Soils of arctic and alpine ecosystems sequester large amount of organic carbon, and the impact of climate forcing on these carbon stocks is a key issue in global carbon cycle studies. However, our understanding of the factors controlling Soil Organic Matter (SOM) dynamic remains incomplete. In alpine ecosystems, topographical variations and its associated changes in snow cover duration have long been recognized as the primary source of ecosystem structure and functioning through direct (i.e. edapho-climatic effects) and indirect effects (i.e. sorting of plant and microbial species). The aim of this work was to identify the main factors controlling SOM dynamics in late (LSM) and early (ESM) snowmelt locations. At the end of September 2008, we sampled the 0-10 cm soil layer and the aboveground plant biomass in LSM and ESM locations in the Vallon de Combe Roche Noire (Massif du Galibier – Hautes-Alpes) at 2550 m. Concentrations of polyphenol, tannin and lignin were measured in soil and vegetation. Soil chemical analyses further included elementary composition, quantification of C compounds adsorbed to the mineral surfaces, and C and N concentration in soil organic matter density fractions. Compared to LSM, soils from ESM were characterized by a higher C and N concentration. Allocation of SOM to micro and macro-aggregates did not differ significantly between both locations. In the dense fraction from the ESM soils a lower percentage of carbon was lost upon demineralisation and was therefore not intimately associated with the mineral phase. There was a significant positive correlation between soil C and N and soil polyphenol concentrations whereas no relationships was detected with polyphenols in the vegetation. SOM had a similar lignin contribution in both locations. These results suggest that the presence of higher amounts of SOM in ESM locations is not related to stabilization processes through sorption onto soil mineral, selective preservation or physical protection through aggregate formation. Rather, it seems that the larger amount of polyphenols may have promoted SOM accumulation in ESM. With plants producing similar polyphenol amounts in EMS and LMS, the reason why polyphenols accumulate to higher levels in ESM soils will desserve further studies.

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