Patterns of soil cover organization within the northern part of the Kondinskaya lowland (Western Siberia)

At the same time, conditions of soil formation in the taiga zone can differ significantly within the vast territory of Western Siberia, especially in the peripheral parts of the lowland, since they have a different history of geological and geomorphological evolution, a more dissected relief, and a greater variety of parent rocks. For example, such a situation is typical of the near Ural regions of the Khanty-Mansi Autonomous Okrug, in particular, in the north of the Kondinskaya lowland [9]. The territory under consideration is characterized by the alternation of boggy plains with ridges (“mineral islands”) covered with pine forests [10]. Soil cover is much more contrasted under such conditions: automorphic and hy- dromorphic soils are clearly delineated without gradual transitions through series of semihydromorphic soils, and the erosional processes are more intensive. Al-Fe humus soils prevail on sandy and, to a lesser extent, on loamy sediments in well- drained positions, while vast floodplains and boggy plains are mainly occupied by organogenic peat soils. Thus, under conditions of well-drained ridges, the organization of soil cover is controlled by lithological-geomorphological rather than hydrological factors.

Another important feature of the territory under consideration is the wide distribution of binomial covering sediments that significantly complicate soil cover within the elevated landscape positions. It is obvious that sandy covering deposits are an integral component of the loamy-sandy lithological framework of soil-forming sediments within the territory of Northern Eurasia. This fact can be explained by landscape peculiarities and the geomorphological evolution of this territory in the Late Quaternary, which significantly contributed to the wide distribution of three types of glacial and periglacial deposits: moraines, loesses, and sandy deposits [11-12]. These types of covering sediments alternate with a gradual increase in the proportion of sands and moraines to the north of the region, along with a decrease in the proportion of subaerial loams [13]. Note that soil formation on different parent rocks representing the lithological framework of covering deposits within Northern Eurasia is significantly uneven, and soils developed on sands as well as on binomial deposits; when loams underlie sands, they are the most unexplored in terms of the geographical aspect. Soils with abrupt textural contacts are most typical of Europe and Scandinavia [14-19], while for Western Siberia, they are less characteristic and can only appear in the middle and northern taiga, in accordance with the spread of moraine-like deposits and silt-dominated periglacial covers [20]. It is also important to mention that soils formed on binomial sediments are much less studied for the middle taiga zone of Western Siberia in comparison with Central and Eastern Europe and Scandinavia. At the same time, they are of significant interest for better understanding how soil formation occurs on substrates with abruptic textural contacts under continental climate conditions.

The studies devoted to the diverisity of soils are especially important for the territory of middle taiga of Western Siberia, as as they can be basis for the protection and monitoring of soil cover in the context of anthropogenic impact due to the exploration of oil and gas fields. Recent studies performed for the reserves within the bogged territories of the East European Plain showed that soils of elevated ridges inside the bog massifs are important for the environmental monitoring of protected sites [21].

The aim of the research was to study the main patterns of soil cover organization within the northern part of the Kondinskaya lowland (Western Siberia) using the example of the territory of the Kondinskie Lakes Natural Park.

Soils of “mineral islands” could be conditionally subdivided into several groups that differ in thickness of the profile, podzolization process intensity, the diversity of soil morphological elements, and the degree of hydromorphism, depending on the specific geomorphological conditions and the lithology of parent rocks. Figure 2 shows photographs of the most representative soil pits for each of the discussed groups.

Folic Albic Podzols (Geoabruptic) represent the soils of the flat tops of the ridges (pits KO-1-18, KO-4-18). In general, the soils of the tops of “mineral islands” are characterized by moderate thickness of the E horizon and a large number of rounded rock fragments varying in size, from coarse to fine pebbles, in the middle part of the profile. Small tongues are the most common morphological elements of the profile, while deep wedge like structures are absent, and the maximum depth of these morphological elements often coincides with bedding depth with loams (40-50 cm). The loamy material lies in the form of separate interlayers with a thickness of up to 50 cm, and often shows signs of cryogenic transformations manifested in the presence of pronounced traces of cryogenic turbations, as well as paleopermafrost gleying. This assumption is also supported by the fact that loamy deposits do not form a continuous cover with consistent thickness: they are often replaced by sandy material from the above and underlying horizons in the form of lenses, interlayers, or individual large structures with signs of turbations. Albic Podzol at the top of the largest “mineral island” (pit KO-9-18), where no bedding with loam was observed, differs from other soils of this geomorphic position. In this case, tongues have a greater depth (up to 80 cm), and thin lamellas are well expressed in the C-horizon, which indicates greater intensity of the podzolization process.

