Biofortification of zinc in onions (Allium cepa L.) and soil Zn status by the application of different organic Zn complexes

• Application of Zn-aminolignosulfonate in Soilacid produced the highest Zn and soluble Zn concentrations in plants.
• Application of Zn-DTPA-HEDTA-EDTA in Soilacid produced the highest Zn and soluble Zn concentrations in plants.
• Both the Zn source and the different soils had a great influence on agronomic Zn biofortification.
• Applying Zn organic complexes to an onion crop improved productivity and Zn concentrations in individual onion plants.

A pot experiment was conducted to determine the influence of different natural and synthetic organic Zn complexes on the agronomic biofortification of onion (Allium cepa L.) in two types of soil: Soilacid (weakly acidic) and Soilcalc (calcareous). Eight different organic Zn complexes were administered at two different Zn application rates. We determined the Zn distribution in different soil fractions and the Zn availability, pH and redox potential in soils with Nil-Zn and Zn treatments. The effectiveness of Zn sources in onion was assessed in terms of total Zn concentration, soluble Zn concentration, plant biomass and chlorophyll and carotenoid contents. Beneficial effects of Zn on onion response (DM plant biomass, total and soluble Zn concentration, utilization of applied Zn, etc.) were observed, with significant increment in the determined plant parameters, in comparison with the Nil-Zn treatment. The rates of increase varied among sources depending on the soil. Applications of Zn-aminolignosulfonate (Zn-AML) at the rate of 10 mg Zn kg−1 in Soilacid and of Zn-DTPA-HEDTA-EDTA at the rate of 10 mg Zn kg−1 in Soilcalc produced the highest Zn and soluble Zn concentrations in plants. In general, the highest concentrations of Zn in labile forms and of potentially available Zn concentrations in both soils were associated with the application of the sources that contained Zn chelated by EDTA and/or DTPA. Zinc source and soil characteristics had a great influence on agronomic Zn biofortification in onions and therefore also on the quality and plant biomass of the crop. Applying Zn organic complexes to an onion crop improved not only productivity, but also Zn concentrations in onion.