Enhancing Organic Selenium Forms in Alfalfa Forage Through Inorganic Selenium Foliar Application: Insights from Laboratory and Field Studies Using X-Ray Spectroscopy

Selenium (Se) is an essential micronutrient, yet its deficiency remains a global concern. This study investigates the biofortification of alfalfa (Medicago sativa cv. ProINTA Super Monarca GR9) via foliar Se application to enhance Se accumulation and transformation into bioavailable organic forms. A controlled environment experiment in a plant growth chamber and a one-season open-field trial (January 2023, Argentina) were conducted. Treatments included sodium selenate (Se(VI)), sodium selenite (Se(IV)), and a 1:1 mixture, applied at 45 and 90 g Se ha−1, with and without the biostimulant BIOFORGE®. Treated plants exhibited increased Se content, correlating with the applied doses. X-ray absorption spectroscopy (XAS) confirmed that most inorganic Se was transformed into organic Se forms, with Se(IV) treatments yielding the highest concentrations of organic Se species such as selenocysteine (SeCys) and selenomethionine (SeMet). Open-field trials showed a complete conversion of Se, though total Se accumulation was lower than in controlled conditions. Se treatments did not affect forage quality or biomass production. The biostimulant slightly reduced Se uptake but did not compromise biofortification. These results highlight Se(IV) as the optimal treatment for alfalfa biofortification, presenting a sustainable strategy to enhance dietary Se intake through functional foods.