Albic Podzols (pits KO-3-18, KO-6-18, KO-8-18, KO-13-18, and KO-14-18) represent the soils of the terraced subhorizontal slopes of the “mineral islands”. The soils of these geomorphic positions are characterized by the presence of a powerful E horizon with separate large tongues penetrating to a depth of 1 m, and a well-defined thick Bs horizon. Rock fragments are much fewer than those in the soils of the flat tops of “mineral islands”. As a rule, large charcoals, confined to soil morphological elements that have characteristic signs of tree-fall-related pedoturbations [32], are present at a depth of 30-40 cm near the border of the E and Bs horizons. In general, this group of soils is characterized by the most diverse structure of the upper part of the profile related to the variability of biogenic structures (tongues and mounds), as well as the highest intensity of podzolization.

The soils of the steep slopes of the mineral islands have a clear horizontal stratification from a depth of about 12 cm (pit KO-11-18). In these positions, soil formation is extremely dynamic and periodically interrupted due to the activation of erosional processes, probably related to fires and the anthropogenic activity of ancient people. Lichen cover in such positions is fragmented and often absent. The soil profile consists of separate interlayers of deluvial or aeolian origin, thin interlayers of loamy material. The differentiation of the profile because of pedogenic processes is weakly expressed; the podzolization process manifests itself slightly in the form of clarification of the upper part of the C horizon. On the basis of the profile's structural features, these soils can be classified as Albic Lamellic Arenosols (Abruptic, Aeolic).

Fig. 2. Photos of the soil profiles representing different landscape and geomorphic positions: A - Folic Albic Podzol (Geoabruptic) at the top of the large ridge (KO-4-18); B - Albic Podzol on the terrace within the gentle slope of the “mineral island” (KO-8-18); C - Albic Lamellic Arenosol (Abruptic, Aeolic) of the steep slope of a large ridge (KO-11-18); D - Albic Gleyic Podzol (T\urbic) in the lower part of the gentle slope of a large ridge (KO-7-18); E - Albic Gleyic Podzol (Turbic) developing over series of buried Albic Podzols under a steep slope (KO-5-18). Photos by Alexandr Konstantinov

The soils of the contact zone of the mineral islands and the boggy plain significantly differ depending on the type of the slope. Albic Gleyic Podzols (Turbic) occupy the lower parts of the gentle slopes of large mineral islands (pits KO-2-18, KO-9-18). These soils have a relatively short profile (groundwater appears at a depth of 1 m) and are practically completely devoid of rock fragments. The upper part of the profile often has signs of turbations, and the E horizon is represented as separate discontinuous patches, sometimes alternating with the slightly developed humus horizon. Signs of gleying appear at a depth of 15-20 cm in the Bs horizon.

Different conditions of soil formation can be observed in areas of contact between mineral islands and the swampy plain under the steep slopes. These landscape positions are characterized by the occurrence of Albic Gleyic Podzols (Tuurbic) developing over series of buried Albic Podzols. In addition to buried soils, the lower parts of soil profiles in this geomorphic position also include separate layers enriched with charcoals and burnt wood (pits KO-5-18, KO-12-18). For example, soil, opened in pit KO-12-18, has the following profile structure: O-E-Bs-[E]-[Bs]-Bc/ [E]-C/[Bs]. Rock fragments are scarce in the buried soils, and the thickness of the E horizon varies in the range of 2-8 cm. The formation of a complex profile with a series of burials is probably associated with the intensification of erosional processes due to fires and the activity of ancient people, as indirectly evidenced by the proximity of soils under consideration to the archaeological sites. The soils of this group were formed under conditions of constant waterlogging, and the lower part of the profile contains Fe-Mn nodules and concretions. It is interesting to note that this group of soils has a rather small and well-defined distribution area with respect to relief and vegetation changes: a contrasting transition from Gleysol to Albic Gleyic Podzol, developed over a series of buried soils, can be observed in soil pit KO-5-18.

Analytical properties

Figure 3 illustrates the particle-size distribution of the most representative soil pits for each geomorphic position. All studied soils are characterized by the predominance of fine (0.05-0.25 mm) and medium sand (0.25-0.5 mm). The content of the coarse sand (0.5-1 mm), as a rule, did not exceed 5-6%, with a tendency toward a slight increase in the C horizons, with the exception of the upper part of the Albic Podzol studied in pit KO-1-18. In almost all horizons of the studied soil, the total content of sand fractions was up to 90-95%, which is generally characteristic of Podzols formed on sandy substrates in the taiga zone of Western Siberia. The content of clay fraction, as a rule, does not exceed 1%, increasing to 5-6% only in pits where bedding with loam was observed.

