Understanding the functioning of soils (i.e. carbon and nitrogen pools) requires a description of the local microorganisms and their metabolism. We characterized the soil microbial communities and their dynamics of two contrasted alpine meadows, snowbed combs and wind-exposed ridges. These weakly anthropized, high altitude ecosytems (2400 m) differ in snow cover duration. Snowbed combs show an 8-10 week delay in snowmelt and spring soil warming. This snow cover maintains soil around 0°C during winter (allowing microbial activity and thus, litter decomposition), whereas wind-exposed ridge soils, without snow cover, undergo -5 to -10°C freezing. Consequently, plant communities of these two ecosystems are entirely different, dominated by fast growing species in the first ecosystem, and by stress tolerant species in the second one. The seasonal successions of microbial communities are therefore shaped by markedly contrasted biotic and abiotic factors. Soils samples were harvested at different seasons, and DNA sample pools were used to amplify a polymorphic segment of the conserved ribosomal DNA using kingdom specific primers. We improved the SSCP method and used it as a fast, highthroughput way of community fingerprinting. Microbial communities\\\\\\\\\\\\\\\' fingerprints were analyzed by computing pairwise distances between normalized electrophoregrams and generating NJ trees. Tree analysis revealed good sampling reproducibility, a seasonal structure of microbial communities, and notable divergences between the two ecosystems at all seasons, except in spring. We then cloned and sequenced rDNA PCR products to ascribe a phylogenetic basis to these variations, and confirmed the large differences in microbial community structure between the two ecosystems. We also were able to identify several microbial clusters that vary widely along the seasonal successions. High-throughput taxonomic sequencing, mesocosm experiments, and metatrancriptomic approaches are now under way to understand the kinetics, extent and biochemical processes of litter degradation in periodically snow covered soils.

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