Soil Structure Characteristics in Three Mountainous Regions in Bulgaria Under Different Land Uses

Abstract

Soil structure has an important role in storing and transporting substances, providing natural habitats for soil microorganisms, and allowing chemical reactions in the soil. A complex investigation on factors affecting soil structure characteristics under herbaceous (H), deciduous (D), mixed (M), and coniferous (SP—Scots Pine and NS—Norway Spruce) vegetation was conducted at three experimental stations—Gabra, Govedartsi, and Igralishte, located correspondingly in the Lozenska, Rila, and Maleshevska Mountains in South-West Bulgaria. The data set obtained includes soil structure indicators and physical, physicochemical, chemical, mineralogical, and microbiological parameters of the A and AC horizons of 11 soil profiles. Under different vegetation conditions, soil structure indicators respond differently depending on climatic conditions and basic soil properties. Regarding the plant available water capacity (PAWC), air capacity (AC), and water-stable aggregates (WSAs), the surface soil layers have an optimal structure in Gabra (H, D), Govedartsi (H, SP, NS), and Igralishte (H). The values for the relative field capacity (RFC < 0.6) showed that the studied soils were water-limited. The WSAs correlated with SOC in Gabra, while in Govedartsi and Igralishte, the WSAs correlated with the β-glucosidase known to hydrolyze organic carbon compounds in soil. The information obtained is important for soil quality monitoring under climatic and anthropogenic changes.

1. Introduction

Through its functions of retaining and transmitting water, soil structure is considered the most important soil characteristic for regulating water balance [1,2]. It also influences aeration, soil biological and chemical processes, and thermal properties [3,4,5,6]. Factors that influence the formation of soil structures include climatic conditions (wetting and drying periods), the presence of organic matter, the physicochemical and mineralogical properties of soil particles, the activity of soil biota, and the development of the root system [7]. Various indicators have been proposed as characterizing soil structures from the point of view of the solid part (aggregation) and from the point of view of the pore system [3,5,8,9]. The stability of the pore space depends on the formation of stable soil aggregates from soil particles, i.e., on the presence of binding agents and mechanisms [7,10,11]. The most popular indirect method for estimating the pore size distribution is the soil water retention curve (SWRC) [5]. The characterization of pore space is usually based on the concept that it is composed of individual pores of different sizes. Soil hydrological properties, as well as indicators of the pore system determined by SWRC, mercury intrusion porosimetry (MIP), nitrogen adsorption, and water desorption, are often used to assess anthropogenic or natural impacts on soil functions [8,12,13,14,15].

Soil structure characteristics of forest soils in lowland are often used as a baseline for comparison after the conversion of natural forest to agricultural land [16,17,18]. The effects of the type of vegetation on the soil hydraulic properties of forest soils in mountain regions are discussed in [19,20,21,22], and the effects of machinery traffic and animal trampling on soil density properties are addressed in [23]. We could not find a complex investigation into factors influencing the soil structure characteristics with different vegetation species in mountain regions.

The aim of the current study was to assess a complex range of factors affecting soil structure characteristics under herbaceous (H), deciduous (D), mixed (M), and coniferous (SP—Scots Pine and NS—Norway Spruce) vegetation at three experimental stations, located correspondingly in the Lozenska, Rila, and Maleshevska Mountains in South-West Bulgaria.

2. Materials and Methods

2.1. Site Description

The investigation was carried out on Cambisols under different land use types at the experimental stations of the Forest Research Institute—Gabra in the Lozen Mountains (herbaceous plantation, deciduous and mixed forest), Govedartsi in the Rila Mountains (herbaceous plantation, Scots pine, Norway spruce), and Igralishte in the Maleshevska Mountains (herbaceous plantation, deciduous—oak forest, and Scots pine forest), all situated in South-West Bulgaria (Figure 1). The information for the location, classification of the soil, and land use is presented in

Table 1. Location and site descriptions of soil profiles and soil types according to [27]. The numbers of soil profiles are indicated in brackets.

