Contribution of remote sensing for spatialized simulation of crop carbon balance components and biogeochemical and biogeophysical mitigation effects of intermediate crops.

Summary Climate change and the demographic growth of the world's population are leading the agricultural world to adapt to meet these two major challenges. While agricultural land, which represents nearly one third of the world's land area, contributes significantly to global greenhouse gas emissions, it also offers the possibility of implementing climate change mitigation levers. In this context, the aim of this thesis is to increase our knowledge of the functioning of agricultural surfaces, to provide tools for assessing the contribution of cultivated surfaces to climate change, and to quantify the biogeochemical (C storage) and biogeophysical (albedo effect) effects of climate change mitigation through the use of intermediate crops. To meet these objectives, two modelling approaches were developed during this work. The first part of this thesis focused on the development of a spatialized modeling approach, allowing to provide estimates of production (biomass and yields), CO2 and water fluxes, these variables being used to quantify the carbon and water balances for field crop plots. To this end, the SAFYE-CO2 agro-meteorological model assimilating satellite products of vegetation index at high spatial and temporal resolutions was developed and applied to different crops (wheat, corn and sunflower) and intercrop vegetation (spontaneous regrowth, weeds, intermediate crops). This approach was validated on a network of plots in southwestern France, using a large number of satellite images and validation data on the Regional Spatial Observatory area. In particular, it has allowed to accurately estimate wheat, sunflower and corn productions, as well as CO2 and water fluxes on wheat and sunflower crops. Vegetation, which can develop on the plots during intercropping periods, was also taken into account in order to improve the estimation of CO2 and water fluxes. In particular, this made it possible to quantify the impact of intermediate crops on the C balance components of plots allocated to field crops in the study area. The second part aimed at developing a model of intermediate crops introduction at the European scale, in order to estimate the radiative forcing induced by the modification of the surface albedo generated by this practice. Thanks to medium resolution albedo products (1/20°), developed by the CNRM (and in collaboration with this laboratory), this modelling approach allowed to provide estimates of the albedo effect relative to intermediate crops. Several introduction scenarios were simulated to account for the impact of certain factors, such as snow or rain. These scenarios were used to highlight the potential negative impact of soil darkening induced by intermediate crops on the radiative forcing of cultivated areas in the long term (via the enrichment of soil organic matter). Finally, as any change in agricultural practices induces biogeochemical and biogeophysical effects on climate, an analysis of these coupled effects was conducted using these two modelling approaches. We conclude that once intercropping is implemented, the soil should be permanently covered so that the soil darkening effect does not result in the loss of other climatic benefits generated by this agricultural practice.