INTRODUCTION

The current state and dynamics of forest ecosystems under climate changes reflect the joint impact of numerous factors, including weather and soil conditions, pathological agents, wildfires, and environmental pollution [22, 26, 34]. The disturbances in forest ecosystems may be to a considerable degree associated with the state of soils, their nutrient status, and pollution level [26]. The changes in soil properties resulting from anthropogenic acidification, nutrient depletion, or pollution can influence the structure, functioning, and dynamics of plant communities. The main changes in soils are confined to the layer of forest litter and upper mineral horizons; correspondingly, the trees with a lateral root system, including spruce, the undergrowth, and the ground vegetation are particularly stressed by the negative impacts. The disturbances of mycorrhiza formation, playing an important role in the nutrient supply of trees, are another contributor to the suppression of a forest phytocenosis [17]. Changes in the edaphic conditions can also interfere with the germination of seeds and, correspondingly, with the natural restoration of plant communities. Thus, soil properties directly or indirectly affect the climate-regulating function of forest ecosystems and their ability to absorb carbon. The attention to the edaphic aspects is reflected in the concept of ecological functions of soils, ecological soil science [5], and edaphology (considering soil as the habitat of living organisms [19]). The applied value of the research into the diversity and current dynamics of the properties of forest soils is determined by the demands in forestry and natural resource management in general [19], maintenance of biodiversity [26, 28], assessment of the consequences of natural and anthropogenic impacts [25–27, 35], and transformation of ecosystems under the conditions of global changes in the environment and climate [19, 34, 36].

Correspondingly, long-term soil monitoring in forest ecosystems becomes most important. So far, 112 countries of the world have assessed the states of their forests via national-wide inventories and/or remote sensing of their resources, totally covering 83% of the global forest areas [29]. However, the attention to forest soils is insufficient. The International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), active in Europe since 1985 under the UNECE Convention on Long-range Transboundary Air Pollution, is the most representative. The ICP Forests comprises the extensive (about 6000 plots with a grid of 16 × 16 km) and intensive (about 860 plots in the main types of forest ecosystems) monitoring [27, 35]. In Russia, with its one-fifth part of the world’s forests [30], the need in environmental monitoring is statutory at federal (Federal Law On the Environmental Protection no. 7-FZ of January 10, 2002; latest edition of December 30, 2021) and regional (On the Environmental Protection in Moscow Oblast, no. 16/202-P of December 13, 2006) levels. The recently adopted Federal science and technology program of the Russian Federation in the areas of environmental improvement and climate change for 2021–2030 allows for the further development of environmental monitoring.

In general, the ecological situation in the central special ecological and economic zone of Moscow oblast, harboring the studied region, was critical at the end of the 20th century. The natural systems there almost completely lost the ability to self-restore; the share of anthropogenically disturbed soils in the land fund increased; and the share of soils in undisturbed natural landscapes decreased [13]. The Zvenigorod Biostation of Moscow State University and the Sima Quarry (ZBC) state nature reserve is a unique natural complex retaining high biological diversity despite its close location to Moscow [15]. The forest ecosystems of this nature reserve are the reference standard for the territory of Moscow oblast. Simultaneously, this nature reserve is an educational and research base of Moscow State University. The intensive soil and ecological monitoring following the ICP Forests recommendations performed in the nature reserve provide the objective and complete information about the state of forest ecosystems meeting the unified international principles and methods [9].

The goal of this work was to analyze the specific ecological and genetic features of forest soils to update and refine their classification and assess their tolerance towards external impacts under changing natural and anthropogenic factors.

OBJECTS AND METHODS

Environmental conditions. The ZBS state nature reserve are found in the southern part of the Smolensk–Moscow Upland beyond the area of the last glaciation. Watershed plateau, ancient terraces, and modern floodplain of the Moskva River are the main landforms. The watershed plateau is covered with a thick mantle of mid-Pleistocene glaciofluvial deposits represented by sands with rare pebble. In the western part, these deposits of the loamy sandy texture in the upper part outcrop to the surface and serve as soil-forming rocks. In the eastern part, they are covered by a thin (30–40 cm) layer of sorted clay loam forming a two-layered soil-forming rock [16].

The ZBS area belongs to the zone of coniferous–broadleaved forests; 83% of the ZBS area (600 ha) represents watershed with coniferous forests, mostly spruce stands [15]. Recently, the role of spruce forests has considerably decreased because of massive spruce dying-off, decline of spruce stands, and sanitation cuttings. Mixed, birch, and aspen forests are abundant.

