The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil-root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include:(1) indirect, fortuitous root exudate recapture as part of the root's C and N distribution network, (2) direct re-uptake to enhance the plant's C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for interroot and root-microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.

International audienceDuring their life, plant roots release organic compounds into their surrounding environment. This process, named rhizodeposition, is of ecological importance because 1/ it is a loss of reduced C for the plant, 2/ it is an input flux for the organic C pool of the soil, and 3/ it fuels the soil microflora, which is involved in the great majority of the biological activity of soils, such as the nutrient and pollutant cycling or the dynamics of soil-borne pathogens, for example. The present review first examines the mechanisms by which major rhizodeposits are released into the soil: the production of root cap cells, the secretion of mucilage, and the passive and controlled diffusion of root exudates. In a second part, results from tracer studies (43 articles) are analyzed and values of C flux from the plant root into the soil are summarized. On average, 17% of the net C fixed by photosynthesis is lost by roots and recovered as rhizosphere respiration (12%) and soil residues (5%), which corresponds to 50% of the C exported by shoots to belowground. Finally, the paper reviews major factors that modify the partitioning of photoassimilates to the soil: microorganisms, nitrogen, soil texture and atmospheric CO$_2$ concentration.La rhizodéposition de C organique par les plantes : mécanismes et contrôles. Au cours de leur vie, les racines des plantes libèrent des composés organiques dans leur environnement proche. Ce processus, nommé rhizodéposition, est d'importance écologique car 1/ c'est une perte de C réduit pour la plante, 2/ c'est une flux d'intrant pour le compartiment de C organique du sol et 3/ il alimente la microflore du sol, qui est impliquée dans la grande majorité de l'activité biologique des sols tels que par exemple le cycle des éléments nutritionnels et des polluants ou encore la dynamique des pathogènes du sol. La présente revue examine en premier lieu les mécanismes par lesquels les rhizodépôts majeurs sont libérés dans le sol : la production de cellules de la coiffe racinaire, la sécrétion de mucilage, la diffusion passive et contrôlée d'exudats racinaires. En second lieu, les résultats d'études de traçage du C (43 articles) sont analysés et les valeurs de flux de C allant de la racine de la plante vers le sol sont synthétisées. En moyenne, 17 % du C net fixé par la photosynthèse est perdu par les racines et il est restitué dans la respiration de la rhizosphère (12 %) et dans les résidus de sol (5 %), ce qui correspond à 50 % du C exporté par les parties aériennes vers le sol. Enfin, l'article répertorie les facteurs principaux qui modifient la répartition des photoassimilats vers le sol : les microorganismes, l'azote, la texture du sol et la concentration en CO$_2$ de l'atmosphère du sol

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In order to understand the effect of the maize rhizosphere on denitrification, the diversity and the activity of the denitrifying community were studied in soil amended with maize mucilage. Diversity of the denitrifying community was investigated by polymerase chain reaction (PCR) amplification of total community DNA extracted from soils using gene fragments, encoding the nitrate reductase (narG) and the nitrous oxide reductase (nosZ), as molecular markers. To assess the underlying diversity, PCR products were cloned and 10 gene libraries were obtained for each targeted gene. Libraries containing 738 and 713 narG and nosZ clones, respectively, were screened by restriction fragment analysis, and grouped based on their RFLP (restriction fragment length polymorphism) patterns. In all, 117 and 171 different clone families have been identified for narG and nosZ and representatives of RFLP families containing at least two clones were sequenced. Rarefaction curves of both genes did not reach a clear saturation, indicating that analysis of an increasing number of clones would have revealed further diversity. Recovered NarG sequences were related to NarG from Actinomycetales and from Proteobacteria but most of them are not related to NarG from known bacteria. In contrast, most of the NosZ sequences were related to NosZ from alpha, beta, and gammaProteobacteria. Denitrifying activity was monitored by incubating the control and amended soils anaerobically in presence of acetylene. The N2O production rates revealed denitrifying activity to be greater in amended soil than in control soil. Altogether, our results revealed that mucilage addition to the soil results in a strong impact on the activity of the denitrifying community and minor changes on its diversity.

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