Biological Soil Crusts
What is Cyanobacteria?
Cyanobacteria occur as single cells or as filaments. The most common form found in Colorado Plateau soils are the filamentous type, which are usually surrounded by sticky, mucilaginous sheaths.
When moistened, cyanobacteria become active, moving through the soil and leaving a trail of sticky material behind. The sheath material sticks to surfaces such as rock or soil particles, forming an intricate web of fibers throughout the soil. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion.
The soil-binding action is not dependent on the presence of living filaments. Layers of abandoned sheaths, built up over long periods of time, can still be found clinging tenaciously to soil particles, providing cohesion and stability in sandy soils at depths up to 4 inches (10 cm).
Nitrogen fixation is another significant capability of cyanobacteria. Vascular plants are unable to utilize nitrogen as it occurs in the atmosphere. Cyanobacteria are able to convert atmospheric nitrogen to a form plants can use. This is especially important in desert ecosystems, where nitrogen levels are low and often limiting to plant productivity.
The sheaths have other functions as well. When moistened, they swell up to ten times their dry size. This ability to intercept and store water benefits both the crustal organisms as well as vascular plants, especially in arid regions with sporadic rainfall.
Sheaths, and the organisms they surround, also contribute organic matter and help make essential nutrients available to vascular plants. Negatively charged clay particles, often found clinging to the sheaths, bind positively charged nutrients, preventing them from being leached out of the upper soil horizons or becoming bound in a form unavailable to plants. Like soil stability, this function is not dependent on the presence of living filaments, but only the presence of sheath material.
Tracks in continuous strips, such as those produced by vehicles or bicycles, are especially damaging, creating areas that are highly vulnerable to wind and water erosion. Rainfall carries away loose material, often creating channels along these tracks, especially when they occur on slopes.
Wind not only blows pieces of the pulverized crust away, thereby preventing reattachment to disturbed areas, but also disturbs the underlying loose soil, often covering nearby crusts. Since crustal organisms need light to photosynthesize, burial can mean death. When large sandy areas are impacted during dry periods, previously stable areas can become a series of shifting sand dunes in just a few years.
Impacted areas may never fully recover. Under the best circumstances, a thin veneer of biological soil crust may return in five to seven years. Damage done to the sheath material, and the accompanying loss of soil nutrients, is repaired slowly during up to 50 years of cyanobacterial growth. Lichens and mosses may take even longer to recover.