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

In the soils and aquatic systems of coniferous forests, selenium is us ually associated with humic substances. To clarify further some of the mechanisms involved, labelled and unlabelled selenite were added to t wo forest floors and to a brown-water lake. Sequential extraction proc edures and chromatographic methods were used to evaluate the resulting association between selenium and humic substances. It was observed th at the forest floors fixed most of the added selenite by means of micr obial reductive incorporation and that selenium was preferentially inc orporated into low-molecular-weight fractions of the humic substances. By contrast, selenium reduction was much slower in the brown-water la ke and instead, inorganic complexation of selenite to metal-humic comp lexes was important during the experiment, provided that the concentra tions of competing ligands were low.