Soil and climatic conditions as well as land cover and land management have been shown to strongly impact the structure and diversity of the soil bacterial communities. Here, we addressed under a same land cover the potential effect of the edaphic parameters on the soil bacterial communities, excluding potential confounding factors as climate. To do this, we characterized two natural soil sequences occurring in the Montiers experimental site. Spatially distant soil samples were collected below Fagus sylvatica tree stands to assess the effect of soil sequences on the edaphic parameters, as well as the structure and diversity of the bacterial communities. Soil analyses revealed that the two soil sequences were characterized by higher pH and calcium and magnesium contents in the lower plots. Metabolic assays based on Biolog Ecoplates highlighted higher intensity and richness in usable carbon substrates in the lower plots than in the middle and upper plots, although no significant differences occurred in the abundance of bacterial and fungal communities along the soil sequences as assessed using quantitative PCR. Pyrosequencing analysis of 16S ribosomal RNA (rRNA) gene amplicons revealed that Proteobacteria, Acidobacteria and Bacteroidetes were the most abundantly represented phyla. Acidobacteria, Proteobacteria and Chlamydiae were significantly enriched in the most acidic and nutrient-poor soils compared to the Bacteroidetes, which were significantly enriched in the soils presenting the higher pH and nutrient contents. Interestingly, aluminium, nitrogen, calcium, nutrient availability and pH appeared to be the best predictors of the bacterial community structures along the soil sequences.

International audienceThe use of next-generation DNA sequencing methods, which produce massive volumes of data, has transformed fungal molecular ecology. These technologies offer the opportunity to describe microbial communities in depth and to investigate fungal diversity at a large geographical scale. In particular, due to the emergence of studies conducted at a large scale (countries, continents), the definition of a compromise between the optimal and representative description of local diversity (plot scale) and the management of a greater number of sites for these studies is required. This work was performed to explore the local richness and the structure of the ectomycorrhizal (EcM) fungal and oomycete communities in a temperate beech plot using 454 pyrosequencing technology. The internal transcribed spacer 1 (ITS1) region was amplified and sequenced from fine roots and soil sampled from the two upper horizons. Our analyses suggest that soil could be a good substitute for fine roots in studying EcM fungi. The fungal assemblage exhibited a vertical distribution in the soil profile. Only four Pythiaceae were identified, which was insufficient to study the spatial distribution of this group. The fungal and particularly the ectomycorrhizal (EcM) richness was not correlated with any soil variable (pH, C, N or P), but the spatial structure was explained by the C:N ratio and phosphorus content. Our data reveal the importance of maintaining a minimal representative sampling at the plot scale for comparative geographical studies

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