The soil cover is diverse. Podzolic, soddy-podzolic, bog-podzolic, and peat bog soils are common on the heterogeneous soil-forming rocks of the watershed [4, 8, 10, 11, 16]. The current appearance, properties, and diversity of soils in this area are determined by the combination of various natural and anthropogenic factors acting in the recent millennia. The biomes of forest-steppe and broadleaved forests, abundant there in the first half of the Holocene, were replaced in the second half by the southern taiga biomes, and dark-humus soils were replaced by gray-humus and soddy-podzolic soils. Alluvial soils have formed on the floodplain in the recent centuries [1]. Forest burning for creating pastures and then, arable land, has been practiced for a long time. The current forest cover in the ZBS area has been influenced by selective and clear cuttings of the 19th century and by agricultural activities, including grazing and annual mowing [2].

The control sites have been monitored in the main types of forest ecosystems of the watershed plateau (Table 1).

Table 1.  Soils in the intensive monitoring sites of forest ecosystems in the ZBS
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Field methods. Soil solid phase was examined following the ICP Forests recommendations [24, 32]. Soil was sampled from each of the three monitoring sites of 30 × 40 m in size divided into square plots of 10 × 10 m in 24 replicates. The sampling points were selected in a stratified random manner. The samples were collected from the litter and depths of 0–5, 5–10, 10–20, 20–40, and 40–80 cm using a borer; these unified depths to a first approximation match the genetic horizons of studied soils. For chemical analyses, 24 individual samples of from horizon were pooled into four mixed samples, six in each. In total, 432 individual soil samples were collected in three types of forest ecosystems to prepare 72 mixed samples for further analysis. Before analysis, the samples were sieved (mesh, 2 mm).

Analytical methods. Particle size distribution was determined by pipette technique according to the ratio of three fractions: <0.002, 0.002–0.063, and 0.063–2 mm. The cementing components were removed with 30% H2O2 solution; 3.3% (NaPO3)6 and 0.7% Na2C-O3 were used for dispersing.

Individual elements (Ca, Mg, K, Na, Al, Fe, Mn, Cd, Cu, Ni, Pb, and Zn) for assaying their “pseudo-total” contents were extracted with a mixture of concentrated hydrochloric and nitric acids at a ratio of 3 : 1 (aqua regia, international standard ISO 11466). The contents of metals were determined after oxidation with the mixture of concentrated acids for 16 h at room temperature and 2 h at boiling. The resulting extracts were filtered and diluted with 0.5 M HNO3 solution to measure the concentrations of metals using inductively coupled plasma mass spectrometry (ICP-MS 7500a, Agilent). The contents of oxalate-soluble Alo, Feo, and Mno were determined after extraction with the mixture of (COONH4)2H2O and (COOH)22H2O (0.2 M oxalate ions, pH 3) with the help of ICP-MS. The contents of exchangeable cations (Ca, Mg, K, Na, Al, Fe, and Mn) were determined in the extracts with 0.1 M BaCl2 solution using the same technique. Soil acidity was determined potentiometrically in water and salt (0.01 M CaCl2 and 1 M KCl) suspensions at the soil-to-solution ratio of 1 : 5 with a Mettler-Toledo Seven Multi ionomer equipped with a Mettler Toledo Inlab 413 electrode. The exchangeable acidity was determined by titration in the extract with 0.1 M BaCl2 and the hydrolytic (total) acidity, in the extract with 1 M CH3COONa at pH 8.2. The effective cation exchange capacity (CECef) was calculated as the sum of exchangeable cations and exchangeable acidity.

The data were processed using descriptive statistics with the assumption that the properties of mixed soil samples were normally distributed. The statistical significance of the differences in the means was assessed using the classical two-sample t-test for independent samples based on Student’s test. An alternative variant of this test was used for the situation with unequal variances as a harder one leading to somewhat lower number (by ≈10%) of the pairs of significantly differing properties. Correlation analysis was also used.

RESULTS AND DISCUSSION

Soil morphological properties and diagnostics. According to the traditional prevalently factor-based diagnostics used in the soil classification in the Soviet Union [6], acid podzolic and soddy-podzolic soils typical of the taiga forest zone are the main soil cover components in the ZBS nature reserve. The surface soil horizon is represented by forest litter; its stock increases from birch–spruce (0.87 kg/m2) to pine–spruce (1.2 kg/m2) and spruce (1.6 kg/m2) stands. The morphology of mineral soil horizons suggests that the soils belong to the podzolic and soddy-podzolic subtypes according to [6].

