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Biological Crusts
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Biological Soil Crusts

What They Are and A Few Interesting Facts

  • Ever walked across the rangeland and heard a little crunch under your feet? If so, you’ve just discovered the biological soil crust. It’s a complex mosaic of living organisms—algae, cyanobacteria (blue-green algae), bacteria, lichens, mosses, liverworts, and fungi—that grow on or just below the soil surface. Biological soil crusts are common worldwide in arid and semi-arid shrublands, grasslands and woodlands.
  • Biological soil crusts are highly variable in appearance. Which organisms dominate the crust is determined by several factors, including soil chemical and physical characteristics, and weather patterns. Biological soil crusts are distinguishable from bare soil by a bumpy appearance, forming sort of a mini-landscape on the soil surface complete with hills and valleys. They tend to be dark in color, especially when dry. However add a little moisture and you may be surrounded by vivid greens, yellows, and oranges as the mosses and lichens come out of dormancy and spring to life.
  • Biological soil crusts are known by lots of names:
    • Microbiotic
    • Cryptogamic
    • Cryptobiotic
    • Microphytic
    • Microfloral

Don’t get confused. The names all refer to the same thing

What They Are Not

Biological soil crusts are not physical or chemical crusts. Physical and chemical crusts are formed by different processes and tend to form a hard, impermeable layer on the soil surface. They also lack the biological characteristics that make biological soil crusts unique.

Why They Are Important

  • Biological soil crusts are, literally, a carpet of photosynthetic life. Remember when we talked about a whole living community being right under your foot? That is one way to describe a healthy biological soil crust. In addition, they are also habitat for fauna that, in turn, contribute to the development of the crust.
  • They stabilize the soil. Some of the organisms secrete sticky substances (polysaccharides), which hold soil particles together.
  • Biological soil crusts make the soil more fertile. Most of the organisms associated with the biological soil crust are photosynthetic, particularly during cold, wet seasons when most plants are dormant. This means that the biological soil crust increases the length of the time during which organic carbon is added to topsoil. In addition, some cyanobacteria and lichens fix atmospheric nitrogen, even during the winter. Biological soil crusts can make other nutrients more available for use by grasses, forbs, and shrubs, as nutrients adhere to the aforementioned sticky substances, and are prevented from leaching
  • Biological soil crusts may help the soil to retain more moisture. The extent to which this function occurs is highly dependent on both the composition of the crust and soil characteristics.
  • The nature of the crust itself can keep unwanted plants, such as exotic weeds, out. Some of these exotic weeds—which include cheatgrass and medusahead wildrye—can invade and dominate rangeland communities within relatively short time periods. Native plants, which evolved with biological soil crusts, may have developed mechanisms to allow seeds to penetrate the crust (e.g. small size, or structures that "drill" them into the crust). Seeds of some exotic species are quite large and may not have a way of getting through the crust.
  • Because of their functions in rangeland systems, biological soil crusts have been adopted by scientists and land management professionals in the U.S., Australia, and South Africa as a visible indicator of rangeland health.

Learn More!

Belnap, Jayne, Kaltenecker, Julie Hilty, Rosentreter, Roger, Williams, John, Leonard, Steve, and Eldridge, David. 2001. Biological Soil Crusts: Ecology and Management. USDI, Bureau of Land Mangement. Technical Reference 1730-2. Denver, CO. 110pp.

Harper, K. T., and J. R. Marble. 1988. A role for nonvascular plants in management of arid and semiarid rangelands in Vegetational Science Applications for Rangeland Analysis and Management. P. T. Tueller (ed.). Kluwer Academic Publishers. Dordrecht, Netherlands.

Johansen, J. 1993. Cryptogamic crusts of semi-arid and arid lands of North America. Journal of Phycology 29: 140-147.

Johnston, Roxanna. 1997. Introduction to microbiotic crusts. U.S. Department of Agriculture, Natural Resources Conservation Service, Soil Quality Institute and Grazing Lands Technology Institute. This is a non-technical information bulletin gear toward the public land user.

Ladyman, J.A.R. and E. Muldavin. 1996. Terrestrial cryptogams of the Pinyon-Juniper woodlands in the Southwestern United States: a review. USDA, USFS General Technical Report RM-GTR-280. Fort Collins, CO. 33 pp.

The Great Basin Naturalist Volume 53 No. 1, March 1993 (Proceedings of the Symposium on Soil Crust Communities). Contains multiple technical papers on biological soil crusts.

West, N.E. 1990. Structure and function of soil microphytic crusts in wildland ecosystems of arid and semi arid regions. Advances in Ecological Research 20: 179-223.

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Examples of
Biological Crusts

biological crust


biological crust


Biological crust dominated by
cyanobacteria and lichens


Physical soil crust


Soil particles
being held together by
filamentous cyanobacteria
Photo credit:
Jayne Belnap, USGS, BRD




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