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CHANGES IN SOIL PROPERTIES OF XEROPHYTIC FORESTS IN SOUTHERN RUSSIA AFTER ANTHROPOGENIC IMPACT
Valeria V. Vilkova* , Kamil Sh. Kazeev , Mikhail S. Nizhelskiy, Sergey I. Kolesnikov , Yulia S. Kozun
Southern Federal University, Russia *e-mail: lera.vilkova.00@mail.ru
Received: 05.03.2024. Revised: 24.04.2024. Accepted: 05.05.2024.
Anthropogenic influences negatively affect the soil properties and woody vegetation in the xerophytic forests and the rare forests of the northern coastal area of the Black Sea. The study was carried out at the properties of soils in the Utrish State Nature Reserve (Russia), which was previously under the influence of anthropogenic impact. Fires, deforestation, and recreational load were selected as the prioritised types of anthropogenic impact in long-term studies with soil (Cambisols) in the Utrish State Nature Reserve, which rapidly lead to the degradation of ecosystems and biodiversity reduction. Soil samples were taken from the 0-10-cm layer in triplicate from each study site. The study site after recreational load has been investigated in 2013, 2015, 2018, 2019, and 2020. The parameters pH, organic carbon content, enzyme activity were studied. Biological properties of soils were found to be more sensitive to changes in characteristics than other soil parameters. For all the studied parameters, there was a decrease in the organic carbon content from 18% to 59% relative to the control. The highest decrease in the indicator was noted for soils after deforestation. Enzymes from the oxidase class recover faster than the hydrolases. Catalase activity for all disturbed soils decreased on average by 13%. Non-significant differences from the control were observed for soils after deforestation and fires. Inhibition of invertase activity was established in soils by an average of 52% after fires and recreational exposure, with an increase in enzyme activity of 77% observed after deforestation. The results can be used to assess effects of the anthropogenic impacts on forests. Biological indicators are good markers of the condition of disturbed soils that could be implicated to determine the prioritised anthropogenic load in the soils of xerophytic forests.
Key words: biological activity, cambisol, deforestation, human activity, pyrogenic impact, recreational activity
Introduction
Every year the influence of anthropogenic factors on ecosystems increases, resulting in a steady trend of deterioration of their ecological status (Teng et al., 2019; Dror et al., 2021; Dar et al., 2022). The impact of anthropogenic factors on natural communities is characterised by a high rate of change in their parameters, because of which the living organisms do not have time enough to adapt rapidly to the changing conditions. As a result, the habitat becomes dangerous for all its inhabitants, including humans. Human impacts can be direct or indirect and include four main factors, namely overexploitation of natural resources, habitat conversion, introduction of alien species, and pollution (Leslie & McCabe, 2013; Kalinitchenko et al., 2022). The transformation of the habitat can be attributed to such phenomena as deforestation, fires, and recreational load. Each of these three phenomena may affect the structure and functions of ecosystems, ecological processes, and biodiversity. Every year types of the anthropogenic impact are becoming more complex. The areas
where the combined effect of various anthropogenic impact factors is observed are becoming more extensive (Dar et al., 2022). Global climate change intensifies this effect (Rosenzweig et al., 2008), which determines the global relevance of such research.
Tree felling or logging is an initial step in the production of timber from the forest. Rapid deforestation is an environmental catastrophe, as it leads to deforestation around the world. Every year, millions of square kilometres of forests and plantations disappear from the face of the Earth because of the expansion of arable land. Restorative succession after deforestation leads to considerable changes in the soils and vegetation (Kuznetsova et al., 2019; Zhang et al., 2020; Kalinitchenko et al., 2021). In post-forest ecosystems, it is possible to note not only soil degradation but also the ecotone effect. Forest productivity and species diversity increase due to ecotone effect and regenerative succession. In recent decades, the fire frequency in natural ecosystems has increased throughout the world, which causes catastrophic environmental dam-
age. In addition, forest fires disrupt the balance of ecosystems and biochemical cycles of the circulation of substances, which leads to changes in atmospheric conditions. As a result of forest fires, the global carbon dioxide (CO2) emissions occur, while CO2 emissions play a vital role in global climate change.
