Nitrogen transfer from root exudates to the rhizobiome: A 15N stem feeding method

Plant roots exude nitrogen (N)-containing compounds, which are assimilated by the rhizobiome to maintain stoichiometric homeostasis. Various 15N-tracing methods exist to estimate the rhizobiome-N derived from root exudation but have never been validated with a full factorial experiment. We exposed ryegrass (Lolium multi-florum) to 15N solutions through different plant organs (stem or leaf feeding), with different 15N-tracers (urea or NH4NO3) and concentrations (64.5, 129 or 193 mmol 15N L-1). Stem feeding with either 15N-source at the highest concentration was the only effective way to quantify root exudate-N assimilation by the rhizobiome, and we recommend this 15N-labeling method for the reliable assessment of microbiome-N dynamics in the rhizosphere.