U.S. Department of the InteriorU.S. Geological Survey

What and Where AreBiological Soil Crusts?

Although the soil surface may look like dirt toyou, it is full of living organisms that are a vitalpart of desert ecosystems. This veneer of life is calleda biological soil crust. These crusts are foundthroughout the world, from hot deserts to polarregions. Crusts generally cover all soil spaces notoccupied by green plants. In many areas, theycomprise over 70% of the living ground cover andare key in reducing erosion, increasing waterretention, and increasing soil fertility. In most dryregions, these crusts are dominated by cyanobacteria(previously called blue-green algae), which are oneof the oldest known life forms. Communities of soilcrusts also include lichens, mosses, microfungi,bacteria, and green algae.

These living organisms and their by-productscreate a continuous crust on the soil surface. Thegeneral color, surface appearance, and amount ofcoverage of these crusts vary depending on climateand disturbance patterns.

Immature crusts are generally flat and the colorof the soil, which makes them difficult to distinguishfrom bare ground. Mature crusts, in contrast, areusually bumpy and dark-colored due to the presenceof lichens, mosses, and high densities ofcyanobacteria and other organisms.

Erosion ControlThe organisms in crusts protect soil from erosion

in a variety of ways. Some organisms, such ascyanobacteria and microfungi, protect themselvesfrom sharp sand grains by secreting sticky mucilagearound their cells. These microbes move throughthe soil when moistened, leaving the mucilagebehind as a trail. These mucilage trails glue soilparticles in place. Mosses and lichens function a bit

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Filamentouscyanobacteriamigrating out ofsheaths at950X

Cyanobacterialsheath clingingto sand grainsat 90X

differently. They cover and protect the soil surfaceas they grow in place, but they also have small root-like anchoring structures that penetrate into the soilsurface. The soil-binding action of crusts is notcompletely dependent on the presence of livingorganisms. Layers of abandoned material, built upover long periods of time, can still be foundclinging tenaciously to soil particles at depthsgreater than four inches (10 cm) in some soils. Soil loss due to rainfall and water movementis increased when cyanobacterial connectionsare broken. This is particularly problematicwhen the impact is in a continuous strip, suchas a vehicle or bicycle track, because channelsfor water flow are quickly formed, especiallyon slopes.

Water RetentionSoil crusts are important in the

absorption of rainfall. This function isespecially important in arid areas thatexperience sporadic, heavy rainfall.When it rains, the organisms andtheir mucilage absorb up to tentimes their volume in water andthen release the water slowlyinto the soil once the rain ends.In cold areas, biologicallycrusted soils frost-heave inwinter and create aroughened surface. Theroughness slowsrainwater runoff, whichincreases waterinfiltration into thesoil.

USGS Fact Sheet FS-065-01July 2001

Reprinted May 2002

Jayne BelnapUSGS Forest and Rangeland Ecosystem Science Center - Canyonlands Field Station2290 S. West Resource Blvd.Moab, UT 84532jayne_belnap@usgs.govwww.soilcrust.org

What Can You Do to Preserve a HealthyDesert Ecosystem?

Biking and driving: Stay on established roadsand trails. Protect trailside vegetation and soilsby searching for wide areas of slickrock whenpassing. At trailheads, do not go beyond piles ofrock or fences. These have been placed toprevent further damage.

Camping: Use designated sites wheneverpossible. Otherwise set up camp inareas where living crusts do notform, such as slickrock, sandybeaches, or under groves oftrees.

Hiking: Stay onestablished trails. Wheretrails do not exist, hike inwashes or on rocks.Consider volunteering fortrail-maintenance crews andlearn techniques forrepairing unmaintained trails.

Soil HealthCrust organisms contribute nutrients and organic

matter to desert soils. Because plant cover is sparse,crusts are an important source of organic matter fordesert soils. Organic matter is an important foodsource for organisms that live below the soil surfaceand who help keep nutrients available for plants bydecomposing plant litter. Cyanobacteria andcyanolichens contribute nitrogen to soils, which isespecially important in desert ecosystems wherenitrogen often limits plant growth. Indirect benefits tosoil health also are evident. Small soil particles oftenstick to trails of microbial mucilage. Soil nutrientsbind to these small particles and are then available toplants. One additional benefit of crusts is thatcyanobacteria secrete compounds that stimulate plantgrowth.

Human ImpactsMany human activities are harmful to biological

crusts. The crusts are no match for the compressionalstress caused by footprints of livestock and people orby the tires of vehicles. Because most living crustbiomass is concentrated in the top 1/8th inch (3 mm)of the soil, even small impacts can have profoundconsequences. Crushed crusts contribute less nitrogenand organic matter to the ecosystem and the soils areleft highly susceptible to both wind and water erosion.

Blowing sediment from disturbed areas can covernearby healthy crusts. Burial can mean death becausecrustal organisms need sunlight to live. In additionsand carried by wind sandblasts nearby crusts andplants, greatly increasing the erosive action of thewind. Windborne air pollutants from nearby urbanareas and coal-fired power plants also damage crusts.

RecoveryEven a single footprint has a long-lasting effect on

desert ecosystems: nitrogen fixation stops, andunderlying soil connections are broken. Damage doneto material underneath the surface cannot be repairedbecause the living organisms are only on the surface.Under good conditions, a thin veneer of cyanobacteriamay return in five years. Recovery may take up to 20years in places of higher rainfall and up to 250 yearsin places of lower rainfall, assuming an area is notagain disturbed.

Fact sheet designed by John Ledges at Ecopress

Partners contributing to second printing are:Bureau of Land Management Idaho State Office, Canyonlands National Park, Death Valley National Park, Joshua Tree National Park