Another anthropogenic factor affecting the natural environment is the recreational load. An increasing number of people are involved in recreational activities, which leads to the expansion of areas covered by these activities around the world. The process of intensifying the use of tourist recreation areas is developing at a no less rapid pace, which leads to an increase in the level of anthropogenic impact on natural complexes. Uncontrolled rest of tourists causes degradation of ecosystems. According to Serengil & Ozhan (2006), the soil density in forest ecosystems after anthropogenic impact increased with the recreation intensity from 1.18 g/cm3 to 1.29 g/cm3. At present, preserving the quality of natural ecosystems is one of the most pressing challenges. There is no doubt that soils are important for maintaining the sustainability of natural communities and the well-being of the population. Soils provide the most important ecological functions as the habitat for living organisms, the central link in the interaction of geological and biological cycles of substances in the biosphere, and the neutralisation of the decay products of animals and plants through the soil (Adhikari & Har-temink, 2016). Some studies (e.g. Dror et al., 2021) indicate other traditional anthropogenic factors of soil formation, namely parent material, time, climate, topography and relief, and organisms. Human activities cause processes in the soil such as erosion, reduced organic carbon, soil pollution, and decreased biodiversity.
Fires, deforestation, and plowing are the main processes depleting the soil organic carbon stocks. In addition, carbon loss increases in erosion-prone soils (Wuepper et al., 2020). Soil enzymology methods are widely used in the diagnosis of the ecological state of the environment, and indicators of the biological properties of soils are also informative (Burns et al., 2013; Thiele-Bruhn et al., 2020). Soil enzymes, which catalyse many biochemical reactions, can exist in soil in various ways. Catalytic enzymes are observed to be present in the cytoplasm or on the membrane surface of viable cells, to be
released into the soil solution, or to form complexes in the soil matrix (Dick & Kandeler, 2005). Extracellular enzymatic activity depends on substrate availability, moisture, temperature, and other. Soil enzyme activity is one of the main indicators of soil health and quality. There is strong variation in soil biogeochemical properties due to multiple factors after deforestation, such as vegetation type, recovery period, enzyme type, degree of disturbance (Soldatov et al., 2020). The enzymes responsible for the carbon cycle in soil were selected for the study. These enzymes from the class of hydrolases influence the decomposition of organic matter, as the main source of energy for these processes. The activity of catalases and invertases can be considered as an indicator of the intensity of organic matter mineralisation processes. A high sensitivity of these enzymes to the anthropogenic impact has been noted in other studies (e.g. Soldatov et al., 2020; Kazeev et al., 2022; Vilkova et al., 2024). Under climate change influence, increases in soil temperature stimulate the carbon mineralisation by soil microbiota, by increasing the magnitude and variability of greenhouse gas emissions to the atmosphere on the short and long term (Masyagina, 2021).
The present study was aimed to assess the impact of priority anthropogenic impacts (deforestation, fires, and recreational load) on the state of forest ecosystems and biological properties of soils in xerophytic forests and rare forests in the coastal area of the Black Sea in the Caucasus region. The results of this study can be used in the development of a system for assessing and rationing anthropogenic impacts on natural communities.
Material and Methods
Site description
In the northern coastal area of the Black Sea in the Caucasus region, in the Krasnodar-sky Krai, Russia, the Utrish State Nature Reserve was founded on the Abrau Peninsula in 2010. This area is characterised by the predominance of low-mountain and foothill forest-type landscapes. The variation of climatic indicators contributes to the formation of typical xe-rophytic forests of the sub-Mediterranean type, occupying mainly the coastal part and broad-leaved fluffy oak forests dominating the area of the Utrish State Nature Reserve. The conditions for the formation of forest stands determine the relief and climate. The climate type of the
Utrish Nature Reserve is defined as transitional between marine Mediterranean and temperate continental (Tkachenko & Denisov, 2013). Summer lasts about five months; winter duration is about three months. The average annual air temperature varies between 9°C and 11°C. The precipitation amount is 550-810 mm, with a peak of 180-210 mm in November - January.
