Biological Survey of Ironwood Forest National MonumentBiological Soil Crusts There is very little exposed soil in this image that spans about 6 inches (15 cm) width. Most of the surface is covered by biological soil crusts. The organisms that form them are nearly invisible except for brief periods when the soil surface is moist. They anchor soil from erosion and some fix atmospheric nitrogen, a limiting nutrient in most desert soils. The upper left quadrant is occupied by a green liverwort. The brown stuff at lower left is probably a lichen. Most of the soil between the pebbles at right is cemented by tiny colonies of cyanobacteria (blue-green algae), algae, mosses, and lichens.
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