Soil Crust
Introduction
Biological soil crusts (BSC) are large complex communities of cyanobacteria, algae, bryophytes, [lichens], mosses, [bacteria], and fungi. These diverse collections of microbes occur all over the world in arid and semi-arid regions, which comprise more than 35% of all terrestrial land. The microbial community that inhabits the upper and bottom few millimeters of soil can comprise up to 70% of the living cover in arid regions. BSC can play an important role in desert environments as they increase the nutrient content of soils, stabilizing soil surfaces, as well as prevent soil erosion. BSC are also known as cryptogamic, microbiotic, cryptobiotic, and microphitic crust. It is important to distinguish BSC as a formation of living organism and by-products. Chemical and physical crust are non organic formations such as salt crust or platy crust.
Environment
BSC are a key component of stabilizing mobile surfaces by binding soil particles together with polysaccharide exudates, and a matrix of fibers. The binding of the surface particles help protect from wind and water erosion as well as cryoturbation. The function these microbe provide is important in arid and semiarid areas with low soil formation, fertility and moisture. With small amounts of vascular vegetation in desert biomes, conditions are ripe for colonization by microbes to create BSC.
Physical and Biological Interactions
Soil Aggregation
Formation of these soil crusts begin with interactions between colonizing fungi and cyanobacteria. The colonizing microbes will often create a surface of compacted soil particles with a thickness up to 10cm thick. Microcoleus is a type of cyanobacteria that will form filaments surrounded by extracellular sheats. The filaments are mobile through the moist soil. Filaments are of this cyanobacteria are in constant regeneration creating stable soil aggregates. Other common cyanobacteria found in BSC are [Nostoc], Scytonema, Calothrix, and Gloeocapsa. Once cyanobacteria and fungi have created a stable platform, other organisms such as lichens, algae, and mosses will begin a secondary succession. Lichens and mosses aid in soil stability by binding soil particles with rhizines further increasing resistance to wind and water erosion.
Indirect Species Interactions
The changes these microbes make to the soil structure create a more suitable habitat and enhance the ability of vascular plant species to reproduce in arid environments. Studies have demonstrated an increase in plant productivity as well as an increase in nutritional content in plant tissue when plants are grown in BSCs. Organisms in the BSC increase moisture, organic matter, and nutrient content. This is important in arid regions as all those variables are the limiting factor in plant growth.
BSC are highly sensitive to water content. Larger concentrations of water increase the diffusion of CO2 and O2 disturbance by surface water. Decrease in amounts of water cause algae to be less productive causing an overall drop of photosynthesis in BSC. An increase in temperature has also shown decreases in photosynthesis production.
Microbial processes
Carbon Fixation
Cyanobacteria, eukaryotic algae, bryophytes and lichens are all labeled as primary producers that provide an additional pathway for carbon to enter the soil.
Nitrogen Fixation
Nitrogen fixation occurs in BSC by lichens containing cyanobacterial photobionts, heterocystic cyanobacteria, and heterotrophic bacteria
BSC that have a large amount of cyanobacteria present are generally expected to have higher nitrogen content due to the N-fixation of cyanobacteria.
Nutrient Cycling
BSCs consume and contribute to carbon or nitrogen nutrient pools. BSCs also alter the chemistry and water status in the soil changing nutrient availability and cycling rates.
Food Web Interactions
Lichens are an important component to the food web of soil crusts. Many types of arthropods use lichens as a food source. Microarthropods are essential to nutrient cycling as the contribution they have in BSC increases the microbial turnover. Microarthropods enhances leaching of soluble materials that promotes microbial colonization by increasing surface area. Some of the more commonly found microarthropods in warm deserts are Acari and Prostigmata.
Major Organism
Protozoa
Nematodes
Cyanobacteria
[Lichens]
Algae
Ahtropods
Bryophytes
Mosses
Fungi
Current Research
Protozoa of biological soil crust
"Photosynthetic components and ecosystem metabolism have been extensively studied, but the microfauna contributing to crust bacterial functioning have receive little attention. This study of five crust in southeastern Utah describes diversity and abundance of the protozoa, which constitute most of the microfauna." (Bamforth, S. 2008)
High Artic glacier foreland
Studies done on the photosynthetic and net primary production of biological soil crust to evaluate the contribution to the carbon cycle in the High Arctic glacier foreland.(Yoshitake, S, et al. 2010)
Microarthropod communities associated with biological soil crust
This study examined the microarthropod community, including mites, collembolans, and tardigrades, associated with early- and late-successional stage biological soil crusts at two locations, Colorado Plateau (southeastern Utah) and Chihuahuan Desert (southern New Mexico)" (Neher, D. et al. 2009)
References
Bamforth, S. (2008). Protozoa of biological soil crusts of a cool desert in Utah. Journal of Arid Environments, 72(5), 722-729. doi:10.1016/j.jaridenv.2007.08.007.
Lalley, J., Viles, H., Henschel, J., & Lalley, V. (2006). Lichen-dominated soil crusts as arthropod habitat in warm deserts. Journal of Arid Environments, 67(4), 579-593. doi:10.1016/j.jaridenv.2006.03.017.
Langhans, T., Storm, C., & Schwabe, A. (2009). Biological soil crusts and their microenvironment: Impact on emergence, survival and establishment of seedlings. Flora, 204(2), 157-168. doi:10.1016/j.flora.2008.01.001.
Langhans, T., Storm, C., & Schwabe, A. (2009). Community Assembly of Biological Soil Crusts of Different Successional Stages in a Temperate Sand Ecosystem, as Assessed by Direct Determination and Enrichment Techniques. Microbial Ecology, 58(2), 394-407. doi:10.1007/s00248-009-9532-x.
Langhans, T., Storm, C., & Schwabe, A. (2010). Regeneration processes of biological soil crusts, macro-cryptogams and vascular plant species after fine-scale disturbance in a temperate region: Recolonization or successional replacement?. Flora, 205(1), 46-60. doi:10.1016/j.flora.2008.12.001.
Li, X., Chen, Y., Su, Y., & Tan, H. (2006). Effects of Biological Soil Crust on Desert Insect Diversity: Evidence from the Tengger Desert of Northern China. Arid Land Research & Management, 20(4), 263-280. doi:10.1080/15324980600940985.
Neher, D., Lewins, S., Weicht, T., & Darby, B. (2009). Microarthropod communities associated with biological soil crusts in the Colorado Plateau and Chihuahuan deserts. Journal of Arid Environments, 73(6/7), 672-677. doi:10.1016/j.jaridenv.2009.01.013.
Sylvia, David M., Fuhrmann, Jeffry J., Hartel, Peter G., Zuberer, David A., (2004) Biological Soil Crust Principles and Applications of Soil Microbiology, 2nd edition (pp. 173-179). Upper Saddle River, N.J. Pearson Prentice Hall
Yoshitake, S., Uchida, M., Koizumi, H., Kanda, H., & Nakatsubo, T. (2010). Production of biological soil crusts in the early stage of primary succession on a High Arctic glacier foreland. New Phytologist, 186(2), 451-460. doi:10.1111/j.1469-8137.2010.03180.x.
