Biological soil crusts are communities of living organisms on the

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

surface in

arid A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ...

and

semi-arid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi- ...

ecosystems. They are found throughout the world with varying species composition and cover depending on

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sc ...

, soil characteristics,

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...

plant community A plant community is a collection or association of plant species within a designated geographical unit, which forms a relatively uniform patch, distinguishable from neighboring patches of different vegetation types. The components of each plant ...

microhabitats In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

, and disturbance regimes. Biological soil crusts perform important ecological roles including carbon fixation, nitrogen fixation and soil stabilization; they alter soil albedo and water relations and affect germination and nutrient levels in vascular plants. They can be damaged by fire, recreational activity, grazing and other disturbances and can require long time periods to recover composition and function. Biological soil crusts are also known as ''biocrusts'' or as ''cryptogamic'', ''microbiotic'', ''microphytic'', or ''cryptobiotic'' soils.

Natural history

Biology and composition

Biological soil crusts are most often composed of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

bryophytes The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants ( embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited ...

, and

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...

in varying proportions. These organisms live in intimate association in the uppermost few millimeters of the soil surface, and are the biological basis for the formation of soil crusts. Cyanobacteria Cyanobacteria are the main photosynthetic component of biological soil crusts, in addition to other photosynthetic taxa such as mosses, lichens, and green algae. The most common cyanobacteria found in soil crusts belong to large filamentous species such as those in the genus '' Microcoleus''. These species form bundled filaments that are surrounded by a gelatinous sheath of

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

. These filaments bind soil particles throughout the uppermost soil layers, forming a 3-D net-like structure that holds the soil together in a crust. Other common cyanobacteria species are as those in the genus ''

Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in var ...

'', which can also form sheaths and sheets of filaments that stabilize the soil. Some ''Nostoc'' species are also able to fix atmospheric nitrogen gas into bio-available forms such as

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

. Bryophytes Bryophytes in soil crusts include

mosses Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hornw ...

and

liverworts The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ...

. Mosses are usually classified as short annual mosses or tall perennial mosses. Liverworts can be flat and ribbon-like or leafy. They can reproduce by spore formation or by asexual fragmentation, and photosynthesize to fix carbon from the atmosphere. Lichens Lichens are often distinguished by growth form and by their photosymbiont. Crust lichens include crustose and areolate lichens that are appressed to the soil substrate,

squamulose A squamulose lichen is a lichen that is composed of small, often overlapping "scales" called squamules. If they are raised from the substrate and appear leafy, the lichen may appear to be a foliose lichen, but the underside does not have a "sk ...

lichens with scale- or plate-like bodies that are raised above the soils, and foliose lichens with more "leafy" structures that can be attached to the soil at only one portion. Lichens with algal symbionts can fix atmospheric carbon, while lichens with cyanobacterial symbionts can fix nitrogen as well. Lichens produce many pigments that help protect them from radiation. Fungi

Microfungi Microfungi or micromycetes are fungi—eukaryotic organisms such as molds, mildews and rusts—which have microscopic spore-producing structures. They exhibit tube tip-growth and have cell walls composed of chitin, a polymer of ''N''-acetylgluco ...

in biological soil crusts can occur as free-living species, or in

symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or para ...

with algae in lichens. Free-living microfungi often function as decomposers, and contribute to soil microbial biomass. Many microfungi in biological soil crusts have adapted to the intense light conditions by evolving the ability to produce

melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...

, and are called black fungi or

black yeast “Black yeasts”, sometimes also black fungi, dematiaceous fungi, microcolonial fungi or meristematic fungi is a diverse group of slow-growing microfungi which reproduce mostly asexually (fungi imperfecti). Only few genera reproduce by budding ...

s. Fungal

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

can bind soil particles together. Free-living green algae

Green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga ...

in soil crusts are present just below the soil surface where they are partially protected from UV radiation. They become inactive when dry and reactivate when moistened. They can photosynthesize to fix carbon from the atmosphere.

Formation and succession

Biological soil crusts are formed in open spaces between

vascular plants Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...

. 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. Recovery following disturbance varies. Cyanobacteria cover can recover by propagules blowing in from adjacent undisturbed areas rapidly after disturbance. Total recovery of cover and composition occurs more rapidly in fine soil textured, moister environments (~2 years) and more slowly (>3800 years) in coarse soil textured, dry environments. Recovery times also depend on disturbance regime, site, and availability of propagules.

Distribution

Geographical range

Cryptobiotic soil crust in Natural Bridges National Monument near Sipapu 20100906 - number 5

Biological soil crusts cover about 12% of the earth's landmass. They are found on almost all soil types, but are more commonly found in arid regions of the world where plant cover is low and plants are more widely spaced. This is because crust organisms have a limited ability to grow upwards and cannot compete for light with vascular plants. Across the globe, biological soil crusts can be found on all continents including Antarctica.Rosentreter, R., M. Bowker, and J. Belnap. 2007. A Field Guide to Biological Soil Crusts of Western U.S. Drylands. U.S. Government Printing Office, Denver, Colorado.

