Black soils in the south side of the Alps : large C stocks, strong weathering degree, genetic issues

Umbrisols are the most common soil type in the Southern side of the Alps, between Piedmont and the Lake District in Lombardy (NW-Italy) and Switzerland, particularly on sialic substrates and glacial till but also on carbonates, if the soil is sufficiently deep. They are best developed on southward aspects, and below chestnut, beech forests or anthropogenic grasslands, at montane elevations. Many of these soils, particularly if developed on stable surfaces above the LGM trimline (i.e., the upper limit reached by glaciers), have 20-40 cm thick A horizons, with fine granular structure and abundant roots, overlying black sombric or sombric-like horizons (80-120 cm of depth) lacking biogenic aggregation, containing very few roots, and characterized by very low bulk densities, even <500 kg/m3. Carbon stocks are very high, up to 40 kg/m2. Egli et al. 2008 found the black horizons to be rich in charcoal, whereas Blaser et al., 1997 interpreted them as spodic horizons dominated by Al and organic matter (OM), and lacking Fe. However, some of their properties were not considered and their pedogenic interpretation is dubious. To better characterize the black horizon, and particularly its OM properties (e.g., age, stability, chemical recalcitrance to decomposition, mineral stabilizing factors) five well-developed Umbrisols were sampled, in different locations in NW-Italy, between Torino metropolitan area (west) and Lake Como (east); soil micromorphological observations, and detailed analysis on mineral weathering and on OM characteristics were performed. The results show that these soils developed during long periods of time, and podzolization is not a primary pedogenic process; strong mineral weathering, associated with temperate temperatures and high rainfall, has been able to release large quantities of Al from the parent materials, creating the conditions for Andosolization-like processes. These soils are highly resistant to erosion and to degradation, due to the strong stabilizing effect of some ions, but their high C contents and old age make them precious and worth of protection.