Diversity and spatial distribution of soil diatoms along a natural altitudinal gradient in the High Atlas (Morocco)

Microscopic algae especially diatoms (Class Bacillariophyceae) are an important component of biological soil crusts (or biocrust) communities in arid and semi-arid environments. In Morocco, a largely arid and semi-arid African Mediterranean country, the soil diatom diversity and ecology remain unexplored, unlike aquatic diatoms. This study aims to understand the spatial distribution of soil diatoms and to investigate the effects of soil properties and land use on their diversity and taxonomic composition in the region of Marrakesh (Morocco). Five sampling sites with different land use and vegetation cover were selected along a natural altitudinal gradient in the Ourika watershed (from the humid mountain highlands of Oukaimeden at 2634 m (High Atlas) to the arid Haouz plain at 765 m). Biological soil crusts were taken and soil physicochemical characteristics (soil moisture and texture, pH, electrical conductivity, total organic carbon, ammoniacal nitrogen, available phosphorus) were measured at five locations and two seasons (February and May). The spatial distribution of soil diatom communities and associated influencing factors were investigated via statistical analysis using Canonical Correspondence Analysis (CCA). A total of 27 diatom taxa belonging to 16 genera were recorded. The results showed that the distribution of soil diatoms was mainly influenced by altitude, bioclimate, substratum type, land use and vegetation cover. These environmental factors induce spatial variability in the soil physicochemical properties. Diatom composition and species richness were correlated with soil moisture, pH, texture, conductivity, nutrient and organic matter content. Diatoms showed higher species richness in siliceous soils of high altitudinal humid grassland dominated by acidophilous and neutral taxa with a preference for low conductivity. In contrast diatom communities of forests and sedimentary lowland arid steppe were relatively species poor, dominated by typically alkaliphilous and halophilous soil species. This exploratory survey provides a first account of the diatom assemblages of biological soil crusts over an altitudinal range in Morocco, necessary for understanding their structure and ecological function, including improving soil condition and sustainable fertility management.