Factors influencing metal bioavailability in soils: preliminary investigations for the development of a critical loads approach for metals

The concept of critical loads, previously applied to acidifying substances, is currently being extended, within the Convention on Long Range Transboundary Air Pollution (CLRTAP), to several metals: Cd? Cu, Hg, Pb and Zn. Receptors in the soil, such as plants and microbiota, are directly exposed to these metals via the soil solution and this should ideally be reflected in environmental quality objectives which, presently, are commonly based on 'total' rather than bioavailable metal contents of soils. An understanding of the factors influencing the partition of metals between the solid and solution phases of soils is an essential prerequisite of any attempt to apply the critical loads approach in a manner which relates to the effects of metals on soil organisms.