leaf beetles grown on poplars with deviant foliar concentrations. However, litter decomposition rates were generally evaluated as normal.