Research reveals desert ecosystems depend on soil crusts
July 18, 2012
Across the western United States, deserts creep outward in all directions. Dry winds shuttle clouds of sand across the landscape, leading a charge to take over surrounding land and advance a growing desertification problem.
The hope for restoring these arid environments and preventing further desertification may exist on the surface of the desert itself. Biological soil crusts (BSC) are a complex blend of microorganisms carpeting arid environments. These organisms fuse with soil particles, stabilizing desert crusts and forming fragile peaks in the soil that influence a variety of processes to allocate important resources. “These crusts kind of act like a living mulch across a desert, by protecting the surface from erosion,” says Mandy Williams (seated in photo to the right), a lab manager in the school of Life Science at the University of Nevada – Las Vegas. “Once you disturb the soil surface, you’re more likely to lose what little resources are available there.”
Williams, along with two other UNLV researchers, performed an in-depth micromorphological investigation of BSC samples from the Mojave Desert to better understand the formation, structure, and significant role soil crusts play in arid environments. Their findings reveal complex internal soil structures suggesting a rich genetic history and a variety of formation processes and will be released in the September-October Soil Science Society of America Journal.
The development of BSC begins with cyanobacteria, a phylum of photosynthetic bacteria. These bacteria form smooth crusts on the desert surface and strengthen soil structure by sealing off the surface to effects of erosion. Wet-dry cycles cause this newly formed soil crust to expand and contract, leaving cracks in the crust that trap dust as it blows over the desert surface. Dust is an important source of nutrients in the Mojave Desert, where organic matter is lacking and much-needed nutrients must migrate to the desert from surrounding environments.
Meanwhile, cyanobacteria weave around particles in the soil, forming thick layers of fine grains by trapping sediments in their sticky casing. Over many years, these bacteria-soil mixtures grow into jagged micro-peaks that accrete more dust and continue to evolve. These peaks are extremely fragile and sensitive to physical impacts such as vehicles, foot traffic, and grazing. BSC's also have implications on water distribution in arid environments. In the Av soil horizon, a thin layer on the desert surface where fine dust particles settle, pockets of air form beneath the soil. These cavities trap water at the surface to be used by soil microbes and desert plants when it’s needed most.
The recovery of these valuable BSCs after disturbance ranges from 10's to 1000's of years, depending on several environmental factors. “I think the real emphasis that should come out of these large timescales is to keep the crust intact in the first place,” says Williams. “If nothing else, it should give us an even bigger reason to protect soil crust.” Williams says there are ideas floating around in the scientific community of ways to encourage the reformation of damaged BSC's in arid environments. One involves restoring the fine-grained Av horizon and introducing microorganisms to begin the crust formation process. However, not much can be done currently to reestablish damaged BSC.
“Soil crusts play an active role in dust capture as well as the formation of important biosedimentary features that influence surface hydrology and the allocation of resources,” says Williams. “These crusts form important features that must be considered not only for the restoration of crusts but entire desert ecosystems in the future.”
View the abstract at https://www.soils.org/publications/sssaj/abstracts/76/5/1685