Effects of Soil Drying On Concentrations and Speciation of Metals and Dissolved Organic Matter in Weak Salt Extracts of Sandy Soils.

Contamination of soils with metals is a serious concern, because metals may adversely affect soil ecology, agricultural production and food quality, and ground and surface water quality. These risks, however, are not related to total metal contents in soil only, but depend also on the partitioning of metals between the soil solid phase and the soil solution and on the speciation of the dissolved metals. In turn, metal concentrations and speciation depend on soil properties such as pH, soil organic matter, reactive Al- and Fe-(hydr)oxides, and dissolved organic matter (DOM). For leaching, total dissolved metal concentrations need to be considered, while plant uptake and toxicity of metals to soil organisms generally correlates best with free metal concentrations. Risks of metal contaminated soils are often assessed via the measurement of metal concentrations and speciation in weak salt extracts used to mimic the soil solution. Before extraction, soils are commonly dried, because sub-sampling of dry soils is much easier and storage of dry soil samples is less complicated. However, drying and subsequent rewetting of soils causes soils to behave differently than soils kept continuously in a field-moist state. For example, the DOM concentration in extracts obtained from dried soils is known to increase significantly in comparison with field-moist soils. Due to an increased DOM concentration, total dissolved concentrations of metals such as Cu and Pb, which bind strongly to humic substances, can increase as well. Until so far, little attention has been paid to the effects of soil drying on free metal concentrations and speciation of DOM. We will present results on the effects of soil drying on the concentrations and speciation of metals and DOM. Free metal concentrations were determined with the Donnan Membrane Technique, while DOM was fractionated according to a recently developed rapid batch procedure.