A characteristic feature of the watershed soils is a pronounced textural heterogeneity resulting from their formation on the two-layered soil-forming rock with the upper silt loamy layer overlying sands and sandy loams forming the “inversion of two layers.” The specificity of these soil profiles is the presence of bleached eluvial horizon under the organic or humus-accumulative horizon; in the absence of middle-profile horizons, this bleached horizon is replaced by the underlying rock. In the substantive-genetic classification of soils in Russia [7], the soils with weak eluvial–illuvial differentiation of the profiles or without such differentiation and developing from two-layered parent materials are classified as eluvial soils, either eluvozems or soddy eluvozems. Note that bleaching is observed in the lower part of the eluvial horizon in the soils under pine–spruce forests. This bleaching results from periodic water stagnation at the interface with the underlying compact layer; the signs of eluvial degradation in the form of whitish tongues appear in its upper part, suggesting that they belong to contact-bleached eluvozems. Along with the presence of gray-humus horizon the soils under birch–spruce stand have an ocherous yellow color of the underlying rock; this indicates the accumulation of iron oxides and allows us to classify these soils as iron-illuvial soddy eluvozems. Along with these soils, soddy eluvozems are developed from somewhat coarser sediments under spruce stands; the presence of rusty ocherous thin and wavy interlayers cemented by iron oxides in the underlying sandy rock is reflected in the name of pseudofibrous soddy eluvozems.

According to WRB [14], the studied soils belong to the same reference group of Cambisols (Dystric Cambisols) (Table 1). This group is dominant (22.9%) in the European forests, as it was shown by the results of the BioSoil soil survey [25], which covered 4928 and 127 monitoring sites in the ICP Forests Level I (extensive) and Level II (intensive)monitoring, respectively [25]. The share of Cambisols in the ICP Forests Level II aggregated forest soil condition database (comprising up to 130 soil properties together with the dynamic characteristics of soil water, precipitation, plant litter, vegetation, atmospheric air, and weather conditions at 286 level II monitoring sites) is approximately the same [27].

The physical and chemical properties of soils are inherited from the soil-forming rock and change during soil formation. Coarse texture and two-layered nature of parent materials determine the morphology and properties of the examined soils. Sand and silt fractions predominate in all horizons (Fig. 1). All the soils developed from two-layered deposits represented by covering silt loam underlain by glaciofluvial sand and sandy loam. The content of sand fractions in the contact-bleached eluvozem and ferruginous soddy eluvozem increases from 45–60% in the upper horizons to 71–81% in the lower horizons; the contents of silt (from 36–50 to 16–25%) and clay (from 3.9–7.0 to 3.3–4.1%) fractions decrease in the same direction. Thus, the upper layers of sand loamy (in some cases, silt loamy) texture are replaced with silty sand or loamy sand at a depth of approximately 40 cm. Pseudofibrous soddy eluvozem has a somewhat coarser texture with its weaker differentiation along the profile. The content of sand fractions increases with depth (from 56–63 to 83%) versus the contents of silt and clay fractions, which decrease down the soil profile from 35–39 to 14% and from 3.9–5.1 to 2.7–3.1%, respectively. In general, the soil texture becomes coarser in the following soil sequence: ferruginous soddy eluvozem—contact-bleached eluvozem—pseudofibrous soddy eluvozem. These data agree well with the data on European forest soils, demonstrating that Cambisols are prevalently associated with silty loams, loams, or sandy loams [25].

Fig. 1.
figure 1

Particle-size distribution data (hereinafter, means and 95% confidence intervals are shown).

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The yields of the elements extracted with aqua regia differ depending on the analyzed element and soil properties. This treatment releases the elements from amorphous and organic compounds, as well as from layered silicates, mainly iron-containing silicates. The digestion with aqua regia is regarded as appropriate for analysis of both macroelements and trace elements extracted from soils and assessment of their availability to plants [23, 33]. Kaasalainen and Yli-Halla [31] report that aqua regia extracts up to 95% of Cu, Ni, and Zn from the coarse sandy and loamy sandy soils, as well as up to 80% of Cd and up to 60% of Cr and Pb. Note that the mixture of HCl and HNO3 extracts a higher amount of elements from the soils with a finer texture [20]. Currently, this method is widely applied for assessing the pseudo-total soil composition in ecological studies [23], geochemical mapping [33], and international monitoring programs [24].

