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Within a landscape, multiple habitats exist for soil microbial communities. But how these habitats shape community composition requires an understanding of the way in which microbial diversity is impacted across a broad range of spatial scales. Mountain ecosystems are excellent systems to study microbial communities, because a multitude of climate and soil variables change within a relatively small distance. We investigated microbial community structure in bulk and rhizosphere soils beneath three plant species, Vaccinium myrtillus, Juniperus communis and Picea abies, that structure local plant communities along an elevation gradient in the French Alps. We examined the impact that climate, soil properties, plant diversity and plant root chemical and morphological traits had on microbial α‐ and β‐diversities. The most abundant bacterial phyla detected in both bulk and rhizosphere soils were Proteobacteria, Actinobacteria, Acidobacteria and Verrucomicrobia. Along the elevation gradient, bacterial phyla did not display a clear distribution pattern between bulk and rhizosphere soils. For fungi, dominant phyla were Ascomycota and Basidiomycota, and contrasting distribution patterns were found between bulk and rhizosphere soils. Overall, bacterial and fungal α‐diversity responded differently to elevation as well to soil compartments (bulk versus rhizosphere soil), revealing no significant patterns in bulk soil beneath any of the structuring plant species, but increasing in the rhizosphere compartment of P. abies just below the treeline. Changes in bacterial β‐diversity with elevation were related mostly to soil physical and chemical properties. Bacterial and fungal α‐diversity in rhizosphere communities were more related to plant species identity, vegetation diversity and belowground plant traits compared to soil properties, whilst the opposite was found for bulk soil. Our results highlight that environmental changes at the landscape scale (e.g. associated to elevation, soil properties or climate), impact significantly soil microbial communities, but vegetation refines communities at a local scale via the rhizosphere niche. [ABSTRACT FROM AUTHOR]

Global warming may accelerate nitrogen (N) transformations in the soil, with potentially large effects in N-poor high-elevation ecosystems. To gain insight into the partitioning of inorganic and organic N inputs within the plant-soil system and how warming influences these patterns, we applied a 15N label (15 NH4Cl or 15N-glycine) shortly after snowmelt during the sixth year of experimental soil warming (+4 °C) at treeline in the Swiss Alps., Seven weeks after labelling, approximately 60% of the applied label remained in the soil organic layer to 10 cm depth, whereas label recovery summed over all measured plant pools was <10% of the added label. Soil warming led to a weaker Δ15N signal in plants and no change in the amount of added label recovered in plants. This 15N dilution resulted from a greater N pool size of some plant species in warmed plots as well as enhanced availability of (unlabelled) N under warming. Temporal dynamics of foliar Δ15N in dominant dwarf shrub species suggested that these plants primarily take up N early in the season. In a subset of plots labelled with 13C-enriched glycine (U-13C2-15N-glycine), the labelled glycine was mineralized rapidly, with approximately 50% of the applied 13C respired as CO2 during the first 99 h, suggesting that effects of warmer soils on N dynamics in this treeline system are only slightly modulated by the preferences of different plant species for inorganic and organic N forms., Synthesis. Plants growing in warmer soils acquired more unlabelled, soil-derived N in the sixth year of treatment, implying a sustained increase in N mineralization and availability in alpine treeline ecosystems with higher soil temperatures predicted for the future. Wide variation in the ability of plant species and functional groups to compete for early-summer N inputs means that there is a feedback between plant community shifts and N dynamics under environmental change at the treeline. [ABSTRACT FROM AUTHOR]