Soil Nutrient Profiles in Three Types of Rocky Fissure Network Habitats of Typical Karst Formations in China: A Maolan World Heritage Perspective

Abstract

Amid global desertification, this study investigates karst ecosystems; analyzing soil’s physiological and ecological properties within intricate fissure networks supporting plant root growth. This study investigates soil nutrients in three types of rocky fissure network habitats in Maolan, through field surveys and experimental measurements. Significant variability was found across habitats. The quantities of organic carbon, total nitrogen, available nitrogen, available phosphorus, and available potassium were highest in Type I soil, followed by Type III; and were lowest in Type II. Total phosphorus was highest in Type III, intermediate in Type I, and lowest in Type II; while total potassium content was highest in Type III, moderate in Type II, and lowest in Type I. Based on nutrient participation in C, N, P, and K cycles, Type I habitats had the highest levels, Type III had moderate levels, and Type II had the lowest levels; while potassium-related nutrients were highest in Type III. The nutrient ratios C/K, N/K, P/K, and N/P were highest in Type I, moderate in Type II, and lowest in Type III. C/N was highest in Type II, moderate in Type III, and lowest in Type I; while C/P was highest in Type II, moderate in Type I, and lowest in Type III. A comprehensive nutrient evaluation ranked Type I as the best, Type III as moderate, and Type II as the worst. Key factors such as average trace length, areal density, and integration significantly influence soil nutrients by impacting humus and soil storage, and the growth space for plant roots within rocky habitats. Additionally, the orientation of fissures primarily impacts nutrient cycling, while both the angles and lacunarity significantly affect the stoichiometric ratios of nutrients. Rocky habitat networks characterize soil quality. A comparison of existing studies reveals stoichiometric differences between karst and non-karst regions. Effective ecological restoration in karst areas requires targeted strategies that consider the specific attributes of different rocky habitats. This study enhances understanding of soil nutrients in karst forest ecosystems and proposes new approaches for soil’s ecological restoration and combating global desertification.

1. Introduction

Karst formations are extensively distributed globally, occupying about 15% of the world’s ice-free terrestrial area. They are primarily found in Southwest China, the Dinaric Alps in Bosnia and Herzegovina, the Central Massif in France, the Ural Mountains in Russia, southern Australia, the central-eastern United States, the Greater Antilles, South America, and the north-central regions of Vietnam, with the broadest distribution in China [1]. The Guizhou karst (karst) region in China is one of the largest and most intensely developed karst areas in the world, and is located in a highland mountainous area in tropical and subtropical zones [2]. Karst areas are characterized by well-developed karstic topography and complex landscape types with rich biodiversity but they are ecologically fragile and typically vulnerable, making them susceptible to damage and difficult to restore [3]. The primary rock types in karst areas are carbonates, and are mostly composed of one or two main carbonate minerals (primarily calcite and dolomite) [4]. Carbonate rocks are prone to dissolution and weathering, which can lead to the formation of unique karst features such as pores, fissures, conduits, sinkholes, and caves [5]; during the geological development, banded geological interfaces with certain directions, scales, shapes, and characteristics formed inside the rock mass, and the dissolution process of carbonate rocks form rock fissure structural surface cracks. Due to the different structures and origins of carbonate rocks, their dissolution processes and mechanisms vary, thus causing differences in the fissure surface of different rock types, leading to the formation of different rocky fissure network morphologies [6]. These rocky fissure networks contain limited soil, and plant roots grow in the soils within these fissure networks. Soil is a vital component of forest ecosystems, facilitating material cycling and energy flow. It actively influences forestland health through its aeration, moisture conditions, nutrient status, and organic matter content [7]. Therefore, studying the soil nutrients in karst forest rocky fissure network stony habitats is of significant theoretical and practical importance for understanding material cycling in karst forest ecosystems, karst soil ecological characteristics, and vegetation restoration.

Current research on karst habitats primarily focuses on qualitative classification and the study of habitat characteristics. Classification studies have largely revolved around six types of exposed karst habitats developed from pure limestone [8]. Regarding the quantification of rock fissure morphological structures, only Fu et al. [9,10]. classified rock fissure inclination into three dimensions, while Wang et al. [6] utilized fractal dimension and porosity to quantify rock fissure morphological characteristics in their study on the influence of rock morphology on plant root systems. Research on habitat characteristics tends to concentrate on micro-habitat climate [11], soil [12,13], and vegetation [14,15]. Studies on soil nutrients in karst regions can be divided into three parts: the first part discusses the spatial variability and causative factors of soil nutrients in karst regions from a spatial perspective [16,17]. The second part involves evaluating soil nutrients in karst areas, focusing on assessments of soil nutrient stocks and values, the effects of converting farmland back to forest on soil nutrients in karst mountainous areas [18]. The third part discusses soil nutrient characteristics under different soil conditions, including the impact of different land use types on soil nutrients, nutrient migration in typical fissure soils, nutrient leaching in shallow arable fissure soils, and the characteristics of soil nutrients at different desertification levels [19,20].

