Selenium

Selenium (Se) is both an essential micronutrient for animals and humans and potentially toxic at relatively low intakes. Total soil Se is usually low(0.01–2 mg/kg), but parts of China, India and the USA have toxic soil Se levels. Available soil Se is poorly correlated with total soil Se and is highly variable, both locally and globally. The plant availability of Se in soil depends on the major Se species present and on soil characteristics, including the quantity of sorption components (aluminium and iron oxide/hydroxides), pH and redox status. Also, the presence of anions competing for the same sorption surfaces (including sulphate, phosphate and organic anions) affects root uptake and retention of Se in soil, and microbial activity is important for Se interactions with organic matter. Depletion of Se and S is common in soils of Sub-Saharan Africa, due to soil erosion, leaching and volatilisation through burning. The only viable long-term solution, especially for farmers who cannot afford commercial fertilisers, is to re-establish agricultural ecosystems that are closer to the natural ecosystems they replaced. Selenium is not considered to be essential for higher plants; however, it has numerous health roles in humans and animals, mostly mediated by Se-dependent enzymes. Although diseases associated with profound Se deficiency (Keshan disease, Kaschin-Beck disease and myxoedema) are rare, suboptimal intake is widespread and may increase risk of heavy metal toxicity, certain cancers, cardiovascular diseases and HIV disease. It is possible to biofortify food crops using selenate, but a high proportion is retained in the soil, and more targeted supplementation may be preferable to conserve this scarce micronutrient.Olav Albert Christophersen, Graham Lyons, Anna Haug and Eiliv Steinne