Biological soil crust
Biological soil crusts are a micro-ecosystem in which inorganic soil particles are connected and thus stabilized by soil organisms such as cyanobacteria , soil algae , mycelium and hyphae of microfungi and mosses and their activities and products, such as extracellularly deposited polysaccharides . These living crusts cover the soil surface as a coherent layer on the soil surface and in the top millimeters of the soil. The structures have also been described under numerous synonymous names such as cryptogamous, cryprobiotic, microbiotic, microphytic or organogenic soil crusts. Biological soil crusts occur in habitats with an open soil surface that is not completely covered by vascular plants; they are of particular importance in arid areas.
Biotic composition
In many arid regions, the most important components of the biological soil crust are cyanobacteria of the genus Microcoleus , whose filaments, encased in a sheath of extracellular matrix, wind through the top soil layers. The living filaments can slide to the surface within the sheaths in damp conditions and move down again in less favorable conditions. The genera Nostoc , Pleurocapsa and Chroococcidiopsis are also significant . In contrast to the cyanobacteria, heterotrophic bacteria play only a subordinate role in the structure of the crust due to their low biomass. Significant among the hundreds of eukaryotic algae involved are mainly green coccal algae, such as the genera Chlorococcum , Macorchloris and Stichococcus . In contrast to these groups, whose most important representatives live inside the soil, the thalli of the lichens and the mosses primarily colonize the soil surface. Among the crust-shaped ground lichens, both those with green algae and those with cyanobacteria are important as partners in the symbiosis for crust formation. These include, for example, the genera Psora , Buellia and Trapelia , which form simple flat crusts, endocarpon and Peltula with shield-shaped, to forms such as Peltigera with complex, leaf-like thalli or stem-forming forms in the genus Cladonia . Typical of arid areas are the so-called "wandering lichens", which lose contact with the ground in drought and are spread by the wind, including Xanthoparmelia , Xanthomaculina and Chondropsis . In addition to those forms with layered thalli, those with unlayered (homeomeric) thalli, usually with a gelatinous structure, are also involved in the coasts. Mosses, for example of the genera Bryum , Campylopus or Gigaspermum, are also important for the biological soil crust . All these forms have in common that they only grow and are biologically active when the soil is damp or wet. Forms that are more sensitive to dehydration and require greater moisture therefore only play a role in northern latitudes or under the protective umbrella of vascular plants and are absent in very arid areas, including most of the deciduous and liverworts. In deserts, the organisms are dependent on dew or the rare rainfall.
The biological soil crusts form the food base and habitat for animal species in the soil. Due to their small size and low productivity, this is especially true for species of the mesofauna such as mites, springtails, tardigrade and nematodes. Larger animals such as land snails, woodlice and, in desert areas, black beetles (Tenebrionidae) can graze on the crusts.
distribution
Biological soil crusts can occur in all climatic zones and on all soil types. However, they are only poorly developed on pure clay soils, as they tend to swell when moistened and thus mechanically destroy the crust. While green algae emerge on acidic soils, cyanobacteria are promoted by alkaline soils, those with a high salt content or very little rainfall. Nitrogen-fixing species emerge on nutrient-poor soils. While the crusts of the soil usually only form a succession stage on moist raw soils and can be displaced by vascular plants in the course of the period, they form permanent communities in extreme habitats. A special feature is the micro-habitat of the "window algae", which form a special community under transparent pebbles, for example made of quartz .
The importance of the biological soil crust increases in areas with low biological productivity due to extreme environmental conditions such as cold, heat and drought. These include arctic and alpine tundra and cold deserts and arid and semiarid semi-deserts and dry deserts , which together cover more than 40 percent of the land surface of the earth. In moderate, temperate latitudes, they are the predominant form of life, restricted to special locations such as some steppes and serpentinite rock corridors. By changing the soil surface on raw soils, however, they can accelerate and facilitate the colonization of the soil by vascular plants.
meaning
In arid areas such as deserts in particular, biological soil crusts are of great importance for the water balance . Cyanobacteria are able to store about ten times their dry volume and eight to twelve times their weight in water. If they appear in the form of smooth coatings, they reduce the pore volume on the surface and thus the seepage capacity, but this counteracts mechanical soil loosening through dying cell threads and soil animals associated with the soil crusts and can have a predominant effect. They also stabilize the soil against soil erosion and effectively intercept dust transported by the wind.
However, these crusts are susceptible to mechanical damage, for example through human activities. If, for example, the extracellular envelopes of the Micrcoleus cyanobacteria are destroyed, the cell threads can no longer slide upwards following the moisture and die. After the destruction, especially in desert areas, the surface runoff of rainwater increases and the wind erosion increases. There are already a few studies on the quantitative influence of these factors, but it cannot yet be reliably quantified. However, the impact can far exceed that of the soil type.
Individual evidence
- ↑ a b c J. Belnap, B. Büdel, OL Lange: Biological soil crusts: Characteristics and distribution. Chapter 1 in Jayne Belnap, Otto L. Lange (Eds.): Biological Soil Crusts: Structure, Function, and Management. Springer, 2013. ISBN 978-3-642-56475-8 .
- ↑ Jayne Belnap (2006): The potential roles of biological soil crusts in dryland hydrologic cycles. Hydrological Processes 20: 3159-3178. doi: 10.1002 / hyp.6325
- ^ Matthew A. Bowker, Jayne Belnap, V. Bala Chaudhary, Nancy C. Johnson (2008): Revisiting classic water erosion models in drylands: The strong impact of biological soil crusts. Soil Biology & Biochemistry 40 (9): 2309-2316. doi: 10.1016 / j.soilbio.2008.05.008