Place	Soil Type	Land Use	Abbreviation
Gabra, Lozen Mountain (23.63 E; 42.53 N; 916–937 m)	Eutric Leptic Cambisols—Ochric	Herbaceous plantation (1)	H
	Dystric Cambisols	Deciduous forest (1)	D
	Dystric Cambisols	Mixed coniferous and deciduous forest (1)	M
Govedartsi, Rila Mountains (23.46 E; 42.22 N; 1503–1579 m)	Dystric Cambisols—Humic	Herbaceous plantation (1)	H
	Dystric Cambisols—Humic	Scots pine forest (1)	SP
	Dystric Cambisols—Humic	Norway spruce forest (1)	NS
Igralishte, Maleshevska Mountains (23.13 E; 41.57 N; 848–869 m)	Eutric Leptic Cambisols—Ochric	Herbaceous plantation (2)	H
	Eutric Cambisols—Ochric	Deciduous forest (1)	D
	Eutric Cambisols—Ochric	Scots pine forest (2)	SP

. The experimental stations differ in climatic conditions. The average annual precipitation (P) in Gabra is 563 mm, and the average annual temperature (T) is 8.7 °C [24]. In Govedartzi, P is 975 mm and T is 4.9 °C [25]. In Igralishte, P is 605 mm and T is 10.6 °C [26].

The bulk soil samples were taken from 11 surface (0–5 cm) and 10 subsurface (10–25 cm) topsoil layers from 11 soil profiles. The sampling depths corresponded to the top A horizon and the underlying AC horizon. Vertically orientated soil cores were sampled in 4 replicates in 100 cm³ metal cylinders for the determination of bulk density (Db) and water retention at suctions from 0.25 to 33 kPa.

2.2. Laboratory Analyses

The laboratory analyses included the determination of soil structure characteristics and physical, physicochemical, mineralogical and microbiological properties.

The particle-size distribution was determined by sieving and the pipette method [28] after the preliminary removal of soil organic matter. Fractions of sand (2–0.063 mm), silt (0.063–0.002 mm), and clay (<0.002 mm) were determined for the application of the textural classification according to [27]. The concentration of organic carbon (SOC, %) was determined by the modified Tjurin method [29,30].

The acidity of soil was measured in a soil/water suspension of 1:2.5 by a pH meter. The acidic and sorption properties of soils were determined according to [31]. The cation exchange capacity (CEC) was determined as the sum of the titratable acidity (pH 8.2) and the extractable Ca by saturation with K malate at pH 8.2 [31]. The mineral composition of sand (>63 μm) and silt + clay (<63 μm) was assessed by X-ray diffraction analysis using a D2 PHASER diffractometer (Bruker).

The amount of the main groups of soil microorganisms was determined by the method of ten-fold dilution by sowing soil suspensions on selective agar nutrient media [32]. The activity of the enzymes acid phosphatase and β-glucosidase was determined according to [33]. CO₂ production was reported by the titrimetric method [34].

The bulk density of fine earth was estimated, taking into account the gravel (2–60 mm) content in each ring. Particle density (Ds) analysis was carried out in water with 100 cm³ pycnometers. The total porosity (Pt) was calculated using the measured bulk density (Db) and particle density (Ds):

Pt = (1 − Db/Ds) × 100%

(1)

The distribution of dry-sieved aggregates in size classes (>10, 10–5, 5–3, 3–1, 1–0.25, <0.25 mm) was determined by the manual dry sieving of air-dried soil using set of sieves arranged from top to bottom with decreasing opening sizes. The proportion of each aggregates class (DSA) was calculated relative to the summed total weights of all the aggregate size classes. The mean weight diameter (MWD, mm) of the fraction less than 10 mm was calculated.

The water-stable aggregates were determined by the Savinov method, with the modification by Vershinin and Revut [35]. Four soil samples were prepared for wet sieving: one sample (20 g) by taking an equal quantity (5 g) of air-dried aggregates from four fractions, 10–5, 5–3, 3–1, and 1–0.25 mm, and three replicate samples (20 g each) with air-dried aggregates from a single fraction, 3–1 mm. The wet sieving was performed using a Savinov device an hour after the direct immersion of the air-dried soil aggregate sample into water (slaked pretreatment). The correction for aggregate-sized sand content was performed according to [9].

The water stability of aggregates is expressed by the ratio (MWDR) of mean weight diameters of aggregates after (MWDwet, mm) and before wet sieving and by the percentage of water-stable macroaggregates >0.25 mm (WSAs > 0.25, %) [17].

