|
CRYPTOBIOTIC SOIL
Introduction
Cryptobiotic soil is found throughout the world. In arid regions,
these living soil crusts are dominated by cyanobacteria, and also
include soil lichens, mosses, green algae, microfungi and bacteria.
These crusts play an important role in the ecosystems in which
they occur. In the high deserts of the Colorado Plateau (which
includes parts of Utah, Arizona, Colorado and New Mexico), these
knobby black crusts are extraordinarily well-developed, and may
represent 70 to 80 percent of the living ground cover.
What Are Cyanobacteria?
Cyanobacteria, previously called blue-green algae, are one of
the oldest known life forms. It is thought that these organisms
were among the first land colonizers of the Earth's early land
masses, and played an integral role in the formation and stabilization
of early soils. The earliest cyanobacteria fossils found are called
stromatolites, which date back more than 3.5 billion years. Extremely
thick mats of these organisms converted the earth's original carbon
dioxide rich atmosphere into one rich in oxygen and capable of
sustaining life.
Cyanobacteria occur as single cells or as filaments. The most
common form found in Colorado Plateau soils are the filamentous
type, which are usually surrounded by sticky, mucilaginous sheaths.
When moistened, cyanobacteria become active, moving through
the soil and leaving a trail of sticky material behind. The
sheath material sticks to surfaces such as rock or soil particles,
forming an intricate web of fibers throughout the soil. In this
way, loose soil particles are joined together, and an otherwise
unstable surface becomes very resistant to both wind and water
erosion.
The soil-binding action is not dependent on the presence of
living filaments. Layers of abandoned sheaths, built up over
long periods of time, can still be found clinging tenaciously
to soil particles, providing cohesion and stability in sandy
soils at depths up to 10cm.
Nitrogen fixation is another significant capability of cyanobacteria.
Vascular plants are unable to utilize nitrogen as it occurs
in the atmosphere. Cyanobacteria are able to convert atmospheric
nitrogen to a form plants can use. This is especially important
in desert ecosystems, where nitrogen levels are low and often
limiting to plant productivity.
The sheaths have other functions as well. When moistened,
they swell up to ten times their dry size. This ability to intercept
and store water benefits both the crustal organisms as well
as vascular plants, especially in arid regions with sporadic
rainfall.
Sheaths, and the organisms they surround, also contribute
organic matter and help make essintial nutrients available to
vascular plants. Negatively charged clay particles, often found
clinging to the sheaths, bind positively charged nutrients,
preventing them from being leached out of the upper soil horizons
or becoming bound in a form unavailable to plants. Like soil
stability, this function is not dependent on the presence of
living filaments, but only the presence of sheath material.
Environmental Impacts
Unfortunately, many human activities are incompatible with the
presence and well-being of cryptobiotic soils. The fibers that
confer such tensile strength to these crusts are no match for
the compressional stress placed on them by footprints or machinery,
especially when the crusts are dry and brittle.
Air pollutants, both from urban areas and coal-fired power
plants, also harm these crusts.
Tracks in continuous strips, such as those produced by vehicles
or bicycles, are especially damaging, creating areas that are
highly vulnerable to wind and water erosion. Rainfall carries
away loose material, often creating channels along these tracks,
especially when they occur on slopes.
Wind not only blows pieces of the pulverized crust away, thereby
preventing reattachment to disturbed areas, but also disturbs
the underlying loose soil, often covering nearby crusts. Since
crustal organisms need light to photosynthesize, burial can
mean death. When large sandy areas are impacted during dry periods,
previously stable areas can become a series of shifting sand
dunes in just a few years.
Impacted areas may never fully recover. Under the best circumstances,
a thin veneer of cryptobiotic soil may return in five to seven
years. Damage done to the sheath material, and the accompanying
loss of soil nutrients, is repaired slowly during up to 50 years
of cyanobacterial growth. Lichens and mosses may take even longer
to recover.
Crypto Tips
- Stay on existing trails.
- When you must go off trail, walk in wash bottoms or on
slickrock.
- Camp on slickrock or in previously disturbed areas.
- Go out of your way, literally, to avoid cryptobiotic
soil.
More information
More information is available on the Internet at www.soilcrust.org.
|