Fabric of topsoil horizons in aridic soils of Central Asia

Two surface microhorizons – vesicular and platy – were found to be typical of the fabric of topsoil horizons in gravely loamy soils located along an arid/continental climatic gradient in the Subboreal zone of Central Asia (from southern Russia to Uzbekistan, Kazakhstan, and Mongolia). Their genesis is related to active vesiculation (for vesicular А1 microhorizon) and cryogenesis (for platy А2 microhorizon) against a background of high clay mobility and/or eolian accumulation of fine material. New mechanisms responsible for the formation of vesicular pores are suggested: (a) the leaching of dense complete crystal infillings (moldic voids) and (b) the development of biogenic capsules typical of iron bacteria that were diagnosed by the microbiological method of fouling Agar-covered glasses under natural conditions. In a series of studied soils, Yermic Regosols are characterized by additional specific microfeatures: (1) iron depletion from the material around the pores and formation of iron concentrations in the intrapedal mass owing to biogenic mobilization of iron with participation of sulfate- and iron-reducing bacteria and (2) the accumulation of organic matter (residues of microbial cells) in the vesicular pores.


Introduction
Arid desert and semidesert ecosystems in Eurasia occupy nearly 18 000 sq. km (Dregne 1976).They are characterized by the great diversity of soils -Kastanozems, Episalic Solonetz, Gypsic Calcisol, Yermic-Gypsic Calcisol, Yermic Regosol, and Yermic-Gypsic Regosol.The separate chapters of the recent monograph (Stoops et al. 2010) show the importance of micromorphological investigations for studying the topsoil horizons in aridic soils throughout the world (Pagliai and Stoops 2010;Gerasimova and Lebedeva-Verba 2010), however the data on the microfabric of virgin aridic soils in the Subboreal zone of Central Asia (in contrast to aridic soils of other continents and other climatic zones) are limited (Gubin 1984;Golovanov et al. 2005;Mees and Singer 2006;Lebedeva-Verba et al. 2009;Lebedeva-Verba and Gerasimova 2009).There are virtually no data on the study of fabric units within thin (from 1 mm to 5-6 cm) microhorizons in the topsoil layer of these soils (Pagliai and Stoops 2010).
A vesicular structure of topsoil horizons in the soils of arid territories is clearly manifested at the macro-and microlevels (Kubiёna 1938;Feofarova 1956;Lobova 1965;Tarchizky et al. 1984;Norton 1987;Pankova 1992;Dixon 2009).At the same time, vesicular horizons are also known in freezing virgin soils of high latitudes (FitzPatrick 1956;Harris and Ellis 1980;Van Vliet-Lanoё 1988, 2010;Gubin and Gulyaeva 1997).In temperate climates, vesicular crusts have only been described for cultivated humus-rich clayey soils (Norton and Schroeder 1987;Pagliai 1987).Micromorphological studies of topsoil horizons are of particular importance in the context of problems relating to the degradation of agrogenic soils and the development of desertification processes (Bishay and Stoops, 1975;Carnicelli et al. 1994;Pagliai and LaMarca 1979).
At present, the formation of a platy or lens-like structure in topsoil horizons of soils freezing at least to the depth of 5-15 cm is explained by the modern cryogenic processes (Rogov 2009;Van Vliet-Lanoё 2010).In the soils of arid territories within the Subboreal zone of Central Asia, topsoil horizons with a platy structure are developed under vesicular horizons (Gubin 1984;Lobova 1965;Pankova 1992;Lebedeva-Verba et al. 2009).Such horizons have not yet been described for warm aridic soils in the south of the Subboreal zone (Dixon 2009).
Diverse species of soil biota have been studied in the topsoil horizons of many aridic soils: algae, cyanobacteria, mosses, liverworts, lichens, bacteria, and fungi (Dunckerley and Brown 1997).In some cases, their amount is so great that a specific kind of topsoil horizonsbiological crusts-has been distinguished (Pagliai and Stoops 2010).However, data on the micromorphological diagnostics of biological crusts are not numerous (Elbridge and Greene 1994).
Topsoil horizons are the most dynamic horizons in the profiles of aridic soils.We argue that their properties explicitly reflect the modern ecological conditions of the soil development.As the environmental conditions and, hence, the soils of arid regions are very diverse, it is important to specify the particular objects of the study.In this paper, we only consider autonomous soils of Subboreal deserts along the aridity/continentality gradient from the south of European Russia to Uzbekistan, Kazakhstan, and Mongolia.
The micromorphological features of aridic soils are important indicators of their genesis because they are taken into account in classification decisions (Allen 1985).