Soil water retention curves (pF curves) were obtained during the drainage of the samples by using the suction plate method, pressure membrane apparatus, and desiccators methods, as described in [22]. The wetting of the undisturbed soil cores in (100 cm³) soil samples at −0.25 kPa on a sand bath lasted more than 20 days. The drainage of the wetted samples at suctions of 1, 5, 10, and 33 kPa was performed by suction type apparatus (G5 shot filters with of pore diameters of 1.0–1.6 μm). A negative matric pressure was applied by means of a hanging water column. Equilibrium for each suction was established for 5–7 days. Soil water retention at suction 1500 kPa (pF = log₁₀(|−cm H₂O|) = 4.2) was determined using fine (<2 mm) earth samples with pressure membrane apparatus. The field capacity was estimated by the water content retained at a suction of 10 kPa (pF = 2.0). The hygroscopic water content at pF 5.6 in the water adsorption part of the pF curves was determined using the vapor pressure method with controlled relative humidity 75% in desiccators containing saturated solution of NaCl.

The effective pore radius r holding water at the applied suction (P) was calculated by Jurin’s formula:

$$P={\displaystyle \frac{2{\gamma }_{{\mathrm{H}}_2\mathrm{O}}}{r}}$$

(2)

where γ_H2O is the surface tension of water (0.0729 J m⁻²) and P is in Pa. The effective diameters (δ) of pores corresponding to 1, 5, 10, 33, and 1500 kPa are 300, 60, 30, 10, and 0.2 μm, respectively. The volume of the air-filled pores at a given suction P was calculated as the difference between the total porosity (Pt) and volume of water content θ (θ = W × Db) retained at this suction. The air capacity, AC, was estimated as the volume of the air-filled pores at pF 2.0.

AC = Pt − θ_2.0

(3)

The relative FC (RFC) was calculated as θ_pF2.0/Pt.

The stability of the soil structure was assessed by the structural stability index (SI) proposed by Pieri [8]:

SI = 1.724 × SOC/(silt + clay) × 100%

(4)

The thresholds of soil structure indicators related to pore space were after [8].

The data set was analyzed based on standard statistical descriptive parameters, correlation and regression analyses, and one-way analyses of variance (ANOVA). The statistical analyses were performed using STATGRAPHICS Centurion 18 software and Excel.

3. Results and Discussion

3.1. Characteristics of Solid Phase

The clay content is a major factor influencing the formation of water-stable aggregates, as well as the pore size distribution. According to [1], the content of clay above 15% is an important precondition for the formation of soil aggregates. The soil texture of the studied soils shows that they have a coarse texture (L, SL, LS). The clay content in the surface (0–10 cm) layer of the Cambisols in Igralishte (Maleshevska Mountains) is very low, at 2–7%, while in the rest soil profiles it reaches 21% (

Table 2. Basic physical and physicochemical properties of surface (0–10 cm) soil layer. CEC≡T_8.2, cation exchange capacity; CEC_SA, cation exchange capacity of soil strongly acidic ion-exchanger; CEC_A, CEC of soil weakly acidic ion-exchanger.

Soil Characteristics	Gabra			Govedartsi			Igralishte
	H	D	M	H	SP	NS	H	D	SP
Clay <2 μm, %	21	20	14	18	19	20	7	2	4
Silt 2–63 μm, %	47	29	32	32	37	32	21	25	14
Sand 63–2000 μm, %	32	52	54	51	43	48	72	73	82
Texture class	L	SL	SL	L	L	L	LS	LS	LS
Gravel, %	11	19	22	26	13	6	11	6	4
SOC, %	1.51	2.73	1.44	3.3	4.2	6.8	2.9	2.4	0.3
SOC:Clay	0.07	0.14	0.10	0.18	0.22	0.34	0.41	1.20	0.08
SI (eq. 5)	4	10	5	11	13	23	18	15	3
W5.6 (75%RH, pF 5.6)	6.2	2.6	2.3	3.8	5.2	6.8	2.6	2.6	3.2
Exchangeable cations and CEC
CEC, cmol kg−1	37.5	20.0	23.0	29.0	28.4	29.0	18.5	21.2	24.5
CECSA, cmol kg−1	28.5	11.7	13.0	13.0	16.9	12.5	12.5	15.5	18.9
CECA, cmol kg−1	9.0	8.3	10.0	16.0	11.5	16.5	6.0	5.7	5.6
Exch. H8.2, cmol kg−1	10.4	15.2	15.0	22.5	15.4	22.8	7.2	7.5	7.1
Exch. Al, cmol kg−1	1.2	6.7	5.0	6.6	3.8	6.3	1.2	1.8	1.5
Exch. Ca, cmol kg−1	20.1	3.1	6.0	5.0	10.2	5.1	10.0	11.0	15.2
Exch. Mg, cmol kg−1	6.9	1.2	2.0	1.8	2.6	1.0	1.2	2.8	2.3
Base saturation, %	72.0	21.5	34.8	23.4	45.1	21.0	60.5	65.1	71.4
pH in H2O	5.2	3.9	4.0	3.8	4.2	3.7	4.6	4.5	4.5

). In these coarse-texture soils, the pore distribution significantly depends on the soil organic matter content, as well as on the gravel content [21].

