An unexpected source of nitrogen for root uptake: positively charged amino acids dominate soil diffusive nitrogen fluxes

Soils typically contain a large variety of nitrogen (N) forms,including inorganic N and a range of organic N compounds ofvarying molecular size (Warren, 2013). Inorganic N was long beenconsidered to constitute the main source of N for plants, but thisview has changed considerably since plants were shown to becapable of directly taking up and metabolizing organic N forms,including amino acids, peptides, proteins and quaternary ammo-nium compounds (N€asholmet al., 2009; Warren, 2013). Aminoacid uptake especially has been demonstrated in every plant speciesstudied thus far and the underlying uptake mechanisms have beeninvestigated extensively (N€asholmet al., 2009; Narcyet al., 2013).Yet even if plants have the potential to take up amino acids, those Nforms first have to be bioavailable and have to be consistentlyreplenished at root surfaces. However, reliably estimating such Navailability is challenging due to the sheer complexity of soils andplant root systems.