Metal and metalloid trace elements at high concentrations generally induce toxicity and reduce fitness of plants. However, some species (metallophytes) have evolved metal tolerance. Excluders maintain low concentrations in aboveground tissue in a large range of soils concentrations, and hyperaccumulators concentrate trace elements in aboveground tissue parts from low and high soil levels.
Among metallophytes, pseudo-metallophytes represent a small proportion of species whose spatial distributions are characterized by multiple and fragmented habitats, including both metalliferous and non-metalliferous soils. In the eastern US, excluder pseudo-metallophytes, but not hyperaccumulators, are common on serpentine barrens. It is not clear whether the presence of pseudometallophytes and their ability to regulate metal uptake in aboveground tissue parts vary with soil properties.
We aim to evaluate the effect of elevated heavy metal concentrations on three dominant grassland species in metalliferous and non-metalliferous soils. We collected plants and their rooting soils from four metalliferous sites: three are serpentine and one is calamine (high concentrations of cadmium and zinc), and one control non-metalliferous site. We measured the major and trace elements concentrations for pseudo-total (HNO3+HCl) and extractible (DTPA and CaCl2) fractions in soils and in leaves by using ICP-AES.
Results/Conclusions
Pseudo-total and extractible concentrations of elements in rooting soils varied from 1 to 5 orders of magnitude. At the site level, all elements showed significant differences between soils for both fractions except for extractible K and Cr. Pseudo-total and extractible concentrations of Co, Cr, Fe, Mn and Ni were higher in serpentine soils. Pseudo-total concentrations of Cd, Cu, Pb, Zn and extractible Al were highest in calamine soil. At the species level, only a few elements differed significantly.
In leaves, concentrations of all elements varied by 1 to 3 orders of magnitude and all elements except Co, Na and Ni varied significantly between sites. Leaf concentrations of Fe, Cd and Zn were higher in the calamine site whereas concentrations of Mn were higher in non-metalliferous site. At the species level, major elements in leaves varied more than the trace elements.
Our study generalized and validated the hypothesis that excluder species can maintain relatively low metal concentrations in their leaves even when the soil concentration of the metals such as Ni and Co are high. But this trend was not found in Zn-contaminated calamine soil. This result indicates the presence of a threshold concentration where metal uptake increases drastically.