Selenium Mobility in Soils and its Absorption, Translocation, and Metabolism in Plants

INTRODUCTION Forms of selenium found in soils influence its mobility, uptake, and metabolism by plants. The major forms in alkaline, oxidizing environments which are available for plant uptake are selenium-VI as selenate, Se0 4 2 -) and selenium-IV (as selenite, $e0,'). The major influences on uptake are soil pH and salinity. High salinit y and pH favor seienium anion adsorption onto clays and metal oxides. Selenite is adsorbed much more strongly than selenate leaving selenate as the major form available for plant uptake. Some soil anions, such as phosphate, increase plant selenium uptake because increased soil-solution anion concentrations compete with selenium anions for adsorption sites. Other anions, such as chloride or sulfate, actually enhance or inhibit uptake by affecting plant metabolism. Inorganic selenides and elemental selenium are mostly insoluble except under conditions of low pH in moist, reducing environments. In these conditions organic selenides may also be found as selenium amino acids, such as selenoglutathione, and in various fractions of humic substances. Although it is unclear whether organic selenides are absorbed from soil by plants, they have been identified in soil solutions as p roducts of bacterial and plant metabolism. Volatilization of organic selenium compounds makes mass balance studies of selenium difficult. Selenate ions are rapidly absorbed and transported in plant xylem sap. Selenite absorption, on the other hand, is slower, but the selenium is more rapidly metabolized to organoselenium compounds and transported into upper portions of the plant. Soil and plant management in seleniferous areas must take into account soil types and the genetic tolerance by plants of high selenium and salt concentrations. For example, plants will tolerate more selenium on high-sulfate soils than on low sulfate soils. Some plants, such as alfalfa, are very sensitive and will show signs of damage at low soil selenium concentrations while others, such as saltbush, may accumulate thousands of milligrams per kilogram of selenium without damage. Some arid and semiarid soils may need to be managed by prudent irrigation practices in order to reduce selenium and salinity to acceptable levels. A number of recent review articles detail the factors influencing selenium mobility, transport, and uptake by plants (Anderson and Scarf, 1983; Brown and Shrift, 1982: Girling, 1984; Gissel-Nielsen and others. 1984: Mavland and others. 1989: Peterson and others, 1981: and Sharma and Singh, 1983). In addition, Fisher and others (1987) examined selenium transport and uptake as it pertains to reclamation management problems and research needs. The purpose of the present report is to survey the literature published since 1983 (confirmation of toxic selenium levels in the Kesterson ecosystem) and discuss how this new information may refine approaches to land reclamation methods and management practices. Much useful information has come from greenhouse and plant culture experiments as well as field studies are included in this review.