Variation throughout range

The species composition and physical appearance of biological soil crusts vary depending on the climate, soil, and disturbance conditions. For example, biological soil crusts are more dominated by green algae on more acidic and less salty soils, whereas cyanobacteria are more favored on alkaline and haline soils. Within a

climate zone Climate classifications are systems that categorize the world's climates. A climate classification may correlate closely with a biome classification, as climate is a major influence on life in a region. One of the most used is the Köppen climate ...

, the abundance of lichens and mosses in biological soil crusts generally increases with increasing clay and silt content and decreasing sand. Also, habitats that are more moist generally support more lichens and mosses. The morphology of biological soil crust surfaces can range from smooth and a few millimeters in thickness to pinnacles up to 15 cm high. Smooth biological soil crusts occur in hot deserts where the soil does not freeze, and consist mostly of cyanobacteria, algae, and fungi. Thicker and rougher crusts occur in areas where higher precipitation results in increased cover of lichen and mosses, and

frost heaving Frost heaving (or a frost heave) is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated in ...

of these surfaces cause microtopography such as rolling hills and steep pinnacles. Due to the intense UV radiation present in areas where biological soil crusts occur, biological soil crusts appear darker than the crustless soil in the same area due to the UV-protective pigmentation of cyanobacteria and other crust organisms.

Ecology

Ecosystem function and services

Biogeochemical cycling

=Carbon cycling

= Biological soil crusts contribute to the

carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...

through

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

and

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

of crust microorganisms which are active only when wet. Respiration can begin in as little as 3 minutes after wetting whereas photosynthesis reaches full activity after 30 minutes. Some groups have different responses to high water content, with some lichens showing decreased photosynthesis when water content was greater than 60% whereas green algae showed little response to high water content. Photosynthesis rates are also dependent on temperature, with rates increasing up to approximately . Estimates for annual carbon inputs range from 0.4 to 37 g/cm*year depending on successional state. Estimates of total net carbon uptake by crusts globally are ~3.9 Pg/year (2.1-7.4 Pg/year).

=Nitrogen cycling

= Biological soil crust contributions to the

nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biolo ...

varies by crust composition because only cyanobacteria and cyanolichens fix nitrogen.

Nitrogen fixation Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...

requires energy from photosynthesis products, and thus increase with temperature given sufficient moisture. Nitrogen fixed by crusts has been shown to leak into surrounding substrate and can be taken up by plants, bacteria, and fungi. Nitrogen fixation has been recorded at rates of 0.7–100 kg/ha*year, from hot deserts in Australia to cold deserts. Estimates of total biological nitrogen fixation are ~ 49 Tg/year (27-99 Tg/year).

Geophysical and geomorphological properties

Soil stability Soils in arid regions are slow-forming and easily eroded. Crust organisms contribute to increased soil stability where they occur. Cyanobacteria have filamentous growth forms that bind soil particles together, and hyphae of fungi and

rhizine In lichens, rhizines are multicellular root-like structures, arising mostly from the lower surface. A lichen with rhizines is termed rhizinate, while a lichen lacking rhizines is termed erhizinate. Rhizines serve only to anchor the lichen to their ...

s/ rhizoids of lichens and mosses also have similar effects. The increased surface roughness of crusted areas compared to bare soil further improves resistance to wind and water

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

. Aggregates of soil formed by crust organisms also increase soil aeration and provide surfaces where nutrient transformation can occur.Belnap, J. (2003). The world at your feet: desert biological soil crusts. Front. Ecol. Environ., 1 Soil water relations The effect of biological soil crusts on

water infiltration Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meter ...

and

soil moisture Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that enter ...

depends on the dominant crust organisms, soil characteristics, and climate. In areas where biological soil crusts produce rough surface microtopography, water is detained longer on the soil surface and this increases water infiltration. However, in warm deserts where biological soil crusts are smooth and flat, infiltration rates can be decreased by

bioclogging Bioclogging or biological clogging is clogging of pore space in soil by microbial biomass; their body and their byproducts such as extracellular polymeric substance (EPS). The microbial biomass blocks the pathway of water in the pore space, formin ...

. Albedo The darkened surfaces of biological soil crusts decreases soil

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

(a measure of the amount of light reflected off of the surface) compared to nearby soils, which increases the energy absorbed by the soil surface. Soils with well-developed biological soil crusts can be over 12 °C (22 °F) warmer than adjacent surfaces. Increased soil temperatures are associated with increased metabolic processes such as photosynthesis and nitrogen fixation, as well as higher soil water evaporation rates and delayed seedling germination and establishment. The activity levels of many arthropods and small mammals are also controlled by soil surface temperature. Dust-trapping The increased surface roughness associated with biological soil crusts increase the capture of

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...

