Zinc lability and solubility in soils of Ethiopia – an isotopic dilution study

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 soil. Improved understanding of factors controlling the phyto-availability of Zn in soils can contribute to potential agronomic interventions to tackle Zn deficiency, although there are major knowledge gaps for many soil types in SSA.

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: pseudo-total (Aqua-Regia digestion; Zn_Tot), available (DTPA-extractable; Zn_DTPA), soluble (dissolved in 0.01 M Ca(NO₃); Zn_Soln) and isotopically exchangeable Zn using the enriched stable Zn isotope ⁷⁰Zn (Zn_E). Soil geochemical properties were assessed for their influence on Zn lability and solubility.

Zn_Tot ranged from 14.1 to 291 mg kg⁻¹ (median = 100 mg kg⁻¹) whereas Zn_DTPA in the majority of soil samples was less than 0.5 mg kg⁻¹ indicating widespread phytoavailable Zn deficiency in these soils. The labile fraction of Zn in soil (Zn_E as %Zn_Tot) 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 Zn_Soln was explained by soil pH.

Zinc distribution between adsorbed Zn_E and Zn_Soln was pH controlled. 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.