The objective of the present study was to evaluate how altitudinal gradients shape the composition of soil bacterial and fungal communities, humus forms and soil properties across six altitude levels in Hyrcanian forests. Soil microbiomes were characterized by sequencing amplicons of selected molecular markers. Soil chemistry and plant mycorrhizal type were the two dominant factors explaining variations in bacterial and fungal diversity, respectively. The lowest altitude level had more favorable conditions for the formation of mull humus and exhibited higher N and Ca contents. These conditions were also associated with a higher proportion of Betaproteobacteria, Acidimicrobia, Acidobacteria and Nitrospirae. Low soil and forest floor quality as well as lower bacterial and fungal diversity characterized higher altitude levels, along with a high proportion of shared bacterial (Thermoleophilia, Actinobacteria and Bacilli) and fungal (Eurotiomycetes and Mortierellomycota) taxa. Beech-dominated sites showed moderate soil quality and high bacterial (Alphaproteobacteria, Acidobacteria, Planctomycetes and Bacteroidetes) and fungal (Basidiomycota) diversity. Particularly, the Basidiomycota were well represented in pure beech forests at an altitude of 1500 m. In fertile and nitrogen rich soils with neutral pH, soil quality decreased along the altitudinal gradient, indicating that microbial diversity and forest floor decomposition were likely constrained by climatic conditions.

Faculty of Natural Resources and Marine Sciences; Tarbiat Modares University Imam Reza Blvd 46614 356 Noor Mazandaran Iran

Human Genetics Research Centre Baqiyatallah University of Medical Sciences Vanak Square Shahid Nosrati Alley P O Box 1435916471 Tehran Iran

Laboratory of Environmental Microbiology Institute of Microbiology of the Czech Academy of Sciences Vídeňská 1083 Praha 4 14220 Czech Republic

Microbial Biotechnology Department Agricultural Biotechnology Research Institute of Iran Fahmideh Blvd P O Box 31535 1897 Karaj Iran

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