Materials and Methods
These soils were classified according to the WRB system (IUSS Working Group WRB 2006).All the studied soils are developed from clay loamy sediments of different origins (Table 1).The groundwater depth at the studied sites exceeded 8 m, except for the Episalic Solonetz in the Caspian Lowland, where the groundwater table was found at the depth of about 3 m.Uzbekistan (Kudrin 1960;Umarov 1975;Sokolov et al. 1962;Lobova et al. 1977;Pankova 1992).Depending on the precipitation, the depth of soil moistening shows a change from 60 cm in the northern deserts to 20-25 cm in extremely arid deserts (Yakunin 1983).
As regards the vegetation one should notice that the Caspian Lowland is characterized by a three member soil-vegetation complex with wormwood-vitex association on Solonetzes, Agropyron-wild rye-chamomile (or chamomilevitex) association on light chestnut soils, and grassy-forb association on Mollic Kastanozems of microdepressions (Kamenetskaya 1952).
Episalic Solonetzes are characterized by sparse vegetation and often have vesicular crust and typical desert subcrust horizons developed under such conditions (Rode and Pol'skiy 1961).
Vegetation subzones of deserts in Kazakhstan and Mongolia are characteristic of decreasing cover (from 16% in northern deserts to 1% in extremely arid deserts).The subzone of northern deserts is specified by predominance of saxaul communities with saltwort on plateaus composed of the Cretaceous-Paleogene rocks.In the southern deserts the higher vegetation is only met in dry valleys, whereas the elevated flats reveal no higher vegetation and represent rocky deserts (hamadas) (Rachkovskaya 1993) digital camera Nikon E 4500.The description of soil fabric followed the guidelines by Stoops (2003).
The chemical and physicochemical analyses were performed in the Analytical Laboratory of the V.V. Dokuchaev Soil Science Institute according to the routine methods accepted in Russia (Vorob'eva 1998).
The soil-pH was determined in water extract (1:2.5)suspension.Texture was determined by pipette method with pyrophosphate pretreatment.
The carbonate content was determined by the acidimetric method, and the humus content, by the method of wet combustion.The exchangeable cations were determined by the Pfeffer method in modification by Molodtsov and Ignatova (1975).The total content of sulfate ions in order to estimate the gypsum content was detected by the method suggested by Khitrov (Khitrov and Ponizovskiy 1990).
Thin sections were prepared by M.A. Lebedev in the Laboratory of Soil Micromorphology and Mineralogy of the V.V. Dokuchaev Soil Science Institute.The area of thin sections prepared from the thin surface horizons was about 3 х 4 cm.In some cases, larger thin sections (5 х 6 cm) comprising both A 1 and A 2 microhorizons were examined.Three to four vertically oriented thin sections from the topsoil horizon were described for each soil profile.
To refine the diagnostics of the biological and biogeochemical activity of microorganisms in hamadas, the method of Rybalkina and Kononenko (1957) was applied for the Yermic Regosol (Prof.3-06).This method allows the study of active microbial communities developing on fouling glasses under natural conditions.Agar-covered glasses were exposed in the soil for 24 h immediately after heavy rain, when the upper soil horizons were moist.In the moist soil, swelling of the agar resulted in a favorable substrate for the activity of diverse viable microorganisms.After the extraction of glasses from the soil, the agar was subjected to dehydration, and the microorganisms became fixed on the glasses in the place they had developed in the moist soil.The glasses were examined under a Biomed 6 optical microscope equipped with Webbers camera.As a result, we obtained photos showing not only the spatial structure of the active microbial cenosis but also separate stages of the functioning of the microorganisms, including their growth, feeding, division, propagation, slime excretion, colony formation, destruction of minerals and synthesis of new minerals, and accumulation of nutrients and mineral substances.

Morphological and physicochemical properties
The topsoil horizons of the studied soil profiles had much in common: they were lighter in color and coarser in texture in comparison with the middle-profile horizons and contained two morphologically distinct surface microhorizons: a compact vesicular crust (A 1 -the upper part of the A horizon) of 1-3 cm in thickness and an underlying platy microhorizon (A 2 -the lower part of the A horizon) with a thickness of 1.5-2.0cm.Some physical and chemical properties of the studied soils are presented in Table 2.

