Desert soil is mostly sandy (90-95%). It is very arid due to the strong wind erosion, sedimentation, daily temperature fluctuations, water deprivation and high levels of solar radiation. Most desert soils are classified as Aridisols (or less commonly Entisols), and possess very low nitrogen content and organic matter (often unevenly distributed), are slightly alkaline, and contain high amounts of salt ions, phosphate, calcium carbonate, and magnesium carbonate. Desert soils are variable in color but often light brown, gray, or yellowish. Life in the desert is profoundly challenged by these cumulative, harsh and abiotic conditions.
Biological soil crusts (or biocrusts) are assemblages of organisms on the soil surface of arid and semi-arid ecosystems; up to 70% of semiarid and arid zones are covered by biocrusts. They are composed of soil particles and filamentous cyanobacteria, lichens, mosses, and fungi in varying proportions. These consortia of soil organisms are present in a cohesive form, providing protection against water and wind erosion and enhancing the soil physical structure via the formation of soil aggregates. Biocrusts also help improving the water holding capacity of the soil and assist with nourishing the soil by capturing nutrient rich dust. In addition, carbon and nitrogen fixed from the atmosphere by microbes are released into the dry surrounding soils in forms that can be utilized by other organisms. The cohesive nature of biocrusts is due to the formation of a polysaccharide sheath on the soil surface by some of the microbial community members (e.g., filamentous cyanobacteria). Microbial activity in desert soils is highly dependent on temperature, moisture and the availability of organic carbon. Of these, it appears that moisture is the major constraint affecting soil microbial biodiversity, community structure and activity.
Did you know? There are 22 deserts in the world. More specifically, 19% (27.6×106 km2) of the global land is arid deserts and 14.6% (21.2×106 km2) is semi-arid deserts (excluding Antarctica). That’s one third of the Earth’s land surface. Estimates based on current global warming trends suggest that drylands will constitute more than half of land surfaces by the end of the century.
Typical biocrust types. (a) Light or thin cyanobacterial crust. Filamentous cyanobacteria (Microcoleus sp. ) dominate this crust, which is only a few millimeters thick. Patches of bare soil are visible. (b) Dark or thick cyanobacterial crust. Besides the filamentous species of Microcoleus, also coccoid species ( Nostoc colonies) and other filamentous cyanobacteria ( Phormidium ) form this up to 5 mm thick biocrust. (c) Crustose “rugose” cyanolichen biocrust. Different Collema species or other cyanolichens dominate this crust type, but free-living cyanobacteria and green algae occur as well. (d) Rugose moss crust. Moss stems grow mainly embedded within the uppermost centimeters of soil. Only the uppermost leaves or the fruiting bodies rise over the soil surface. (e) Rolling chlorolichen crust. Mainly crustose and squamulose chlorolichens on top of the soil with rhizines penetrating deep into the soil dominate this crust type. Other components like cyanobacteria or green algae are also free living in this biocrust. (f) Rolling “thick” moss crust. Up to 5 cm thick moss carpets and cushions with cyanobacteria and green algae living on top of or in between the stems. (g) Pinnacled crust. Turret-like structures are elevated over the ground surface where organisms prevent soil erosion.
Colesie et al (2016) Biological Soil Crusts: An Organizing Principle in Drylands (https://link.springer.com/chapter/10.1007%2F978-3-319-30214-0_9)