Evaluating soil formation processes in Mediterranean granitic soils

Laburpena

Soil is one of the most important natural systems that support the development of life and is part of fundamental processes that make our planetary ecosystem work. It is well known that the factors that intervene in the formation of the soil are climate, bedrock, relief, biota and time. However, despite the numerous studies related to soil formation, little is known about these processes due to the immense complexity in which they are integrated. This study identifies and investigates in detail the different soil-forming processes in the study area located in a rocky Mediterranean landscape. Specifically, the processes that have been studied have been biological such as bioturbation, physical such as erosiondeposit and physical weathering. These processes are related to the spatial distribution of carbon stored in the soil, which has also been part of this study. The spatial distribution of carbon stocks in the soil is highly variable in the study area, as well as bulk density and stoniness. The variable that controls the spatial distribution of the stoniness is the slope, demonstrating the influence of the erosion. The spatial distribution of the carbon content and carbon stocks in the soil depend on vegetation cover and insolation. Bioturbation has been studied in soil profiles at different depths along a steeply inclined slope using the IRSL (Infrared Stimulated Luminescence) luminescence technique used on sand sized feldspar grains for the first time for this purpose. This technique has made it possible to identify the age of the sample at different depths and an effective soil mixing rate that takes into account only those grains that have ever been exposed to the soil surface. This has allowed, for the first time, to identify the border between the mobile regolith and the saprolite because the grains close to the border have never visited the surface.The soil age at different depths previously calculated has been used to determine the bioturbation, erosion and deposition rates along the slope through a new analytical solution of the advection-diffusion equation. Physical weathering has been assessed in the horizons of ten soil profiles by comparing rock fragmentation field data with different models. In this way, the patterns governing rock fragmentation were established in six profiles. The rest of the profiles did not fit any of these models because they are characterized by armouring effect on the surface, i.e. the surface horizon contains larger size of rocks than the lower one due to the removal of fine particles produced by erosion. The study presented here is a great advance in the knowledge of these soil-forming processes and can be implemented in other studies of the same type. In the future, this study could be compared with the results that would be obtained from erosion rates using other methods (10Be or 137Cs), completed with an estimation of the chemical processes in the study area and, finally, useful for the calibration and validation of both the new soil formation model that will be developed linked to the work developed in this thesis, as explained in chapter six, and other soil formation and landscape evolution models to better understand the connection between pedology and geomorphological processes.