In soil profiles where the natural sequence of genetic horizons is not strongly disturbed by tongues and other biogenic pedoturbations, there is a tendency for Bs horizons to be enriched with a clay fraction, which is typical of Podzols [33-34]. In all studied soils, coarse silt (0.01-0.05 mm) predominates over fine and medium silt. The content of silt fraction for the soils of the tops and gentle slopes of large ridges without bedding with loam, as a rule, does not exceed 5-7%. Higher contents of the silt fraction are characteristic for horizons and lenses composed of loamy material (up to 70%), and for soils with series of burials formed in the foot of steep slopes (up to 20%). In general, a nearly twofold increase in the contents of silt fraction in the Bs in comparison with the overlying E and underlying C horizons is typical of soils of flat tops and the terraced slopes of ridges.

Soils under consideration are characterized by an acidic reaction: from strongly acidic in O horizons, to acidic and weakly acidic in mineral horizons (Fig. 3). Only in the C horizon of Folic Albic Podzol (Geoabruptic) (pit KO-11-19) the reaction of the medium was close to neutral. On average, pH H₂O values varied from 3.7 in O to 5.6 in C, and pH KCl from 2.6 to 4.7, respectively. It can be noted that for all the soils studied, there is a tendency to increase pH values (both H₂O and KCl) with depth. The local maximum of pH values is also characteristic for Bs horizons. In Albic Gleyic Podzols (Turbic) developed in the contact zones of the mineral islands and the boggy plain, both under gentle and steep slopes, pH H₂O values were generally lower than those in soils of well-drained geomorphic positions; even in C horizons and did not exceed a value of 5.0. A rather uniform distribution of pH H₂O values along the profile was observed in Albic Gleyic Podzols with burials. Moreover, in buried E horizons, and layers enriched with charcoal and burnt wood, a noticeable decrease in pH KCl values was observed. A significant decrease in pH values is also characteristic for layers and lenses of loamy material in Albic Podzols (Geoabruptic). For example, in the pit KO-1-18, pH H₂O decreased from 5.7 in Bc, composed of sandy material, to 4.9 in the underlying loam, while pH KCl decreased from 4.6 to 3.4, respectively.

Fig. 3. Texture and chemical properties of soils representing different landscape and geomorphic positions: A - Folic Albic Podzol (Geoabruptic) of the large ridge top (KO-4-18); B - Albic Podzol on the terrace within the gentle slope of the “mineral island” (KO-8-18); C - Albic Lamellic Arenosol (Abruptic, Aeolic) of the steep slope of a large ridge (KO-11-18); D - Albic Gleyic Podzol (Turbic) of the lower part of the gentle slope (KO-7-18); E - Albic Gleyic Podzol (Turbic) developing over series of buried Albic Podzols under a steep slope (KO-5-18)

On average, pH H₂O values varied from 3.7 in O to 5.6 in C, and pH KCl from 2.6 to 4.7, respectively. It can be noted that for all the soils studied, there is a tendency to increase pH values (both H₂O and KCl) with depth. The local maximum of pH values is also characteristic for Bs horizons. In Albic Gleyic Podzols (Turbic) developed in the contact zones of the mineral islands and the boggy plain, both under gentle and steep slopes, pH H₂O values were generally lower than those in soils of well-drained geomorphic positions; even in C horizons and did not exceed a value of 5.0. A rather uniform distribution of pH H₂O values along the profile was observed in Albic Gleyic Podzols with burials. Moreover, in buried E horizons, and layers enriched with charcoal and burnt wood, a noticeable decrease in pH KCl values was observed. A significant decrease in pH values is also characteristic for layers and lenses of loamy material in Albic Podzols (Geoabruptic). For example, in the pit KO-1-18, pH H₂O decreased from 5.7 in Bc, composed of sandy material, to 4.9 in the underlying loam, while pH KCl decreased from 4.6 to 3.4, respectively.

The content of organic carbon in the studied soils was rather small and sharply decreased with depth (Fig. 3). The content of organic carbon close to 1% is typical only of fragmentary humus horizons, formed under conditions of waterlogging in Albic Gleyic Podzols (pit KO-2-18), developed under the gentle slopes of mineral islands, as well as for buried soils and layers enriched with charcoal in Albic Gleyic Podzols developed under steep slopes. In most soils of well-drained positions, TOC was 0.1-0.2% in E horizons, 0.3-0.4% in Bs horizons, and less than 1% in C horizons. A small increase in TOC up to 0.2% is typical of interlayers composed of loamy material. LOI values varied from 90% in O horizons of Albic Gleyic Podzols to 70% in Albic Podzols of well-drained landscape positions; in mineral horizons, as a rule, TOC do not exceed 1%.

