SOIL CRUSTS
A note published in the MGS Science Newsletter, 2011 described the value of mulches in fynbos soils (these occur in the Cape region of South Africa) and explained that some mulches encouraged the build-up of damping-off diseases leading to plant failure. Unmulched soils appeared not to have these problems.
MGS member G. Klein has now sent new research by Mandy Williams of the American Society of Agronomy and Soil Science which describes the complex blend of microorganisms carpeting arid environments as biological soil crusts (BSC). These organisms fuse with soil particles, stabilizing desert crusts and forming fragile peaks in the soil that influence a variety of processes. Williams says, "These crusts…act like a living mulch across a desert, by protecting the surface from erosion. Once you disturb the soil surface, you're more likely to lose what little resources are available there."
Micromorphological investigations of BSC samples from the Mojave Desert show that a crust 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 crust to expand and contract, leaving cracks in the crust that trap dust as it blows over the desert surface. This dust is an important source of nutrients.
Meanwhile, cyanobacteria weave around particles in the soil, forming thick layers of fine grains by trapping sediments in a 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 also impacts water distribution in arid environments. Fine dust particles settle and 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.
This new research reveals, that recovery of valuable BSC after a disturbance, can take years, depending on several environmental factors. Williams.says, "These crusts form important features that must be considered, not only for the restoration of crusts, but for entire desert ecosystems in the future."
(Note for mediterranean gardeners: keep cultivations to an absolute minimum in order to conserve soil moisture)