Amino acid concentration in the rhizosphere results from fluxes between plant roots, soil and microorganisms. In this context, root amino acid exudation process, composed of both efflux and influx, remains unclear. One main issue is to understand the selectivity of amino acid exudation resulting mainly in high proportions of glycine and serine in exudates compared to low proportions inside the root. To reach this point, a quantitative analysis of exudation with dissociated measurements of efflux from influx is needed. We measured efflux and influx by supplying 15 N-labelled glycine or serine for a short time of exposure at ecologically relevant concentrations to plants of white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.), maize (Zea mays L.), oilseed rape (Brassica napus L.), tomato (Lycopersicon esculentum Mill.) and alfalfa (Medicago sativa L.). Efflux was estimated by the increase of 14 N content of amino acids in root exudates and influx was estimated by the increase of 15 N content in plant tissue. Glycine efflux exceeded influx for all six species and was much higher in Fabaceae than in Poaceae. Serine efflux exceeded influx in alfalfa, white clover and rape. We conclude that presence of glycine and serine in root bathing solutions results from high glycine and serine efflux rates, observed in all six species studied here. The physiological and ecological significances of these high efflux rates are discussed in the context of N metabolism and plantsoil-microorganisms interactions.

The overuse of classical N fertilisers contributes substantially to environmental degradation by pollution of groundwater by nitrates. This leaching of N in waters is also an economic flaw for farmers because only a part of the fertiliser is used by the plants. Here, systems involving mixtures of legumes and grasses represent a sustainable alternative because legumes can fix atmospheric N 2 using symbiotic microbes. N transfer in those mixtures has been thoroughly investigated but little is known concerning the effect of N fertiliser on N transfer between N-fixing legumes and companion grasses. In white clover (Trifolium repens L.) -perennial ryegrass (Lolium perenne L.) associations, N is transferred mostly through rhizodeposition into the soil by clover followed by re-uptake by ryegrass. Rhizodeposition of N occurs through senescence and decomposition of legume tissue or through exudation of N compounds by living cells. Ammonium and amino acids are the main compounds exuded and their exudation is thought to occur by passive diffusion attributed to a concentration gradient from root to soil. In this study, we test the hypothesis that greater N transfer from clover to grass, as seen in N-rich soils or nutrient solutions, is due to greater N rhizodeposition brought about by higher ammonium and amino acid content of roots. The relations between N input, root N content, N net exudation and N transfer between legumes and grasses were investigated using 15 N by growing white clover and perennial ryegrass with increasing N application in axenic microlysimeters or in pots. Ammonium and amino acid concentrations were measured in root tissues, in root bathing solutions and in soils. We found that mineral N application strongly reduced atmospheric N fixation by clover, from 3.0 to 0.9 mg per plant, and root amino acid content, from 164 to 49 nmoles per g dry weight, but had no effect on ammonium and amino acid concentrations in sterile exudates, showing for the first time that amino acid net exudation is independent of root content. In contrast, ammonium and amino acid concentrations in clover soils increased with N fixation, showing the link between N fixation and N rhizodeposition in soils. Nitrate application increased ryegrass root growth by 7-8 times, and transfer of N between clover and ryegrass (by 3 times). It is concluded that N fertiliser does not modify N exudation but decreases N fixation and ammonium rhizodeposition in soil by clover. N fertiliser increases N transfer between clover and ryegrass by increasing soil exploration by ryegrass and giving a better access to different available N sources, including the N compounds exuded from clover.

The nitrogen isotope 15 N was used to quantify and determine the direction of nitrogen transfer between white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.). A micro-lysimeter system allowed the determination of the nature of the compounds exuded. During N privation, transfer was one-way from clover to ryegrass and was mainly explained by the large amounts of ammonium exuded by clover. N compound exudation by legume followed by uptake by companion grass is a highly significant pathway for inter-specific N transfer between young plants. Ammonium was the major N compound exuded. Amino acids were released in lower amounts, serine and glycine being the most abundant in leachates. In the micro-lysimeter, the amounts of ammonium, serine and glycine in the exudate were not altered by the presence of bacteria, providing evidence of the plant origin of these compounds. mixed culture / exudation / ammonium / amino acid Résumé-Transfert d'azote du trèfle blanc vers le ray-grass anglais, via l'exsudation de composés azotés. Le 15 N a été utilisé pour quantifier et déterminer le sens du transfert de l'azote entre le trèfle blanc (Trifolium repens L.) et le ray-grass anglais (Lolium perenne L.). Un système de micro-lysimetre a permis de déterminer la nature des composés exsudés. Pendant une privation en azote, le transfert est monodirectionnel, du trèfle vers le ray-grass, et est dû principalement à une exsudation importante d'ammonium par le trèfle. L'exsudation de composés azotés par une légumineuse, puis l'absorption par la graminée compagne constitue une voie importante du transfert interspécifique de l'azote entre jeunes plantes. L'ammonium est le composé majoritairement exsudé. Des acides aminés sont exsudés en quantités moindres, la sérine et la glycine étant les plus abondants dans les exsudats. En micro-lysimètre, les quantités d'ammonium, de sérine et de glycine dans les exsudats ne sont pas modifiées par la présence de bactéries, ce qui met en évidence l'origine végétale de ces composés. culture associée / exsudation / ammonium / acide aminé