Objectives The breeding of high-yielding and high-zinc wheat cultivars is an important approach to meet the dual demands of grain supply and zinc nutritional quality for residents. We investigated the differences in grain zinc content response to nitrogen application between high- and low-zinc wheat cultivars, as well as the underlying mechanisms.

Methods A pot experiment was conducted using calcareous soil with a pH of 8.55 and available zinc content of 0.58 mg/kg. Four high-yielding wheat cultivars, which had no significant difference in yield but significantly different grain zinc content, were selected as experimental materials. Two treatments were set: no nitrogen application and nitrogen application at 200 kg/hm². Rhizosphere soil, non-rhizosphere soil, and plant samples were collected at the jointing, heading, grain-filling, and maturity stages. Soil pH, different forms of zinc, and available zinc content were measured; biomass and zinc content in roots, stems and leaves, spikes, and grains were analyzed; and indices such as rhizosphere zinc activation index, root zinc uptake capacity, root-to-shoot and stem-leaf-to-spike zinc transfer coefficients, maturity-stage zinc harvest index, and their responses to nitrogen application were calculated.

Results After nitrogen application, the grain Zn content of high-Zn cultivars increased from 29.1 mg/kg to 42.8 mg/kg (a 48% increase), while that of low-Zn cultivars rose only from 21.1 mg/kg to 23.4 mg/kg (an 11% increase). Compared with no nitrogen application, the high-Zn cultivars exhibited 101%, 151%, and 108% higher Zn uptake in the shoot at heading, grain-filling, and maturity stages, respectively. Root Zn acquisition efficiency increased by 26.1% and 53.1% at heading and grain-filling stages, and the root-to-shoot Zn transfer index increased by 78.6% during grain-filling—all significantly greater increases than in low-zinc varieties. N application reduced rhizosphere soil pH, with decreases of 0.08−0.12 in high-Zn cultivars and 0.07−0.09 in low-Zn cultivars. The rhizosphere Zn activation index of high-Zn cultivars increased by 17.1%, 33.5%, and 26.0% at jointing, heading, and grain-filling stages, whereas in low-Zn cultivars it increased only by 1.6% and 8.8% at jointing and heading, and decreased by 9.2% during grain-filling. The available Zn content in the rhizosphere of high-Zn cultivars increased by 30.9%−67.4%, far exceeding the 3.1%−26% increase in low-Zn cultivars. In addition, after nitrogen application, the rhizosphere ammonium and nitrate nitrogen content in high-Zn cultivars increased by 1.96−3.65-fold and 24.7−30.9-fold, respectively, both greater increases than in low-zinc varieties.

Conclusions On zinc-deficient calcareous soils, nitrogen application significantly enhances grain zinc content in wheat, and high-zinc wheat varieties are more responsive to nitrogen fertilizer. The primary mechanism lies in the more pronounced rhizosphere acidification in high-zinc varieties after nitrogen application, which enhances zinc mobilization capacity—especially during heading and grain-filling—greatly improving the supply of bioavailable zinc. Meanwhile, the greater improvements in root zinc acquisition and root-to-shoot translocation capacity lead to significantly higher zinc accumulation in the shoot and ultimately in the grain.