“The sustainable role of agricultural residues in future bioeconomy strategies and its dependency upon carbon returns to arable soils”
This thesis is partly financed by the project “Fonds de Solidarité pour Projects Innovants FSPI” by the French Ministry of Foreign Affairs and the French Embassy in Ecuador, the Technical University of Manabí and the project Cambioscop (https://cambioscop.cnrs.fr – ANR-17-MGPA-0006), and was developed at INSA Toulouse under the direction of Dr. Lorie Hamelin (INRAE) and Dr. Ezequiel Zamora-Ledezma (Technical University of Manabí).
The defense will take place the Friday 23rd June at 9h00 at the room 401 of the Toulouse Biotechnology Institute, 135 AV de Rangueil, 31400 Toulouse. The defense can also be attended on videoconference through the link: https://zoom.us/j/91020750385 (ID 910 2075 0385).
- Jorgen Eivind Olesen, Professor, Aarhus University (Rapporteur)
- Birka Wicke, Professor, Radboud University (Rapportrice)
- Sonja Keel, Senior researcher, Agroscope (Examiner)
- Jean-François Soussana, Vice-president of International Policy, INRAE (Examiner)
- Florent Levavasseur, Researcher, INRAE (Examiner)
- Etienne Paul, Professor, INSA Toulouse (Examiner)
Crop residues are a key feedstock to supply renewable carbon due to their relative abundance, flexibility to supply several bioeconomy pathways, and no land or food competition. Nevertheless, when left on the field, crop residues contribute to maintaining the soil carbon balance, which creates a tradeoff between supplying the bioeconomy and maintaining the soil carbon stocks. In France, it is recommended to maintain between 41 and 96% of the technically harvestable residue on soils to prevent soil organic carbon (SOC) losses, which precludes the supply of biomass to the bioeconomy. However, these restrictions have not considered the end-use of the residues or foreseen that bioeconomy processes generate, along with the main product, a residual biomass known as coproduct, which is rich in recalcitrant carbon that can be returned to soils.
This thesis aims to understand the interactions between the return of recalcitrant bioeconomy coproducts to soil and long-term SOC dynamics in a C-neutral harvest context. The term “C-neutral harvest” is defined as a situation where the harvesting of crop residues for a given bioeconomy management does not decrease the long-term SOC stocks, compared to a reference situation (BAU) where crop residues are not harvested. The thesis, thus, determines the amount of crop residues that could be harvested to supply the bioeconomy, in a spatially explicit manner and function of the bioeconomy pathway, while preserving or enhancing SOC stocks. The interrelation between soil organic carbon dynamics and the recalcitrance of bioeconomy coproducts was investigated to provide a set of tools that integrate bioeconomy-derived coproducts within soil C models. The conversion rates of crop residues to bioeconomy coproducts, as well as the inherent recalcitrance of these coproducts, were exhaustively reviewed for five bioeconomy technologies (coproducts in brackets): i) pyrolysis (biochar), ii) gasification (char), iii) hydrothermal liquefaction (hydrochar), iv) anaerobic digestion (digestate), and v) lignocellulosic alcohol production (solid cake and liquid molasses). A harmonized database of over 600 data records was compiled to report average C conversion (CC) and recalcitrance (CR) coefficients for the coproducts under study.
A novel framework based on four interconnected modules that integrate the CC and CR coefficients of the five coproducts in the AMG and RothC soil models was developed. The models were applied to spatially explicitly assess the C-neutral harvest potential of crop residue to supply the bioeconomy in France (AMG) and Ecuador (RothC). In the case of RothC, the priming effect exerted by the input of the bioeconomy coproducts on crop-dedicated soils was also considered. While returning the bioeconomy coproducts could allow exporting all the technically harvestable crop residues without imposing pressures on SOC stock (compared to a BAU situation), it could impact other environmental variables disregarded by the soil C models (e.g., climate change, eutrophication, toxicity, among others). To shed light on the trade-off between carbon sequestration and environmental mitigation, a full life cycle assessment (LCA) was performed. Maritime biofuel production derived from residual crop biomass was studied for two processing pathways (biogas to liquefied gas, and bio-oil produced by pyrolysis) and was compared to a no-use scenario (BAU), including counterfactual biomass use. The integration of the SOC modeling results with the LCA conclusions allows the identification of the most suitable scenario for the valorization of crop residues.