macro- and micro-biota. The distribution of metals among these phases and components, and their physical relation to the soil, control dissolution properties of the metals and hence their bioavailability.

One of the main differences between soils and aquatic sediments is that the latter are in intimate contact to an overlying water body, while soils are directly exposed to the atmosphere. As a consequence, soils are the result of natural weathering processes, while sediments are build up by solutes and particulates entering or formed in the water column. Although geochemically rather similar, this difference makes that soils and sediments are subjected to almost completely different physical, chemical and biological conditions, with the result that the properties of solid phases and the kinetics of reactions at the solid-liquid interface dominate soil solution chemistry. However, their underlying similar geochemistry and mineralogy was one reason, why methods developed and used for metal speciation are rather the same for both types of substrates, and why soil scientists working on metal speciation often are also involved in corresponding studies on sediments, or vice versa. Beside some more classical soil extraction procedures, which are almost exclusively used to estimate plant-uptake and bioavailability of certain trace elements (like the CaCl2-, EDTA-, or DTPA-extraction), current speciation methods are almost equally used for both soils and sediments. In contrast, techniques to obtain soil or sediment porewater obviously differ from each other, as do laboratory-based bioassays developed to assess the ecological risk of metal contamination, due to obviously different habitat conditions (i. e. soil/air versus sediment/ water) existing in either systems. A enhancing cooperation between soil and sediment scientists in the field of trace metal speciation would surely contribute to improve our present understanding of the influence of processes and factors that control the intricate interaction between metal species, solid substrate and organisms, as well as of existing analytical and modelling approaches designed to assess and predict their potential impact, if environmental conditions change. In the following, we will focus on those soil parameters and speciation methods that have evolved as relevant during recent years to successfully characterize the bioavailability of metals.

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