Soil organic matter formation along a chronosequence in the Morteratsch proglacial area (Upper Engadine, Switzerland)

Abstract

Global warming leads to the melting of ice caps and glaciers and, consequently, the exposure of new areas of land to the atmosphere and weathering. These areas usually have a high reactivity to both biotic and abiotic changes. Proglacial areas in the Alps usually have a deglaciation time span of around 150 years (time since the end of the “Little Ice Age” in the 1850's). We investigated a chronosequence of very young soils in the proglacial area Morteratsch (Swiss Alps) to derive time-trends of soil organic matter accumulation and evolution. Total organic C and N contents, C and N contents of the various organic matter (OM) density fractions and of the labile (oxidised by H2O2) and stable (H2O2-resistant) fractions were measured. Further characterisation of OM and the various fractions was performed using Diffuse Reflection Infrared Fourier Transform (DRIFT). Soil organic matter has been accumulated over 150 years at very high rates, values lay between 7 and 36 g C/m2/year. This led to a soil organic matter abundance of about 1–5.5 kg C/m2 after 140 years. Even at the start of soil formation, a very stable fraction of soil organic matter was detectable. Stable organic matter (resistant to the H2O2 treatment) comprised about 6% of the total soil organic carbon and 10% of the total nitrogen. At the start of soil formation, a very high proportion of soil organic matter was present in the density fractions < 1.6 g/cm3. After about 140 years, 15% of soil organic carbon and 35–40% of the nitrogen was already present in the highest density fraction (> 2 g/cm3). With time, the quality of soil organic matter changed: a decrease of hydrophobicity, an increase in aromatic compounds in the bulk soil and a decrease in phenolic functional groups in the heaviest density fraction were detectable with increasing age. In general, stable organic matter as well as the density fraction > 2 g/cm3 had a low C/N ratio and were enriched in proteinaceous materials. The adsorption of proteinaceous materials points to a strong organo-mineral association. This process has existed since the very beginning of soil formation.