Microphytic Soil Crusts and Desert Ecosystems
Communities of micro-organisms create crusts on soils throughout semi-arid and arid regions of the world. These microphytic (also called cryptogamic) crusts are formed when all or some of a diverse array of photosynthetic cyanobacteria (blue-green algae), fungi, bacteria, lichens and mosses, bind together with inorganic particles in the first few millimeters of a soil.
Microphytic crusts are dominant feature in desert soils; they are estimated to represent approximately 70% of desert soil biomass world wide (Belnap 1993). Un-restricted human activity (farming, livestock grazing, recreation) results in the denigration or destruction these prominent crusts. Many claim that soils and soil mechanisms are at the base of other ecosystem functions (Vitousek, Walker, Syers in Gillis 1994). In order to better understand and manage desert ecosystems, it is important to begin to understand how cryptogamic crusts form, what role crusts play in shaping desert soil properties, and further, how crust removal might effect soil quality and ecosystem stability.
It is generally thought that the formation of microphytic crusts begins with the establishment of cyanobacteria or agal communities on the soil surface (Campbell et. al. 1989). There are many different types of algae and cyanobacteria which exist in the new crusts, however it is difficult to ascertain which types of organisms are responsible for which processes of early crust formation. Johansen postulates that crusts begin to form when filamentous cyanobacteria (as opposed to diatomic and nonfilamentous cyanobacteria or other algae) colonize the surface of soils in a period of moist weather (1993). As cyanobacteria begin to metabolize in the moist and light conditions, they excrete polysaccharides which cause them to adhere to other soil particles. During periods of limited moisture, the filaments dehydrate and become brittle, and the organisms stop photosynthesis. When water is available to the organisms again, they expand out into new areas, leaving behind their old filaments. This process creates a complex of soil particles bound together by old filaments. Eventually the soil surface is covered by a mat, and the soil below the mat has been consolidated by a net of polysaccharides.
After the initial establishment of agal mats, other organisms may take advantage of biomass created by cyanobacteria. Generally, the diversity of organisms present in crusts increases over time. Lichens form when fungi associate with cyanobacteria and/or algae. This association requires long periods of time (on the order of 50+ undisturbed years to establish an well developed crust), and takes place in soils which are relatively silt and clay rich (Skujins ). Lichen hyphae increase the structural integrity of soil crusts. This leads in one way or another to the formation of pinnacles (microtopography on the surface of the soil). It is not...