Drying effects on selenium and copper in 0.01M calcium chloride soil extractions

The study aimed to understand the effects of drying and rewetting the soils on soluble selenium (Se) and copper (Cu) concentrations in 0.01M CaCl2 soil extraction, and the mechanisms leading to these changes. Soluble Se and Cu concentrations were measured in the extractions of 58 soil samples taken from grassland and arable land in The Netherlands before and after oven drying at 40°C. Selenium speciation and dissolved organic matter (DOM) fractionation were performed using Donnan membrane technique (DMT) and DAX-8 resin. Drying and rewetting the soils increased soluble Se and Cu concentrations in CaCl2 extraction. The increase of both elements was associated with the increase of dissolved organic carbon (DOC). Compared to the magnitude of DOC increase (1.3 to 11 fold), the increase of soluble Se (1.3 to 34 fold) was more pronounced, whereas the increase of soluble Cu (1.1 to 4.6 fold) was less strong. The DMT analysis confirmed the dominance of colloidal sized DOM-associated Se both in the moist and dried soil extractions, showing that the colloidal sized organic Se is mainly responsible for the increase of total soluble Se upon drying. The Se to DOC ratio increased upon drying in most of the soils, indicating enrichment of Se in DOM after drying. No consistent difference in Se to DOC ratio was found between the fulvic acids (FA) and hydrophilic (Hy) fraction of DOM after drying. Fractionation of DOM showed that Hy and FA fraction accounted for respectively 74% and 26% of total DOC in the dried soils, which corresponds to the Se fraction in Hy and FA of DOM. The decrease of SUVA (0.3 to 1.0 fold) after drying implied a decrease of FA:Hy ratio in DOM and a decrease of average DOM aromaticity. Upon drying, a positive correlation between the change of Cu loading on DOC (Cu:DOC ratio) and the change of aromaticity (SUVA) of DOM was found, showing that the increase of Cu complexed with (mainly) aromatic DOM explains soluble Cu increase upon drying. The decrease of Cu loading on DOC upon drying was attributed to a decrease of the aromaticity and affinity to Cu of the DOM. Colloidal sized organic Se and Cu–DOM complex are the major species of Se and Cu, respectively, in the extractions of moist and dried soils, which may not directly be available for plant uptake. •Soluble Se, Cu and DOC in 0.01M CaCl2 soil extraction increased upon drying.•The Se increase is larger and the Cu increase is smaller than the DOC increase.•DOM is enriched in Se after drying and Se:DO