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

Summary � 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 15 N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either 15 NH þ 4 or 15 N-glutamate and traced the movement of 15 N over 24 h. � Imaging revealed that glutamate was rapidly depleted from the rhizosphere and that most 15 N was captured by rhizobacteria, leading to very high 15 N microbial enrichment. After microbial capture, approximately half of the 15 N-glutamate was rapidly mineralized, leading to the excretion of NH þ , which became available for plant capture. Roots proved to be poor competitors for 15 N-glutamate and took up N mainly as 15 NH þ . Spatial mapping of 15 N revealed differential patterns of 15 N uptake within bacteria and the rapid uptake and redistribution of 15 N 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.