Cryptobiotic
soils, also known as biological soil crusts, or just biocrusts, are according to
Wikipedia:
“…communities of living organisms inhabiting the surface
of soils in arid and semi-arid ecosystems, which form stable aggregates of soil
particles in a thin layer millimeters to centimeters thick.[1] They are found
throughout the world with varying species composition and cover depending on
topography, soil characteristics, climate, plant community, microhabitats, and
disturbance regimes. An estimated 12% of Earth's surface is covered by
biocrusts.”
Biocrusts fix carbon and nitrogen and stabilize soil. They
affect soil albedo, water infiltration, and nutrient uptake in vascular plants.
They can be damaged by fire, recreational activity, and grazing and may take
decades to recover. Fungi, lichens, cyanobacteria, bryophytes (mostly mosses
and liverworts), and algae in varying proportions compose cryptobiotic soils.
Formation and succession of biocrusts are given below – from Wikipedia:
“Biological soil crusts are formed in open spaces between
vascular plants. Frequently, single-celled organisms such as cyanobacteria or
spores of free-living fungi colonize bare ground first. Once filaments have
stabilized the soil, lichens and mosses can colonize. Appressed lichens are
generally earlier colonizers or persist in more stressful conditions, while
more three-dimensional lichens require long disturbance-free growth periods and
more moderate conditions.”
Biocrusts are
prevalent in dry regions, holding the soil together and preventing erosion. If
these biological soil communities are damaged or degraded as has become more
common, the soil becomes less fertile and more susceptible to wind erosion.
Wikipedia
explains the hydration/dehydration cycling of cryptobiotic soils:
“The biological soil crust is an integral part of many
arid and semi-arid ecosystems as an essential contributor to conditions such as
dust control, water acquisition, and contributors of soil nutrients. Biocrust
is poikilohydric and does not have the ability to maintain or regulate its own
water retention. This causes the biocrust's water content to change depending
on the water in the surrounding environment. Due to biological soil crust
existing in mostly arid and semi-arid environments with the inability to hold
water, the crust is mainly dormant except for short periods of activity when
the crust receives precipitation. Microorganisms like those that make up
biological soil crust are good at responding quickly to changes in the
environment even after a period of dormancy such as precipitation.”
A biocrust is basically
an arid-region surficial soil biological community. According to Jude Coleman
in an August 2024 article in Knowable Magazine biocrusts can be quite variable
by region:
“…in desert areas with more moisture, like Moab, Utah,
biocrusts tend to feature mosses. In gypsum-rich soils, such as near Lake Mead,
Nevada, lichens take center stage. Some crusts feature all components, and in
other crusts, multiple components are missing. But regardless of their
community lineup, the crusts all serve as a living skin for desert land.”
Modeling suggests that biocrusts are at risk due to
temperature increases and rainfall changes. They are quite sensitive to warming
and to rainfall changes – either more or less.
Soil microbiology
is vast as many different species occur in different areas. New microbes are
found often. The microbes in arid regions are more limited. Biocrusts require a
plant-free surface or a plant litter-free surface so that they can access light
to photosynthesize. The study of biocrusts is relatively new. They can inform our
ideas about early life on land as some of these microbes were the earliest terrestrial
organisms. Scientists are now recognizing different types of biocrusts. Thye
can be differentiated based on soil texture, soil chemistry, and the type of
primary producer. According to Ferran Garcia-Pichel in a September 2023 paper
in Annual Review of Microbiology:
“…the most consequential distinction involves the type of
primary producers: cyanobacterial (or microalgal), lichen, and moss crusts are
the most important types. In terms of range in relative vertical scaling, the
differences are tantamount to those found among grasslands and boreal forests,
biomes hardly considered ecologically equivalent ( Figure 1 ). Some basic
properties are certainly shared among them: They all arm the soil surface
effectively against erosion (54), modify soil surface albedo (42, 144), interact
with soil hydrological properties (31, 46, 83), and increase soil organic
carbon content (44), but they do differ in some key biological properties; some
authors (34) have wisely warned that lumping all biocrusts together brings
about a tangible risk of oversimplification.”
Garcia-Pichel gives types as cyanocrusts, lichen crusts, and
moss crusts, based on the type of primary producer. The paper explores
biocrusts chemical activity including the fixing of carbon and nitrogen and
metabolic chemical reactions. It also explores effects on soil structure and water
retention and the different activities in wet and dry periods. There is a
succession pattern established that goes from cyanocrusts to moss crusts.
