Effects of combined application of nitrogen and zinc on the availability and fraction of zinc in root-soil interface of winter wheat

Objectives Nitrogen (N) is critical for zinc (Zn) absorption by roots and Zn accumulation in grains of winter wheat. This paper investigated the effect of combined application of N with Zn on the availability and chemical fractions of Zn in root-soil interface of winter wheat, which helped to investigate a possible mechanism for increase of Zn uptake by winter wheat through combined application of N with Zn, and to provide a theoretical basis for improving Zn contents in grains of winter wheat through reasonable N application.

Methods A rhizobox experiment with three N application rates (N 0, 0.2 and 0.4 g/kg) and two Zn application rates (Zn 0 and 10 mg/kg) was conducted to analyze the Zn concentrations in aboveground parts, available Zn, pH and Zn chemical fractions in rhizosphere and non-rhizosphere soil of winter wheat.

Results Combined application of N with Zn (N_0.2Zn₁₀ and N_0.4Zn₁₀) significantly increased the dry matter biomass and Zn concentrations in aboveground parts of winter wheat. At Zn₀, N_0.4 treatment significantly increased the available Zn contents in the rhizosphere soil. At Zn₁₀, N_0.4 and N_0.2 treatments decreased the available Zn contents in the rhizosphere soil, while increased the available Zn concentrations in the non-rhizosphere soil. Regardless of Zn application, N_0.4 and N_0.2 treatments significantly decreased the rhizosphere soil pH, but did not affect the non-rhizosphere soil pH. Meanwhile, at Zn₀, N_0.4 and N_0.2 treatments significantly decreased exchangeable Zn and carbonate bound Zn in rhizosphere soil, Fe-Mn oxides bound Zn in non-rhizosphere soil, and increased exchangeable Zn in non-rhizosphere soil, and residual Zn in rhizosphere and non-rhizosphere soil. At Zn₁₀, N_0.4 and N_0.2 treatments significantly increased exchangeable Zn in rhizosphere and non-rhizosphere soil, loose and tight organic bound Zn in non-rhizosphere soil, and residual Zn in rhizosphere soil, but decreased loose organic and carbonate bound Zn in rhizosphere soil and residual Zn in rhizosphere and non-rhizosphere soil.

Conclusions Combined application of N with Zn increased Zn concentration and enhanced the absorption of Zn by winter wheat, possible because of the combined influence of wheat root and the combination of N and Zn on the reduction of rhizosphere soil pH, promoting transformation of Zn from loose organic bound and carbonate bound fraction to exchangeable fraction to improve the bio-availability of Zn in soil.