Soil structure and nutrients

Soils in arid lands are usually poorly developed, with little profile structure (Dregne 1976). Evaporation rather than leaching is the primary influence of water in the soil, at least for small or moderate rainfall events. Because most precipitation does not penetrate very far, biological activity (and thus organic matter and available nutrients) is usually concentrated near the surface. However, the surface is exposed to wind and water movement, which can dislodge and remove particles; thus erosion and transport processes are critical in the loss of nutrients from a site.

The nature of vegetation is critical in influencing these processes. Vegetation influences the "roughness" of the surface, which in turn influences the movement and erosive power of wind and water (e.g. Abrahams et al. 1994). The soil-binding properties of plant roots are especially critical in dune areas, where species that can stabilize sediments serve a keystone role in altering ecosystem structure (Klopatek and Stock 1994).

The predominance of evaporation in desert regions means that caliche or carbonate pans often develop (Dregne 1976). The rooting depth and transpiration rate of plant species exert a significant control on the moisture content of soil at various depths, and hence can influence the rate and location of caliche development (Schlesinger et al. 1987). In turn, caliche layers constrain the rooting depth for most plants, and exert a direct limitation on water storage and thus on plant productivity (Cunningham and Burk 1973).

Several groups of desert organisms play critical roles in the weathering of rock and other mineral surfaces, thus influencing nutrient availability for the entire system. Lichens and soil algae are obvious examples (see references cited in Evenari 1985), but invertebrates also play a surprisingly important role. Both snails and isopods process substantial amounts of soil during the few days a year that they feed on surface algae in the Negev (Shachak et al. 1976; Shachak and Steinberger 1980). Some desert snails are even capable of consuming rock and its lichens directly, thus contributing significantly to nutrient availability in the ecosystem (Shachak et al. 1987; Jones and Shachak 1990).

Much or most nutrient cycling is mediated by biological activity; thus the modification of soil microclimate by individual plants, and the reliance of microbes on conditions in the rhizosphere, mean that plant rooting zones will be regions of high localized microbe populations and high activity (Gallardo and Schlesinger 1992). This localization of microbial populations, together with the physical impact of plants on infiltration and organic matter inputs, is responsible for the well-known phenomenon of "islands of fertility", or localized areas of biological activity surrounding individual shrubs in arid systems (Garner and Steinberger 1989). Annual plants and other ephemeral species are distributed along the gradients of soil nutrients and conditions established by these shrub islands (e.g. Guiterrez et al. 1993). Differences in root system morphology can cause differences among shrub species in the gradients of soil nitrogen they enforce on surrounding soil (Garcia Moya and McKell 1970). Shrubs are also critical as sites of protection from erosion; they trap and protect dust and organic matter inputs (as svell as serving as a source of organic matter, of course). In this way they serve as reservoirs of nitrogen for the system (Garcia Moya and McKell 1970).

Another feature of arid and semi-arid lands is the importance of macro-organisms in decomposition. Shachak et al. (1976) found that processing of surface soii by isopods in the Negev had a significant positive effect on the rate of decomposition of organic matter in the system. Termites (Wood and Sands 1978; Whkford 1991), and the presence of vertebrate burrows and nests, can have dramatic effects on the rate of decomposition (Sehaefer and Whitford 1981; Wbitford and Parker 1989) by moving material below-ground and/or by moderating temperature and humidity. Thus the removal of one or more groups of species (e.g. all termites, or all burrowing rodents)

will have direct impacts on the rates and the spatial location of nutrient cycling processes.

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