Abstract
Abstract. Zinc (Zn) deficiency is a widespread nutritional problem in human populations, especially in sub-Saharan Africa (SSA). The Zn concentration of crops consumed depends in part on the Zn status of the soil. Improved understanding of factors controlling the phyto-availability of Zn in soils can contribute to potential agronomic interventions to tackle Zn deficiency, but many soil types in SSA are poorly studied. Soil samples (n=475) were collected from a large part of the Amhara Region of Ethiopia, where there is widespread Zn deficiency. Zinc status was quantified by measuring several fractions, including the pseudo-total (aqua regia digestion; ZnTot), available (DTPA (diethylenetriamine pentaacetate) extractable; ZnDTPA), soluble (dissolved in 0.01 M Ca(NO3); ZnSoln) and isotopically exchangeable Zn, using the enriched stable Zn isotope 70Zn (ZnE). Soil geochemical properties were assessed for their influence on Zn lability and solubility. A parameterized geochemical assemblage model (Windermere Humic Aqueous Model – WHAM) was also employed to predict the solid phase fractionation of Zn in tropical soils rather than using sequential chemical extractions. ZnTot ranged from 14.1 to 291 mg kg−1 (median = 100 mg kg−1), whereas ZnDTPA in the majority of soil samples was less than 0.5 mg kg−1, indicating widespread phyto-available Zn deficiency in these soils. The labile fraction of Zn in soil (ZnE as % ZnTot) was low, with median and mean values of 4.7 % and 8.0 %, respectively. Labile Zn partitioning between the solid and the solution phases of soil was highly pH dependent, where 94 % of the variation in the partitioning coefficient of 70Zn was explained by soil pH. Similarly, 86 % of the variation in ZnSoln was explained by soil pH. Zinc distribution between adsorbed ZnE and ZnSoln was controlled by pH. Notably, Zn isotopic exchangeability increased with soil pH. This contrasts with literature on contaminated and urban soils and may arise from covarying factors, such as contrasting soil clay mineralogy across the pH range of the soils used in the current study. These results could be used to improve agronomic interventions to tackle Zn deficiency in SSA.
Highlights
Zinc deficiency is a widespread nutritional disorder affecting ∼ 17 % of the global population and rising to 25 % of the population in countries within sub-Saharan Africa (SSA; Kumssa et al, 2015; Wessells and Brown, 2012)
The aim of the present study was to investigate the status of Zn lability in soils from a large part of the Amhara Region of Ethiopia, which represent a diverse range of soil types from SSA (Gashu et al, 2020), in which Zn deficiency is thought to be widespread (Hengl et al, 2017)
There was a significant but weak positive correlation (r = 0.25) between COrg and the percent of ZnDTPA, possibly indicating that Zn bound to soil organic matter is in a form that is accessible to diethylenetriaminepentaacetic acid (DTPA) extraction
Summary
Zinc deficiency is a widespread nutritional disorder affecting ∼ 17 % of the global population and rising to 25 % of the population in countries within sub-Saharan Africa (SSA; Kumssa et al, 2015; Wessells and Brown, 2012). Several interlinked causes contribute to the prevalence of Zn deficiency issues in SSA, including lack of access to animal source foods. This can lead to inadequate Zn intake if the diet is heavily reliant on staple crops, which are inherently low in mineral micronutrients (Joy et al, 2014; Kumssa et al, 2015). Soil degradation and a lack of access to micronutrient fertilizers can contribute to the production of staple crops with poor nutritional quality (Kihara et al, 2020). The search yielded 24 publications, none of which involved SSA soils. This is potentially a serious omission because Zn geochemistry in SSA soils is likely to differ from that in temperate soils because of differences in geo-colloidal mineralogy, organic C content and the soil pH at which agriculture is practiced
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