Effects of different application method of ZnSO₄ and EDTA-Zn on wheat grain zinc biofortification and soil zinc availability in the next year

ObjectivesHuman Zn deficiency has been concerned in potentially Zn deficient soil, especially in areas where wheat is the staple food. Increasing Zn concentration in wheat grain to meet human Zn requirement is critical for alleviating human Zn deficiency.

MethodsA two-year field experiment with the split block design was conducted using ZnSO₄ and Zn-EDTA as Zn sources. The main factor was Zn application methods, which included foliar application and soil application, and the soil application was splited into mixed and banded application methods during the first cropping year. Only foliar Zn application was conducted during the second cropping year. All data in this paper were collected in the second cropping year.

ResultIn the second season when Zn was foliar applied, the grain Zn concentration was significantly increased by 11.13 mg/kg (33%) by foliar spray of ZnSO₄ alone, not by foliar Zn-EDTA application. Without foliar Zn application, the grain Zn contents were increased by 6.05 and 3.51 mg/kg (20% and 11%) under the mixed and banding ZnSO₄ fertilization, compared with the no Zn treatment in the second cropping year, respectively. With the foliar Zn application, the wheat grain Zn contents were increased by 28.59 mg/kg (100%) and 21.59 mg/kg (76%) under the mixed and banding ZnSO₄ fertilization, but these increases were much less than those of applying foliar Zn alone. Soil DTPA-Zn after wheat harvest in the second cropping year maintained above 1 mg/kg when Zn fertilizer was added to soil at the first season. Specifically, soil DTPA-Zn contents were respectively 1.99 and 1.65 mg/kg under the mixed and banding ZnSO₄, while the contents were respectively 1.23 and 1.01 mg/kg under the mixed and banding Zn-EDTA fertilization. Under condition of the foliar Zn fertilization, the contents of DTPA-Zn were 1.44 and 2.22 mg/kg respectively under mixed and banding ZnSO₄ fertilization, and 1.16 and 1.10 mg/kg respectively under the mixed and banding Zn-EDTA fertilization. The contents of soil Zn fractions were in the order of:weakly bounded organic matter Zn (Wbo-Zn) > carbonate bound Zn (Car-Zn) > manganese oxides bound (MnOZn) > tightly bounded organic Zn (Tom-Zn) > exchangeable Zn (Ex-Zn). Specifically, the contents of Ex-Zn, Wbo-Zn, and Car-Zn were significantly increased by 184%, 75% and 53%, respectively with mixed ZnSO₄, while the increases were respectively 116%, 85%, and 43% with banding ZnSO₄. However, only the Ex-Zn and Wbo-Zn were increased under the mixed and banding Zn-EDTA, and the increases relative to the mixed Zn-EDTA were respectively 232% and 18%, while the increases were 132% and 10% respectively in the treatment of banding Zn-EDTA. Compared with the non Zn treatment, the Zn use efficiency in the mixed Zn-EDTA fertilization treatment was 0.27%, while in the banding treatment, the efficiency was 0.70%, which was three times higher than that of the mixed one. No differences were observed between the mixed and banding ZnSO₄.

ConclusionFoliar application of ZnSO₄ alone can significanlty increase grain Zn concentration of wheat grown on potentially Zndeficient soil, while foliar Zn-EDTA has little effect. Although it can lead to high level of soil DTPA-Zn and Zn fractions with higher mobility (Ex-Zn, Wbo-Zn), mixed or banded Zn-EDTA and ZnSO₄ has little effect on grain Zn concentration in the next cropping year. Soil application combined with foliar spray of ZnSO₄ is satisfactory in increasing Zn concentration in wheat grain.