SELENIUM BIOAVAILABILITY IN BRAZILIAN SOILS BY TOTAL REFLECTION X-RAY FLUORESCENCE

Selenium (Se) is a naturally occurring metalloid element that is essential to the health of humans and other animals in trace amounts, but is harmful in excess. Of all the elements, Se has one of the narrowest ranges between dietary deficiency (< 40 μg day−1) and toxic levels (> 400 μg day−1)1. Se toxicity and deficiency are influenced by bioavailability of Se in the environment2. In this study, Se concentrations were determined in fractions of nine soil classes to evaluate bioavailability, as well as to correlate it with soil physicochemical properties (SPP). In this way, total reflection X-ray fluorescence (TXRF) and three extractants (ultrapure water, phosphate buffer and sodium hydroxide) were used to assess water-soluble Se (W-Se), exchangeable Se (E-Se) and Se bound to soil organic matter (OM-Se), respectively. Subsequently, an OM-Se extract was fractioned (using nitric acid) into Se associated with fulvic acids (FA-Se) and humic acids (HA-Se). The soil samples showed different levels of Se bioavailable. OM-Se was the dominant fraction, accounting for 15.2% to 98.2% Se content. The combination of W-Se, E-Se and FA-Se fractions is an indicator of Se bioavailability for plants. These fractions presented ranges from 0.9 to 7.1%, 1.4 to 19.4% and 1.5% to 64.4% of the total Se (concentration varied from 0.31 ± 0.03 to 5.97 ± 0.20 mg kg-1), respectively. HA-Se ranged from 5.4% to 54.6% of total Se. Most soil samples showed residual Se fraction under 50%. Significant correlations (P < 0.05) between these fractions and SPP were obtained, mainly for W-Se fraction. This fraction was positively correlated with pH (r = 0.49) and sand content (r = 0.57), and negatively with Al2O3 (r = - 0.50). The Fe2O3 was strongly correlated with HA-Se (r = 0.78), residual Se fractions (r = 0.79) (fractions generally considered to be unavailable for plants3), FA-Se (r = 0.86) and total Se (r = 0.97). These results have significant implications for Se bioavailability for tropical soils, which typically have high acidity and Al2O3 and Fe2O3 oxide contents.