| Summary: | [Departement_IRSTEA]GT [TR1_IRSTEA]SET / DYMON [Encadrant_IRSTEA]Brun, J.J. (trad auto)Soils in mountain regions are facing strong climate and use changes that make them particularly vulnerable to three threats: erosion, loss of organic matter and biodiversity. This thesis addresses the issue of soil quality from the perspective of key processes (decomposition and aggregation of organic matter) within different compartments of the surface part of the living soil (epipedon). These key processes are involved in the intensity of soil services such as carbon sequestration, soil fertility and the maintenance of biological activity. Among the indicators used in soil quality assessment, we chose to test simple and composite indicators related to the key processes of two types of ecosystems under severe environmental constraints. Two study sites located in the Dévoluy (Isère) and Maures (Var) mountains were used. Both are marked by a strong vegetation gradient induced by natural (permafrost soils for the Dévoluy site) or anthropogenic (fires for the Maures site) constraints. In this work, we formulate three hypotheses: (i) there is an original signature at the soil level of environmental changes influencing the functioning of forest ecosystems; (ii) this signature is related to soil quality and covers a largely underestimated biological part; (iii) the reflectance of the soil summarizes this signature and allows its characterization under contrasting conditions. The results of the thesis show that constraints related to soil climate and fire induce significant changes in the variables describing decomposition and aggregation processes in soils. The biological compartment of the soil (microflora or microfauna) is particularly affected by these constraints, revealing changes in processes related to soil food chains (preferential pathways of biological decomposition and aggregation, potential nitrification and denitrification). Near infrared spectroscopy is proving to be a relevant tool for reporting changes in the ...
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