. These Aeolian deposits of dust are often enriched in plant-essential nutrients, and thus increase both the fertility and the

water holding capacity Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This usually takes place 2–3 days after rain or irrigation in pervious soils of u ...

of soils.

Role in the biological community

Effects on vascular plants

Germination and establishment The presence of biological soil crust cover can differentially inhibit or facilitate plant seed catchment and

germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fe ...

. The increased micro-topography generally increases the probability that plant seeds will be caught on the soil surface and not blown away. Differences in water infiltration and soil moisture also contribute to differential germination depending on the plant species. It has been shown that while some native desert plant species have seeds with self-burial mechanisms that can establish readily in crusted areas, many exotic invasive plants do not. Therefore, the presence of biological soil crusts may slow the establishment of

invasive plant species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...

such as cheatgrass (''

Bromus tectorum ''Bromus tectorum'', known as downy brome, drooping brome or cheatgrass, is a winter annual grass native to Europe, southwestern Asia, and northern Africa, but has become invasive in many other areas. It now is present in most of Europe, south ...

''). Nutrient levels Biological soil crusts do not compete with vascular plants for nutrients, but rather have been shown to increase nutrient levels in plant tissues, which results in higher

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...

for plants that grow near biological soil crusts. This can occur through N fixation by cyanobacteria in the crusts, increased trapment of nutrient-rich dust, as well as increased concentrations of micronutrients that are able to chelate to the negatively charged clay particles bound by cyanobacterial filaments.

Effects on animals

The increased nutrient status of plant tissue in areas where biological soil crusts occur can directly benefit

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...

species in the community. Microarthropod populations also increase with more developed crusts due to increased microhabitats produced by the crust microtopography.

Human impacts and management

Human disturbance

Biological soil crusts are extremely susceptible to disturbance from human activities. Compressional and shear forces can disrupt biological soil crusts especially when they are dry, leaving them to be blown or washed away. Thus, animal hoof impact, human footsteps,

off-road vehicles An off-road vehicle, sometimes referred to as an overland or adventure vehicle, is considered to be any type of vehicle which is capable of driving on and off paved or gravel surface. It is generally characterized by having large tires with dee ...

, and tank treads can remove crusts and these disturbances have occurred over large areas globally. Once biological soil crusts are disrupted, wind and water can move sediments onto adjacent intact crusts, burying them and preventing photosynthesis of non-motile organisms such as mosses, lichens, green algae, and small cyanobacteria, and of motile cyanobacteria when the soil remains dry. This kills remaining intact crust and causes large areas of loss. Invasive species introduced by humans can also affect biological soil crusts. Invasive annual grasses can occupy areas once occupied by crusts and allow fire to travel between large plants, whereas previously it would have just jumped from plant to plant and not directly affected the crusts.

Climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

affects biological soil crusts by altering the timing and magnitude of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

events and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

. Because crusts are only active when wet, some of these new conditions may reduce the amount of time when conditions are favorable for activity. Biological soil crusts require stored carbon when reactivating after being dry. If they do not have enough moisture to photosynthesize to make up for the carbon used, they can gradually deplete carbon stocks and die. Reduced carbon fixation also leads to decreased nitrogen fixation rates because crust organisms do not have sufficient energy for this energy-intensive process. Without carbon and nitrogen available, they are not able to grow nor repair damaged cells from excess radiation.

Conservation and management

Removal of stressors such as

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and ot ...

or protection from disturbance are the easiest ways to maintain and improve biological soil crusts. Protection of relic sites that have not been disturbed can serve as reference conditions for restoration. There are several successful methods for stabilizing soil to allow recolonization of crusts including coarse litter application (such as straw) and planting vascular plants, but these are costly and labor-intensive techniques. Spraying

polyacrylamide Polyacrylamide (abbreviated as PAM) is a polymer with the formula (-CH2CHCONH2-). It has a linear-chain structure. PAM is highly water-absorbent, forming a soft gel when hydrated. In 2008, an estimated 750,000,000 kg were produced, mainly fo ...

gel has been tried but this has adversely affected photosynthesis and nitrogen fixation of '' Collema'' species and thus is less useful. Other methods such as fertilization and inoculation with material from adjacent sites may enhance crust recovery, but more research is needed to determine the local costs of disturbance. Today, direct inoculation of soil native microorganisms, bacteria and cyanobacteria, supposed as a new step, biologic, sustainable, eco-friendly and economically-effective technique to rehabilitate biological soil crust.Kheirfam, H., Sadeghi, S. H., Darki, B. Z., & Homaee, M. (2017). Controlling rainfall-induced soil loss from small experimental plots through inoculation of bacteria and cyanobacteria. CATENA, 152, 40-4

/ref>

References

External links

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Biological soil crusts
USGS Canyonlands Research Station {{Authority control Soil biology Lichens Mycology