Environmental Engineering Reference
In-Depth Information
and Belnap, unpub. data; Evans and Ehleringer, 1993). Biological soil crusts are also
important sources of fixed carbon on sparsely vegetated areas common throughout
the arid West (Beymer and Klopatek, 1991). Plants growing on crusted soil often
show higher concentrations and/or greater total accumulation of various essential
nutrients when compared to plants growing in adjacent, uncrusted soils (Belnap and
Harper, 1995; Harper and Pendleton, 1993).
Cryptobiotic soil crusts are highly susceptible to soil-surface disturbance such as
trampling by hooves or feet, or driving of off-road vehicles, especially in soils with
low aggregate stability such as areas of sand dunes and sheets in the southwest, in
particular over much of the Colorado Plateau (Belnap and Gardner, 1993; Gillette
et al., 1980; Webb and Wilshire, 1983). When crusts in sandy areas are broken in
dry periods, previously stable areas can become moving sand dunes in a matter of
only a few years.
Cyanobacterial filaments, lichens, and mosses are brittle when dry, and crush
easily when subjected to compressional or shear forces by activities such as tram-
pling or vehicular traffic. Many soils in these areas are thin and are easily removed
without crust protection. As most crustal biomass is concentrated in the top 3 mm
of the soil, even very little erosion can have profound consequences for ecosystem
dynamics. Because crustal organism are only metabolically active when wet, re-
establishment time is slow in arid systems. Although cyanobacteria are mobile and
can often move up through disturbed sediments to reach needed light levels for pho-
tosynthesis, lichens and mosses are incapable of such movement and often die as a
result. On newly disturbed surfaces, mosses and lichens often have extremely slow
colonization and growth rates. Assuming adjoining soils are stable and rainfall is
average, recovery rates for lichen cover in southern Utah have been most recently
estimated at a minimum of 45 years, while recovery of moss cover was estimated at
250 years (Belnap, 1993).
Because of such slow recolonization of soil surfaces by the different crustal com-
ponents, underlying soils are left vulnerable to both wind and water erosion for at
least 20 years after disturbance (Belnap and Gillette, 1997). Because soils take 5000
to 10,000 years to form in arid areas such as in southern Utah (Webb, 1983), acceler-
ated soil loss may be considered an irreversible loss. Loss of soil also means loss of
site fertility through loss of organic matter, fine soil particles, nutrients, and micro-
bial populations in soils (Harper and Marble, 1988; Schimel et al., 1985). Moving
sediments further destabilize adjoining areas by burying adjacent crusts, leading
to their death, or by providing material for “sandblasting” nearby surfaces, thus
increasing wind erosion rates (Belnap, 1995; McKenna-Neumann et al., 1996).
Soil erosion in arid lands is a global problem. Beasley et al. (1984) estimated
that in the rangelands of the United States alone, 3.6 million hectares have under-
gone some degree of accelerated wind erosion. Relatively undisturbed biological
soil crusts can contribute a great deal of stability to otherwise high erodible soils.
Unlike vascular plant cover, crustal cover is not reduced in drought, and unlike rain
crusts, these organic crusts are present year-round; consequently, they offer stabil-
ity over time and under adverse conditions that is often lacking in other soil surface
protectors.
