Selenium and nano-selenium in agroecosystems

Selenium (Se) is an essential health element becoming rare in food as a result of intensive plant production. Indeed, several enzymes contain selenium in the form of the unusual selenocysteine amino acid. Selenium was found an essential nutrient in the late 1950s, when selenium was found to replace vitamin E in the diets of rats and chicks for the prevention of vascular, muscular, and hepatic lesions. At that time, selenium was considered solely as a toxic element in the northern Great Plains of the USA, because selenium was associated with the 'alkali disease' of grazing livestock. The major source of Se in soils is the weathering of Se-containing rocks. Secondary sources are volcanic activities, dusts such as in the vicinity of coal burning, Se-containing fertilizers, and some waters. Se cycles through the food system; Se is first removed from soils by plants and soil microorganisms, which can take up Se into their proteins and produce volatile forms such as dimethylselenide. Dimethylselenide enters the atmosphere to be brought down with precipitation and airborne particulates. Here, we review Se in agroecosystems. We focus on the production, biological effects, and use of nano-selenium particles.