Wheat grain zinc concentration as affected by soil nitrogen and phosphorus availability and root mycorrhizal colonization

Nitrogen (N) fertilization increases zinc (Zn) acquisition in cereal crops. However, little is known about the effects of N fertilization on grain Zn concentration in relation to availability of soil N and phosphorous (P) and root mycorrhizal colonization. The present study used a 12-year location-fixed field experiment with winter wheat grown at five N fertilization rates (0, 80, 160, 240 and 320 kg N ha⁻¹). The objective was to assess effects of phytoavailability of soil N and P and root mycorrhizal colonization on wheat Zn nutrition during three-year field measurements. Zinc concentration in wheat grains was increased with N fertilization in a linear-plateau fashion, and it reached the plateau of 30.3 ± 0.4 mg kg⁻¹ when soil mineral N (nitrate-N) levels were 8.5 ± 1.0 mg kg⁻¹ and above at maturity. Similarly, total shoot Zn uptake and Zn remobilization from vegetative tissues to grains were also increased significantly and then attained a plateau as the N rates increased. The enhancements in grain Zn concentration by N fertilization mainly occurred in the crease tissue, embryo and aleurone fractions of the grain. Long-term N fertilization was also associated with increases in root mycorrhizal colonization up to 8.8 ± 2.9 mg kg⁻¹ soil available mineral N at maturity. Shoot Zn uptake at maturity increased quadratically with the increase in shoot N uptake and root mycorrhizal colonization rate, whereas there was an inverse relationship between shoot Zn uptake and soil P availability. The results suggest that grain Zn concentration of wheat plants grown under long-term N fertilization was closely related to the soil mineral N status and showed clear increases with decreased soil available P concentration and improved root mycorrhizal colonization rates.