Quantifying the influence of soil properties on the solubility of metals by predictive modelling of secondary data

The concept of critical loads, previously applied to acidifying substances, is currently being extended, within the Convention on Long Range Transboundary Air Pollution (CLRTAP) under the UNECE, to several metals: Cd, Cu, Pb and Zn. Soil organisms such as plants and microbiota are exposed to these metals via the soil solution, and critical loads should therefore be based on metal concentrations in soil solution rather than total concentrations. Whilst such data do not exist on a large scale, empirical models may be able to predict bioavailable metal concentrations from existing data on total metal concentrations and those soil factors thought to influence the partition of metals between the solid and solution phases of soils. In this study we attempt, by undertaking statistical analysis of previously published data, to quantify the influence of sail factors on this partition. A major limitation to the approach is the availability of suitable data. However, analysis shows that, amongst the soil factors investigated, pH alone consistently accounts for a large percentage of the variance in metal extractability. Verification of this finding may enable models to be developed which can use existing and commonly available data on total metal concentrations, and pH, to predict approximations of bioavailable concentrations of metals which can subsequently be used in the derivation of critical loads.