Appropriate Soil Fertilization or Drone-Based Foliar Zn Spraying Can Simultaneously Improve Yield and Micronutrient (Particularly for Zn) Nutritional Quality of Wheat Grains

To better understand the effects of agronomic practices on yield–nutrition relationships in wheat (Triticum aestivum L.) grains for Zn biofortification while improving yields simultaneously, effects of different soil fertilization and different drone-based foliar spraying treatments were investigated in calcareous soils. For soil fertilization, the incorporation of Zn or increasing the N/P ratio in compound fertilizers proved to be effective in enhancing grain Zn concentrations and yields. However, the overall effects of soil fertilization are limited, with a maximal yield increase of only 7.0% and a maximal increase of the grain Zn concentration from 19.4 to 27.0 mg/kg, which is far below the target biofortification value of 40–50 mg/kg. Unfortunately, there was a negative side effect, which decreased Fe and Mn concentrations and the Fe bioavailability. Notably, drone-based foliar Zn sprayings increased grain yields from the control 7.5 t/ha to 8.6 t/ha at ZnO treatment by 12.0% and 8.8 t/ha at ZnSO4·7H2O treatment by 17.3%. Meanwhile, grain Zn concentrations were increased from the control 33.5 mg/kg to 41.9 mg/kg at ZnO treatment by 25.1% and 43.6 mg/kg at ZnSO4·7H2O treatment by 30.1%. Treatments with ZnSO4·7H2O increased grain Zn concentrations and accumulation more so than ZnO, indicating the importance of chemical Zn forms in determining the effectiveness of foliar spraying. Moreover, foliar Zn sprayings simultaneously increased grain concentrations and accumulation of Fe, Mn and Cu, demonstrating multiple benefits. There were positive correlations between Zn and Fe, Mn or Cu, indicating synergistic interactions. Compared to micronutrients, concentrations of grain macronutrients (N, P, K, Ca and Mg) were less affected. Thus, a dual-benefit in both grain yields and micronutrient (particularly for Zn) nutrition could be effectively achieved through appropriate soil fertilization and foliar Zn spraying. These findings provide a better understanding of the yield–nutrition relationship among wheat grain yields, Zn and other nutrient elements for a better integrated manipulation to achieve a win–win situation in yield and nutrition.