Competition between plant and bacterial cells at the microscale regulates the dynamics of nitrogen acquisition in wheat (Triticum aestivum)

The ability of plants to compete effectively for nitrogen (N) resources is critical to plant survival. However, controversy surrounds the importance of organic and inorganic sources of N in plant nutrition because of our poor ability to visualize and understand processes happening at the root-microbial-soil interface. Using high-resolution nano-scale secondary ion mass spectrometry stable isotope imaging (NanoSIMS-SII), we quantified the fate of ¹⁵N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either ¹⁵NH₄⁺ or ¹⁵N-glutamate and traced the movement of ¹⁵N over 24 h. Imaging revealed that glutamate was rapidly depleted from the rhizosphere and that most ¹⁵N was captured by rhizobacteria, leading to very high ¹⁵N microbial enrichment. After microbial capture, approximately half of the ¹⁵N-glutamate was rapidly mineralized, leading to the excretion of NH₄⁺, which became available for plant capture. Roots proved to be poor competitors for ¹⁵N-glutamate and took up N mainly as ¹⁵NH₄⁺. Spatial mapping of ¹⁵N revealed differential patterns of ¹⁵N uptake within bacteria and the rapid uptake and redistribution of ¹⁵N within roots. In conclusion, we demonstrate the rapid cycling and transformation of N at the soil-root interface and that wheat capture of organic N is low in comparison to inorganic N under the conditions tested.