In the litters, Ca is the leader in the amount extracted with aqua regia followed by Mn, Fe, Al, K, and Mg (Table 2). Note that the total contents of elements in the litter and their amounts extracted with aqua regia are very close. The litter of the soddy eluvozem in the birch–spruce forest is the richest in macroelements (Ca, Mg, and K) and trace elements (Mn, Zn, Cu, and Ni) versus the litter of soddy eluvozem under the spruce stand, which is enriched with K, Fe, Al, and Pb.

Table 2.   Contents of the elements extractable with aqua regia in the soils of ZBS forest ecosystems: M, mean and CI1/2, half width of 95% confidence interval for mean, n = 4 (each pooled of six individual samples)
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In the acid extracts from all mineral horizons, Fe and Al predominate; ferruginous soddy eluvozem, which is relatively fine-textured, is the richest in Fe and Al. The differentiation of Fe and Al along the soil profile is weakly pronounced because of a relatively weak differentiation of parent materials. The distribution of Fe and Al along the profiles of the remaining two soils first and foremost reflects the heterogeneity of the two-layered deposits. The Fe content in the profile of contact-bleached eluvozem gradually increases with the depth in the upper loamy layer and sharply drops in the underlying loamy sandy glaciofluvial deposits. Within the upper loamy layer, Al is depleted from the horizon of 5–10 cm and accumulates at a depth of 20–40 cm to return to the minimum in the underlying sandy material. Similar distributions of Fe and Al are also characteristic of pseudofibrous soddy eluvozem.

The distributions of other metals in the mineral layer reflect the effects of both the biotic factor and the lithological and textural heterogeneity. In all soils, biogenic accumulation of Ca and K is observed not only in the litter but also in the underlying layer. The content of Mg in mineral layers is close to or somewhat higher than that in the litter, whereas the Na content is almost the same in all layers, which is also characteristic of the forest soils in Europe [25]. As for the potentially toxic metals, the maximum concentrations of Pb, Cd, Zn, and Mn are observed in the upper horizons and decrease with depth, which reflects the potential input of Pb and Cd with atmospheric fallout and the mainly biogenic accumulation of Mn and Zn. On the contrary, the contents of Ni and Co increase with depth, suggesting their geogenic origin. The concentrations of most metals in eluvozem sharply decrease at a depth of 50–80 cm, where the soil texture becomes coarser. The changes along the profile are less pronounced in soddy eluvozems.

As compared with the forest soils of Europe [25], litters contain a moderate amount of biophilic elements (Ca, Mg, K, Fe, and Zn), while the coarse-textured mineral horizons are depleted of these elements. All the soils are enriched in Na and Mn and depleted of Cu, Ni, and Pb. The content of all potentially toxic metals—Cd, Cu, Ni, Pb, and Zn—is very low, even taking into account their incomplete extraction, and matches their total content in the reference soils of the region [13].

Data on the distribution of the oxalate-soluble Alo, Feo, and Mno (as the youngest freshly deposited compounds that characterize current soil formation processes) along the soil profiles make it possible to refine the classification position of the soils. The minimum amount of these compounds is recorded in the organic horizons. The Feo content is up to 2200–2500 mg/kg in the upper mineral soil layers and up to 850–1030 mg/kg in the lower layer (50–80 cm), or 27–33 and 12–17% of the pseudo-total Fe content, respectively. The absolute and relative yields of Alo are halved as compared with Feo in the upper part of the profile and are the same in the lower part.

