Factors controlling dissolved organic carbon in soils: A database analysis and a model developmen

Carbon is constantly displaced along the land-ocean aquatic continuum, but, despite its crucial role in the global carbon cycle, this lateral transport of dissolved organic carbon (DOC) is usually neglected in Land Surface Models (LSM). Including these lateral DOC fluxes in global carbon cycle models is necessary for two main reasons: 1) Neglecting these fluxes can lead to an overestimation of terrestrial ecosystem carbon sequestration, and 2) In order to better understand and predict Earth's carbon cycle, we need to move towards a "boundless carbon cycle", that integrates terrestrial, freshwater and marine ecosystems. Therefore this thesis has a twofold objective: First, to compile and analyze a database on DOC concentrations in soil solution with the aim of better understanding which drivers determine their spatial and temporal variability. Second, to implement the processes of DOC. production, retention and transport in the LSM ORCHIDEE. The database analysis revealed that, as expected, DOC concentrations were greater in organic soils than in mineral soils. I also found that DOC concentrations were lower in broadleaved forests than in coniferous forests, probably due to the slower decomposition rate of coniferous litter. There was evidence of an overall increasing trend in DOC concentrations in the organic layers, buf no consistent temporal trend in concentrations of DOC in soil solution in the mineral soil was detected. The database analysis revealed a key role of site N availability and pH in controlling both the spatial and the temporal variability of DOC. Regarding the modelling work, the resulting new model, ORCHIDEE-SOM, was evaluated against field data of DOC concentrations in a temperate forest site. The model-data agreement was very satisfactory for the dynamics and soil profiles of DOC concentrations, indicating that the most important processes of DOC production and transport are already well-represented in ORCHIDEE-SOM. However, the ORCHIDEE-SOM simulations of DOC and SOC dynamics at site level are not perfect because the physico-chemical soil properties that are important in determining DOC and SOC dynamics are not yet included in the model.