Objectives Application of nitrogen (N) fertilizer or a combination of nitrogen and zinc (Zn) can significantly increase the availability of soil Zn, improve the Zn nutrition and grain yield of winter wheat. Root exudates have been widely recognized for their critical role in mobilizing and activating nutrients within the rhizosphere, thereby facilitating their uptake by plants. In this study, we investigated the combined effects of nitrogen-zinc (N-Zn) fertilizer application and exogenous winter wheat root exudates on the transformation of different Zn forms in calcareous soils.

Methods A hydroponic culture trial was carried out, using winter wheat cultivar Bainong207 as the test material. Two Zn application levels (0, 10 µmol/L) and two N application levels (0, 7.5 mmol/L) were set up to compose 4 treatments. The root exudates were collected, and the composition and concentration of organic acids in the root exudates were analyzed. Subsequently, a rhizobox experiment was carried out using the same wheat cultivar, as well as the same Zn and N application levels. Each treatment was then divided into two parts: one was added with 50 mL distilled water, and the other with 50 mL root exudates collected in the hydroponic trial. The biomass, N and Zn concentration of various parts of the wheat plant, and the pH, available Zn content, and the contents of Zn fractions in rhizosphere and non-rhizosphere soil were determined.

Results Co-application of N-Zn fertilizers markedly elevated the concentrations of aconitic acid, fumaric acid, and malic acid in root exudates, whereas it significantly depressed the levels of pyruvic acid, α-ketoglutaric acid, and succinic acid. Irrespective of root exudate addition, N-Zn application notably boosted grain yield, Zn concentrations in roots, stems, leaves, and grains, as well as N concentrations in glumes and grains. In both rhizosphere and non-rhizosphere soils, this combined application significantly increased available Zn concentrations, along with the contents and proportions of exchangeable Zn, carbonate-bound Zn, and Fe-Mn oxide-bound Zn, while dramatically reducing the concentration and proportion of residual Zn. Under the regime of combined N-Zn application, the supplementation of root exudates further increased grain yield and Zn content, as well as N content in roots, stems, leaves, and glumes. It significantly elevated available Zn and exchangeable Zn concentrations in non-rhizosphere soil, raised the content and proportion of carbonate-bound Zn in rhizosphere soil, but significantly lowered the pH and decreased the content and proportion of residual Zn in rhizosphere soil.

Conclusions Under the condition of combined N and Zn application, exogenous root exudates from winter wheat facilitate the conversion of residual Zn to carbonate-bound Zn in rhizosphere soil, increase the contents of carbonate-bound Zn and Fe-Mn oxide-bound Zn in both rhizosphere and non-rhizosphere soils, thereby effectively improving the bioavailability of soil Zn, leading to enhanced Zn uptake by winter wheat.