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Activities > Biological Soil Crust

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[ || Soil Moisture || Geology & Soil Texture || Biological Soil Crusts || Fire || Vegetation Dynamics || Soil Compaction || Wind Erosion || Climate || Repeat Photography || Land Use History || Spatial Modeling || ]

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Dark lichens in dense arrays on light-colored, grussy, granitic alluvial fan deposit. Photograph by USGS staff.

Biological soil crusts are found throughout the world, from the hottest deserts to polar regions. In arid regions, these soil crusts are generally dominated by cyanobacteria and include soil lichens, mosses, green algae, microfungi, and bacteria. These crusts play many important roles in the ecosystem.

When moistened, cyanobacteria become active, moving through the soils and leaving a trail of the sticky, mucilaginous sheath material behind. This sheath material connects otherwise loose soil particles and erosion-prone surfaces so they become resistant to both wind and water erosion. Sheath material can be found as deep as 10 cm in soils, thus providing stability in soils at depth.

Crusts also influence hydrologic cycles and vascular plants. When moistened, the sheaths absorb up to 10 times their volume of water. In areas that freeze, the soil surfaces roughened by crusts slow rainfall runoff and increase infiltration into the soil. This is especially important in arid areas that have sporadic, heavy rainfall.

Because biological soil crusts stabilize soils, they can prevent the germination of large-seeded plants whose seeds require burial. Most native plants have burial mechanisms. The biological crusts can also act to prevent germination of exotic annual grasses.

Crustal organisms also contribute nitrogen and organic matter to the ecosystem. This process is especially important in desert ecosystems where nitrogen levels often limit plant productivity. Vascular plants growing in crusted areas have higher levels of many essential nutrients than plants growing in areas without crusts.

Unfortunately, many activities are incompatible with the well-being of these soil crusts. Cyanobacterial fibers have great tensile strength, but little or no resistance to compressional stress, especially when dry. Crushed crusts contribute less nitrogen and organic matter to the ecosystem. Impacted soils are left highly susceptible to both wind and water erosion. Raindrop erosion is increased, and overland water flows carry detached material away. Wind erosional rates increase dramatically, leading to both soil loss and the burial of nearby crusts. Since crustal organisms need light to photosynthesize, burial can mean death.

Close-up of lichen soil crust in the Mojave Desert. Photograph by USGS staff.

The objectives of this activity are two-fold:

• To develop models that predict soil crust distribution (cover of mosses, lichens, and cyanobacteria) and to use these models to create spatially explicit maps

• To use these maps to inform land managers about where different crust communities are located and the vulnerability of each community type. Using this information, managers can then make more informed decisions about the siting of land use and restoration activities

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Preliminary biological soil crusts prediction map.

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