Agronomic biofortification of leafy vegetables grown in an Oxisol, Alfisol and Vertisol with isotopically labelled selenium (77Se)

Abstract

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 77 Se VI ) 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 ( 77 Se and 78 Se). The concentration of 77 Se 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 77 Se concentration in plant grain but the low yield in the single conventional treatment reduced 77 Se uptake. Residual 77 Se 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 77 Se 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.Highlights. Selenium at 20 g ha -1 raises dietary levels above the recommended dietary allowance Selenium fixation into humus-bound forms eliminates residual availability Conservation agriculture treatments have little effect on selenium uptake by maize Legumes (e.g. groundnuts) accumulate greater concentrations of selenium than maize Keywords.

Increasing zinc concentration in maize grown under contrasting soil types in Malawi through agronomic biofortification: Trial protocol for a field experiment to detect small effect sizes

“…These soil types were selected because of their significance for staple food crop production in Malawi and because there is evidence that Zn grain quality differs between maize crops grown on these two soils (Chilimba et al., 2011). Vertisols are naturally more fertile and productive than Lixisols and they are an agriculturally important soil type in Malawi despite occupying only a small proportion (1.5%) of the land area (Dijkshoorn et al., 2016; Ligowe, Young, et al., 2020). For example, they cover almost all (99%) of the area used by smallholder farmers for wetland‐irrigated farming (known locally as dambo farming) to produce green maize and vegetables (MoAFS, 2019).…”

Selenium deficiency risks in sub-Saharan African food systems and their geospatial linkages

“…Vertisols represent about 0⋅5 % of the total land area of Malawi (30,43) although they are agriculturally significant, e.g. they represent about 11 % of the cultivated arable soils of Blantyre agricultural development division, which is one of the eight agricultural development divisions covering Malawi (44) .…”

“…they represent about 11 % of the cultivated arable soils of Blantyre agricultural development division, which is one of the eight agricultural development divisions covering Malawi (44) . Vertisols form from Ca-and Mg-rich parent materials, such as limestones and basalts, and in topographic depressions where leached elements collect from higher elevations (43) . Vertisols have a predominantly 2:1 clay mineralogy, whereas the acidic Oxisols and moderately acidic Alfisols, which dominate most arable systems in Malawi, are characterised by larger concentrations of hydrous oxides of aluminium, iron and manganese, which can adsorb inorganic Se anions much more strongly than the Vertisols (43) .…”

“…(30) also noted that the chemical stability of selenate, which is taken up more rapidly than selenite by plants under most soil conditions, is greater in the soil solution at higher soil pH values. Recent studies have similarly shown that fertiliser-applied selenate is more bioavailable in Vertisols than in acidic soils such as Oxisols and Alfisols (43) .…”

“…Vertisols form from Ca- and Mg-rich parent materials, such as limestones and basalts, and in topographic depressions where leached elements collect from higher elevations (43) . Vertisols have a predominantly 2:1 clay mineralogy, whereas the acidic Oxisols and moderately acidic Alfisols, which dominate most arable systems in Malawi, are characterised by larger concentrations of hydrous oxides of aluminium, iron and manganese, which can adsorb inorganic Se anions much more strongly than the Vertisols (43) . The cycling of Se between organic and inorganic forms is also likely to have a strong influence on crop Se uptake and it is noteworthy that Vertisols have a variable, but generally much larger, soil organic matter content than the Alfisols and Oxisols.…”