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Introduction to Microbiotic Crusts United States
Department of
Agriculture
Natural
Resources
Conservation
Service
Soil Quality Institute
Grazing Lands
Technology
Institute
Introduction to
Microbiotic Crusts
July 1997 2
Foreword
Introduction to Microbiotic Crusts provides information on a soil-
associated component of many plant communities that has not
been widely recognized or characterized. The majority of the
research on these crusts is limited to the Great Basin and Colo-
rado Plateau regions of the United States. There is validity in
generalizing the basic functions of the crusts to wherever crusts
are found, given that their gross compositions are similar
(cyanobacteria, algae, mosses, lichens, etc.). However, it would
not be valid to estimate the general importance of these functions
in other regions because species composition does differ between
crusts, particularly within the larger components (i.e., lichens,
mosses)
(39). In addition, the plant composition and functions of
associated plant communities where crusts occur differ between
regions. Understanding the role of microbiotic crusts in total
resource management is an ongoing challenge.
This document was written by Roxanna Johnston, botanist, and
includes the comments of numerous reviewers.
Cover
Top photo - mature crust in the Colorado Plateau
Bottom photo - Area without crust
Credits: Jayne Belnap / USGS-Biological Research Division
More information is needed about the functions that crusts perform and the effect
of crust disturbance or elimination on the total plant community and production,
the soil and the environment.
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To file a complaint, write the Secretary of Agriculture, U.S. Department of Agriculture, Washington, D.C. 20250 or call
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Introduction to
Microbiotic Crusts
Microbiotic crusts are com-
monly found in semiarid and
arid environments throughout
the world. Areas in the United
States where crusts are a
prominent feature of the
landscape include the Great
Basin, Colorado Plateau
(19),
Sonoran Desert
(12), and the
lower Columbia Basin
(23).
Crusts are also found in
agricultural areas
(21), native
prairies
(36), and sandy soils
in Glacier Bay, Alaska
(42).
Outside the United States,
crusts have been studied in the
Antarctic
(13), Australia (33),
and Israel
(28), among other
locations. In fact, microbiotic
crusts have been found on all
continents and in most habi-
tats, leaving few areas crust
free
(39).
Microbiotic crusts are
formed by living
organisms and their
by-products, creating
a surface crust of soil
particles bound
together by organic
materials.
Many names and many forms
Microbiotic crusts are also known as cryptogamic,
cryptobiotic, and microphytic, leading to some
confusion. The names are all meant to indicate
common features of the organisms that compose
the crusts. The most inclusive term is probably
microbiotic
(38), referring to the small size of
the organisms and not limiting crust components
to plants. Whatever name used, there remains
an important distinction between these formations
and physical or chemical crusts.
Microbiotic crusts are formed by living organisms
and their by-products, creating a crust of soil
particles bound together by organic materials.
Chemical and physical crusts are inorganic
features, such as a salt crusts or platy surface
crusts.
Figure 1Utah
The general appearance of crusts in terms of color, surface topography and surficial
coverage varies in different regions.(Jayne Belnap / USGS-Biological Research Division) 4
Characteristics and
formation
Microbiotic crusts are formed
by living organisms and their
by-products, creating a surface
crust of soil particles bound
together by organic materials.
Aboveground crust thickness
can reach up to 10 cm
(39).
The general appearance of the
crusts in terms of color, surface
topography, and surficial
coverage varies (figs. 1-4).
Mature crusts of the Great
Basin and Colorado Plateau are
usually darker than the sur-
rounding soil. This color is
due in part to the density of the
organisms and to the often dark
color of the cyanobacteria,
lichens, and mosses. The
presence or absence of a crust
is partly determined by soil
texture and conductivity, pH,
moisture, and possibly tem-
perature
(15, 21, 22). Crust
coverage varies greatly, from
less than 10 percent to nearly
100 percent
(39).
Figures 2, 3, and 4
The general appearance of crusts in terms
of color, surface topography and surficial
coverage varies in different regions.
Fig. 2 Santa Barbara Island, California;
Fig. 3 southern Arizona;
Fig. 4 Salmon, Idaho)
(Figs 2 and 3: Jayne Belnap / USGS-
Biological Research Division.
Fig 4: Julie Kaltenecker/USDI-BLM) 5
Some crusts are characterized
by their marked increase in
surface topography, often
referred to as pinnacles or
pedicles
(3). Other crusts are
merely rough or smooth and flat
(22). The process of creating
surface topography, or
pinnacling, is due largely to the
presence of filamentous
cyanobacteria and green algae
(fig. 5). These organisms swell
when wet, migrating out of
their sheaths. After each migra-
tion new sheath material is
exuded, thus extending sheath
length. Repeated swelling
leaves a complex network of
empty sheath material that
maintains soil structure after the
organisms have dehydrated and
decreased in size
(7). A con-
tributing mechanism is frost
heaving and subsequent uneven
erosion, leaving soil mounds
bound by crust organisms.
Lack of frost heaving has been
used to explain the absence of
pinnacles in warmer regions
(39).
Figure 5
Pinnacles are formed by sheaths of cyanobacteria as they extend in length and bind soil
particles together. Frost-heaving also causes sheath-bound particles to rise.
(Jayne Belnap / USGS-Biological Research Division) 6
Composition
Microbiotic crusts are predomi-
nantly composed of
cyanobacteria (formerly blue-
green algae), green and brown
algae, mosses, and lichens (figs.
6-8). Liverworts, fungi, and
bacteria can also be important
components. Cyanobacteria or
green algae make up a large
component of microbiotic
crusts in semiarid and arid
regions of the United States.
Glossary
algae
nonvascular photosynthetic plant-
like organisms, they are informally
divided into groups by their
dominant pigments (i.e., green,
brown, red, etc.).
bacteria
microscopic, single celled
organisms.
cyanobacteria
photosynthetic bacteria formerly
called blue-green algae, their
growth forms tend to be filamentous.
fungi
nonphotosynthetic multicellular
organisms that are either
saprophytic or parasitic.
hyphae
single strands of a fungus.
lichen
a composite plant consisting of fungi
living symbiotically with algae or
cyanobacteria.
liverworts and mosses nonvascular
plants of small stature, the two are
similar with the exception of
reproductive methods.
rhizines/rhizoids root-like structures of lichens
and mosses respectively, they
are used for attachment.
sheaths
external coating formed by some
filamentous cyanobacteria, those
discussed in the article are formed
from polysaccharides. 7
In the Great Basin and the
Colorado Plateau,
Microcoleus
vaginatus (a cyanobacteria)
composes the vast majority of
the crust structure
(10, 3).
Lichens of the genera
Collema
spp. and mosses from the
genera
Tortula spp. are also
common
(3, 4, 26). In hot
deserts, such as the Sonoran,
Schizothrix species (another
cyanobacteria) are more com-
mon
(12). Lower Columbia
Basin crusts tend to be domi-
nated by green algae
(23).
Shifts between green algal and
cyanobacterial dominance have
been attributed to changes in
pH, with decreasing alkalinity
(pH) favoring green algae
(23,
27). Crusts from other regions
can be dominated by lichens
and/or mosses. The organism
that dominates the crust is
partly determined by microcli-
mate and may also represent
different successional stages
(39).
Figures 6, 7, and 8
Microbiotic crusts may include
cyanobacteria, green a