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What makes a soil landscape robust? Landscape sensitivity to land use changes in a southern Alpine valley (Ticino, Switzerland)

Subject Area Physical Geography
Soil Sciences
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 409808488
 
Final Report Year 2023

Final Report Abstract

Land use changes induced by agriculture constitute the ecologically most intense and, in terms of surface area affected, largest anthropogenic impact into natural soil landscapes. Alpine soil landscapes can be considered as particularly sensitive to land use changes because their effects tend to be enhanced by often extreme climatic and topographic conditions as well as intense geomorphological activity. Within the scope of the present project the sensitivity of a southern Alpine soil landscape in Ticino (Switzerland) to a set of different land use changes will be assessed. These land use changes are represented in six different land use-topography units that can be differentiated regarding: (i) the type of land use (pasture; meadow; agriculture; forest), (ii) the status of land use (active; abandoned/extensified), and (iii) the topography (artificial terraces; natural slope). Firstly, we analyzed the effects of the different land use changes on particular soil physical and chemical properties that serve as indicators for the sensitivity of the soil landscape. The results show a generally very high aggregate stability as an indicator of the soil’s susceptibility to erosion, independent of land use. In contrast to aggregate stability, soil water repellance is most sensitive to land use changes. In addition, saturated hydraulic conductivity (Ksat) served as a proxy for the soil’s infiltration capacity. By comparing Ksat with the precipitation intensity, the susceptibility of the studied land uses to the formation of Hortonian surface runoff (HSR) was assessed. According to this, the soil landscape of Valle Onsernone seems to be stable up to precipitation intensities of a return period of 10 years. However, this approach does not take into account all the controlling factors for the development of HSR. Therefore, secondly, it was examined whether there was a significant increase in HSR and soil erosion due to use-related changes in these sensitivity indicators, since soil erosion in mountainous landscapes is the main trigger for soil degradation. The quantification of HSR and soil erosion by means of rainfall simulation experiments showed a very high land use-related variability of HSR with lowest values on meadows on north-facing slopes (MSN) and highest values on (re-)forested, abandoned terraces (FTS) on south-facing slopes. Nevertheless, soil erosion rates were very low regardless of land use, indicating a high stability of the soil landscape. The only exception is FTS where erosion and thus sediment transport was increased by one order of magnitude, particularly in areas where, due to lack of terrace maintenance and tree growth, the terrace walls had collapsed and bare soil was exposed. This resulted in a local increase in slope gradient and erosion of the bare soil, which may lead to a loss of valuable and limited soil resources, especially in this alpine landscape. Consequently, FTS can be considered an unstable soil landscapes, which poses a serious risk due to the enormous spatial extent of terraces in the Valle Onsernone. To avoid or mitigate this situation, terrace walls should be maintained and preserved and the bottom of the terraces should be covered with dense vegetation to provide natural protection against soil erosion.

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