A specific feature of the examined soils is an accumulative distribution of oxalate-soluble Fe and Al along the profile with a maximum in gray-humus and eluvial horizons, i.e., in the upper loamy layer of the two-layered parent material (Fig. 2). The Feo content in the AY, AYel, Elf, and ЕL horizons is 1.6–2.6 times higher as compared with the D horizon; the Alo content in the upper horizons is 1.5-times higher than in the D horizon. This specificity prevents the studied soils from meeting the criteria for texturally differentiated soils and suggests their attribution to the orders of eluvial and ferruginous metamorphic soils [7]. The most pronounced progressive-accumulative distribution in the soil profile is characteristic of the amorphous and weakly crystallized Fe compounds. A low Alo accumulation in the layer of 10–40 cm l (horizons Elf and EL) as compared with the topmost layer of 0–10 cm (horizons AY and AYel) is observed within the eluvial layer of eluvozem and soddy eluvozems (this layer coincides in its thickness with the upper loamy layer of parent material). However, this differentiation is weakly pronounced and does not definitely suggest an eluvial–illuvial redistribution of Alo resulting from the development of an Al–Fe-humus process during the superimposed evolution, as it takes place in loamy texturally differentiated podzolic soils [18]. Any considerable qualitative differences in the Feo and Alo distributions along the profile in the soils formed under different phytocenoses are unobservable.

Fig. 2.
figure 2

Contents of the oxalate-soluble aluminum, iron, and manganese in soils.

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The uptake by plants considerably influences the distribution of Mno, which is confirmed by its pronounced accumulation in litters, where the Mno content is several times higher as compared with Feo and Alo. The Mno content smoothly decreases with depth.

Soil acidity influences plant growth and development. Concurrently, it determines many soil properties by affecting the mobility of elements and their availability to plants, as well as the CECef, composition of exchangeable cations, composition and activity of soil biota, and soil physical properties.

The soils of forest ecosystems of the ZBS nature reserve are acidic. The litters display the highest pHH2O values (5.2–5.7) and the eluvial horizons, the lowest (4.6–4.8); in the deeper mineral horizons, the pH values increase to 5.0–5.1 (Fig. 3). The pH values increase in the following order: pseudofibrous soddy eluvozem—contact-bleached eluvozem—ferruginous soddy eluvozem. The \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) values are by 0.4–0.7 units higher than \({\text{p}}{{{\text{H}}}_{{{\text{CaC}}{{{\text{l}}}_{{\text{2}}}}}}}\) values and by 0.8–1.0 units higher than pHKCl (except for the litters of spruce and birch–spruce forest stands, where pHKCl values are by 0.5–0.6 units lower than \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\)).

Fig. 3.
figure 3

Values of pH (left) and exchangeable and total acidities (right) in soils.

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High acidity in the upper part of the soil profile and its decrease with depth are typical of the podzolic and soddy-podzolic soils of the taiga zone [12, 13]. The large-scale surveys of European forest soils demonstrate that most of these soils are highly acidic, especially in the upper part of the profile. In particular, average \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) values change from 5.1 and 4.5 in the litter horizons OL and OFH to 4.8, 5.1, 5.3, and 5.5 in the layers of 0–10, 10–20, 20–40, and 40–60 cm, respectively [25]. The average \({\text{p}}{{{\text{H}}}_{{{\text{CaC}}{{{\text{l}}}_{{\text{2}}}}}}}\) is 4.8 and 4.15 in the litter horizons and 4.5, 4.2, 4.7, and 4.9 in the mineral horizons, respectively. Earlier, the \({\text{p}}{{{\text{H}}}_{{{\text{CaC}}{{{\text{l}}}_{{\text{2}}}}}}}\) values below 3.5 have been recorded in 42% of the litters, 22% of the upper horizons, and only 4% of the lower mineral horizons [37]. The pH of organic horizons varies within a large range because of diverse natures of organic compounds; 80% of the pH values change in the range of 3.0–5.4 [37]. The type of vegetation has a considerable effect on soil acidity because of the differences in the chemical composition of plant litter. The soils under coniferous forest stands are usually more acidic as compared with the soils under deciduous forests. However, these differences are not always statistically significant and the cause-and-effect relationships are vague. Plants have different tolerance to soil acidity; correspondingly, soil properties determine the composition of plant communities to a higher degree than plant communities influence soil acidity.

Litters of the examined ecosystems have maximum values of exchangeable and total acidities (3.0–3.5 and 22–28 cmolc/kg, respectively). The exchangeable acidity increases in the following order: pseudofibrous soddy eluvozem—contact-bleached eluvozem—ferruginous soddy eluvozem; total acidity, in the order: ferruginous soddy eluvozem—pseudofibrous soddy eluvozem—contact-bleached eluvozem. The exchangeable acidity of mineral horizons is considerably lower, with the minimum values in humus-accumulative horizons (0.4–0.8 cmolc/kg). Its values somewhat increase down the profile in eluvial horizons (1.5–2.0 cmolc/kg) and smoothly decease in the underlying rock (0.8–1.0 cmolc/kg) depending on the texture and content of organic matter. The exchangeable acidity is mainly represented by hydrogen ions in the litter and aluminum ions in the mineral horizons. Total acidity gradually decreases in the mineral part of the profile (from 7.5 to 1.6 cmolc/kg) without any distinct maximums. Sandy texture determines low acidity values in the mineral horizons; the differences in acidity of lower horizons between the examined soils are statistically insignificant.

