Effect of permafrost on the formation of soil organic carbon pools and their physical–chemical properties in the Eastern Swiss Alps

Abstract

Abstract Current climatic conditions and the occurrence of discontinuous and sporadic permafrost in the Alps result in a low turnover rate and therefore accumulation of organic matter (OM) in soils. Alpine soils are thus highly sensitive to global warming that potentially promotes the mineralisation of soil organic matter (SOM). This process might increase the release of CO2 to the atmosphere. Our aim was to investigate the potential effect of permafrost thawing by the analysis of the physical–chemical soil properties of permafrost versus non-permafrost sites. Specifically, we i) quantified the SOM stocks at such sites, ii) characterised SOM and its physical and chemical fractions and iii) estimated the age range of the bulk soil and stable C-fraction (radiocarbon dating). In south-eastern Switzerland, two areas above the timberline and one below the timberline (where isolated permafrost was verified) were investigated in detail. At each site, the experimental set-up consisted in the comparison of nearby soils that were either influenced or not by permafrost. The C-stocks (down to the C horizon or rock surface) did not show a significant difference between permafrost and non-permafrost soils and were in the same range of 10–15 kg/m2 in alpine (grassland) and subalpine (forest) sites. Above the timberline, the bulk SOM showed a distinct higher age at permafrost sites compared to non-permafrost sites. This higher age was even more evident in the stable C-fraction (resistant to an H2O2 treatment), where ages of up to 11 ky in permafrost soils were recorded. The highest age obtained in the stable C-fraction in non-permafrost soils was around 4 ky. Consequently, climatic conditions and the occurrence of discontinuous permafrost resulted in a very low turnover rate of SOM. At the subalpine site, the difference between permafrost and non-permafrost sites was less. At both sites (alpine and subalpine), DRIFT (Diffuse Reflection Infrared Fourier Transform) was used to determine the functional groups in the bulk soil and in the stable C-fraction. In general, the stable C-fraction had a different composition compared to the bulk SOM at non-permafrost sites; this was mostly not the case at the permafrost sites. This confirms that different decomposition processes occur between permafrost and non-permafrost sites. Furthermore, permafrost sites accumulated more the low-density physical fractions of SOM that are potentially easily degradable. The obtained results suggest that a warmer climate may not necessarily lead to an increased CO2 release from SOM-degradation in permafrost soils compared to non-permafrost soils. High-alpine soils and OM furthermore integrate a multi-facetted response to the past and ongoing surrounding conditions. The melting of permafrost will most likely enhance vegetation growth, which to a certain degree will probably compensate for carbon losses on the long-term.