The variation of climatic indicators contributes to the formation of typical xerophytic forests of the sub-Mediterranean type, occupying mainly the coastal part of the Utrish State Nature Reserve. In the central part of the Utrish State Nature Reserve, the mesophytic rock oak and oak-pine forests are common in the middle and high-altitude belts. Local mesophytic plant communities are formed along the slits due to wetter and milder conditions (Bocharnikov et al., 2019). The Utrish State Nature Reserve is a unique Protected Area, where the vegetation cover is characterised by a high species saturation of plant communities, reaching the maximal values in the seaside belt of pistachio-juniper and fluffy oak forests and rare forests. On the Abrau Peninsula, there is a high number of species with a Mediterranean distribution and a high level of endemism. Among them, the following dominant species of forest communities occur: Pistacia mutica Fisch. & C.A. Mey, Juniperus excelsa Bieb., Pinuspallasiana D. Don, P. pityusa Stev., Ruscus aculeatus L. (Ogureeva et al., 2020).
Of particular interest, this study is not only about unique vegetation but also about the cambisols (lUSS Working Group WRB, 2015), which are rare in Russia (Kazeev et al., 2015). Cambisols differ from other soil types by the following features: brown colour of the profile, intensive textural flaming of the middle part of the soil profile, eluvial-illuvial type of de-carbonisation, close to neutral reaction of the medium, richness of soil mineral nutrition elements (Kazeev et al., 2010). More often there are leached varieties of cambisols, the soil profile of which is completely free from calcium carbonates. They are formed in humidified elevated places on non-carbonate dense rocks, less often on eluvium of carbonate sandstones. At the same time, most soils are referred to incompletely developed types of brown soils due to their formation on dense rocks of various compositions (Kazeev et al., 2010). The diversity of the soil cover, and the variation in the biological
activity indicators are caused by various variants of exposure, steepness of slopes, as well as rockiness and amount of crushed stone. The formation of these deposits was considerably influenced by limestones, erosion, and reddening of the weathering crust of erupted and metamor-phic rocks, as well as the entry of dusty material through the atmosphere (Lemos et al., 1997).
Until 2010, when the Utrish State Nature Reserve was established, this area was annually a subject to various types of anthropogenic impacts. In the Utrish State Nature Reserve, there are sites affected by fires, deforestation, and recreational load. Therefore, three study sites are considered in this research (Fig. 1).
Study sites
Study site №1 was affected by fire (44.747650° N, 37.410667° E). The altitude of this study site in the surroundings of the Vodopad-naya Shchel is 112 m a.s.l. The slope steepness is 20-25° of south-eastern exposure. The soils of study site 1 were damaged by the fire of 2009. Dense grassy-grain vegetation occurs; tree-shrub undergrowth reaching 3 m in height was observed 10 years after the fire impact (Fig. 2a).
Study site №2 was impacted by deforestation (44.752367° N, 37.457300° E). Study site №2 is a clearing (5-7 m wide and 20-30 m long) with 0.5-m in height herbaceous vegetation, shrubs (Vaccinium spp., Rosa sp.), with green-moss cover. The slope steepness is 5° of western exposure. Deforestation was carried out in the 1970s; a few decades later, the young forest was cut down again (Fig. 2b).
Study site №3 was influenced by the recreational load (44.747750° N, 37.407683° E). Study site №3 was used for a long time by spontaneous tourists for the location of tented parking lots. Recreational activities were discontinued with the foundation of the Utrish State Nature Reserve in 2010. Morphological changes in soil and vegetation cover have so far been observed at former tourist tent sites. Now, the natural restoration of the vegetation began; there is a thick grassy cover and traces of the presence of vacationers (Fig. 2c).
The control forest site, №4 (44.747500 ° N, 37.410183° E), is the background forest area, which has not been affected by anthropogenic factors. The vegetation of the control forest site is represented by Juniperus-dominated thin forest with young forest growth of broad-leaved trees and shrub undergrowth (Fig. 2d).
Soil sampling
Studies of ecosystems and soil cover in the Utrish State Nature Reserve have been carried out in 2012-2020. About 1000 soil samples have been studied. During this time, extensive material has been accumulated on the influence of relief, climate, and vegetation on the biological properties of soils. The degree of knowledge of all study sites varies depending on the research period. Biological indicators highly vary depending on environmental conditions. To make it possible to compare the biological properties of soils of various anthropogenic impacts, this study presents data obtained in the autumn of one year, when most of the samples were taken. For the long-term study of study site №3, it is possible to trace the dynamics of changes in properties over time. To minimise seasonal changes and increase the reliability of data when studying the biological properties of soils in autumn (during one sampling season), soil samples were taken from three study sites with various anthropogenic impacts and a control forest site in triplicate for each study site. Soil samples were taken from the 0-10 cm layer in triplicate. Soil samples were collected 1-3 m away from tree trunks. Study site №3 is given the most at-
tention, due to the better accessibility of this area. Since the establishment of the Utrish State Nature Reserve, study site №3 has been studied in 2013, 2015, 2018, 2019, and 2020.