The content of exchangeable bases in soils allows us to judge the nutrient supply of plants. The coarse-textured soils of the ZBS nature reserve are poor in exchangeable bases (Table 3). Their content is minimal in the eluvozem under pine–spruce forest (30 cmolc/kg in the litter and 5.6 cmolc/kg in the humus-eluvial horizon). An increase in the share of birch in a forest stand is accompanied by enrichment of the upper horizons of the formed soddy eluvozem with exchangeable bases (52 cmolc/kg in the litter and 7.5 cmolc/kg in the gray-humus horizon). Although the texture of soddy eluvozem under complex spruce stand is somewhat coarser, the content of exchangeable bases in this soil is relatively high (50 and 6.4 cmolc/kg, respectively) because of significant admixture of broadleaved species.

Table 3. Cation exchange properties of the soils in ZBS forest ecosystems: CEC, potential cation exchange capacity; BS, base saturation; M, mean and CI1/2, half width of 95% confidence interval for mean, n = 4 (each pooled of six individual samples); DL, detection limit
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Calcium is prevalent among the exchangeable bases (24–40 cmolc/kg in litters and 4.7–6.2 cmolc/kg in the upper mineral layers); as for magnesium and potassium, their contents do not exceed several cmolc/kg and several tenths of cmolc/kg, respectively. Note that the soddy eluvozems under complex spruce stand and birch–spruce forest are somewhat enriched with potassium as compared with eluvozem. Litters and sublitter horizons are the richest in exchangeable bases; their concentrations sharply decrease down the profile to reach the minimum in eluvial horizons. Note that the differences in the enrichment with exchangeable bases in the upper horizons are also preserved in the lower part of the profile.

The contents and distribution of acid exchangeable cations—Al, Fe, and Mn—differ in the soils of the ZBS nature reserve. The Al content, which to a considerable degree determines the exchangeable acidity, increases from the litter to accumulative-eluvial and eluvial horizons and decreases in the deeper part of the profile. The content of exchangeable Fe is very low, frequently below the detection limit. An increased content is characteristic of exchangeable Mn, with its accumulative distribution along the profile. The organic soil horizons typically contain increased amounts of trace elements, vitally important to plants, because of their biological accumulation.

The CEC values are similar in all examined soils because of their close textures and amount to 37–59 cmolc/kg in litters and 1.9–8.7 cmolc/kg in the upper mineral horizons, decreasing to 1.7–2.1 cmolc/kg at a depth of 40–80 cm. The degree of base saturation is minimal in the eluvozem under pine–spruce forest stand (82% in the litter and 18% in the eluvial horizon) and maximal in the soddy eluvozem under birch–spruce forest (88% in the litter and 85% in the gray-humus horizon); the differences smooth over with depth.

The differences between soils are analyzed by comparison of their averaged characteristics. This approach focuses on the characterization of an object based on a relatively large set of measurements of a group of properties in different locations, that is, the characterization of a spatially extended object as an integral whole. Such approach conceptually matches a statistical approach in physics and population studies in biology and fits well the study of complex variable objects, such as soils under different plant communities.

The properties of soils under different forest types growing in a limited area of the ZBS nature reserve are rather close and not contrasting. However, the pattern of vegetation cover is sensitive to soil properties and represents a good integral indicator of their differences. The mean values of a number of soil properties differ in a statistically significant manner at a confidence level of 95% for these three forest types and all assayed soil layers in total by more than 25% for all the examined layers–properties and by more than 30% for the upper sublitter layer (Fig. 4).

