EDTA alone enhanced soil zinc availability and winter wheat grain Zn concentration on calcareous soil

- Wheat grain Zn can be markedly increased by enhancing soil Zn availability.
- EDTA addition alone increased soil available Zn without decreasing soil pH.
- Soil Zn availability is controlled by transformation among different fractions.
- A mechanistic map was proposed to illustrate Zn transformation in calcareous soil.

Zinc (Zn) biofortification of staple food crops is essential for addressing worldwide human malnutrition problem, but its success has been restricted by low soil Zn availability. With ethylenediaminetetraacetic acid (EDTA) addition, soil Zn availability is able to be increased in contaminated soils, but whether and how it will behave in uncontaminated calcareous soil with low available Zn, and its effect on winter wheat grain Zn need further research. In a series of pot experiments, winter wheat was grown on a low-Zn calcareous soil, with EDTA applied at different stages or in different rates, and then soil pH, soil available Zn, Zn fractions, and Zn concentration of different wheat organs were determined. Obtained results showed that grain Zn concentration was increased by 8%-46%, but grain yield was not affected by EDTA addition. With EDTA applied at sowing, straw Zn uptake was increased by 69%-120% during heading to filling, and after anthesis it significantly decreased and contributed 61% of the grain Zn uptake. Meanwhile, soil available Zn (DTPA-Zn) in rhizosphere and nonrhizosphere was increased by 185%-377%. Correspondingly, soil exchangeable Zn (Ex-Zn) was dramatically increased; Zn weakly bounded with organic matter (Wbo-Zn) and Zn bounded with carbonate (Carb-Zn) were increased by 4%-54% and 2-18 fold respectively; while Zn bounded with amorphous iron oxide (AmoFe-Zn), Zn bounded with crystal iron oxide (CryFe-Zn), and Zn in residue mineral (Res-Zn) were decreased by 78%, 10%-31%, and 6% respectively, and soil pH kept stable or increased by 0.1-0.2 unit. Among Zn fractions only Ex-Zn, Wbo-Zn, and Carb-Zn were significantly and positively correlated with DTPA-Zn. With EDTA application, the enhanced soil Zn transformation from unavailable Zn fractions (AmoFe, Cry-Fe, and Res) to available Zn fractions (Ex, Wbo, Carb), and the enhanced straw Zn uptake at anthesis and its remobilization to grain during filling period was responsible for the increase of grain Zn concentration. Also, without decreasing soil pH the unavailable soil Zn fractions can be still transformed to available Zn in calcareous soil.