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Fig. 1. Location of study sites in the Utrish State Nature Reserve, Russia. Designations: 1 - study site after fire impact; 2 - study site after deforestation; 3 - study site after recreational load; 4 - control forest site.
Fig. 2. Study sites in the Utrish State Nature Reserve, Russia. Designations: (a) - study site №1 after fire impact; (b) - study site №2 after deforestation; (c) - study site №3 after recreational load; (d) - control forest site.
Analysis of soil properties
The study has been performed using traditional methods for biology and soil sciences based on Kazeev et al. (2016). The biological and chemical properties of soils were studied in triplicate for each soil sample. On each study site, nine biological and chemical properties were obtained. The aqueous pH of the soil was measured potentiometrically in the soil suspension at the soil/water ratio of 1.0/2.5. Total organic carbon (TOC) was determined by the K2Cr2O7-H2SO4 oxidation method. Soil catalase activity was determined by the volumetric method and based on the amount of hydrogen peroxide processed during the reaction. Invertase activity was determined calori-metrically by the amount of reduced inverted copper glucose from Fehling's reagent. In the field, the temperature of the soil surface was measured using an electronic thermometer HANNA CHECTEMP; soil moisture was measured with a moisture meter (Field-scout TDR 100) in 10-fold replicates.
Statistical analysis
Statistical processing of data was performed using analysis of variance (ANOVA) to assess the validity of differences and correlation analysis (Pearson correlation coefficient) to study the cramping and the form of relationship between indicators of the biological state of soils using the software Sta-tistica 10.0 (https://www.statsoft.com). Statistically significant differences were considered with a significance level of 5% (p < 0.05).
Results
Environmental changes
Based on the species composition, the spectrum of life forms, and stratification of plant communities, the following succession stages are quite clearly distinguished: 1) the grouping of annuals; 2) the com-
munity of ruderal perennials; 3) the community of perennial cereals; 4) shrub communities; 5) stage of small-leaved forests; 6) the climax community. Study site №1 is a plant community of perennial cereals. Study site №2 is at the stage of small-leaved forests, marked by a dense juvenile Fraxinus tree with a height of up to 4.5 m and Rosa sp. thickets. Study site №3 after the recreational load is a community of ruderal species, characterised by sparse herbaceous vegetation.
After anthropogenic disturbance in addition to changes in plant communities, other environmental conditions have changed like temperature and moisture. On study site №1 and study site №3, the soil surface temperature exceeds the air temperature by 2-4°C. On study site №2, the soil surface temperature differs slightly from the air temperature (Table 1). Soil moisture is considerably reduced compared to the control forest site, namely by 38%, 32% and 15% on study sites №2, №3 and №1, respectively.
Changes in soil chemistry
Data on the gross chemical composition in soils on the study sites showed typical values for the main chemical elements in the Utrish State Nature Reserve. There was a high variation in the chemical composition of soils for each type of impact: silicon oxide (SiO2) (decrease by 7% and 5% on study site №1 and study site №3 respectively, and increase by 19% on study site №2), calcium oxide (CaO) (increase by 57% and 43% on study site №1 and study site №3 respectively, and decrease by 70% on study site №2) (Table 2). The content of other basic chemical elements, including heavy metals, differs marginally from the reference values. The content of all studied heavy metals (MnO, Sr, Cr, Zn, V, Ni, Cu, Pb, Co, As) does not exceed the maximum permissible concentration values.
