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 <1 in both years, except for the velvet beans. Differences in CA management had minimal influence on ⁷⁷Se concentration in plant grain but the low yield in the single conventional treatment reduced ⁷⁷Se uptake. Residual ⁷⁷Se in the soil (35% of the applied) measured at harvest in 2017 was still present at harvest in the residual year (2018) but was completely unavailable to any of the crops. Almost none of the remaining ⁷⁷Se was present in soluble or phosphate-extractable forms and virtually all was present in the ‘organic’ (TMAH-extractable) fraction. Thus, annual Se applications to maize would be necessary to maintain concentrations which could improve dietary supply and reduce current Se deficiency in Malawi.