In arid and semi-arid regions, the ground itself is alive. Often appearing as a dark, bumpy layer, biological soil crust, or biocrust, is a community of organisms living on or just beneath the soil surface. This living skin carpets about 12% of the Earth’s land surface, shaping the environments where it thrives.
The Living Components of Biocrusts
Biocrusts are communities built by microscopic and macroscopic organisms. The foundational builders are cyanobacteria, a photosynthesizing bacteria. When wet, these bacteria move through the soil, leaving behind sticky sheaths that bind loose particles together. This creates a stable structure for other organisms to colonize, forming a crust from millimeters to centimeters thick.
This cyanobacterial framework provides a home for lichens, mosses, algae, and fungi. Lichens, a partnership between fungi and an alga or cyanobacterium, contribute to the crust’s stability and diversity. They often give older crusts their lumpy and dark appearance. Mosses and liverworts, known as bryophytes, add to the surface roughness and structural complexity.
The composition of a biocrust varies with local climate, soil type, and other environmental factors. In wetter arid regions, mosses may be more prominent, creating a greener surface. In soils rich with gypsum, certain lichens may dominate. In the driest areas, the crust might be smoother and composed almost entirely of cyanobacteria.
Ecological Role in Arid Environments
Biocrusts perform functions that support desert ecosystems, with a primary role in soil stabilization. The web-like network of cyanobacterial filaments holds soil particles in place, creating a cohesive surface that resists wind and water. By anchoring the soil, this stabilization reduces erosion, preventing the loss of topsoil and lessening the intensity of dust storms.
Biocrusts also manage scarce water resources. The crust’s organic matter and rough texture absorb and hold rainwater that might otherwise run off or evaporate. This retained moisture helps plant seeds germinate and seedlings survive. The crust’s surface slows water movement, allowing more time for it to infiltrate the soil.
These living crusts act as natural fertilizers. Certain species of cyanobacteria and lichens perform nitrogen fixation, converting atmospheric nitrogen into forms like ammonia that plants can use. By adding this nitrogen to the soil, biocrusts enrich the nutrient-poor desert environment and support the surrounding plant life and ecosystem productivity.
Vulnerability to Disturbance
Despite their strength against erosion, biocrusts are fragile under compressional stress. Their structure, woven by microscopic organisms, is easily shattered under pressure, especially when dry. The threats come from human activities that crush the soil, including off-road vehicles, grazing livestock, and repeated foot traffic.
When the crust is broken, the web of filaments and organisms is destroyed. This damage negates the ecological functions the crust performs. The soil loses its stability and becomes vulnerable to wind and water erosion. The water retention capability vanishes, and nutrient cycling processes are halted, compromising the foundation of the local ecosystem.
The impact of these disturbances extends beyond the immediate footprint. A path through a crust can channel water, leading to further erosion and gully formation. The crust’s loss makes it difficult for native plants to reestablish, which can allow invasive species to take hold. Its absence also alters soil temperature, as the dark surface is no longer present to absorb sunlight.
Recovery and Restoration Processes
The natural recovery of a damaged biocrust is a slow process. First, cyanobacteria must recolonize the disturbed soil, which can take several years to a decade depending on the climate. For a complex crust with lichens and mosses to fully reform and regain its ecological function can take from several decades to centuries, especially in very arid environments.
Due to the slow pace of natural regeneration, scientists are exploring active restoration methods. One technique involves salvaging intact biocrust from areas slated for development and relocating it to damaged sites. This introduces a mature community of organisms for a head start on recovery.
Another approach is cultivating biocrust organisms in a lab. Scientists can grow large quantities of cyanobacteria and other components to create an inoculum, or a liquid slurry of organisms. This mixture can be sprayed onto degraded soils to seed the area and begin recovery. While still in development, these restoration techniques offer hope for mending the fragile skin of the desert and rebuilding these unique ecosystems.