Biological Soil Crusts and their ability to trap dust, add nutrients to soil and affect the hydro cycle, are critical factors of soil pedogenesis within a desert region. As pioneer organisms, BSC's are able to colonize extreme environmental conditions well before vascular plants take root. Without the soil crusts, a desert can and will turn into desert pavement, an impervious surface, which supports no biological processes.
The microorganisms add a significant amount of organic carbon to an otherwise void mineral topography (sandy deserts!). They are shown to significantly affect podilization, or the soil forming process, where each of materials accumulates into lower horizons (like creating those pesky fragipans!).
(Figure 1 Soil Science Society of America)
Because the predominate BSC organisms are photosynthesizes, the crusts are found at the surface or within 4 mm. Some of the organisms have pigments for UV protection, protection that larger species lack, which as part of the biological community of BSC's, will only surface on cloudy days and retreat on dry days (NEAT!), creating turbation (mixing) of minerals and organic materials. This turbation develops intermingle layers which further develop the A horizon (think topsoil).
Biological Soil Crusts, bury seeds in vast micropores (soil holes) and help in the process of germination of native species. (Belnap 2004) Overall, BSC's are a key component of arid pedogenesis and desert morphology.
BSC microorganisms such as cyanobacteria, green algae, lichen, mosses, fungi and heterotrophic bacteria can produce an organic layer a centimeter thick on what would normally be a climate poor/bio-poor region of a desert. BSC's cyanobacteria binds the soil, increasing soil surface roughness and will fix nitrogen once buried, while they move back towards the surface to continue photosynthesizing.
Green Algae are photosynthesizers that are well adapted to arid regions by resting during droughts and continuing metabolic function during a wet event.
Microfungi are beneficial as decomposers, often associated with vascular roots that bind soil particles and increase water holding capacity. Lichens form a symbiotic relation with the Bryophytes, by capturing and cultivating algae resulting in new morphological entities. (Belnap 2004)
Biological Soil Crusts can be found on every continent. They are most prevalent in arid and alpine habitats, being the dominant or most common feature. BSC's are also found in semi-arid regions, woodlands, steppe formations, gaps in evergreen shrub stands, and forests of the Xeric temperature regime (winters are moist and cool and summers are warm and dry), open ground between tundra vegetation and can reach a few meters squared in temperate climates of xerothermic conditions. (Belnap 2004)
Biological Soil Crusts, are a vital influential factor in the arid and semi-arid landscapes. BSC's consist of a complex bio-sedimentary structure that stabilizes the surrounding soils to suspend erosion, add to surface mineral precipitation, capture dust for nutrients and form vesicular horizons (Av horizon consisting of dust and larger micropore space). (Williams)
They are considered living mulch because they retain moisture and discourage annual weed growth. This living protective membrane, in arid soils, controls the movement of soil gases, water, and solutes. BSC's infiltrate more water from pulse rain events, due to their micro-topographical changes, and reduce run-off. They impact particle detachment, keep and energy, balance, influence soil fertility and plant community establishment, even though BCS's are extremely fragile and sensitive to disruption by vehicle, grazing, and human activities. (Williams) Possibly, small portions of BSC's could be blended with nutrients and sprayed on sensitive disturbed landscapes.
(All other pictures are my own from in Utah's amazing Grand Staircase-Escalante National Monument.)