Weber State University      Department of Botany

                                                    BOTANY  LS1203 - PLANT  BIOLOGY

Cryptobiotic Soil Crusts

also called Microbiotic Crusts or Cryptogamic Soils

Mostly composed of cyanobacteria, lichens, mosses, and algae.  
Might also have fungi or liverworts.
The exact species compositions of the crusts vary from place to place, as do the vascular plant communities that grow in the crusts.

Cryptobiotic crusts are very important ecosystems in arid and semi-arid regions.  In the USA, these crusts have been studied in the Colorado Plateau, the Great Basin, and the Sonoran Desert.
These areas include portions of NV, UT, CO, NM, and AZ.

The crust that forms can be up to 10 cm thick, with the still living organisms in the upper few mm of the soil.

The cryptobiotic crust can account for >70% of the ground cover in arid and semi-arid environments, with vascular plants providing the remainder.
The organisms of the crust increase the stability of the soil, decreasing soil erosion.
The organisms increase the organic matter in soils.
Free-living cyanobacteria and cyanobacteria living as lichens with fungal partners increase the available nitrogen content of the soils by nitrogen fixation.
The availability of other soil minerals improves.
The amount of available water also improves.

Most of the cyanobacteria found in cryptobiotic crusts are filamentous.  As they grow, they are covered by a sheath of mucous.  As the cyanobacteria move through the soil, they leave a trail of the mucous as they go (think snail trail).  The mucous acts like a glue to hold soil particles together.  Clumps of soil resist wind and water erosion better than the small particles do.  Even after the cyanobacteria die, the mucous is still in the soil, holding the particles together.

The filaments of green algae and the hyphae of the fungi also hold the soil particles in place, as do the rhizoids of the lichens, mosses, and liverworts.

Binding of the soil particles increases the amount of P available as this mineral is often high in soil fines.

The organic carbon put into the soils as the cryptobiotic organisms die improves the water and mineral holding capacity of the soil:  fewer minerals are leached from the soil when it rains.

The nitrogen fixation is a very important contribution by the cyanobacteria.  They serve as the primary source of nitrogen in the Colorado Plateau.

The crust organisms are very good at absorbing water from dew and water vapor; they are also good at retaining water even under very low relative humidity.  Thus, they slow water evaporation from the soil surface, benefitting the vascular plants that grow in the crusts.

The crusts are highly susceptible damage when they are disturbed by hooves, feet, and vehicles (motorized or not).  The damage is worse when the soils are dry because of the brittleness of the living organisms when they are dry.   (The organisms in the crusts survive drying due to a mechanism called poikilohydry.)
The organisms are only active when wet.  Therefore, in an arid environment, re-establishment of the crust is slow.  In southern Utah, the estimated recovery time for lichens is 45 years and for mosses is 250 years.
Not only are the crust-free soils now subject to erosion, the soil particles now removed from these areas can cover adjacent crusts that were not disturbed.
The crusts can suffer substantial damage, even when there is no apparent damage to the vascular plant vegetation.

As a result of damage to the crusts:
runoff increases
soil loss increases
if runoff brings in N, the presence of that N will decrease natural nitrogen fixation
often, rangeland shrubs increase; some are allelopathic for nitrogen fixing bacteria

Review

In what types of habitats do you find cryptobiotic soil crusts?  In which US states have they been most extensively studied?
What are the main types of organisms found in cryptobiotic soil crusts?
What specific advantages do the crust organisms provide to their ecosystem?  What specific features of the organisms contribute to their ability to provide these advantages?  (For example, what are the various factors provided by the cyanobacteria?)

What is poikilohydry?  How does it contribute to the survival of organisms in soil crusts?

What are the possible consequences of damage to cryptobiotic soil crusts?  What are estimated recovery times for damage to crusts in southern Utah?

Links

The Microbial World: Cyanobacteria   (by Jim Deacon at the University of Edinburgh)

Cryptobiotic Crusts  (The National Park Service)

An Introduction to Biological Soil Crusts  (from the USGS Canyonlands Field Station)

Return to Botany 1203 Home Page.

Return to Botany 1203 Syllabus

Return to Harley Home.

6 September 2002.  Links checked 20 March 2011.