Table 1. Air temperature and soil surface temperature, and soil moisture on the study sites in the Utrish State Nature Reserve (Russia) based on data of 27.09.2019
Study sites t, air, °C t soil surface, °C Moisture, %
Control forest site 22.8 22.3 4.0
Study site №1 after fire impact 25.6 29.6 3.4
Study site №2 after deforestation 17.2 17.5 2.7
Study site №3 after recreational load 20.2 22.4 2.5
Study sites рн SiO, A1A FeA CaO MgO K2O TiO7 PA
Control forest site 7.9 63.4 8.1 4.2 3.0 1.1 1.9 0.6 0.2
Study site №1 after fire impact 7.7 59.2 7.1 3.5 4.7 1.3 1.9 0.4 0.2
Study site №2 after deforestation 6.8 75.3 7.0 4.0 0.9 0.7 1.7 0.6 0.2
Study site №3 after recreational load 8.0 60.5 6.6 3.6 4.3 1.2 1.8 0.4 0.3
Table 2. Gross chemical composition (%) and pH values of Cambisols of anthropogenically disturbed forests in the Utrish State Nature Reserve, Russia
The pH values of soils in disturbed sites do not have significant differences from ones on the control forest site, №4 (Fig. 3 a). There was a variation in pH values between the study plots, ranging from 6.8 to 8.0. It was found that the pH of the environment to all studied soils was close to neutral (pH = 7.2-7.4) on study site №3 in 2013. Down the soil profile, there was an increasing tendency of the pH to 7.6. Such indicators distinguish brown soils from mesophilic brown forest soils, which are characterised by low pH values (< 5.5). Repeated studies of study site №3 in 2019 and 2020 showed that pH values of the soil did practically not differ from values on the control forest site. Therefore, the differences in pH values in the surface horizons over research time were in the range of 6.5-7.1 on study site №2 in 2013 (Fig. 3a). In 2019, the soil pH on study site №2 still differed from pH values on the control forest site by 14% (Fig. 3a). On study site №1, a decrease in the soil pH by 3% compared to control was found (Fig. 3 a).
We found an increase in the TOC in the surface soil layer (0-10 cm), namely 8.7% on study site №1, 4.7% on study site №2, and 9.4% on study site №3, while on the control forest site TOC content was 11.6% (Fig. 3b). The dynamics of changes in the TOC content in soils on study site №3 demonstrated the highest decrease (68%) in the TOC content values relative to the control forest site in 2013 (Fig. 4). On study site №3, the TOC values decreased by 46% in 2018 (i.e. eight years after the restriction of tourism), by 18% in 2019, and by 26% in 2020 (Fig. 4).
Changes in biological properties of soils
The significant inhibition of enzyme activity by two classes of hydrolases and oxidases was observed. On study site №1, the invertase activity of in soils was found to be 37% lower than the relative control (Fig. 3d). Enzyme activity was lower by 11% on study site №3 (Fig. 3d). On study site №2, the invertase activity increased by 77% (Fig. 3d). Catalase activity decreased slightly (by 3%) on study site №1 (Fig. 3c). Also, minor differences from the control were found on study site №2. Several decades after the deforestation, catalase activity has been reduced by 5% (Fig. 3c). A significant (by 30%) decrease in enzyme activity was found on study site №3 (Fig. 3 c).
On study site №3, we found a decrease in catalase activity by 16% in 2015 compared with their respective controls, 46% in 2018, and 30% in 2019. However, there was a significant increase in the catalase activity by 45% with their relative control in 2020 (Fig. 5).
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Fig. 3. The range of pH values (a), total organic carbon content (b), catalase activity (c), invertase activity (d) in soils on the study sites in the Utrish State Nature Reserve, Russia. Designations: 1 - study site №1 after fire impact; 2 - study site №2 after deforestation; 3 - study site №3 after recreational load; 4 - control forest site.
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Fig. 4. Changes in the soil organic carbon content from 2013 to 2020 on the site impacted by the recreational load (№3) compared to the control forest site (№4) in the Utrish State Nature Reserve, Russia.
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Fig. 5. Changes in the soil catalase activity from 2015 to 2020 on the site impacted by the recreational load (№3) compared to the control forest site (№4) in the Utrish State Nature Reserve, Russia.