The southern part of European Russia
Episalic Solonetz (Profile 69-07).In the field, the Episalic Solonetz of the solonetzic soil complex has a crusty topsoil with the upper dense A 1 microhorizon with vesicular porosity underlain by the A 2 microhorizon with platy structure.The topsoil horizon is characterized by a relatively high (for an aridic soil) humus content (2.1%).It does not, however, contain soluble salts, and has a very low carbonate content.Gypsum is absent in all of these topsoil horizons.The platy A 2 microhorizon has an increased content of exchangeable sodium in comparison with the vesicular A 1 microhorizon (Table 2).The soil texture is silty light loam.
Gypsic Calcisol (Profile 1B).The upper vesicular part of the topsoil A horizon is more compact than the lower part with the platy structure; the boundary between them is diffuse.The A horizon does not contain soluble salts and gypsum; it has a low humus content and contains carbonates (2.1%).
The soil-pH is neutral in these topsoil horizons, which is different from that in the other soils the properties of which are considered below (Table 2).
[ FABRIC OF TOPSOIL HORIZONS IN ARIDIC SOILS OF CENTRAL ASIA ] The samples were very friable and easily parted to small crumb aggregates and silty particles.
3 and gypsum; the calcium carbonate content reached 5.1-5.7%, and the humus content was less than 1% (Table 2).The soil texture was coarse silty loam.
The topsoil contains some amount of soluble salts with the predominance of chlorides and sulfates; the content of calcium carbonates reaches 8.1-9.4%,whereas the gypsum content is low (0.12-0.17%).The humus content in the A 2 microhorizon is higher than that in the A 1 microhorizon (0.39 and 0.18%, respectively) (Table 2).The soil texture is heavy: fine sandy clay in the A 1 microhorizon and fine sandy heavy loam in the A 2 microhorizon.
3.1.3.2.Soils developed from the Neogene deposits Yermic-Gypsic Regosol (Profile 5-06).The topsoil horizon in the field was relatively compact and had distinct vesicular porosity.The content of soluble salts is higher in the A 1 microhorizon, whereas the gypsum content is higher in the A 2 microhorizon (A 1 -2.6%,A 2 -11.57%); the calcium carbonate content is about 6.5% (Table 2).Soluble salts and gypsum, as well as the heavy texture of the topsoil (coarse silty light clay), are inherited from the saline Neogene clay.
Yermic-Gypsic Calcisol (Profile 28).With respect to the chemical and physicochemical properties, the topsoil horizon is generally similar to that in the Gypsic Calcisol.However, the humus and gypsum contents are somewhat higher: 0.44% and 0.25%, respectively (Table 2).The soil texture is loamy sand.
Yermic Regosol (Profile 10).A comparative chemical analysis of the upper vesicular and lower platy parts of the topsoil attests to certain differences between them (Table 2).The A 2 microhorizon is richer in humus (0.52%), gypsum (0.51%), and soluble salts (1.71%).The topsoil horizons in this soil profile have higher contents of soluble salts and exchangeable Na + in comparison with the other investigated aridic soils on Quaternary deposits of Mongolia (profiles 28, 30).This may be related to the presence of the derivatives of red-earth saline clay in the parent material (sandy clay loam).

Micromorphology
Table 3 illustrates the main micromorphological results.

The southern part of European Russia
Episalic Solonetz (Profile 69-07).The basic distinctive fabric of topsoil horizons in this soil is the gradual change in the microstructure from vesicular (Figure 2A) through platy-vesicular (Figure 2B) to platy (Figure 2C).Under higher magnification, the zones of iron depletion from the material around the pores are clearly seen.
The silt-size grains of quartz and feldspars are tightly packed and do not have coatings.
Upon the examination in reflected light the numerous iron nodules and the great number of the biogenic filaments (probably, algae and/or cyanobacteria) were clearly seen in the reddish brown groundmass (Figure 2D).The vesicular and platy porosity was developed in the siltyclay material characterized by a higher content of fine dispersed humus microforms without carbonates.
[ FABRIC OF TOPSOIL HORIZONS IN ARIDIC SOILS OF CENTRAL ASIA ] Gypsic Calcisol (Profile 1B).In the microfabric of the A 1 horizon, the fine sand particles are arranged in a nested pattern.The c/f related distribution is uneven in separate microzones, which are single spaced and double porphyric.Vesicular pores are allocated to the microzones with a higher content of phyllosilicates and micrite (Figure 2E).
Table 3.The main micromorphological features of the topsoil horizons In A 2 microhorizon the platy structure is poorly pronounced.Thin clayey coatings cover the walls of fissures provides evidence for clay mobility (Figure 2F).

