The association between selenium and humic substances in forested ecosystems—laboratory evidence

Abstract

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 brownwater 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 lowmolecular-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.

Selenium speciation and bioavailability in Dutch agricultural soils: the role of soil organic matter

“…This association is possibly due to either selenium-metal-organic matter complexation, such as Se-Fe(III)-organic matter complexes, or due to biotic formation via microbial reduction of Se and further incorporation into soil organic matter structure (Gustafsson and Johnsson, 1994;Fernández-Martínez and Charlet, 2009). Most studies indicated that the association relies more on biotic formation than on abiotic complexation (Fernández-Martínez and Charlet, 2009).…”

“…Methodology to determine Se speciation in soils has been explored intensively either using various single step soil extractions (Yamada and Hattori, 1989;Abrams et al, 1990a;Kang et al, 1991b;Gustafsson and Johnsson, 1992;Johnsson, 1992;Gustafsson and Johnsson, 1994;Alam et al, 2000;Keskinen et al, 2009;Tolu et al, 2011;Qin et al, 2012;Stroud et al, 2012) or sequential soil extractions (Chao General introduction and Sanzolone, 1989;Tokunaga et al, 1991;MacLeod et al, 1998;Mao and Xing, 1999;Wang and Chen, 2003;Hagarová et al, 2005). In general, these soil extractions represent different Se pools or Se fractions present in soils that are removed by the corresponding extractants, such as soluble Se, exchangeable Se, Se adsorbed to metal-(hydr)oxides or carbonates, and Se bound to organic matter or humic compounds, instead of specific Se species (Tokunaga et al, 1991).…”

“…The results of DMT analysis indicate that the increase of total soluble Se upon drying is mainly due to the increase of colloidal sized organic Se. Adsorption to the reactive groups on soil organic matter (directly or indirectly via metal ions) or as part of the structure of organic matter molecules are two possible mechanisms for Se association with soil organic matter (Gustafsson and Johnsson, 1994;Qin et al, 2012). A study carried out by Gustafsson and Johnsson (1994) suggests that Se incorporated into soil organic matter through microbial activity is the major mechanism for Se-organic matter association.…”

“…Gustafsson and Johnsson (1994) suggested that Se is mainly incorporated into the hydrophobic processes. It is interesting to point out that in clay soils the Se to organic C ratio in the FA + HON fraction is in general higher than in the other soil types (Table 8).…”

“…In the solid organic matter, organic Se is present as Se associated with humic acids and fulvic acids, which may be in the forms of selenoamino acid incorporated into protein or peptides (Kang et al, 1991b). Through microbial activity, organic Se can be transformed into inorganic forms upon mineralization, and vice versa inorganic Se, mainly selenite (+4) can be reduced and incorporated into soil organic matter (Gustafsson and Johnsson, 1994). In addition, plants or soil microorganisms can transform inorganic Se into volatile Se compounds, such as diselenides (RSeSeR) or dimethylselenide ((CH 3 ) 2 Se) (Pyrzyńska, 2002;Hartikainen, 2005).…”