Correlation of the studied parameters The correlation analysis has been carried out to identify the relationships between indicators of chemical and biological properties of anthropogeni-cally disturbed soils. A strong negative correlation between the pH values and the activities of invertase and catalase (r = -0.79 to -0.60) was found on study
site №1. A mean positive correlation between TOC content and catalase activity (r = 0.56) was also noted on study site №1. A negative correlation between pH values and catalase activity (r = -0.35) was identified on study site №2. The relationships between the remaining indicators were weak, where the correlation coefficient ranged from -0.19 to 0.25. Mean positive correlation between catalase activity, pH, and TOC content (r = 0.39-0.53), and between invertase activity, TOC, and catalase activity (r = 0.48-0.59) were found on study site №3. Strong negative correlation was found between invertase activity and pH (r = -0.85). Moderate correlation was noted between the activities of catalase and invertase (r = 0.3), and a weak correlation between other indicators on the control forest site. Fig. 5 shows the graph plot of the relationships between all variables. There, positively correlated variables are grouped together, and negatively correlated variables are located on opposite sides of the graph. Variables that are away from the origin are well represented on the factor map (Fig. 6).
Discussion
Fire as a threat to forest ecosystems
Undoubtedly, fire causes remarkable damage of ecosystems, including the soil conditions. Many plants and animals die during wildfires, and the consequences of pyrogenic exposure are noticeable even after decades (Alcañiz et al., 2018; Chungu et al., 2020; Fernández-García et al., 2020). Not only the composition of plant communities but also other environmental conditions are changed. Due to the bareness of study site №1 and the absence of high tree and shrub vegetation, which shade the soil cover, the soil surface temperature was found to be increased. The soil darkening due to the charring of the soil particles also leads to a change in the albedo and temperature regime of post-fire soils (Ngole-Jeme, 2019). As shown by the studies carried out 10 years after the fire impact in subtropical forests of Russia, significant differences in the chemical and biological properties of disturbed soils were noted in comparison with the control area of unburned forests (Kazeev et al., 2019). Most often, immediately after the fire, an increase in pH values is noted, which is associated with the release of alkaline cations during the combustion of plant materials and organic substances on the soil surfaces, such as Ca2+ and Mg2+, while the higher combustion temperature, the more decrease in pH values, which has been confirmed by some other studies (Ngole-Jeme, 2019; Kazeev et al., 2020). Alkaline cations are predominantly present in ash. Ten years after the
fire on the soil surface, ash is completely gone due to the occurrence of water and wind erosions. That is why now there is a slight decrease in pH values of 3% compared to the control. However, these values are close to the reference values. The content of P, K, and MgO is slightly different from the control, and the increase in CaO content has not demonstrated any significant effects on soil pH changes.
A decrease in the TOC content by 24% was noted ten years after fire impact. Soil carbon decline during the wildfire depends on the combustion temperature, amount and distribution of organic materials, and decomposition rate of the organic matter remaining after fire action. For a long time, study site №1 was deprived of the protective effect of the forest litter. Therefore, a high flushing speed, and increased water and air permeability caused losses of organic matter. However, immediately after the fire, many studies showed an increase in the TOC content because of the accumulation of ash and coal on the soil surface in various forest ecosystems (Liu et al., 2014; Nichols et al., 2021; Vilkova et al., 2022). Due to the introduction of an additional carbon source, a positive effect was observed in the first post-fire year, when the soil biological activity increases, and there is a rapid change in stages of the recovery succession. This is mistakenly perceived as a beneficial effect of fires. But after several years, the toxic effect has manifested itself, as demonstrated in the present study.
Fig. 6. Projection of the variables on the factorial plane of the study sites in the Utrish State Nature Reserve, Russia. Designations: (a) - study site №1 after fire impact; (b) -study site №2 after deforestation; (c) - study site №3 after recreational load; (d) - control forest site.
During the present study, a prolonged effect of high temperatures on the soil biological activity was found. Significant inhibition of the enzyme from the hydrolase (invertase) and oxi-doreductase (catalase) classes was noted. At the same time, inhibition of invertase activity was observed to a higher extent. Several factors affect the decrease in enzyme activities, such as a decrease in water permeability with an increase in the hydrophobicity of soils, increased temperatures on the exposed surface, changes in the vegetation cover (Alcaniz et al., 2018; Ngole-Jeme, 2019; Dadwal et al., 2021). Invertases act on water-soluble cellodextrins from the reducing end, by resulting in glucose molecules that can then be absorbed by microbial cells (Wallenstein & Burns, 2011). This enzyme does not provide nutrients to plants, although the produced glucose is necessary for the growth of soil micro-organisms, which, in turn, control the availability of nutrients. Micro-organisms subsequently control the availability of N, P, S and other nutrients and promote plant growth (Dotaniya et al., 2019).
