We examined how biological soil crust (BSC) communities are affected by micro-geomorphology and soil characteristics in an arid ecosystem in northeastern Iran. Sampling was carried out systematically in the summer of 2016 along a geomorphic gradient within an alluvial fan by using micro-scale plots (0.25 m2) and soil samples from the top soil layer (0-5 cm). According to the geomorphologic features and particle size distribution, the landform surfaces were divided into three units across the topographic gradient. From top downstream: Unit 1 involved coarse particles, Unit 2 included medium, and unit 3 comprised fine deposits., A total of 16 samples were taken for each unit (48 samples in total) along the alluvial fan from the apex to the base sector. The results indicated that micro-geomorphic and soil characteristics play an important role in the development of biological soil crust (BSC) micro-habitats. Decreasing content of calcium carbonate, pH, and soil salinity versus increasing soil moisture and clay content along the gradient of the alluvial fan showed a relevant correlation with increased BSCs coverage. BSCs increased along the landform gradient, although their diversity tended to decrease; in that way complex communities in the apex (Unit 1) involved cyanobacteria, lichen, moss-es, and algae, while the dominant BSCs in the base (Unit 3) included moss species.

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