To mitigate climate change, increasing soil organic carbon (C) stocks and enhancing chemical weathering in croplands have been proposed as CO₂ removal strategies. Here, we investigate the role of belowground organic C inputs, namely roots and rhizodeposition, in these approaches. First, we quantified the rhizodeposition of 12 crop species and assessed its decomposition in soil. We found that rhizodeposition accounts for a significant C pool (42% of root inputs) and decomposes more slowly than roots, making it a relevant C input to consider.

Next, we examined its role in regulating the chemical weathering of crushed basalt using a new experimental setup with 15 lysimeters in a climate chamber. Our results showed that while plants enhanced solute release, they also reduced seepage, limiting dissolved inorganic C export. Altogether, this highlights the need for a thorough understanding of belowground C inputs to optimize CO₂ removal strategies.

Terrestrial ecosystems contain vast amounts of carbon (C) and are a hub for C exchanges. They remove CO₂ from the atmosphere by sequestrating organic C in biomass and soils via photosynthesis. They also consume CO₂ through chemical weathering of minerals. To mitigate climate change, it has been proposed to increase these 2 fluxes in agricultural ecosystems. Belowground organic C inputs, namely roots and rhizodepostion, represent around 46% of net primary production, and greatly contribute to shape their surroundings and to drive the processes controlling organic C sequestration and chemical weathering. This thesis explores how belowground carbon inputs should be considered when addressing CO₂ removal through increased plant inputs and weathering.

We carried out a first 13C-CO₂ labelling mesocosm experiment in a climate chamber to quantify the C inputs attributed to aboveground biomass, roots and net rhizodeposition for 12 crop species. We highlighted a positive correlation between rhizodeposition and aboveground biomass and found no negative correlation among any of the 3 pools. This suggests that increasing inputs by targeting a specific C source will not be at the extent of the others. We then assessed root decomposition and net rhizodeposition through a litterbags incubation experiment in the field over a year. We found that rhizodeposition had a decomposition rate slightly smaller to that of roots. These 2 experiments suggest that net rhizodeposition, that accounted for 22 to 38% of the C allocated belowground, is a significant C input to consider for soil organic carbon increase. We conducted a second experiment designed to facilitate the simultaneous study of the organic and inorganic C cycles. For this purpose, we constructed a new experimental platform composed of 15 instrumented lysimeters in a climate chamber. This notably enabled the monitoring of water flow, which connects most biogeochemical processes in the critical zone. We used this setup to study the chemical weathering of a crushed basalt substrate, on which we grew different genotypes of alfalfa. We found that plants, mostly by increasing the pore CO2 concentration, increased the concentration of solutes in the discharge water.

However, through evapotranspiration, they significantly reduced seepage, thereby limiting the export of dissolved inorganic carbon from chemical weathering. This highlighted a duality between C storage strategies and water management. Altogether, our results confirm that belowground C inputs are a major lever for sequestering organic carbon and that their interaction with the inorganic carbon cycle should also be considered.

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https://cnrs.zoom.us/j/91480217183?pwd=7xU8caudDqKO0IsuCadBtaf93HEroY.1

• Samuel ABIVEN (Directeur de thèse), Professeur junior, ENS-PSL
• Pierre BARRÉ (Directeur de thèse), DR, ENS-PSL-CNRS
• Marc BENEDETTI (Président), Professeur, IPGP-UPC
• Safya MENASSERI (Rapporteuse), Professeure, Institut agro Rennes Angers
• Sara VICCA (Rapporteuse), Associate pr., University of Antwerp
• Philippe HINSINGER (Examinateur), DR, INRAE
• Simon CHOLLET (Invité), IR, Cereep-ECOTRON-CNRS