Fig. 4.
figure 4

Polar histograms of the statistical significance of the differences between the properties of (1) contact-bleached eluvozem, (2) ferruginous soddy eluvozem, and (3) pseudofibrous soddy eluvozem in the litters (O) and mineral layers at depth of 0–5, 5–10, 10–20, 20–40, and 40–80 cm. Confidence levels for differences of the properties (P ≥ 0.07) between soil pairs (see the legend in histogram O) are represented by the radii of the corresponding pie wedges with the centers marked with plus. One asterisk denotes the pie wedges with the probability of differences P ≥ 0.95 and two asterisks, with the probability range of 0.75 ≤ P < 0.95. The elements extracted with aqua regia are shown with their chemical symbols; the oxalate-soluble compounds are denoted with subscript “o”; and exchangeable cations, with subscript “ex”; EA, exchangeable acidity; TA, total acidity; CEC, cation exchange capacity; BS, base saturation; and BD, soil bulk density.

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Indeed, these estimates of differences are conditional, because they are determined by an agreed threshold confidence level by itself rather than an agreed general procedure of assessment of this level. A natural question here is to what degree the threshold level fits the problem being solved; perhaps, it is possible to select another value of the threshold that better matches the analyzed data. In physics and other natural sciences, the regions of sharp changes in properties or breaks in a series of quasi-continuously changing discrete values of properties may be selected as thresholds. In this paper, we also used this approach. We constructed some histograms of the probability of differences for the pairs of properties for the same data (shown in Fig. 4); increasing the number of bins, we observed gradually emerging gaps, or empty bins. This behavior is characteristic of relatively small probability values; this is the so-called fluctuation catastrophe when none of the values falls into a classification bin because of some accidental causes. However, the gaps also appear in the case of higher probabilities in the range of the values when they cannot emerge in the case of a quasi-continuous change in probabilities. In other words, the series of observed probabilities of differences contains the breaks—gaps of a non-fluctuating nature. The upper boundary of the largest solitary non-fluctuating gap can well be used as the threshold probability characteristic of the corresponding data. In this context, a threshold boundary of P = 0.73, natural for the data, is rather reasonable; this threshold value is close to the boundary of the upper quartile, the concept widely used in statistics, which facilitates the comparison with published data.

Turning to the difference in the chemical properties of the soils under different forest types in the ZBS nature reserve, we see that the properties considerably differ more often in the case of the above-discussed value of natural confidence threshold as compared with a commonly used confidence interval of 95%. In particular, the properties for the upper sublitter layer differ in 65% of the cases; for the remaining layers, in 60% of the cases (with an accuracy of 2%), except for the 5–10- and 20–40-cm layers, where the differences are 55 and 43%, respectively. As for the soils in general, differences are observed in 60% of the cases. In this view, a high sensitivity of vegetation to soil properties becomes clearer. Most likely, when studying the association of vegetation and soils, it is reasonable to rely on the confidence levels that are closer to reality than the widely used level of 95%.

The most informative property that characterizes the distinction between soils under different forest types is their bulk density. The differences are less pronounced in the contents of the Mg, Al, and Ca extractable with aqua regia; oxalate-soluble Alo and Feo; exchangeable Ca, K, and Al; CEC; degree of base saturation; as well as sand and clay contents. The examined soils almost do not differ in their acidity.

Comparison of the properties of soil horizons/layers in different forest types demonstrates that the most pronounced differences are characteristic of the upper mineral layer (0–5 cm; horizons AY and AYel) and the deepest layer (40–80 cm; horizons Del and Df). Smaller differences are observed for the organic horizon and 5–10-cm layer of forest soils. The eluvial horizons of different soils, especially their lower part (20–40 cm), are comparatively close in their properties. The similarity between forest litters is most likely determined by a close composition of tree stands with the prevalence of spruce. These patterns are quite natural and are interpretable as the manifestation of mutual influence of vegetation and soil properties against the background of pedological and lithological differences. The differences in chemical properties are most pronounced in the forest soils in the pair of contact-bleached eluvozem—ferruginous soddy eluvozem; to a lesser degree, in the pair of pseudofibrous soddy eluvozem—ferruginous soddy eluvozem; and minimally, in the pair of contact-bleached eluvozem—pseudofibrous soddy eluvozem.

