Mobility and bioavailability of Cr, Cu, Ni, Pb and Zn in a basaltic grassland: Comparison of selective extractions with quantitative approaches at different scales

Bulk Cr, Cu, Ni, Pb and Zn contents were analyzed in a Cr–Ni rich basalt, derived andic soil and in grass subaerial and aerial components to assess natural trace metal mobility in soil. Phytoavailability and relative mobility of elements were quantified by three different approaches: selective extractions, calculation at mineral scale after XRD, SEM-EDS and EMP analyses and measurement of pools at profile scale. Pool variations from rock to soil horizons give a view of the long-term mobility, while selective extractions and comparisons of pools measured in plants and top soil provide estimations of the current mobility and bioavailability. Major amounts of trace metals were held in the solid constituents of the soil. Examination of the solid residues following extractions clarified the potential mobility of these metal containing solids. Water soluble and exchangeable fractions never exceed 3% of the total trace metal amounts, which is of similar magnitude to pools found in grass roots and shoots. Zinc is the most easily extracted element followed by Cu, Ni and Cr. Over the short term, Zn also occurs in greater amounts in grass. Its involvement in organic matter turnover explains its greater accumulation in the soil profile over the long term. Chromium and Ni which are predominantly held in stable minerals (spinels) are less available for plants in the short term. They are both relatively depleted in soil, compared to initial pools in rock, over the long term. From a methodological point of view, the results demonstrate the necessity of comparing selective extraction data with other quantitative methods for assessing both short-term and long-term trace metal mobility and bioavailability.