Selenium in Arid and Semiarid Soils

Abstract

Selenium is an essential element for many life forms but can cause toxicity when present at high levels in animal diets. The chemistry and particularly the geochemistry of selenium is similar to that of sulfur. Selenate, the selenium analog of sulfate, is the key to understanding selenium toxicity in arid and semiarid areas. Selenate salts of calcium, magnesium, sodium, and potassium are generally more soluble than those of sulfate. Also, selenate is not significantly adsorbed in neutral and alkaline soils (a common characteristic of arid and semiarid soils). Therefore, selenate is highly mobile, causing it to be transferred readily from place to place dissolved in water, where it also enters biological food webs by plant uptake. Within plants (including phytoplankta in surface water), selenium is incorporated into organic compounds by forming C‐Se bonds. Three other factors that control selenium in soil‐water‐plant systems are soil parent material (sedimentary rocks are generally higher in selenium), pH (the more soluble selenate is favored at high pH), and rainfall (higher rainfall can deplete the soil of selenium by leaching).