Effect of Natural Organic Matter and pH on the Bioavailability of Metal Ions in Soils

Metal ions may be essential elements for soil biota in trace amounts and may become toxic at elevated levels. The relationship between the total metal content in the soil and its positive or negative effect on biota is not straightforward. It is well recognized that the speciation of metal ions, that is the distribution over different chemical species both in solid and liquid phase, plays an important role in natural systems with respect to bioavailability. The binding of metal ions to abiotic surfaces and ligands like particulate organic matter (POC), clay, and metal(hydr)oxides may strongly influence the bioavailability. In this chapter we focus on the role of soil organic matter. The binding of metal ions to soil organic matter is influenced not only by the amount of organic matter present and the metal ion content of the soil, but also strongly by the pH and possibly the concentration of other metal ions present that may compete for the same binding sites. In addition, binding to dissolved organic carbon (DOC) and formation of inorganic complexes in solution may increase the total amount of metals in solution.