Selenium biofortification of crops on a Malawi Alfisol under conservation agriculture
Introduction
Selenium (Se) is a trace element with an essential nutritional role in human and animal health. Selenium deficiency in humans has been linked to thyroid gland dysfunction, irreversible brain damage, peripheral vascular diseases, chronic and degenerative osteoarthropathy (Kashin–Beck disease), impaired immune response to viral infections, male infertility, pre-eclampsia in women, heart diseases and higher risks for several types of cancers (Cardoso et al., 2015, Fairweather-tait et al., 2011, Riaz and Mehmood, 2012). According to the Institute of Medicine of the USA National Academy, the Se recommended dietary allowance (RDA) for Se is 55 µg day−1 for adults while the tolerable upper intake for adults is 400 µg day−1 (Bendich, 2001).
Dietary Se intake can be strongly related to the availability of Se in soil (Fairweather-Tait et al., 2011; Rayman, 2008), especially where populations depend on local food production in Se-deficient regions. The bioavailability of selenium in soil depends on supply factors, such as parent material and atmospheric inputs, and on soil factors that affect the strength of Se sorption such as pH and the concentration of soil organic matter and hydrous oxides of Fe, Al and Mn (Fordyce et al., 2000, Lopes et al., 2017, Rayman, 2008). It is recognized that selenite ions (HSeO3−, SeO32−) are adsorbed strongly on hydrous oxides at low pH (3.5–6.5). However, most Se in soils is usually organically bound in humus and there is evidence that different forms of humus can give rise to differences in Se bioavailability (Qin et al., 2012). It has also been shown that soil microbial processes have some involvement in the control of inorganic Se availability (Tolu et al., 2014). In highly weathered, acidic, oxide-rich tropical soils, as found in Malawi, low bioavailable Se presents a serious restriction to dietary Se supply with estimates of 70% of the population eating insufficient Se with an average daily intake range of 27–45 μg capita−1 day−1 (Chilimba et al., 2011, Hurst et al., 2013, Joy et al., 2015a, Joy et al., 2015b) which is in close agreement with national plasma Se data for adult women (Phiri et al., 2019).
Dietary intake of Se can be increased through agronomic ‘biofortification’ of crops (Chilimba et al., 2012a, Mathers et al., 2017) with application as Se-containing fertilizers applied to soil or as foliar sprays (Lopes et al., 2017). In particular, the success of agronomic biofortification of staple cereal crops is well recognized (Broadley et al., 2010, Chilimba et al., 2012a, Chilimba et al., 2014). The use of selenium-enriched fertilizers was the public-health solution successfully adopted in 1984 by Finland, that resulted in an increase in Se intake from 38 µg d−1, before fortification, to 80 µg d−1 in 2000, assuming a daily energy intake of 10 MJ (Broadley et al., 2006, Eurola and Hietaniemi, 2000, Hartikainen, 2005).
The use of enriched Se isotopes as tracers in field experimentation permits discrimination between soil-derived and fertilizer-derived Se in crops and, potentially, allows the examination of residual effects of Se application in soil and crops. The approach has been made possible by advances in ICP-MS technology, the commercial availability of several enriched stable Se isotopes and the fact that comparatively small Se additions are required on a field plot scale (c. 1 mg m−2) for realistic biofortification results. To date, there have been relatively few studies that have utilized this approach in field experiments. Chilimba et al. (2012b) studied uptake of 74Se in maize in Malawi; Mathers et al. (2017) used 77Se to audit the fate of Se applied to wheat in U.K. soils. More recently, Ligowe et al. (2020) used enriched 77Se to characterize uptake and residual availability to green vegetables grown in an Oxisol, Alfisol and Vertisol from Malawi (Ligowe et al., 2020).
Conservation agriculture (CA) that focuses on minimum soil disturbance, the retention of crop residues and crop diversification, is one of the cropping systems which has been heavily promoted in recent years in southern Africa (Thierfelder and Wall, 2010, Thierfelder et al., 2017, Kassam et al., 2009). In addition to providing a coping strategy for climate change (Branca et al., 2011, Lipper et al., 2014, Steward et al., 2019), conservation agriculture is widely reported to improve soil health and crop yield through partly increased soil organic matter (Ligowe et al., 2017, Ngwira et al., 2012, Powlson et al., 2016).
The aim of this study was to examine the viability of Se biofortification in an Alfisol, managed under CA, and representing typical agronomic circumstances in Malawi and, more broadly, in sub-Saharan countries. We used application of 77Se-enriched selenate (77SeFert) to sub-plots of maize and selected legumes (cowpea, ground nuts, pigeon peas and velvet beans) within an established CA rotational and intercropped trial. The objectives were (i) to assess Se availability, uptake and translocation within crops and (ii) to quantify residual effects of Se application in the following season.
Section snippets
Study location
The study was carried out within a long-term CA trial situated at Chitedze Research Station (CRS), Malawi. The CRS is located on the Lilongwe-Kasungu plains (13.973 S, 33.654 E) at 1145 m above sea level. The soils are Ferruginous Latosols, classed as Alfisols under the USDA Soil Taxonomy (USDA, 1975) which are deep and free-draining with a well-developed structure. In the first year of the trial establishment (2007), baseline topsoil (0–10 cm) analysis showed the following average values for
Maize
In the year of 77SeFert biofortification (2017) the concentrations of 77SeFert in maize grain had a restricted range across all CA treatments (217 ± 27 µg kg−1), including the conventionally cultivated maize plots, and showed no difference between treatments (Table 3, Table 4). The results confirm the potential of fertilizer to increase Se in staple grain as has been demonstrated in previous studies (Broadley et al., 2010, Broadley et al., 2006, Chilimba et al., 2012a, Mathers et al., 2017).
In
Conclusions
A single application of 77Se to crops grown either under CA or conventional cropping systems, at the grain filling stage, provides a viable approach to Se biofortification. Application of 20 g ha−1 Se produced sufficient grain Se enrichment in maize and legumes to provide the recommended dietary Se requirement. The additional organic inputs to the soil through CA cropping systems has no apparent influence on the rapid fixation of applied 77Se. The transformation of applied selenate was almost
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding and support for this study were provided by the Royal Society - Department for International Development (RS-DFID) Africa Capacity Building Initiative Grant AQ140000 “Strengthening African Capacity in Soil Geochemistry to inform Agricultural and HealthPolicies”, The Malawi Government (Ministry of Agriculture and Food Security – Department of Agricultural Research), Lilongwe University of Agriculture and Natural Resources, the University of Nottingham and the British Geological Survey.
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