Selenium biofortification of crops on a Malawi Alfisol under conservation agriculture

Biofortification with selenium (Se) may rely on rapid uptake by crops, following application, to offset progressive fixation into unavailable organic forms of Se in soil. A biofortification study was conducted on an Alfisol within a long-term conservation agriculture (CA) field trial at Chitedze Research Station, Malawi. The aim was to assess the dynamics of selenium bioavailability to a staple cereal (Zea mays) and a range of legumes (cowpeas, groundnuts, pigeon peas and velvet beans) under CA management, as well as residual Se effects in the year following biofortification. Isotopically labelled selenate (>99% enriched 77SeVI) was applied to each plot, in solution, at a rate of 20 g ha−1, at maize flowering (75 days after planting), in February 2017. Samples of grain and stover from maize and legumes, and topsoil, were collected at harvest in May 2017 and May 2018. Plant and soil samples were analyzed by ICP-MS for selenium isotopes (77Se and 78Se). The concentration of 77Se in the grain of maize and single-cropped legumes exceeded 200 µg kg−1 in all the treatments. This would contribute approximately 56–64 µg day−1 to the Malawi diet, as refined maize flour. The fertilizer derived Se concentration ratio of maize grain-to-stover Se were >1 in 2017 but <1 in 2018; which followed the same trend as the native soil-derived Se in the residual year. In legumes the grain-to-stover concentration ratio was consistently