Desert-Steppe ecosystems

We will consider both Steppe and Desert ecosystems as deficient in atmospheric humidity (evapotranspiration exceeds precipitation). The Sub-Boreal, Semi-arid and Arid Steppe and Desert ecosystems occupy a significant part of the global area. This territory includes Sub-Boreal zones (Steppe, Arid Steppe, and Desert Steppe ecosystems) with total area of 9.23 x 10 km, as well as subtropical zones (Shrub Steppe and Desert Steppe ecosystems) with total area of 7.04 x 10 km. These areas do not include Subtropical and Tropical Sandy Desert ecosystems (5.77 x 10 km-) and Stony Desert ecosystems (8.96 x 10 km ). Thus the extra-tropical arid area takes about 20% of the World s terrestrial ecosystems. Most of this area goes to the inter-continental regions of Eurasia and, partly, of North and South America. The biogeochemistry of semi-arid and arid ecosystems shows distinctive parameters, which allow us to consider the quantitative features of element turnover and dynamics in natural fluxes. 

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

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 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. 

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). 

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). 

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. 

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

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. 

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). 

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. 

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. 

The landlocked Caspian Sea is the largest inland body of water on Earth. Surrounded by Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan, the Caspian Sea is home to myriad ecosystems. At the meeting point of the Middle East, Europe, and Asia, the Caspian region includes steppe land in the north, cold, continental deserts and semi-deserts in the northeast and east, and warmer mountain and highland systems in the south and southwest. The coastal wetlands of the Caspian basin include many shallow, saline pools, which attract a variety of bird life and biodiversity over 400 species are unique to the Caspian. In addition, the sea s native sturgeon is famous the world around for the roe it produces sturgeon from the Caspian Sea accounts for approximately 90% of the world s caviar industry. 

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 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... 

The smaller Cw values are also connected with water deficiency in Steppe and Desert ecosystems. However, the concentration of various chemical species in rainwater of background regions is higher that that in the Forest ecosystem belt. The major reason is the wind deflation of the soil s surface owing to lack of tree species and only a lean protective layer of grasses and half-shrubs. A large mass of soil particles becomes entrained into the air migration. The most characteristic example is connected with the Yellow sand phenomenon (see Box 7). 

What are the general features of Steppe and Desert ecosystems Emphasise the role of humidity and aridity in the formation and proceeding biogeochemical cycles of various elements. 

Discuss the role of various plant species in migration and accumulation of trace metals in Steppe and Desert ecosystems. What parts of herbaceous species are the main accumulators of different elements ... 

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