Assessing residual availability of selenium applied to maize crops in Malawi

The environmental fate and possible residual effects of applied selenium (Se) are important issues when considering sustainable biofortification of food crops to improve human health. Our objectives were to assess residual effects of Se biofortification of maize in Malawi using a combination of 74Se-labelling studies and measurements of KH2PO4-extractable soil Se. Stable 74Se isotope trials showed recovery rates in maize grain of 6.5 and 10.8% from 10 g Se ha−1 applied to two contrasting sites, Chitedze and Mbawa; the 74Se label comprised 69 and 81% of the grain Se content respectively. However, in the following ‘residual’ year, only 2.0 and 0.78% of Se uptake in grain came from the 74Se-labelled fertiliser originally applied, amounting to <0.1% of the original fertiliser application. In a second trial, residual phosphate-extractable Se in the soil measured at harvest in the year of application (0–100 g ha−1) at six sites across Malawi varied with soil type but comprised 3.5 and 4.8% of that applied, at Chitedze and Mbawa respectively. However, a second maize crop grown in the same plots in the following cropping season accessed only 0.23 and 0.19% of the original Se application, constituting less than 5% of the phosphate-extractable fertiliser Se measured in the soil at harvest in the previous (fertilised) year. Thus, residual benefits from Se application were measureable but extremely small, probably due to rapid loss of soluble selenate after application or fixation into unavailable organic forms.