In summary, research on the classification of karst rocky habitats rarely adopts a topological and geometrical perspective. Studies focusing on the morphological characteristics of internal rock fissure networks are particularly scarce, and no reports have been found on soil nutrient studies based on rock fissure network characteristics. Building on preliminary research by our team, this study selects three types of rocky fissure network habitats within the Maolan National Nature Reserve to investigate soil nutrient dynamics. This research aims to reveal the depositional characteristics of soil nutrients in different rocky fissure network habitats and explore how the morphology of these fissure networks affects soil nutrients, providing a theoretical basis for understanding material cycling in karst forest ecosystems, maintaining karst soil fertility, and vegetation restoration.

2. Materials and Methods

2.1. Study Site Selection and Overview

This research was conducted in the Maolan National Nature Reserve in Guizhou, China, which was designated as a World Natural Heritage Site by UNESCO in 2007. Located in Libo County of Qiannan Buyi and Miao Autonomous Prefecture in Guizhou Province, the reserve spans coordinates (107°52′–108°45′ E, 25°09′–25°20′ N). The area, covering 21,285 square kilometers, sits at an elevation ranging from 430 to 1078 m above sea level and is situated within a mid-subtropical monsoon climate zone. It experiences an average annual temperature of 15.3 °C, annual rainfall of 1752.5 mm, and average humidity of 83%. The forest cover in the reserve is exceptionally high at 87.4%, contributing to its rich biodiversity, which includes 2086 animal species and 2406 plant species (refer to Figure 1).

The Maolan area hosts one of the most complete, unique, and stable karst forest ecosystems globally, known for its distinctive karst peak-cluster depressions and doline landscape [21]. The region is characterized by various successional landscape sequences ranging from peak clusters to karst valleys and basins, representing a complete process of cone karst morphological evolution and climate variation, making it an outstanding example of cone-like karst landscape evolution globally.

The karst topography of Maolan is complex, with a high rock exposure rate of 80% [22], primarily consisting of carbonate rocks such as limestone and dolomite. These rocks have low insoluble material content, which influences the soil environment, leading to slow soil formation rates and a fragile soil environment. The karst process has fully developed karst features such as karren, grooves, and dissolution fissures, creating a diverse range of rocky fissure structures. The exposed rocks vary significantly in elevation, with large boulders frequently collapsing and accumulating, contributing to the complex micro-topography and a variety of habitats. This makes the Maolan karst region a typical and representative area for studying soil nutrients in karst stony habitats.

2.2. Types and Characteristics of Three Karst Rocky Fissure Network Habitats

Building on preliminary research [6], three types of rocky fissure network habitats within the Maolan Nature Reserve were studied. Rock structural surveys were conducted for these habitats (see Figure 2A). Seven rock fissure network indicators were selected, namely: Average Trace Length (ALT), Dip Angle (Dag), Areal Density (Ads), Fractal Dimension (Fra), Lacunarity (Lac), Integration (Int), and Connectivity (Con). These seven rock indicators comprehensively summarize the spatial characteristics of the entire rock fissure network, such as the length, density, angle, and connectivity of the fissures. Specifically, Ads represents the intensity of fissure distribution; the larger the Ads value, the higher the intensity. Fra reflects the number of surface fissures; the higher the Fra value, the greater the number of surface fissures. ALT represents the length of fissures; the larger the ALT value, the longer the average trace length of surface fissures. Dag represents the extension direction of fissures. Con represents the degree of connection between fissures. Int reflects the number of turns required to connect two fissures; the higher the Int value, the more turns are needed to connect two fissures. Lac represents the uniformity of the fissure network; a higher Lac value indicates a more random distribution of rock fissures. The following steps were employed to obtain rock structural data: 1, Field Survey: Directly measured ALT and Dag of the rock fissures. 2, Image Analysis: Captured high-resolution images and used CAD 2016 and Minitab 19 software to construct planar network models of the rock fissures (see Figure 2B). 3, Calculated the Ads, Fra, Lac, Int and Con, by using techniques such as space syntax, fractal dimension, and lacunarity, various metrics. These parameters help in understanding the spatial organization and complexity of the fissure networks, providing insights into the physical structure and potential ecological functions of these karst formations.

Table 1 presents the specific values for rocky habitat indicators. Based on the values of the rocky fissure indicators, the characteristics of each type can be described as follows. Type I: fissures in this habitat primarily extend along an inclined direction, with moderate lengths and medium turning paths between adjacent fissures. The connectivity between fissures is relatively low, resulting in a less dense overall fissure network with fewer surface fissures, which are randomly distributed. This habitat is henceforth referred to as Type I. Type II: fissures here mainly extend vertically with shorter lengths. The turning paths between adjacent fissures are relatively high, and the connectivity between fissures is moderate. This results in a higher density of the overall rock fissure network, with more surface fissures, which are patchily clustered. This habitat is referred to as Type II. Type III: in this type, single fissures primarily extend horizontally, with longer lengths and fewer turning paths between fissures. The overall density of the fissure network is moderate, but connectivity between fissures is high. The number of surface fissures is moderate, and their distribution is more uniform. This habitat is referred to as Type III. These descriptions help in understanding the spatial arrangement and connectivity of fissures within each type of karst rocky habitat, providing insights into their ecological dynamics and potential challenges in conservation and management.