Correlation analysis demonstrates a close connection between many soil properties in all types of forest ecosystems. Statistically significant strong positive correlations between the pH of water and salt suspensions (r = 0.95–1.00, p ≥ 0.95) and their negative correlations with the content of exchangeable Al (r = –0.55 to ‒0.87) are characteristic of all horizons. The degree of base saturation in the majority of cases tightly correlates with the content of exchangeable bases (r = 0.64–0.70). Of interest is the positive correlation of the contents of acid-soluble and oxalate-soluble Fe (r = 0.70–0.95) and Al (r = 0.67–0.89) with the enrichment for fine fractions, namely, silt and clay. Among the potentially toxic metals, the contents of Co (r = 0.69–0.91) and often Ni (r = 0.32–0.43) extracted with aqua regia positively correlate with the contents of silt and clay fractions, whereas the contents of Mn, Zn, and Cd (r = –0.53 to –0.81) display negative correlation with the contents of silt and clay. This difference reflects the prevalent affinity of metals to the mineral phase or organic matter. Opposite patterns of the correlation between the contents of exchangeable cations and clay fraction suggest its insignificant role in the creation of the exchangeable pool of metals in coarse-textured soils.

The natural factors in forest ecosystems, first and foremost, edaphic and climatic, in many respects determine the diversity and productivity of plant communities. On the other hand, soil is the final receiver of most technogenic chemical compounds entering the biosphere. Soil as the most important geochemical barrier to the migration of pollutants protects the adjacent media from technogenic impact. Knowledge about the soil potential in neutralization, accumulation, and transformation of the pollutant components of atmospheric fallout near the largest metropolis is necessary for the timely identification and assessment of the potential negative impact on ecosystems. The results of integrated soil and ecological monitoring make it possible to objectively estimate the soil tolerance towards the anthropogenic impact. Eluvozems and soddy eluvozems under spruce and mixed forests, forming the basis of the soil cover in the ZBS nature reserve, have a coarse texture, high acidity, low content of exchangeable bases, and low base saturation. These soils are sensitive to anthropogenic impact, including the pollution with nitrogen, sulfur, and potentially toxic metals. Under the ever-increasing anthropogenic load and climate changes, the ecological state and stability of soils to a considerable degree can influence the structure and functioning of plant communities in forest ecosystems, suggesting the need in soil and ecological monitoring.

CONCLUSIONS

The comprehensive characterization of morphological, physical, and chemical properties of soils has made it possible to clarify their specific ecological and genetic features and to update and refine their place in the classification system of Russian soils [7]. The soils under complex pine–spruce, birch–spruce, and complex spruce forests on two-layered deposits belong to contact-bleached eluvozems, ferruginous soddy eluvozems, and pseudofibrous soddy eluvozems of the order of eluvial soils.

Eluvozems and soddy eluvozems predominating in the soil cover of the ZBS nature reserve have a low content of clay fraction (3.3–7.0%), acid reaction (pH 4.6–5.7), low CEC, low content of exchangeable bases (30–52 and 0.6–7.5 cmolc/kg in litter and mineral horizons, respectively), and low base saturation (49–67 and 11–51%, respectively). The concentrations of potentially toxic metals (Pb, Cd, Cu, Ni, and Zn) in the soils of forest ecosystems in the ZBS nature reserve match the regional background levels. The contents of fine fractions and mineral nutrition elements increase and the acidity decreases in the following order: contact-bleached eluvozems—pseudofibrous soddy eluvozems—ferruginous soddy eluvozems; these changes result in the improvement of the soil ecological state.

Coarse texture, low pH, and depletion of exchangeable bases suggest a low tolerance of the forest soils in the ZBS nature reserve to external impacts under an ever-increasing anthropogenic load and changing climate. Further studies are necessary to assess the potential of forest soils to sequester and store carbon and mitigate climate change.

Analysis of the breaks in the quasi-continuous series of observed discrete probabilities of the differences in soil properties allowed us to propose the threshold confidence level for the probabilities of differences determined by the analyzed data per se. This level is close to the boundary value of the upper quartile; as a consequence, both levels almost equally and, most likely, better characterize the differences in soil properties. This natural level (rather than the widely used conditional confidence level of 95%) better explains the high sensitivity of plant communities to soil properties, reported by many authors. The question on whether the used approach is appropriate in other situations is open and requires further studies.

Testing of the system of intensive soil and ecological monitoring according to the ICP Forests recommendations in the ZBS state nature reserve confirmed the feasibility of systematic integrated long-term studies of soils in the protected areas with due account for the international experience. The use of unified methods allows for adequate comparison of the state of soils in forest ecosystems on both the regional and global scales. The obtained array of unique data forms the basis for long-term systematic monitoring of the forest ecosystems in the nature reserve. The used methodological approaches are beneficial for foundation of the national system for monitoring of carbon pools and fluxes of climatically active gases in the forest zone of Russia with respect to climate change.