Uzbekistan
Yermic-Gypsic Calcisol (Profile 111).The A 1 microhorizon is characterized by the nested distribution of fine sand particles; vesicular pores are confined to the zones with the fine monic c/f related distribution (Figure 2G).In the series of studied soils, this particular profile is characterized by the presence of recent eolian sediments above the buried crusty horizon.The eolian material contains the rounded porous crumb aggregates typical of Haplic Calcisols on the surrounding plain, the major soil type in this part of Uzbekistan.
The A 2 microhorizon is characterized by the crumb-platy aggregates; oval-shaped excrements of soil mesofauna are present in the pores; clay particles display banded orientation (Figure 2H).The platy aggregates are not very distinct.As well as the covering eolian layer, they include rounded microaggregates from Haplic Calcisols inherited from the previous stage of the eolian deposition.
3.2.3.Kazakhstan 3.2.3.1.Soils developed from the Quaternary deposits Gypsic Calcisol (Profile 1-06).According to the soil structure and character of pores in thin sections, the A horizon cannot be definitely subdivided into separate microhorizons.Porous crumb aggregates and compound packing voids predominate.A characteristic feature of the topsoil horizon in this soil is the great amount of filaments of desert mosses (Figure 3 A, B).
In separate zones without filaments, isometric pores can be seen.According to Pagliai and Stoops (2010), this horizon can be classified as a biogenic crust.The aggregating role of moss filaments is clearly seen in thin sections.In the lower part of the horizon, abundant organic residues are present.The peculiar feature of the A 2 microhorizon is the presence of micrite hypocoatings around large channels.
Yermic Regosol (Profile 3-06).Vesicular pores of two types are distinguished in thin sections from the A 1 microhorizon: (1) small pores (0.1-0.3 mm) within the light-colored slightly compact calcareous groundmass and (2) large pores (0.8-1.2 mm) open to the surface and partly filled with eolian material (Figure 3E).The eolian infillings contain very small fragments of plant tissues, algal cells and filaments visible under high magnification.Rock fragments are covered by dark iron-manganic coatings.Under them, there are abundant oval-shaped excrements of the soil microfauna.Platy aggregates appear in the A 2 microhorizon; on their upper faces, matrix pedofeatures composed of striated fine material are seen (Figure 3F).
The high diversity developed on the fouling Agarcovered glasses attests to the high activity of the soil biota under conditions of the strong and fast moistening of the surface horizons.The frequency of occurrence of different species of microorganisms as seen on the glasses was estimated.Special attention was paid to their functional specificity, including the formation of slime, as well as to the composition and location of mineral compounds on the surface of the microorganisms or inside (Zavarzin 1972).We suppose that the formation of microaggregates of fine material detected under rock debris in the A 1 microhorizon is related to the gluing action of the slime of microorganisms (Kutovaya et al. 2012).
Green, yellow-green (Figure 4A), diatomaceous algae (Figure 4B), and dinoflagellates were typical representatives of protists; the domain of Bacteria was represented by cyanobacteria  (Figure 4C), iron bacteria, and some others.
On the filaments of neutrophile Gallionella iron bacteria, membrane sacs and dilatations were partly or fully filled with amorphous iron compounds (Figure 4 D).Leptothrix ochracea, Leptothrix discophora, Kusnezovia polymorpha, and Siderocaps were also identified.At the same time, no fungal hyphae were detected on the fouling Agar-covered glasses and in inoculations from the same samples (Kutovaya et al. 2012).
In the zone with vesicular porosity and close c/f related distribution, separate lens-shaped gypsum crystals of different sizes are seen in the groundmass (Figure 3G).In the zone with the predominance of channel pores, gypsum intergrowths with features of their destruction and recrystallization take place; coatings and internal quasi-coatings are observed in large channels (Figure 3H).These coatings may have different compositions; namely, pure clayey coatings alternate with clayey-salt coatings.
The latter are virtually isotropic (XPL).As judged from the analysis of water extracts, the salt component is mainly represented by sodium chloride.Micritic coatings are seen on some of the rock fragments.
The A 2 microhorizon does not display the platy structure: crumb aggregates predominate.
The microzonality of the optical orientation of clayey material is observed: zones with crystallitic, granostriated and speckled b-fabric can be distinguished.Impregnative micritic pedofeatures are clearly seen.Vesicular pores occur in the zones enriched in clayey plasma (Figure 5B).
Yermic-Gypsic Calcisol (Profile 28).The A 1 microhorizon is characterized by the predominance of vesicular pores (Figure 5C), though there are separate zones with compound packing voids.Among the vesicular pores, there are pores with smooth and with nipple-shaped walls.In the zones enriched in clay material, vesicular pores with layered Fe-clay coatings are seen (Figure 5D).
The A 2 microhorizon in thin sections does not display a distinct platy structure.
Yermic Regosol (Profile 10).The vesicular A 1 microhorizon in the Yermic Regosol (profile 10) differs from those in the Gypsic Calcisols and Yermic-Gypsic Calcisols in (1) the predominance of the zones with clayey-calcitic fine material having crystallitic b-fabric; (2) the presence of infillings with the high content of the cells of microorganisms visible under high magnification (Figure 5E) in the rounded pores; and (3) the presence of the iron-depleted zones around the pores (Figure 5F) with the formation of iron concentrations in the intraped mass (Figure 5F).
The A 2 microhorizon is distinguished from the A 1 microhorizon by the abundance of micritic calcite (carbonate impregnation), the predominance of platy peds, and fine planar pores.However, very fine vesicular pores are present inside the platy peds.