Contents of Fe_o and Fe_d have slightly different distribution patterns (Fig. 3). In the Albic Podzols of flat tops and gentle terraced slopes of mineral islands, the maximum values of Fe_o were observed in Bs horizons (0.3-0.4%), sharply decreasing in C, while in E horizons these values were less than 0.1%. Differences in Fe_o content in E and Bs horizons of Folic Albic Podzols (Geoabruptic) were less pronounced in comprehension with Albic Podzols developed on sands without bedding with loams. In such soil profiles, the Fe_o content in Bs horizons did not exceed 0.1-0.2%, but there was a slight increase in the content of oxalate-soluble iron in the underlying loamy sediments. Higher Fe_o values are characteristic for C horizons in Albic Gleyic Podzols. A similar distribution type characterizes dithionite extractable iron: maximum Fe_d values were characteristic for interlayers and lenses of loamy material in the Folic Albic Podzols (Geoabruptic) of flat tops, which is directly related to the texture of the bedding material. In Bs horizons of Albic Podzols developed on terraced slopes, these values reached 1.3%, while in E horizons, these values did not exceed 0.1%, which is probably related to the highest intensity of the podzolization process. In Albic Gleyic Podzols in the bottom parts of slopes, the Fe_d content in Bs horizons was also rather high (up to 1.29%), which can be explained as a result of lateral podzolization [35-36]. It is interesting to note that Fe_d values were higher in the buried Bs horizons of Albic Gleyic Podzols developed under the steep slopes in comparison with Bs horizons of modern soils. This fact can be an indirect sign of higher-intensity podzoliza- tion during the previous stages of soil development. It can also be noted that the Fe_d content in the studied soils, as a rule, coincides with the chromatic maximum, and strong relations of these parameters were reported for similar soils in other regions [37].

For Albic Lamellic Arenosols (Abruptic) of steep slopes, all studied soil properties and their variability along the profile (Fig. 3) are strongly determined by the lithological heterogeneity of slope sediments, as well as soil-forming processes that are frequently interrupted by erosional processes. Therefore, the analytical properties of these soils slightly reflect current pedogenic processes.

Relationships between lithological and geomorphological factors, and morphological features of soils within the Kondinskie Lakes Natural Park

The nature of the parent rocks and their position in the relief have a significant effect on the intensity of the main pedogenic processes and, first of all, podzolization. The main morphological and chemical parameters for E and Bs horizons in Podzols are presented in Table. Available analytical and morphological data showed that the highest intensity of podzolization is characteristic for Albic Podzols of terraced slopes.

The results of morphological and analytical studies allowed us to conclude that the nature of the parent rocks and their position in the relief determine the variability of the soils within this territory (Fig. 4). The most obvious pattern of soil cover organization is a direct relationship between the number and size of rock fragments, and the presence of bedding with loams on the one hand and the depth and variability a extrahorizontal morphons (tongues) on the other. Strong influence of the underlining lithology on the morphology and properties of soils was reported for slope sequences of Podzols in Poland [17]. The close occurrence of dense loamy sediments and numerous large rock fragments at the border between E and Bs horizons most likely limits tongue thickness, the depth of which, as a rule, does not exceed 40-50 mm in the Folic Albic Podzols (Geoabruptic) of flat tops of ridges. In such landscape- geomorphic positions, small tongues prevail, while deep wedge-like tongues are almost absent. In addition, it is remarkable that soils at the tops of large ridges are characterized by the most diverse mineralogical composition of rock fragments. On the contrary, in Albic Podzols of gentle terraced slopes, numerous large tongues penetrate to a depth of more than a meter. In such geomorphological context, the upper and the middle parts of the profiles are often complicated by spotty or streaky structures caused by treefalls, pit-and- mound complexes, and thin tongues developed over root channels with different deposits associated with the bottom boundaries of old pits [32]. Charcoals often present in the forms of interlayers displaced by treefalls are more common in soils of terraced slops in comprehension with flat tops.

Fig. 4. Scheme illustrating relationships between lithological and geomorphic conditions and morphological parameters of soils: I - steep gentle slopes of large ridges with Albic Gleyic Podzol (Turbic); II - terraces within the gentle slopes of ridges with Albic Podzols; III - flat tops of ridges with Folic Albic Podzol (Geoabruptic) or Folic Albic Podzol (Lamellic); IV - steep slopes of ridges with Albic Lamellic Arenosols (Abruptic, Aeolic); V - bottom parts of steep slopes with Albic Gleyic Podzol (Turbic) over series of buried Albic Podzols

Main morphological parameters and properties of E and Bs horizons in soils representing various geomorphic positions and parent rocks