Deforestation as a threat to forest ecosystems
Deforestation leads to fundamental changes in natural ecosystems. The ecological state of forest soils changes remarkably with deforestation, which in turn leads to the soil cover degradation. Consequently, erosion and other processes occurred in conditions of mountainous relief, and a large amount of precipitation was observed (Nichols et al., 2021). A significant decrease in pH values and TOC is associated with the forest stand thinning, which leads to a higher intake of precipitation on study site №2; however, the retarding capacity decreases which leads to the washing-off of the upper humus layer (Teng et al., 2019). Additionally, due to the lack of shading, the soil is heated. As a result, there is a decrease in air humidity and an increase in water evaporation from the soil surface. So, there is a decrease in soil moisture compared to the control forest site by 32%. The deterioration of water permeability in disturbed soils is also affected by over-compaction. In addition, the microclimate of the undergrowth forest contrasts sharply with the climate outside the forest due to the presence of trees and shrubs. It is known that forests have microclimate, regulated by the upper forest layer. This leads to changes in the forest microclimate (den Ouden & Mohren, 2020).
All these factors also directly affect the soil micro-organism activity; in this regard, a decrease in the activity of enzymes was noted (Ma et al., 2022). The catalase activity in soils on study site №2 was close to the reference values, and the invertase activity exceeded the reference values by 77%. The invertase activity was stimulated by the processes of restorative succession and the formation of a variety of herbaceous vegetation on areas exposed by deforestation. This results in favourable hydrothermal conditions and development of a turf process, accompanied by an improvement in soil structure and an increase in organic matter content. Due to the decrease in content of basic alkaline cations, a decrease in pH values of the studied soils was also observed. Even though several decades have passed since the deforestation, the properties of the disturbed soils still differ from the control, which may be caused by the repeated deforestation and the compaction influence. Soil compaction resulting from the use of heavy machines in forest operations can seriously limit the productivity of the forest for a long time (Cambi et al., 2015). The decrease in forest productivity found in our study is considered in the context of changes in activity of the studied enzymes up to 2019.
The high variation in the chemical composition of soils is caused by differences in the soil-forming rocks, which are changing significantly even at short distances because of the layering and rock topography. Differences in soil composition are associated with various levels of calcium enrichment because of the release to the surface of layers rich in calcium carbonates. The anthropogenic impact has a lesser effect on the chemical composition of the studied soils compared to the soil-forming rocks and terrain conditions.
Recreational load as a threat to forest ecosystems
For a long time (until 2010), the study area of the subtropical forests was subject to uncontrolled recreational tourism, when tent camps were located there. Besides direct trampling, vacationers polluted this area with household waste and used plant material as fuel for fires. Evaluation of the soil sustainability, as a component of ecosystems, is necessary to predict and analyse changes in the ecological status of ecosystems during human economic activity.
During the present study, it was found that the values of the soil reaction of the medium practically did not differ from the control. How-
ever, there was a 30% decrease in TOC in soils on study site №3. This can be associated with destruction of herb and shrub vegetation in the trampled area and an increase in rockiness. With a long-term absence of vegetation in conditions of mountainous relief and steep slopes, the soils on study site №3 were subjected to the water erosion. There was also an increase in the soil surface temperature (by 20°C relative to the air temperature) due to the absence of shading from herb vegetation. After the recreational load influence, the plant species composition has changed, and ruderal species have appeared. All these factors inhibited the activity of plants and micro-organisms, which had a negative effect on the enzyme activity. In soils under high recreational load, catalase activity was reduced to a greater extent than invertase activity. Studies of the microbial communities of camping soils in the sub-alpine belt showed that microbial communities changed in response to disturbance in the upper 6 cm of the soil. In soil layers below 6 cm, no noticeable differences were observed between disturbed and undisturbed natural communities. There was also a more remarkable decrease (by 54%) in carbon on disturbed sites than on undisturbed sites (95%) (Zabinski & Gannon, 1997), which is reflected in the results in our present study.