The soil organic matter content is a major factor in the formation of soil structures. At the same time, stable soil aggregates protect organic matter from mineralization [10]. According to Greenland [36], at a soil organic carbon (SOC) content below 2.3%, the soil structure is susceptible to deterioration during cultivation. The highest SOC in the surface layers were obtained at all sites in Govedartsi—between 3.3 and 6.8% (Table 2). Studies on the influence of the content and composition of organic carbon on the stability of soil micro and macroaggregates show that it is achieved through various cohesive and binding mechanisms [10,11,15,37]. According to a number of authors, the SOC/clay ratio is more indicative when assessing soil structure [37,38,39,40]. An indication of good physical quality is represented by SOC/clay values of 1:10, where clay is the content of particles with a size of <0.002 mm [37,38]. In most surface layers of the mountain soils studied, this ratio is above 0.10, with the exception of the Cambisols below the herbaceous plantation (H) and under mixed forest in Gabra and under pine forest in Igralishte (Table 2). Assessment is similar through the structural index (SI, Equation (4)), which is above 10 for all sites, except for those already mentioned, where it indicates a high risk of degradation (Gabra) or degraded soil (Igralishte).

Studies on the role of soil physicochemical properties in soil structure show that a decrease in pH has a positive effect on the water stability of aggregates, which is explained by the increased coagulation of particles covered with organic matter [11,40]. The soils studied in the three mountainous regions are very strongly acidic, with the exception of the soil under grass in Gabra (Table 2). According to the basic constitution of the humus horizon, all studied soils have a pH < 6.0 and a sum of basic cations less than the capacity of the strongly acidic ion-exchanger (CEC_SA), which defines them as podzolic soils. The strongly acidic ion-exchanger of the soil colloids is not completely neutralized with strong bases, which leads to the occurrence of a strong acidic effect, which causes the onset of thermodynamic instability in the clay structures in these soils. With the exception of the Eutric Leptic Cambisols—Ochric under grass in Gabra, which has a moderately colloidal reactivity (CEC = 37.5 cmol kg⁻¹), the remaining soils belong to the moderately low colloidal category [41], in which the CEC varies from 29.4 to 16.5 cmol kg⁻¹. A slightly higher exchange capacity is observed in the samples from Govedartsi than in those from Igralishte. According to the prevailing clay mineralogy, the studied Eutric Cambisols—Ochric soils are of the illite type (CEC_SA = 70–60% CEC) and the Dystric Cambisols are of the illite–kaolinite type (CEC_SA = 60–40% CEC) [41].

The main characteristic of these soils include very advanced acidification (pH 4.8–3.6), exchange H_8.2 = 60–79% CEC, and exchange Al = 29% CEC, which covers both the weakly acidic surfaces of the soil colloids and a significant part of their strongly acidic positions. As a result of the existing strongly acidic system in the soil colloids, processes of intensive destruction of clay materials occur in these soils, which is also confirmed by the significant amounts of exchangeable Al appearing due to the destruction of the octahedral layer of the clay crystal lattice. The second acidic component in these soils is the acidic buffer systems of water-soluble organic acids (fulvic acids, tannic acids, etc.) and their salts, which decrease the soil reaction.

Linear dependences of pH with the degree of saturation with bases (Figure 2a) and with exchangeable aluminum (Figure 2b) with a high coefficient of determination (R² = 0.84 and 0.82) have been established. The small slope of this dependence is associated with the predominance of the strongly acidic exchange positions of the soil adsorbent.

The mineralogical composition of selected samples shows a predominance of quartz and muscovite in the Cambisols in Gabra in both soil fractions (>0.063 mm and <0.063 mm), with the quartz content in the sand increasing with depth (

Table 3. XRD mineralogical composition of selected samples (<0.063 mm and >0.063 mm).