Soils developed from the Cretaceous-Paleogene deposits
Yermic-Gypsic Regosol (Profile M43).The A 1 microhorizon of these soils is especially peculiar due to the "vesicular porosity" and diverse salt and gypsiferous pedofeatures.Its fine material is clayey or carbonate-clayey.The soils reveal the dissolution of rounded nodules of soluble salts (thenardite) with the development of moldic voids (Figure 5G).The diagnostics of thenardite was based on optical characteristics of its crystals (Mees and Tursina 2010).As a result, such a "vesicular porosity" occurs.

A B C D E F G H
The A 2 microhorizon is characterized by the high porosity and the predominance of matrix pedofeatures in the form of thin lens-like gypsum crystal intergrowths (Figure 5H).The platy aggregates have not been found.

Vesicular porosity
The obtained data indicate that the formation of vesicular porosity in the studied soils is no less active than in the soils of the other arid regions of the world (Springer 1958;Lobova 1965;Evenari et al. 1974;Dregne 1976;Dixon 2009).
The vesicular pores have been found in the horizons with different compositions of the soil micromass: humus-clayey, clayey, or calcitic.The presence of vesicular pores in the horizons with different contents of carbonates allows us to suppose that their origin in the topsoil horizons may be related to different processes, which are the following: 1.In case of soda-saline soils, the phase transition of sodium bicarbonates into sodium carbonates with the release of CO 2 can take place upon the rapid drying of the soil.In particular, this may happen in spring in the westernmost and the least aridic of the studied soils-the Episalic Solonetzesthe surface horizons of which are depleted of soluble salts, and the high ESP (>10%) attests to the possibility of the soda formation.This increases the mobility of the clay material and results in the formation of the middle-profile solonetzic horizon with abundant clay coatings (Bouza and Valle 1993).
2. In case of the increased content of calcium carbonates, shifts in the calcium bicarbonate-carbonate equilibrium toward calcite precipitation with the release of CO 2 into the gaseous phase may take place upon changes in the soil moisture and temperature.This mechanism is particularly important during the spring maximum of precipitation, which is typical of all the studied territories except for the Trans-Altai Gobi in Mongolia, where rains occur in the summer, winters are snowless.
3. In weakly structured bare soils that are virtually devoid of the organic matter (e.g., extremely arid Yermic Regosols), the formation of vesicular pores may be due to the rapid displacement of air adsorbed on the solid soil particles by the water of summer showers; this may also take place upon the rapid soil freezing (Rogov 2009;Van Vliet-Lanoë 1985, 1988, 2010).The freezing depth in the studied soils of Mongolia reaches 1 m and more.
4. Some of the vesicular pores in the crust horizon may have the biological origin; they represent membrane sacks of iron bacteria that have been identified in the soils by the method of fouling Agar-covered glasses in natural conditions (Rybalkina and Kononenko 1957).Such configurations are physiologically important for the iron bacteria; they serve as reservoirs for concentration of amorphous iron compounds (Figure 4D).It can be supposed that some smallest (about 50-100 μm) rounded pores may represent the biogenic capsules (Lengeler et al. 1999;Kutovaya et al. 2012).Such small pores can be found in different types of arid soils (Figure 2E; 3G; 5A, C, E).
5. Rounded pores were also diagnosed in the Yermic Gypsic Regosols developed from gypsiferous Cretaceous-Paleogene deposits (Figure 5G).We suppose that their origin is related to the local leaching of rounded thenardite nodules (the remains of this salt can be diagnosed under high magnification).Thus, rounded pores may be the result of leaching of these nodules.
Thus, the micromorphological study of the Subboreal aridic soils along the gradient of aridity and continentality allows to suppose the polygenetic origin of the vesicular topsoil horizon typical of all of these soils.The contributions of particular processes to the origin of this horizon differ in dependence on the local conditions.

