Résumé
The elevational patterns of plant and animal diversity have been extensively described in ecological theory. However, how soil-temperature gradients and spatial heterogeneity of the forest and its associated root, soil and litter properties affect the diversity of soil microbial communities remains unclear. We studied the effects of a soil-temperature gradient and spatial heterogeneity of forests on taxonomic and phylogenetic patterns of bacterial and fungal communities in plant roots, rhizosphere and bulk soil. Soil samples were collected in closed (tree islands) and open (gaps) forest at three altitudes in the French Alps (1400, 1700 and 2000 m). Samples were separated into three fractions: plant roots (at the community level, as roots were not sorted by species), rhizosphere and bulk soil. Bacterial (16S) and fungal (ITS) diversity was determined, along with soil aggregate stability, litter and root traits. Phylogenetic clustering processes were observed for bacterial but not for fungal communities. Forest spatial heterogeneity significantly influenced taxonomic and phylogenetic diversities and divergence (phylogenetic relatedness) for bacterial communities. The soil temperature gradient and forest spatial heterogeneity affected fungal phylogenetic divergence, even though no over-dispersion or clustering processes were found. Soil aggregate stability was a significant factor explaining the bacterial and fungal communities’ structure and was correlated with bacterial taxonomic diversity and fungal divergence. Bacterial communities’ composition was better explained by root traits than in fungal communities, particularly for rhizosphere and root communities. Our findings highlight the importance of root traits describing the taxonomic and phylogenetic diversity of bacterial communities. Conversely, fungal communities did not show clustering processes and the variance of their structure was not significantly explained by the root or litter traits measured. Additionally, our results highlight the importance of spatial heterogeneity of forests to study the soil biodiversity of forests