Fate of selenium in biofortification of wheat on calcareous soil: an isotopic study.

Selenium (Se) biofortification of staple cereal crops can improve the Se nutritional status of populations. A field trial employing an enriched stable isotope of Se (Se) was undertaken over three consecutive cropping seasons in a coarse-textured, calcareous soil in Gilgit-Baltistan, Pakistan. The objectives were to (1) assess the feasibility and efficiency of Se biofortification, (2) determine the fate of residual Se, and (3) assess the consequences for dietary Se intake. Isotopically enriched Se (Se) was applied, either as selenate or as selenite, at three levels (0, 10, and 20 g ha) to a wheat crop. Residual Se availability was assessed in subsequent crops of maize and wheat without further Se addition. Loss of Se was c.35% by the first (wheat) harvest, for both selenium species, attributable to the practice of flood irrigation and low adsorption capacity of the soil. No Se was detectable in subsequent maize or wheat crops. The remaining Se in soil was almost entirely organically bound and diminished with time following a reversible (pseudo-)first-order trend. Thus, repeat applications of Se would be required to adequately biofortify grain each year. In contrast to native soil Se, there was no transfer of Se to a recalcitrant form. Grain from control plots would provide only 0.5 µg person day of Se. By contrast, a single application of 20 g ha Se could provide c. 47 µg person day Se in wheat, sufficient to avoid deficiency when combined with dietary Se intake from other sources (c. 25 µg day).