Terrestrial ecosystems absorb, at present, about one-fourth of anthropogenic CO2 emissions, which is accumulating in the carbon (C) stocks of vegetation and soils. Land-surface models are used to project the 21st century evolution of this CO2 sink, which mitigates the expected increase in atmospheric CO2 concentration and, thus, climate change. However, classical land-surface models neglect that a fraction of the anthropogenic C absorbed by terrestrial ecosystems is not accumulating on land but is instead exported through the river network. It was shown that this negligence may lead to signification biases, underestimating the CO2 uptake by terrestrial ecosystems while overestimating the amount of anthropogenic C sequestered within vegetation and soils (Lauerwald et al. 2020). The first step to include these riverine C transfers had been achieved with the model branch ORCHILEAK (Lauerwald et al. 2017), which featured a representation of dissolved organic C and CO2 leaching from soils to the river network, and the reactive transport of these allochthonous carbon loads through the river network, including river-to atmosphere CO2 emissions and exchanges of C between the water column and soils in inundated floodplains. The model was first developed and tested for the Amazon basin, but later also applied to other regions such as the Congo basin (Hastie et al. 2021), Europe (Gommet et al., 2022) and, after a merge with the high-latitude branch ORCHIDEE-MICT, to the Lena river (Bowring et al., 2020). The next important step was taken with the model branch ORCHIDEE-Clateral (Zhange et al. 2022), which combined the developments of ORCHILEAK with a representation of soil erosion related riverine fluxes of particulate organic carbon, its decomposition in transit, and its deposition onto floodplains. This model has so far been tested and applied at the regional scale of Europe. Now, further developments are carried out to add a representation of the coupled cycling of C and nitrogen along the river network. These developments will not only improve the representation of terrestrial C and greenhouse gas budgets, but also provide spatially and temporally resolved estimates of riverine C and nutrient exports to the coast that can then be used as boundary conditions for ocean biogeochemistry models. This presentation will give an overview of these model developments and the major findings from the associated studies.

Mardi 11 avril à 11h • ENS – salle Claude Froidevaux – E314 • 24, rue Lhomond 75005 PARIS