Desert ecosystems

On the basis of variations in the habitat characteristics (landform and soil characteristics), the floristic composition and the dominant species, Omar [9] suggested six ecosystems coastal plain and lowland ecosystem desert plain ecosystem alluvial fan ecosystem escarpment, ridge and hilly ecosystem wadi and depression ecosystem and burchan sand dune ecosystem. Each of these ecosystems is characterized by a dominant plant community and associated with several other species. 

Holmes RM, Fisher SG, Grimm NB (1994) Parafluvial nitrogen dynamics in a desert stream ecosystem. J North Am Benthol Soc 13 468-478... 

In many ecosystems, plants tend to pattern themselves as pure stands or as individuals spaced in rather specific densities or configurations. Many desert species show obvious zones of inhibition around which few, if any, alien species are able to invade. These patterns often cannot be adequately explained by competition alone, and are probably caused by a combination of factors including allelopathy. The phenomenon happens with herbaceous plants as well as woody shrubs and trees. 

Table 1. The annual biogeochemical fluxes and pools in Steppe and Desert natural ecosystems (after Rodin et al., 1975).

The biomass of arid ecosystems is significantly less than that of Forest ecosystems and changes from 10 to 25 ton/ha, by dry weight, in Steppe, from 4.0 to 4.5 ton/ha in Desert and from 2 to 3 ton/ha in Extra-Desert Ecosystems of the Central Asia. The overall biomass of Arid Steppe and Desert ecosystems is an order of magnitude less than that of Forest ecosystems (Rodin et al., 1975). 

The ash content of Arid Steppe and Desert ecosystem vegetation is about 2 times higher than that of forest species. Accordingly, the biogeochemical fluxes of elements are similar to those in the forest ecosystems, in spite the smaller biomass (see above). The compartments of biogeochemical turnover in Steppe and Desert ecosystems are shown in Table 1. 

The characteristic biogeochemical feature inherent in all Steppe and Desert ecosystems is the most intensive cycling of different chemical species in comparison with forest ecosystems. For a Steppe ecosystem the biogeochemical cycle is 2-3 years and this means that the complete renewal of all ecosystems biomass takes place over this period. Remember that in Forest ecosystems the biogeochemical cycling is about 3->25 years and even about 50 years in Forest Swamp ecosystems. The turnover is the highest in Ephemeral Desert and gradually decreases to the north. 

The vast area of Arid and Semi-Arid ecosystems of Central and East Asia is subject to wind erosion. The major natural sources of dust emission are Gobi desert (Xinjiang... 

Role of Humidity in Soil Exposure Pathway Formation in Steppe and Desert Ecosystems... 

The water deficiency in Arid ecosystems is the main restricting factor for biogeochem-ical exposure processes. We know that many links of the biogeochemical food web are connected in Steppe soils with invertebrates. Their population varies very much in Steppe ecosystems depending on the moisture conditions (Table 6). For instance, the wet biomass of soil invertebrates in the Meadow Steppe and Forest Steppe ecosystems exceeds that for the Extra-Dry Rocky Desert ecosystems by 150-300 times. 

The content of heavy metals in Steppe soils is tightly connected with their contents in geological rocks. In formation of soil exposure pathways in Desert ecosystems, water-soluble forms of these metals play the most important role. We can see an analogy between the increasing content of elements in soil dead organic matter as a function of decreasing water excess in Forest ecosystems and the increasing content of water-soluble species of chemical elements in the soils of Dry Steppe and Desert ecosystems as a function of enhanced aridity. The accumulation of water-soluble species occurs in the upper horizon for almost all elements, with exception of strontium. The main factor responsible for the accumulation of water-soluble forms is connected with evapotranspiration. 

The existence of an evapotranspiration barrier in the upper soil horizon of Dry and Extra-Dry Desert ecosystems favors the accumulation of alkalinity and alkaline reaction of soil solution. In turn this accelerates the mineralization of organic matter and mobilization of finely dispersed mineral and organic suspensions. This fact provides a plausible explanation of the occurrence of some heavy metals, like Zr, Ti, Ga, Yt and their congeneric elements in the aqueous extracts from soil samples of Dry Desert ecosystems. 

The extraction by 1N NCI yields 5-10% of total heavy metal content. In case of Fe and Mn, these values are even higher. The maximum contents of mobile fractions of trace elements are monitored in the upper horizon. Thus, the role of evapotranspiration barrier in biogeochemical migration of elements in Dry Desert ecosystems pays a very important role in pollutants exposure. 

In Desert ecosystems similar to Steppe ecosystems the plants distinctly exhibit their biogeochemical specificity. We can consider the distribution of heavy metals in Dry Desert ecosystems of the Ustyurt Plateau, Kazakhstan, with predominance of wormwood (Artemisia terrae albae) and saxaul (Anabasis salsa). In rubble stone territories, of common occurrence is the dense shrubbery of Sasola anbuscula. Most elements found in the wormwood occur in their highest concentrations. In the roots of the wormwood and saxaul, higher contents of Mn, Cu, Mo, and Sr have been monitored, whereas the aerial parts contain more Ti, V, and Zr. We can see that the root elements are most biologically active and those in aerial parts, more inert. Possibly their presence was related to the dust exposure and deposition on the plant exterior (see above). 

