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Keywords:

  • amino acids;
  • dissolved organic nitrogen;
  • NanoSIMS;
  • nitrogen cycling;
  • nutrient uptake;
  • rhizobacteria;
  • rhizosphere architecture

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 15N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either inline image 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 inline image, which became available for plant capture. Roots proved to be poor competitors for 15N-glutamate and took up N mainly as inline image. 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.