Platy horizon
Soil freezing is a typical process affecting the topsoil horizons of all the studied soils.Platy aggregates with a gradual change of particle sizes from the top to the bottom of plates (which is opposite to the contrast sorting of particles in sedimentary crusts) serve as a diagnostic feature of the process of cryogenic structuring (Rogov 2009;Van Vliet-Lanoë 2010).The manifestation degree of platy aggregates depends on the texture of surface horizons and on the soil water content before freezing.The most distinct platy aggregates are developed in well-sorted silty clay loams in arid regions with a precipitation maximum in autumn.In the studied soils a well-pronounced platy structure is typical of the A 2 microhorizon in the Episalic Solonetz.Micromorphological studies indicate that the vesicular A 1 microhorizon is gradually transformed into the layered A 2 microhorizon, what can be clearly traced in thin sections (Figure 2A-2C).A distinct platy structure is also typical of the aridic soils developed from the silty clay loamy deposits in Kazakhstan (Figure 3D, F).In both cases, the microhorizon with a platy structure (A 2 ) forms a paragenetic association with the overlying vesicular microhorizon (A 1 ).Good cryogenic structuring of the A 2 microhorizon is observed in the aridic soils subjected to freezing in a relatively moist state after autumn rains.The freezing of aridic soils in Mongolia takes place in a virtually dry state, so the platy structure is not pronounced in them.Also, we suppose that a relatively high content of soluble salts and gypsum inherited from the ancient saline deposits retards the formation of platy aggregates.
The development of the banded distribution of clay-iron-humus fine material in the A 2 microhorizon may be the result of two processes: (1) the cryogenic sorting of sandy and fine silty and clayey particles and (2) the activity of cyanobacteria that considerably decrease the water infiltration through the plates (Fox et al. 2009).

Iron mobility
Some fabric units in the studied soils attest to the mobility of iron in the vesicular horizons of aridic soils.Thus, in Episalic Solonetzes developing under moderately arid conditions (with the MAP reaching 30 mm), the zones of iron depletion are diagnosed around the vesicular pores, and fine iron nodules are formed in the intraped mass of A 1 and A 2 microhorizons (Figure 2A, D).The small sizes of iron nodules and the sharp form of boundaries between them and surrounding groundmass testify to the rapid formation of nodules due to the contrasting redoximorphic conditions in the processes of wetting and drying.
We suppose that the major role in the mobility of iron oxides belongs to the soil biota; in particular, to iron bacteria.It is interesting that similar fabric units have been diagnosed in the soils of extremely arid deserts in Kazakhstan and Mongolia.The study of soil microbiota in Yermic Regosols of Kazakhstan by the method of fouling Agar-covered glasses under natural conditions allowed the identification of alive and active species of neutrophile Gallionella iron bacteria (Figure 4D) and different species of Leptothrix iron bacteria (Kutovaya et al. 2012).
The activity of these bacteria is greatly enhanced during short periods of the soil moistening and may contribute to the formation of iron-depleted zones around the vesicular pores and iron concentration in the matrix pedofeatures.The possibility of periodical local hydromorphism in the extremely aridic soils is also proved by the presence of diatom algae in them (Figure 4B).

Migration of salts, gypsum, and carbonates
A comparative micromorphological study of the same type of soils (Yermic Gypsic Regosols) developed from the similar parent materials in the areas with different climatic parameters attests to different intensities of the migration of soluble salts, gypsum, and calcium carbonates in them.
In particular, the different amount and character of carbonate pedofeatures may be explained by differences in the seasonal distribution of precipitation.The highest amount of diverse calcitic pedofeatures (coatings, nodules, micritic impregnation of the groundmass) was identified in the deserts of Uzbekistan and Kazakhstan and in the northern deserts of Mongolia.In these regions, the maximum precipitation is typical of the cold seasons, which specifies the higher mobility of carbonates.

