The influence of Alpine soil properties on shallow movement hazards, investigated through factor analysis

Abstract. Mountain watersheds are particularly vulnerable to extreme meteorological events, such as high intensity rainfall, and mountain soils often show pronounced fragility and low resilience due to severe environmental conditions. Alpine soil vulnerability is partly intrinsic but in part related to climate change (mainly precipitation regimes), and is enhanced by the abandonment of rural mountain areas that reduced the land maintenance actions traditionally carried out by farmers and local populations in the past. Soil hazards are related to different processes such as water erosion, loss of consistency, surface runoff and sediment transport, often occurring simultaneously and interacting with each other. Therefore, the overall effects on soil are not easy to quantify as they can be evaluated from different soil chemical and physical properties, referring to specific soil loss phenomena such as soil erosion, soil liquefaction, loss of consistency etc. In this study, we focus our attention on a mountain region in the NW Italian Alps (Valle d'Aosta), which suffered from diffuse soil instability phenomena in recent years, as a consequence of extreme rainfall events and general abandonment of the agricultural activities in marginal areas. The main effects were a large number of shallow landislides involving limited soil depths (less than 1 m), affecting considerable surfaces in the lower and middle part of the slopes. These events caused loss of human lives in the year 2000 and therefore raised the attention on land maintenance issues. Surface (topsoil: 0–20 cm) and subsurface (subsoil: 20–70 cm) samples were characterised chemically and physically (pH, carbon and nitrogen contents, cation exchange capacity, texture, aggregate stability, Atterberg limits etc.) and they showed very different soil properties. Topsoils were characterised by better stability, structure, and consistency. The differences between the two depths were potential trigger factors for shallow soil movements involving the upper soil horizons. We assessed a great number of soil properties that are known to be related to vulnerability to the main hazards present in the area. These properties were evaluated at the two depths and a factor analysis was performed to simplify the dataset interpretation, and to hypothesise the most decisive parameters that were potentially related to vulnerability. The factors (soil structure, aggregation, consistency, texture and parent material, cation exchange complex and other chemical properties) were a first step towards identifying soil quality indexes in the studied environment.