Effects of soil N application rate and foliar application of Zn and K on Zn concentration and its bioavailability in wheat grains

Objectives Wheat grains are generally low in Zinc (Zn) content which does not meet human needs. We studied the optimum nitrogen application rate and Zn fertilizer foliar application way that is effective in improving Zn content and its bioavailability in wheat grain and its milling fractions (especially in consumed flour).

Methods Two field experiments were conducted in Yangling and Sanyuan City, Shannxi Province in 2013 and 2014. The soil type in two locations was potential Zn-deficient calcareous soil, with DTPA-Zn (soil available Zn) content of 0.67 and 0.90 mg/kg and wheat–maize rotation planting system. The N rates (i.e., main factor) were N 0 (control, CK), 120 (N₁) and 240 (N₂) kg/hm² in Yangling, and 150 kg/hm² (Nc), –15% N (N_R1) and –30%N (N_R2) in Sanyuan. The foliar application treatments (i.e., the subfactor) in the two sites were spraying of water (control, CK), 0.5% K₂SO₄ (K), 0.3% ZnSO₄·7H₂O (Zn) and both Zn and K (Zn+K) at the grain filling stage. The Zn fractions in whole grains and processed flour were determined.

Results In Yangling, the N₁ (120 kg/hm²) treatment significantly increased Zn contents in whole grain, flour and bran by 13.2%, 17.3% and 8.78% respectively, but N₂ (240 kg/hm²) treatment did not increase Zn contents. N₁ (120 kg/hm²) treatment also significantly reduced the phytate concentration in grain milling fractions. Compared with Zn treatment, Zn+K treatment significantly increased grain Zn content by 13.1% on average. Under N₁ treatment, Zn+K treatment increased flour Zn content, especially the water-soluble Zn content, and decreased the ratio of phytate to Zn in whole grain and processed flour. N rate and foliar application showed significant interaction on Zn fractions and Zn bioavailability of whole grain and milling products. In Sanyuan, N_c treatment recorded the highest Zn contents in whole grains, flour and bran, while N_R1 treatment did reduce the Zn contents significantly. Compared with Zn treatment, Zn concentrations in whole grains and its milling fractions were slightly affected by Zn+K treatment. N rate and foliar Zn and/or K aplplication had significant interactive effects on flour Zn concentration in both cropping year.

Conclusions Both the two experiments showed significant increment in Zn content and bioavailability with N rate and Zn foliar application. Moreover, the results revealed that the optimum N rate was 120 kg/hm², and the effective foliar application way was the combination of Zn and K in the potentially Zn-deficient calcareous soil.