Effect of north and south exposure on organic matter in high Alpine soils

The present work focuses on the subalpine range of the Italian Alps to determine the influence of aspect and consequently climate on soil humus properties and chemistry. This was done by comparing soils developing in north- and south-facing sites on siliceous parent material. The soils were investigated with respect to the total organic C and N content, C and N contents of organic matter (OM) density fractions and of labile (oxidised by H2O2) and stable (H2O2-resistant) fractions. Further characterisation of OM and the different fractions was performed with Diffuse Reflection Infrared Fourier Transform (DRIFT), Scanning Electron Microscopy/Electron Diffuse Scattering (SEM/EDS) and radiocarbon dating. The soils at north-facing slopes had a higher OM content. Density measurements and the H2O2 fractionation proved that this higher content was primarily due to a more pronounced accumulation of weakly degraded, labile organic materials when compared to the south-facing slopes. Independent of the sites, a large part of OM (around 50% of OM) was fixed in the densest fraction (> 2 g/cm3) which correlated well with the abundance of the H2O2-resistant fraction. The 14C ages of the latter were up to 17,000 years, reflecting the presence of stable, refractory OM and the effectiveness of organo-mineral interactions in protecting OM from decay. Podzolisation has been more intense at north-facing sites. Due to the accumulation of weakly degraded OM, eluviation of Fe and Al is strongly enhanced. High-mountain ecosystems like the Alps are very sensitive to changing environmental conditions such as global warming. Especially at cooler sites, the obtained results suggest that a warmer climate, and thus more favourable conditions for biological activity, will lead in the short- to mid-term to an increased loss of accumulated, weakly degraded OM.