Biological soil crusts and wind erosion

Wind is an important erosive force in deserts, where limited cover of vascular plant material offers little soil-surface protection. Dust deposition by wind often exceeds that of fluvial deposition in these drier regions (Goudie 1978; Williams et al. 1995). Sediment production from soil surfaces occurs when wind forces exceed soil threshold friction velocities (TFV: the wind velocity needed to detach particles from soil surfaces). Decreased TFVs are directly associated with increased sediment movement. Soil fine particles are preferentially lost over larger sand particles (Leys 1990; Williams et al. 1995). Increased sediment movement can result in many direct and indirect problems for semiarid and arid ecosystems. Soils weather slowly from parent rock in deserts, often taking 5000-10000 years (Webb and Wilshire 1983). Much of the soil fine material found in these regions is from atmospheric deposition (Danin and Yaalon 1982). Soils from the Colorado Plateau deserts show that most fine particles are derived from surrounding igneous mountains. Current deposition rates in these regions are low and when soils are disturbed, loss rates may far exceed deposition (Gillette et al. 1980; Offer et al. 1992; Belnap and Gillette 1997,1998; Reynolds et al. 1998). Most of the soil photosynthetic productivity and nitrogen (N) fixation in desert soils is concentrated within 3 mm of the surface (Garcia-Pichel and Belnap 1996; see Chap. 16). Thus, only a little soil loss can significantly reduce C and N inputs from these organisms (see Chaps. 18, 19). In addition, the top few mm of soil contain a much higher percentage of soil fine particles than underlying soils (Danin and Ganor 1991; Verrecchia et al. 1995). Loss of soil fines can reduce site productivity, as plant -essential nutrients are often bound to these particles. Burial of nearby biological soil crusts from windblown sediments generally means death for the photosynthetic components of the soil crusts, further reducing fertility. Reduced fertility of systems is one of the most definitive, and problematic, aspects of desertification. Worldwide increases in windborne sediments are amply documented (Goudie 1978; Kovda 1980; Tsoar and Pye 1987). Both plant and soil characteristics influence wind erosion. In deserts, vascular plants are generally short with sparsely vegetated stems, and very large spaces occur between individuals. Plant litter cover is also very low. Thus, plant materials in semiarid and arid regions offer limited protection to soils from wind erosion. In such environments, biological soil crusts can playa critical role in soil stabilization