Effects of soil oven-drying on concentrations and speciation of trace metals and dissolved organic matter in soil solution extracts of sandy soils

Weak salt extracts can be used to assess the availability of trace metals for leaching and uptake by soil organisms and plants in soil. Before extraction, the International Organization for Standardization recommends to dry soils in an oven at a temperature of 40 °C. Effects of soil oven-drying on different fractions of dissolved organic matter (DOM) and the effect thereof on total concentrations and speciation of trace metals in weak salt extracts have, however, not been quantified yet. In this study, free metal concentrations and DOM speciation were determined in 2 mM Ca(NO3)2 extracts obtained from twelve field-contaminated soils in their field-moist state and after drying at 40 °C. Free metal concentrations were measured with the Donnan Membrane Technique. DOM was fractioned into humic acid (HA), fulvic acid (FA), and hydrophilic (Hy) compounds. Soil oven-drying led to significant increases in the concentrations of total DOM and total dissolved Cu and Ni. For the measurement of total dissolved Cu and Ni concentrations, it is, therefore, better to use field-moist soils. The release of Hy compounds was mainly responsible for the increase in DOM, which accounted for 64 to 77% of the increase in total dissolved organic carbon. Soil oven-drying left the free Cd2+, Cr3+, Cu2+, Ni2+, and Zn2+ concentrations unchanged. Both field-moist and oven-dried soils can, therefore, be used for the measurement of free metal concentrations. Free Cd2+, Cu2+, Ni2+, and Zn2+ concentrations were predicted very well for both field-moist and oven-dried soils using ORCHESTRA, which includes the NICA-Donnan model. However, poor predictions were obtained for Cr3+, for which better NICA parameters need to be derived.