Despite the quantitative variability of salts and silicate dust particles in the plants of Arid ecosystems, we can easily discern a trend towards the selective uptake of trace elements. The calculation of coefficient of biogeochemical uptake (Cb) shows the rates of exposure to heavy metals in biogeochemical food webs. One can see that the elements contained in the plant species of both Steppe and Desert ecosystems are in equal measure susceptible to the influence of environmental factors. The most extensively absorbed are Sr, Cu, Mo, and Zn. Their values of Cb are more than unit. The group of other elements, like Ti, Zr, and V, are poorly taken up, with their values of Cb often dropping below 0.1 (see Figures 4 and 5). 

Figure 4. Coefficients ofbiogeochemical uptake of trace metals by plant species of the Ustyurt Plateau Dry Desert ecosystems. 1 —wormwood (Artemisia terrae albae), aerial parts 2—roots 3—saxaul (Anabasis salsa), aerial parts and 4—roots (Dobrovolsky, 1994).

Figure 5. Coefficients of bio geochemical uptake of trace metals by cenospecific plant species of Gobi Extra-Dry Desert ecosystems, Central Asia. 1—Flaloxylon ammodendron 2—Iljina regeli 3—Ephedra Przewalskii 4—Anabasis brevifolia (Dobrovolsky, 1994).

The general trend towards increase of ash elements in the plants of steppe ecosystems from Dry to Extra-Dry Desert ecosystems does not seem to affect the Cb values appreciably (see Box 2). 

Elements (desert plains) (desert depressions) (desert depressions) ecosystem)... 

In plain autonomous ecosystems the fluxes of sodium are less 40 g/ha/yr and those of Mg are less than 10 g/ha/yr. For iron these values are close to 1 g/ha/yr, and for all heavy metals, are between 0.01 and 0.04 g/ha/yr. In the geochemically subordinate landscapes (Naloxylon ammodendron and Ephedra przewalskii ecosystems) which receive additional moisture and chemical elements, the biogeochemical exposure fluxes are 360-912 g/ha/yr for Mg and Na, and from 0.44 to 6.65 g/ha/yr for heavy metals. In the periphery of the Gobi desert, Anabasis brevifloria and Graminaceae Dry Desert ecosystems show the overall increase of biogeochemical fluxes. The turnover for some elements (Mg, V, Cr) rises but slightly in comparison to their turnover in Extra-Dry ecosystems, whereas the turnover for other elements (Sr, Zn, Cu) increases several times. 

Biogeochemical cycling of elements and pollutants exposure pathways in the tropical ecosystems, which occur between 30°N and 30°S, are both intensive and at high probability of risk for human and ecosystem health. The tropical belt receives about 60% of solar radiation inputting on the Earth s surface. The total area of tropical ecosystems is about 40 x 106 km2, with exception of the High Mountain and Extra-Dry Sandy Deserts with strongly depressed life processes. 

The proportion of areas with different precipitation rates varies from continent to continent. For instance, different arid ecosystems, from Dry Savanna to Extra-Dry Desert, are predominant in India and Australia. To a lesser degree these ecosystems occur in Central and South America. In an equatorial belt of Africa, the distribution of areas with different precipitation is shown in Table 1. 

We can see that the Tropical Rain Green Forest ecosystems occupy about 1/5 of the African equatorial belt, whereas about 1/2 of this area is Woody and Tall Grass Savanna ecosystems. The rest of the area are occupied by various Dry Steppe and Dry and even Extra-Dry Desert ecosystems, like the Sahara, with annual rainfall less than 200 mm. As it has been mentioned above, the amount of precipitation is of high significance for exposure pathways of pollutants. 

Biogeochemical Cycling and Pollutant Exposure in Dry Desert Tropical Ecosystems... 

Dry desert tropical ecosystems occupy 4.5 x 106 km2, or 3.0% of total land area of the Earth. These ecosystems have dry periods during 7-10 months a year. Not only trees, but also numerous grasses cannot grow in such severe conditions. The vegetation... 

Table 10 characterizes the plant biomass of the Tar Dry Desert ecosystems. 

Table 10. Biomass and net primary productivity of the Tar Dry Desert ecosystem, India.

Eurasia is the biggest continent of the World. Because of the huge size of Eurasia, all types of ecosystems and climatic belts are represented, from arctic deserts up to tropical rain forests. Accordingly, some characteristic examples will be given here with special attention to biogeochemical provinces where biogeochemical exposure pathways induce the relevant diseases. 

The low-Se ecosystems occur mainly in and near the temperate forest and forest steppe landscapes as an axis in China, and the relatively high-Se content in the ecosystems usually appear in the typical humid tropical and subtropical landscapes and typical temperate desert and steppe landscapes. 

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