REDOX TRANSFORMATIONS AND PLANT UPTAKE OF SELENIUM RESULTING FROM ROOT-SOIL INTERACTIONS

Batch studies were conducted with Mn oxides (birnessite-hausmannite mixture, BHM) and samples of four soil series from the Mid-Atlantic region of the USA to determine effects of reducing organic acids, similar to those found in the rhizosphere, on the SeO3/SeO4 distribution. Jackland (Typic Hapludalf), Myersville (Ultic Hapludalf), Christiana (Aeric Paleaquult), and Evesboro (Typic Quartzipsamment) A and B horizon soil samples with and without prior Mn oxide reduction were incubated aerobically for 10 d with 0.1 mmol kg(-1) SeO3 and 0 or 25 mmol kg(-1) of ascorbic acid, gallic acid, oxalic acid, or citric acid. Selenite was also added to BHM (10 mmol kg(-1)) with 0 or 0.1 mmol kg(-1) ascorbic acid. The availability of Se for plant uptake as a result of root-soil interactions was examined using growth chamber studies with barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seedlings grown in 150-mL cone-shaped containers to maximize root-soil surface interactions and to create 'rhizosphere' soil throughout the root zone. In the BHM system ascorbic acid increased oxidation of SeO3 to SeO4 to 33% of added SeO3. In the presence of ascorbic and gallic acids and Mn oxides, oxidation of SeO3 to SeO4 occurred in the B horizons of all the soils and in the A horizons of Jackland and Myersville soils. Removal of Mn oxides decreased the oxidation in some samples. Wheat and barley plants were able to accumulate up to 20 mu mol Se kg(-1) from the Jackland soil when soluble Se was not measurable. The root-soil interactions in the Jackland soil with barley and wheat provided the plant with Se from insoluble sources. The results also indicate that Mn oxides coming in contact with reducing root exudates have a greater ability to oxidize SeO3 to SeO4. Thus, rhizosphere processes play an important role in the availability of Se for plant uptake.