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Dissolved organic matter (DOM), as sources of nutrient or pollutant dissemination pathway are implied in numerous environmental issues. Although DOM have been the subject of numerous studies for several decades, the mechanisms implied for their solubilization and their transport from soils to aquatic systems are still a matter of discussion. Based on DOM molecular composition determined using thermally assisted hydrolysis and methylation –gas chromatography – mass spectrometry (THM-GC-MS), this thesis aims to provide a better understanding of their solubilization and transfer mechanisms at the scale of an agricultural headwater catchment. This work was conducted on the experimental headwater catchment of Kervidy-Naizin (France, Environmental Research Observatory AgrHys) in order to determine the processes implied at two temporal scales. At the scale of a rain event, this work has clarified the impact of hydrological conditions on the DOM dynamics. At annual scale, the use of carbon isotope signature (δ13C) and DOM molecular composition allowed to clarify the DOM transfer mechanisms at the slope scale. The use of THM-GC-MS appears to be a suitable tool for the study of DOM dynamics. The results thus obtained allowed to highlight the role of hydrological conditions and in particular the water-table level in the solubilization and transfer of DOM., Les matières organiques dissoutes (MOD), en tant que sources de nutriments ou potentiels vecteurs de pollution, sont impliquées dans de nombreuses problématiques environnementales. Bien qu'elles fassent l'objet de nombreuses études depuis plusieurs décennies, les mécanismes gouvernant leur solubilisation et leur transfert depuis les sols vers les systèmes aquatiques demeurent sujets à discussion. En s'appuyant sur l'étude de la composition moléculaire des MOD par hydrolyse et méthylation assistée par température et couplée à la chromatographie en phase gazeuse et à la spectrométrie de masse (HMT-CPG-SM), cette thèse a pour objectif d'apporter une meilleure compréhension de leurs mécanismes de solubilisation et de transfert à l'échelle d'un bassin versant agricole. Ce travail s'est appuyé sur le bassin versant expérimental de Kervidy-Naizin (Morbihan, Observatoire de Recherche en Environnement AgrHys) afin d'observer les processus mis en jeux à deux échelles temporelles différentes. A l'échelle de la crue, ce travail a permis de préciser l'impact des conditions hydrologiques spécifiques sur la dynamique des MOD. A l'échelle annuelle, l'utilisation conjointe de la signature isotopique du carbone (δ13C) et de la composition moléculaire des MOD a permis de préciser les mécanismes de transfert de MOD impliqués à l'échelle du versant. L'utilisation de la HMT-CPG-SM s'est avéré un outil adéquat pour l'étude de la dynamique des MOD. L'ensemble des résultats ainsi obtenus ont permis de souligner l'importance des conditions hydrologiques et en particulier de la dynamique de nappe dans les processus de solubilisation et de transfert des MOD

National audience; dissolved organic matter (DOM) over hydrological year. Duringthese hot moments, the increased flow at the outlet is accompaniedwith an increase of DOM concentration and a modification ofDOM composition, implying the mobilization of additional DOMsources regard to base-flow conditions. Molecular analysis havebeen performed on outlet samples of a French agricultural catchment(Kervidy-Naizin) which belongs to the Critical Zone ObservatoryAgrHyS (SOERE Réseau des Bassins Versants). Previousresults shown a clear modification in the distribution of lignincompounds during flood events and evidence an heterogeneity inthe intensity of molecular changes. This DOM, less biodegraded,was assumed to be produced by partition between particulate anddissolved phases when the soil/water ratio is low (soil particlesisolated in water). However, in the outlet, modifications of DOMcomposition is still recorded even after the return to pre-eventturbidity. To explain these results we hypothesize that molecularchanges could come from DOM produced in soil solutions. So howsoil erosion could induce production of less biodegraded DOM? Isthe intensity of molecular composition changes impacted by intensityof the flood event?Those questions were investigated during flood events, by samplingwith high frequency stream DOM at the outlet of the watershedand soil solutions in riparian soils equipped with zero-tensionlysimeters. Samples were filtered, analyzed for DOC and freezedriedfor molecular analysis (thermally assisted hydrolysis andmethylation–gas chromatography/mass spectrometry). The hydraulicgradient was monitored every 15 minutes, using piezometersimplemented along the slope. The long-term monitoring programof ORE AgrHyS on this site allowed a good understandingof hydrological functioning of this catchment, which is essentialto link hydrology and biogeochemistry.At the beginning of the event, hydraulic gradient increased rapidlyand stayed high during several days. Modification of DOM compositionin soil solutions were recorded during the hydraulic gradientrise with an increase in the proportion of less degraded moleculesthan during base-flow conditions. In river, modification of DOMquality is also recorded short time after its appearance in soil solutions.Intensity of the flood events seems also to impact intensityof the modification with intense flood events responsible for highvariations in DOM composition.For the first time, composition of soil DOM has been investigatedduring flood events. Results evidence the concomitant evolutionof hydraulic gradient, DOM concentration and composition in soilssolutions and stream outlet. Results highlight the contribution ofsub-surface erosion in the production of storm stream DOM, andthe key role of hydrological conditions in these modifications