Minerals	Gabra				Govedartsi						Igralishte
Land Use	D		D		H		H		NS	NS	H	H
Soil texture fraction	>63 μm		<63 μm		>63 μm		<63 μm		>63 μm	<63 μm	>63 μm	<63 μm
Depth, cm	0–5	15–20	0–5	15–20	4–9	20–25	4–9	20–25	10–20	10–20	4–24	4–24
Quarz (SiO2)	37	41	34	23	17	33	23	14	14	14	19	9
Plagioclase [(Na,Ca)(Si,Al)4O8]	6	11	16	15	25	28	21	24	48	24	25	15
K-feldspar (KAlSi3O8)	14	22	10	16	35	14	9	12	11	12		9
Muscovite {KAl2[AlSi3O10](OH)2}	38	22	30	28	11	13	22	20	15	20	53	51
Amphibol {Ca2[Mg4(Al,Fe)]Si7AlO22(OH)2}	4		3		6	5	11	8	4	8
Chlorite {[Mg,Al,Fe]6[Si,Al]4O10(OH)8}		3	4 *	7 *	4	6	10 *	19	5	14	3	15
Hematite (Fe2O3)	1	1		1				4
Talc [Mg3Si4O10(OH)2]					3	2			2	4
Montmorillonite [(Na,Ca)0,3(Al,Mg)2Si4O10(OH)2•n(H2O)]			1	2			0.3	0.3		2		2
Kaolinite [Al2Si2O5(OH)4]			2	8			5

* clinochlore.

).

The feldspars predominate in the Dystric Cambisols—Humic from Govedartsi: plagioclase and K-feldspar in the site under grass vegetation, and plagioclase under spruce. The dominant mineral is muscovite in the Eutric Leptic Cambisols—Ochric in Igralishte. The kaolinite was determined in the finer fraction only under the deciduous forest in Gabra and under grass vegetation in Govedartsi, but it should be kept in mind that the clay fraction was not studied separately and, according to the physicochemical indicators, these Cambisols have a mixed type of illite–kaolinite mineralogy. A number of authors point out the positive influence of kaolinite on the water resistance of soil aggregates [7].

An indirect indicator of the specific surface area of soil particles is the hygroscopic water content at a relative air humidity of 75%, corresponding to water retention at a potential of pF5.6 (Wh_5.6). This also explains the well-pronounced relationship between Wh_5.6 and the cation exchange capacity (CEC) (Figure 3).

3.2. Soil Microbiological Characteristics

Soil biota is important factor for soil structure formation [6,42]. According to the data on soil microbiological characteristics, the grass vegetation from Igralishte stood out from the other studied soils with the best microbiological indicators, containing the most heterotrophic and cellulose-decomposing microorganisms, enzyme activity, and total biological activity [43]. The heterotrophic microorganisms were also higher under grass than under deciduous and coniferous vegetation for the other two sites, Gabra and Govedartsi, which can be explained with the more intensive processes of decomposition for organic substances that are hard to mineralize, such as humus. Compared to the other two mountain regions, the soil at Gabra site had the highest number of microscopic fungi [43], which are often considered a factor in the water stability of soil aggregates [10].

The differences between grass and forest vegetation can be explained by the predominant formation of fulvic acids under forests, which are more mobile, as well as the spread of microbial activity to greater depths through macro bio-pores [42].

3.3. Soil Aggregation

The size distribution of dry aggregates, presented in Figure 4, shows that the soils at the Gabra station contain the lowest relative amount (6–8%) of microaggregates (<0.25 mm). At this site, as well as in the soils at Govedartsi, agronomically valuable aggregates (0.25–10 mm) are predominant—on average, 76 and 75%. In Gabra, the share of clods (>10 mm) is also significant, which could be associated with their finer texture. In the Eutric Leptic Cambisols—Ochric, in Igralishte, the proportion of microaggregates reaches 42% (Figure 3). At this site, the predominant (72–82%) textural fraction of sand (2–0.063 mm) contains an average of 50% coarse sand (0.25–2 mm), which means that these particles contribute to aggregate fractions at 0.25–1 and 1–3 mm. Dry sieving shows that the amount of aggregates with a size over 3 mm in the soil samples from Igralishte is minimal, which is also reflected in the small MWD of 0.9–1.2 mm and in comprising a smaller percentage of agronomically valuable aggregates—64% (Figure 4).

After sieving in water and excluding the skeletal content in the soil aggregate samples, it was found that the water resistance of the soil aggregates was highest at the Igralishte station, including the macroaggregates with a size of 3–10 mm participating in the composite sample (Figure 5).