On disturbed sites, carbon substrates could not be metabolised by microbial communities. Study site №3 was the most investigated area among the three considered in the present study. However, it is not yet possible to trace clear trends in soil recovery. As early as 2013, the pH values of the studied soils slightly differed (by only 3%) from the soils, on the control forest site. Nine years later, i.e. after the recreational load, pH values differ just by 1%, which indicates a rapid return of the soil pH values to the natural. It was observed that there was no clear trend towards the recovery of TOC. There was an increase in the TOC content from 2013 to 2020, which is associated with the improvement of soil aeration. Aeration improvement was caused by the emergence of herb vegetation after the cessation of anthropogenic impact and restoration of organogenic horizon.
Conclusions
Unique xerophytic forests were investigated after human activity influence. Changes in soil properties were studied after such impacts as fire, deforestation, and recreational load. In the present study, the long-term negative impact of
the mentioned anthropogenic impacts on the soil properties was shown. Even after a long period, there was no clear trend towards restoring the properties of disturbed soils. At the same time, the degree of changes in chemical and biological properties of soils depends not only on the influence type but also on conditions of the terrain and the duration of the recovery succession. The highest negative effect on organic content was caused by deforestation impact. The soil biological activities were strongly affected negatively by the recreational load. Due to the over-compaction of soils disturbed by the recreation, environmental conditions were still changing after a long period. The duration and success of the soil recovery processes are related to which components have been modified. The partial restoration of these natural communities takes several dozens or hundreds of years. Dynamic observations of the restoration processes of various soil types in the course of succession after anthropogenic disturbances are necessary. The obtained data can be applied in the research actions of environmental institutions, as well as in normalisation and assessment of the impact of anthropogenic factors on forests. Moreover, studying the functional activity of micro-organisms, in particular the enzymatic activity, of soils will allow us predicting greenhouse gas fluxes.
Acknowledgements
The research was financially supported by the Ministry of Science and Higher Education of the Russian Federation (№FENW-2023-0008).
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ИЗМЕНЕНИЕ СВОЙСТВ ПОЧВ КСЕРОФИТНЫХ ЛЕСОВ ЮГА РОССИИ ПОСЛЕ АНТРОПОГЕННОГО ВОЗДЕЙСТВИЯ
В. В. Вилкова , К. Ш. Казеев , М. С. Нижельский, С. И. Колесников , Ю. С. Козунь
Южный федеральный университет, Россия *e-mail: lera.vilkova.00@mail.ru
Влияние антропогенного фактора вызывает существенные изменения свойств почв и древесной растительности ксерофитных лесов северной части Черноморского побережья. Представлены результаты исследования свойств почв на территории государственного природного заповедника «Утриш» (Россия), ранее подверженные влиянию антропогенных факторов. В качестве приоритетных видов антропогенного воздействия в долгосрочных исследованиях коричневых почв (Cambisols) государственного природного заповедника «Утриш» были выбраны пожары, вырубки и рекреационная нагрузка, которые быстро приводят к деградации экосистем и сокращению биоразнообразия. Пробы почвы отбирали из слоя 0-10 см в трех повторностях с каждого участка исследований. Площадь после рекреационной нагрузки изучалась в 2013, 2015, 2018, 2019, 2020 гг. Изучены следующие параметры: pH, содержание органического углерода, активности каталазы и инвертазы. Биологические свойства почв проявили большую чувствительность к изученным видам антропогенного воздействия. По всем изучаемым показателям наблюдалось снижение содержания органического углерода с 18% до 59% относительно контроля; наибольшее снижение показателя отмечено для почв после вырубки лесов. Ферменты класса оксидаз восстанавливаются быстрее, чем гидролазы. Активность каталазы для всех нарушенных почв снизилась в среднем на 13%. Незначительные отличия от контроля наблюдались для почв после вырубок и пожаров. Ингибирование активности инвертазы установлено в почвах в среднем на 52% после пожаров и рекреационного воздействия, при этом после вырубки лесов наблюдается повышение активности фермента на 77%. Результаты могут быть использованы для оценки последствий антропогенного воздействия на лесные экосистемы. Биологические индикаторы являются хорошими маркерами состояния нарушенных почв и могут быть использованы для определения последствий влияния антропогенного воздействия на почвы ксерофитных лесов.
Ключевые слова: Cambisol, антропогенное воздействие, биологическая активность почв, вырубка, пирогенное воздействие, рекреационное воздействие
