Speciation of Se and DOC in soil solution and their relation to Se bioavailability.

A 0.01 M CaCl2 extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl2 extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67−86%) in the extractions (15 samples) are colloidal-sized Se. Only 13−34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47−85% of DOC is colloidal-sized and 15−53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content i...