Microbiotic Soil Crust
DB: This is Earth and Sky. If you go for a hike in Utah’s Arches National Park, the park ranger will probably tell you to be careful where you step.
JB: That’s because the bare soil in this arid desert supports what scientists call a “biological crust.” A soil crust is a complex, living community. The exact composition varies from place to place, but a soil crust can include algae, bacteria, lichens, mosses, fungi, liverworts, and, especially, cyanobacteria – the organisms formerly known as blue-green algae.
DB: Soil crusts form in dry climates, where other plants ? grasses, shrubs and trees – may be scarce. They play a very important role in the ecosystem. Soil crusts prevent erosion. They act like a sponge, to retain precious rainwater. And they make the soil more fertile.
JB: But soil crusts are fragile. A footstep can kill the micro-organisms. Then erosion begins. Soil crusts have been damaged by cattle, hikers, or off-road vehicles. And recovery is slow. General Patton sent troops to practice maneuvers in the Mojave Desert 60 years ago. The tank tracks still look fresh, and experts say it may take a thousand years for the soil crust to recover.
DB: Today’s program was made possible by the National Fish and Wildlife Foundation, in cooperation with the U.S. Forest Service. We’re Block and Byrd for Earth and Sky.
Our thanks to the following individual who assisted in the preparation of this script:
Jayne Belnap
Research Ecologist
US Geological Survey
Moab, UT
The following articles were used in preparing this script:
“In fear of mountain bike impact.” Malaysia: New Straits Times, June 6, 1995, p. 29.
Abstracts:
Belnap, Jayne. “Surface disturbances: Their role in accelerating desertification.” Environmental Monitoring and Assessment, 37: 39-57, 1995.
Lange, Otto L., Jayne Belnap, Hans Reichenberger, and Angelika Meyer. “Photosynthesis of green algal soil crust lichens from arid lands in southern Utha, USA: Role of wataer content n light and temperature responses of CO-2 exchange.” Flora Jena 192: 1-15; 1997.
Garcia, Pichel Ferran, and Jayne Belnap. Microenvironments and microscale productivity of cyanobacterial desert crusts. Journal of Phycology, 32: 774-782, 1996.
West, N.E. “Structure and function of soil microphytic crusts in wildland ecosystems of arid and semi-arid regions. Advances in Ecological Research 20: 179-223.
Web sites to visit:
Jayne Belnap’s web site. Click on “Crusts 101” for basic info, or select “Gallery” to see pictures of different kinds of soil crusts
Author’s notes:
You may be more familiar with physical or chemical soil crusts than you are with biological soil crusts. Chemical and physical crusts form when soil that is rich in clay or salt dries up. In contrast, biological crusts are made up of living organisms.
Scientists have a number of different names for biological soil crusts. They are referred to as microbiotic crusts (because the life forms are so small); cryptogamic soil crusts (same reason); or even microphytic or microfloral crusts. Soil crusts can be as much as 10 centimeters thick and often have a bumpy appearance.
Soil crusts tend to form in areas where the climate is dry and the “vegetative cover” is sparse. So you often find soil crusts in desert habitats; you don’t often find soil crusts in the eastern United States, where rainfall is plentiful, although you will see them on eastern mountaintops, above the treeline, or in the New Jersey Pine barrens, where vegetation is sparse.
Different kinds of crusts made up of different organisms form in different places, depending on whether the environment is arid or just semi-arid, depending on the physical and chemical characteristics of the soil (Big particles? Small particles? Mostly clay? Mostly sand?) and also depending on whether winter is cold enough to bring a cycle of freezing and thawing.
Where soil crusts are damaged, more water runs off the surface of the land after a rain event, so that soil loss through erosion can increase dramatically. As the soil blows around, nearby developed areas may experience dust storms.
The soil fertility also decreases when a crust is damaged, in two ways. Intact soil crusts are sticky?they catch and hold windblown dust particles, which act like fertilizer. So without an intact crust, the soil doesn’t get this nutrient boost. Also, without the nitrogen-fixing action of the crust, nearby vascular plants?trees, grasses, and shrubs?will have a hard time getting enough nitrogen. Experiments have showed that plants growing near damaged crusts have less nitrogen in their tissues than plants growing near intact crusts.
Cyanobacteria are usually be the first microorganisms to recolonize after a crust is damaged; moss and lichens take much longer to grow back in a damaged area. In moister systems, damaged crusts may recover in a few years to 50 years, but in dry environments, it may take 100 to 1000 years for a damaged crust to return to its original thickness and functioning.
Researchers have found one unusual benefit of soil crusts – they seem to discourage exotic and undesirable plant species from taking root. Specifically, the introduced grass called “cheatgrass,” which provides poor habitat for native wildlife, does not germinate successfully in places where the soil crust is intact. That’s because cheatgrass seeds need a light covering of loose soil to germinate?the soil retains moisture that the seeds need to sprout. However, in areas with intact soil crusts, there’s no loose soil lying around, so the seeds dry out before they can germinate.
Additional Teacher Resources
U.S. Geological Survey: Soil and Cryptobiotic Crusts in Arid Lands
Cryptobiotic crusts are important features of arid and semiarid ecosystems throughout the Southwest. Living soil crusts are found throughout the world, from the hottest deserts to the polar regions. In arid regions, these soil crust are dominated by the blue-green algae. This article explains the make up of this biological crust and the role it plays in the desert ecosystem.
U.S. Bureau of Land Management National Science and Technology Center: Biological Soil Crusts?What They Are and A Few Interesting Facts
This site is wonderfully set up for students. It covers what biological soil crusts are, what they do, and why they are important. It also contains links to more resources at the end of each page as well as accompanying photo images.
University of California, Berkeley: Introduction to the Cyanobacteria? Architects of Earth’s Atmosphere
An easy to understand scientific overview of cyanobacteria, or ?blue-green algae’. Most likely suited to secondary students this site contains links to various sites on cyanobacteria as well.
