Mitigating phosphorus–zinc antagonism in calcareous soils through the interaction of high–zinc wheat and the rhizospheric microbiome

Context: Selecting high–yielding wheat cultivars with increased zinc (Zn) concentration is a sustainable approach to mitigating the reduction in grain Zn nutrients caused by phosphorus (P) application in high–pH soils. Objective: It is important to understand how high–Zn (H_Zn) wheat enhances Zn uptake under P applications by optimizing the rhizosphere, particularly through the recruitment of beneficial bacteria, root colonization by arbuscular mycorrhizal (AM) fungi, and modification of root morphology. This knowledge is essential for the biofortification of wheat with Zn. Methods: We analyzed Zn rhizo–mobilization, root morphology, Zn uptake, and the microbial composition in the rhizosphere and roots of four high–yielding wheat cultivars with contrasting grain Zn levels. The study was conducted under two P fertilizer rates, 0 and 44 kg P/ha, on the southern Loess Plateau, China. Results: Bacteria potentially alleviating P–Zn antagonisms, such as Rhizobium sp., Sphingomonas sp., and Pseudomonas spp., were specially enriched in H_Zn cultivars with P application and demonstrated the ability to promote Zn rhizo–mobilization by decreasing soil pH, resulting in a 69.1 % increase in available Zn concentration. P application reduced root colonization by AM fungi Diversispora densissima in Low–Zn (L_Zn) cultivars but not in H_Zn cultivars, allowing H_Zn cultivars to maintain higher root Zn acquisition efficiency. The P–induced increases in total root length and surface area per plant in H_Zn cultivars were 40 % and 7 % higher, respectively, compared to L_Zn cultivars, while the increase in average root diameter of H_Zn cultivars was 62 % lower than that of L_Zn cultivars. This suggests that the longer, thinner roots with larger surface areas were advantageous for H_Zn cultivars in capturing more Zn from the soil. Conclusion: Therefore, recruiting more beneficial rhizobacteria, maintaining stable root colonization by AM fungi, and optimizing root growth are crucial strategies for H_Zn cultivars to enhance shoot Zn uptake and mitigate P–Zn antagonism. Implications or significance: Combining high–Zn wheat with specific bacteria and fungi at the soil–root interface, along with appropriate P application, holds significant potential for achieving wheat biofortification with Zn.