| Summary: | Climate change and its consequences stand before us as the greatest challenge of our generation. The Arctic is warming at unprecedented rate and permafrost thaw is leading to irreversible changes with a global influence on climate. Permafrost carbon reservoir is twice as large as what is stored in the atmosphere. Upon thawing, permafrost organic carbon is vulnerable to be degraded by microorganisms resulting in additional greenhouse gas emissions. How fast and how much carbon will be released to the atmosphere with permafrost thaw remains highly uncertain. Mineral surfaces and cations can stabilize organic carbon in soils and sediments and mitigate its degradation as greenhouse gases. Yet, the interactions between minerals and organic carbon in permafrost and the consequences of changing soil conditions upon thawing are still poorly understood. This Ph.D. thesis tackles the influence of permafrost thaw on mineral organic carbon interactions and investigates how changing soil conditions may promote or mitigate organic carbon stabilization mechanisms. We found that up to 59% of total organic carbon is potentially mineral-protected. We observed that abrupt thaw increases the supply of stabilizing surfaces and cations. Upon gradual thaw, we highlighted that redox interface greatly influences the location of hotspots for mineral organic carbon interactions. These findings highlight that mineral organic carbon interactions can protect organic carbon despite permafrost thaw. We argue that mineral organic carbon interactions should be investigated together with other factors and included in the next generation of Earth System models to better predict climate trajectories. (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2022
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