Three types of crusts form on the soil surface: biological, chemical and physical.
Biological crusts are known by many different names including biocrusts and cryptobiotic, microbiotic, cryptogamic or microphytic crusts. I use the term microbiotic crusts. These biological crusts contain communities from diverse taxa: cyanobacteria (blue-green algae), algae, fungi, bacteria, lichens and/or bryophytes. Different ecological sites favor certain taxa thus crusts differ in composition depending on their location. Microbiotic crusts are found in arid and semiarid locations throughout the world.
In the Great Basin, where Leonard and I live, microbiotic crusts are composed primarily of cyanobacteria and green algae. Well developed crusts here are usually darker than the soil beneath and have a sponge-like texture. The density and color of the organisms forming the crust determine their color and physical properties. Microbiotic crusts can cover 10% to 100% of the soil surface.
Microbiotic crusts perform several functions:
*Stabilize the soil: The filamentous nature of cyanobacteria and green algae bind soil particles and aggregate on the soil surface helping to prevent erosion.
*Nutrient contributions: Cyanobacteria can fix nitrogen, particularly useful in nitrogen poor soils of the Great Basin. The soil fines that are bound into the crust are high in phosphorous. The crust organisms may also make carbon and other nutrients available.
*Water regulation: Microbiotic crusts hold water and benefit surrounding vegetation by slowing evaporation. On the other hand, some crusts impede water absorption with the surrounding plants benefiting from water runoff.
*Seed germination: Some seeds experience reduced germination if they fall on a microbiotic crust while others benefit. The surface of the crust holds seeds and the dark color of the crust increasessoil surface temperature promoting germination.
*Plant growth: As with seed germination, some plants do poorly if they germinate on microbiotic crusts. Others exhibit increased growth because of water regulation by the crust and more available nutrients.
*Build up soil: The microbiotic crust captures dust, which builds up the soil. The organic matter from dead crust organisms accumulates and also enhances the soil.
Microbiotic crust organisms are well adapted to very severe environmental conditions. However they and their resultant crusts are poorly adapted to disturbances caused by human activity such as livestock grazing, hiking, biking and off road vehicles. These disruptions cause decreased diversity, the loss of soil nutrients and organic matter and the erosion of the soil itself. Once disrupted, the microbiotic crust is very slow to recover. Care should be taken to protect beneficial microbial crusts.
There is a low area in Ash Creek Wildlife Area (Lassen County CA) containing both microbiotic crusts and the chemical crusts I will discuss in my next post.