In Gabra, the water stability of aggregates in the surface horizons is high (MWDR = 0.77–0.91) and decreases with depth (MWDR = 0.52). In Govedartsi, although the water stability assessment under grass is high (MWDR > 0.6), it is significantly lower than under SP and NS. Grass vegetation is known to be a factor in the formation of water-resistant macroaggregates [18], but in this case, the higher SOC under SP and NS (Table 2) has a greater influence.

Smaller aggregate fractions are usually more water-resistant than larger ones. According to the pore exclusion principle [44], the smaller aggregates will have closer packing (or less pore space) compared to larger aggregates, which also coincides with the higher energy binding the elementary soil particles and microaggregates than that gluing macroaggregates [10]. Data from the studied sites show that the water resistance of the composite samples (F_0.25–10 = F_0.25–1 + F_1–3 + F_3–5 + F_5–10) and the single fraction (F_1–3) in the samples from Gabra is the same, while at the other two sites, F_1–3 is more water-resistant than the composite sample (Figure 6).

3.4. Characteristics of Porous System

The total porosity of the surface layers varies between 49%vol. and 75%vol. (Figure 7). The other factors affecting this parameter are the presence of skeletal fraction (Figure 8) and SOC content [44]. The total porosity of the soils in Gabra and Govedartsi is influenced by the presence of gravel fraction (2–60 mm). As can be seen from Figure 8, with an increase in gravel, pores below 300 μm, which constitute a large part of the total porosity, decrease, and the volume of the largest pores (>300 μm) increases slightly (Figure 8).

It is known that the pore distribution depends on the size of the aggregates [10,15]. The volume of pores with a slow drainage function (effective pore diameter of 10–30 um) is higher under herbaceous cover (6–11%). Under coniferous cover, the volume is between 4 and 6%. The total volume of pores with drainage functions is the highest out of all sites in Igralishte, where the soil has a coarser texture.

The physical quality indicators are summarized in

Table 4. Soil structural indicators.

Soil Structural Indicators	Gabra			Govedartsi			Igralishte
	H	D	M	H	SP	NS	H	D	SP
PAWC, %vol.	26.0	22.9	16.2	20.5	27.5	28.2	24.7	19.9	12.8
RFC	0.6	0.5	0.4	0.5	0.6	0.5	0.5	0.4	0.4
AC = Pt−θpF2.0, %vol.	23.9	31.5	30.3	29.0	28.1	34.6	28.0	43.1	30.0
WSAF1–3 > 0.25 mm, %	59.7	68.7	57.9	77.1	69.7	80.6	82.0	83.8	68.5
Skeleton in fraction F1–3, %	28.1	22.1	30.8	19.5	7.4	2.2	14.4	12.1	27.1

. The data show optimal physical conditions for the Dystric Cambisols—Humic in Govedartsi. The available water capacity of the Dystric Cambisol under a mixed forest in Gabra and under forest plantations in Igralishte are lower than the optimal values (Table 4). The relative field capacity (RFC) is below or at the border of the optimal range (0.6–0.7), which defines the soils as limited in terms of water storage. The water resistance of the 1–3 mm fraction is high for all sites.

After the statistical analysis of the entire data set, no significant correlations were found between the indicators for water stability of aggregates and the other soil indicators studied. In addition to the different aggregation mechanisms mentioned, another reason for this involves the close, high values for water resistance obtained for the soil aggregates.

The separation of the sites allowed the obtention of the statistically proven dependencies (Figure 9) of the WSAs of the single fraction (1–3 mm) on the organic carbon content in the samples from Gabra and on the microbiological enzyme activity for the remaining two sites. The microbial enzyme β-glucosidase is known to hydrolyze organic carbon compounds in soil and thus contribute to the stabilization of soil aggregates [45].

4. Conclusions

A complex set of physical, physicochemical, mineralogical, and microbiological indicators that are relevant to the formation of soil structure in mountainous regions under grass and deciduous and coniferous vegetation were studied. It was established that the surface soil layers were characterized by optimal values for available water capacity, aeration capacity and water-resistant soil aggregates under grass and deciduous vegetation in Gabra, under all studied land use types in Govedartsi, and under grass vegetation in Igralishte. The relative soil field capacity is below or at the border of the optimal range, which defines the soils as limited in terms of water storage capacity. The water resistance of soil aggregates depends on various factors in the areas studied. In Gabra, WSAs correlated with soil organic carbon (SOC), while in Govedartsi and Igralishte, WSAs correlated with the soil enzyme β-glucosidase. The obtained information is important for soil quality monitoring under climate and anthropogenic changes. The data can also be used in models for soil water and heat balance, organic matter turnover, and other ecological analyses.