Clay mobility
The presence of clayey coatings in the vesicular pores of Yermic Regosols and Yermic Gypsic Regosols in Kazakhstan and Mongolia (Figure 5D) can be explained by the lateral migration of clay particles under conditions of the increased alkalinity in the topsoil horizons.

Eolian input of silty material
This may play a significant role in the development of the topsoils.In some cases, rounded soil aggregates (0.2-0.3 mm) typical of Sierozemic loess soils subjected to wind erosion are found in the topsoil horizon of aridic soils of Uzbekistan.The admixture of eolian material is also seen in the vesicular pores of the aridic soils of Kazakhstan (Figure 3E).These rounded soil aggregates are clearly identified in the soil mass because of the increased content of silt particles, distinct boundaries, and specific fabric differing from the fabric of the enclosing groundmass.
The boundary between the eolian material and the crust surface is also clearly traced (Gerasimova and Lebedeva-Verba 2008).If the soil skeleton consists of nonsorted fine sandy and silty particles, their distribution in the soil mass has a clustered (porphyric) pattern.In this case, the A 1 microhorizon contains not only the vesicular pores but also packing voids shaped by the aggregates enriched in such particles.
The great aggregation role in the soils developed from the eolian sediments belongs to the soil biota and, in particular, to mosses (Figure 3A).Elbridge (1994) also described how mosses collect eroded and transported aggregates and grains contributing to the surface stability.

Conclusions
(1) The results of chemical and physicochemical analyses of aridic soils with similar morphologies of topsoil horizons indicate that the A 1 and A 2 microhorizons are formed under a relatively wide range of bioclimatic conditions; they are formed from parent materials differing in their genesis, age, and salinization degree.
(2) Vesicular pores in the topsoil of aridic soils are formed in the zones with the fine monic c/f related distribution and with different mineral compositions.The composition of the fine material (the presence of carbonates, clay and silt particles, soluble salts, and gypsum) specifies the particular process of the development of vesicular porosity that can be considered as an annually reproducible element of the soil fabric.With increasing aridity, the density and thickness of the vesicular horizon is increased against the background of the mobility of clayey particles.
The major process of their development is related to the emission of gases (in the form of bubbles) upon the active biochemical decomposition of organic matter with active participation of the soil microorganisms and upon phase transition of sodium and calcium bicarbonates into carbonates during the topsoil drying.In the extremely aridic soils developing from the ancient saline clayey sediments, the formation of rounded moldic pores may be due to the local dissolution of salt nodules.Some part of the rounded (vesicular) pores may be created by the microorganisms and represent specific biogenic capsules and bulges that are physiologically important for iron bacteria.
(4) The platy structure of the freezing aridic soils in the Subboreal zone of Central Asia is created by the cryogenic structuring.
(5) The soils of extremely arid deserts have fabric units attesting to the periodical mobility of iron oxides in the anaerobic conditions.Such conditions may be periodically created in the soils after strong rainfall.A significant role in these processes is played by iron-reducing and iron-oxidizing bacteria.

Acknowledgments
This study was supported by the Russian Foundation for Basic Research, project no.12-04-00990.

Figure 5 .
Figure 5. Fabric of topsoil horizons in aridic soils of Mongolia (XPL).(A, B) Gypsic Calcisol, profile 30: (A) vesicular pores in the calcite-rich material and calcite pendants on quartzite fragment in the A 1 microhorizon; (B) vesicular porosity in the clayey material of the A 1 microhorizon.(C, D) Yermic-Gypsic Calcisol profile 28: (C) zone of abundant vesicular pores, and (D) vesicular and nipple-shaped pores with layered iron-clayey coatings.(E, F) Yermic Regosol, profile 10: (E) infilling of fine gray material with the high content of humus and microbial cells (1) and (F) iron depletion from the material around the pores and iron concentration in the intraped mass.(G, H) Yermic-Gypsic Regosol, profile 43M: (G) the high porosity with a predominance of moldic pores and the remains of thenardite infillings and (H) openwork porosity; matrix pedofeatures with fine lenticular gypsum intergrowths.

Table 1 .
Location of the profiles, topography and parent materials