ENVIRONMENTAL VARIATION BETWEEN HABITATS AND UPTAKE OF HEAVY-METALS BY URTICA-DIOICA

The observation from previous surveys, that Urtica dioica plants that had grown in metal contaminated soil in the floodplains of the former Rhine estuary in different habitats, but at comparable total soil meta l concentrations, showed significant differences in tissue metal conce ntrations, led to the hypothesis that variation in other environmental characteristics than soil composition and chemical speciation of meta ls between habitats is also important in determining uptake and transl ocation of metals in plants. A field survey indicated that differences in root Cd, Cu and Zn concentrations might partly be explained by var iation in speciation of metals in different habitats. However, shoot c oncentrations showed a different pattern that did not relate to variat ion in soil metal concentrations. In a habitat experiment Urtica dioic a plants were grown in artificially contaminated soil in pots that wer e placed in the four habitats (grassland, pure reed, mixed reed, osier bed) that were also included in the field survey. After seven weeks t he plants showed significant differences in Cu and Zn concentrations i n roots and aboveground plant parts and in distribution of the metals in the plants between habitats. It was concluded that variation betwee n habitats in environmental characteristics other than soil compositio n can explain as much variation in plants as can variation in soil met al concentrations and/or speciation. The implications for assessment o f soil metal contamination and uptake by plants are discussed.