However, moss is more sensitive to drought and will not grow in drier areas. The
figures from the paper are shown below.
Scientists from
the U.S. Geological Survey (USGS) are studying cryptobiotic soils in Utah
basically by cultivating them. A property of biocrusts is that they are
totipotent, which means that pieces can be scattered to spread to new areas,
sort of like spreading seeds. The spreading takes time. The ability to
facilitate this could be a key to restoring degraded biocrusts and keeping them
and their ecosystem services – carbon and nitrogen storage and soil structure
stability. They found that they are more difficult to cultivate than hoped.
Sprinkling pieces of biocrust onto a fabric proved to be a solution. Unfortunately,
spreading fabric over large areas is not really feasible. Another thing they
are trying is transporting biocrusts from drier regions to less dry regions to
study any changes.
Recent research has shown that the parts of the Great Wall of China built with rammed earth have been better preserved due to the formation of biocrusts as shown below. This is another example of humans engineering biocrust, albeit in this case it as accidental.
The USGS outfitted
unmanned aerial systems (UAS) to map biocrusts in the U.S. West. These can also
be used to remote monitor for damage which can come from grazing animals, recreation,
and energy development.
The goal of biocrust restoration is to re-establish ecosystem function and build climate change resilience across ecologically disturbed drylands, according to the USGS scientists. Biocrusts are key to any dryland restoration efforts. Another type of biocrust restoration they are trying involves the use of a liquid cyanobacterial slurry to disperse inoculum for biocrusts on a larger scale. They want to try this method for restoring biocrust damaged by oil & gas development in the Southwest, areas deemed too large for dry inoculation.
A May 2022 paper
in Biological Reviews sought to refine the definition of biocrust and
characterize the science of biocrust. They show that there are four distinct elements
of biocrusts: physical structure, functional characteristics, habitat, and
taxonomic composition. They favor the definition given by Belnap, Büdel &
Lange:
“Biological soil crusts (biocrusts) result from an
intimate association between soil particles and differing proportions of
photoautotrophic (e.g. cyanobacteria, algae, lichens, bryophytes) and
heterotrophic (e.g. bacteria, fungi, archaea) organisms, which live within, or
immediately on top of, the uppermost millimetres of soil. Soil particles are
aggregated through the presence and activity of these often extremotolerant
biota that desiccate regularly, and the resultant living crust covers the
surface of the ground as a coherent layer.”
The abstract and figures from the paper demonstrating the classification
scheme and showing different biocrust types in different regions around the
globe are shown below.
References:
The
dirt on biocrusts: Why scientists are working to save Earth’s living skin. Jude
Coleman. Knowable Magazine. August 26, 2024. What are biocrusts and why are they
important? | Knowable Magazine
Biological
soil crusts. Wikipedia. Biological soil crust - Wikipedia
Biological
Soil Crust ("Biocrust") Science. Southwest Biological Science Center.
USGS. January 10, 2022. Biological Soil Crust
("Biocrust") Science | U.S. Geological Survey
What
is a biocrust? A refined, contemporary definition for a broadening research
community. Bettina Weber, Jayne Belnap, Burkhard Büdel, Anita J. Antoninka,
Nichole N. Barger, V. Bala Chaudhary, Anthony Darrouzet-Nardi, David J.
Eldridge, Akasha M. Faist, Scott Ferrenberg, Caroline A. Havrilla, Elisabeth
Huber-Sannwald, Oumarou Malam Issa, Fernando T. Maestre, Sasha C. Reed, Emilio
Rodriguez-Caballero, Colin Tucker, Kristina E. Young, Yuanming Zhang, Yunge
Zhao, Xiaobing Zhou, and Matthew A. Bowke. Biological Reviews. Volume97, Issue 5.
October 2022. Pages 1768-1785. What is a biocrust? A refined,
contemporary definition for a broadening research community - Weber - 2022 -
Biological Reviews - Wiley Online Library
The
Microbiology of Biological Soil Crusts. Ferran
Garcia-Pichel. Annual Review of Microbiology. Volume 77, (September 2023). The
Microbiology of Biological Soil Crusts | Annual Reviews
Mapping
and Monitoring Biological Soil Crusts. U.S. Geological Survey. Mapping
and Monitoring Biological Soil Crusts | Land Imaging Report Site
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