THE ASSOCIATION BETWEEN SELENIUM AND HUMIC SUBSTANCES IN FORESTED ECOSYSTEMS - LABORATORY EVIDENCE

In the soils and aquatic systems of coniferous forests, selenium is usually associated with humic substances. To clarify further some of the mechanisms involved, labelled and unlabelled selenite were added to two forest floors and to a brown-water lake. Sequential extraction procedures and chromatographic methods were used to evaluate the resulting association between selenium and humic substances. It was observed that the forest floors fixed most of the added selenite by means of microbial reductive incorporation and that selenium was preferentially incorporated into low-molecular-weight fractions of the humic substances. By contrast, selenium reduction was much slower in the brown-water lake and instead, inorganic complexation of selenite to metal-humic complexes was important during the experiment, provided that the concentrations of competing ligands were low.