The objective of this study was to clarify the response of soil available zinc (Zn) and wheat grain Zn concentration to soil Zn fertilization under different Zn supply field conditions, and to explore the Zn fertilizer regulation measures for grain Zn fortification based on soil available Zn, so as to provide a scientific basis for optimizing Zn fertilizer application and achieving wheat grain with high-yield and high-quality.

The two-year location-fixed field experiments with five Zn fertilizer application rates of 0, 6, 12, 18, and 24 kgZn·hm^-2 were carried out at Taigu (high-Zn field) and Wanrong (low-Zn field) of Shanxi Province in the eastern Loess Plateau, respectively. The wheat grain yield and Zn concentration, Zn uptake and its translocation and distribution in the aerial part, as well as soil available Zn were investigated in the high- and low-Zn fields.

Grain yield was not affected by Zn fertilizer rates in both high- and low-Zn fields. In high-Zn field, a slight increase in grain Zn concentration was observed with the increase of Zn fertilizer rates. For grain Zn concentration, no significant difference existed among all treatments in the first year, while it was increased by 2.4%-11.0% for Zn fertilization treatments as compared with that of no Zn fertilization in the second year. The grain Zn concentration was higher than 40 mg·kg^-1 for all treatments. Compared with Zn application, the Zn transfer factor from straw to grain and grain Zn portioning index were decreased by 23.9%-37.9% and 4.3%-13.1%, respectively, and more than 20% of shoot Zn still remained in the stems and leaves at wheat harvest. In low-Zn field, the grain Zn concentration and Zn uptake in each organ increased with increasing Zn rates, whereas the opposite trend was observed for Zn transfer factor from straw to grain. Compared with no Zn application, the grain Zn concentration averaged two years increased by 9.4%-23.1%, while the Zn transfer factor from straw to grain decreased by 13.5%-24.5%, but no obvious difference was found for Zn portioning index among five Zn rates. In both high- and low-Zn fields, the soil available Zn increased significantly with the added Zn fertilizer. The regression analysis showed that soil available Zn slightly increased grain Zn concentration, and the increase with available Zn could be described by a quadratic function in high-Zn field, and the linear-with-plateau model showed that the grain Zn plateau of 34.76 mg·kg^-1 was reached at the soil available Zn of 4.12 mg·kg^-1.

Therefore, for the purpose of achieving desirable grain Zn concentration of 40 mg·kg^-1 in the wheat monoculture aera of eastern Loess Plateau, it could be considered that higher soil available Zn played a critical role in the high-Zn field, and soil Zn fertilization could be considered to increase soil available Zn up to 4 mg·kg^-1 first, and then other agronomic measures such as foliar Zn application should not be ignored to address the gap between the current grain Zn concentration and the recommended value in the low-Zn field.