2.3. Community Ecology Survey

Building on previous research, three types of rocky fissure network habitats with distinctive structural characteristics were selected for soil sampling and plant community surveys. Following the protocols established by Zhu et al. [5] from extensive plot surveys in the Maolan karst forests, the minimum representational area was set at 900 m². In each of the three different rocky habitats, three plots of 30 m × 30 m with similar site conditions were randomly established. Plant community surveys were conducted in each plot, and basic information such as elevation, slope, and dominant species was recorded. Details of these surveys are presented in Table 2.

2.4. Field Soil Sample Collection and Testing

2.4.1. Field Soil Sample Collection

In each plot, soil samples are collected using the plum-blossom-shaped mixed sampling method from a depth of 0–15 cm [2]. Multiple samples are combined into a representative test soil sample, which is then refrigerated and preserved for subsequent determination of soil nutrients and related indicators.

2.4.2. Selection and Testing Methods for Soil Nutrient Indicators

This study selected seven soil nutrient indicators: organic carbon, total nitrogen, alkaline hydrolysable nitrogen, total phosphorus, available phosphorus, total potassium, and available potassium. Based on their properties and primary functions, these indicators were categorized into four elemental cycles—C, N, P, and K—according to the elements they predominantly participate in. To eliminate dimensional differences, the values were standardized using Equations (1) and (2), and the average value for each elemental cycle was calculated. Furthermore, soil nutrient stoichiometric ratios were computed based on experimental measurements of organic carbon, total nitrogen, alkaline hydrolysable nitrogen, and total phosphorus. Details of the seven indicators’ measurement methods and classifications by property are provided in Table 3.

(1)$\mathrm{Increasing} \mathrm{Distribution} \mathrm{Function}: SEI=(x_{ij}-x_{i min})/(x_{i max}-x_{i min})$

(2)$\mathrm{Decreasing} \mathrm{Distribution} \mathrm{Function}: SEI\left(x_i\right)=(x_{max}-x_{ij})/(x_{i max}-x_{i min})$

2.4.3. Comprehensive Evaluation of Soil Nutrients

To better understand the soil nutrient characteristics of different rocky habitats, this study calculated the Integrated Fertility Index (IFI). Initially, to minimize the differences in dimensions and avoid large variations in absolute values between indicators, the raw data were normalized before analysis using the following formula:

(3)$x_{ij}^{’}=(x_{ij}-{\overline{x}}_j)/s_j$

where i represents the number of samples, j represents the number of indicators, ${\overline{x}}_j$ is the average value of the measurements for indicator j, and $s_j$ is the standard deviation for indicator j.

Next, the soil nutrients were subjected to a Principal Component Analysis (PCA) to calculate the contribution rate of each component and the scores of the principal components. The factor impacts were then calculated to determine the weights, using the following formula:

(4)$a_i=C_i/C$

where $a_i$ represents the weight of each soil nutrient indicator, $C_i$ is the variance of the common factor, and C is the sum of the variances of all common factors.

The IFI was then calculated using the weighted sum method, as follows:

(5)$IFI=\sum _{n=1}^n{a}_i{y}_i/w$

In this formula, a_i is the contribution rate of the i principal component, y_i is the score of the i principal component, and w is the cumulative contribution rate of the principal components.

2.5. Data Processing

As shown in Figure 3, after the field survey, the collected rock fissure networks and soil samples were processed differently to obtain the values for selected indicators: 1. In addition to Dag and ALT obtained on-site, the remaining indicators were calculated using space syntax, fractal mathematics, and porosity; 2. Soil nutrient data analysis was carried out using SPSS 26.0 to calculate the means and variances, and perform Principal Component Analysis (PCA) of soil nutrients, one-way ANOVA (Analysis of Variance) was also conducted within SPSS 26.0 (significance level α = 0.05) to analyze the differences in soil nutrient levels across various rocky habitats; 3. The Integrated Fertility Index (IFI) was computed using Excel 2016; 4. Redundancy analysis (RDA) was performed using Canoco 5.0, with soil nutrients as the response variables and rocky habitat indicators as explanatory variables. This approach helps to discern the influence of physical habitat characteristics on soil nutrient profiles and to understand how these factors interact within the specific environmental context of each habitat.

3. Results

3.1. Analysis of Soil Nutrients in Three Types of Rocky Fissure Network Habitats

3.1.1. Distribution of Soil Nutrients in Three Types of Rocky Fissure Network Habitats

Figure 4 indicates that the contents of organic carbon, total nitrogen, alkaline hydrolysable nitrogen, available phosphorus, and available potassium are highest in Type I habitats; intermediate in Type III; and lowest in Type II. Total phosphorus content is highest in Type III, intermediate in Type I, and lowest in Type II. Total potassium content follows a similar pattern, with the highest values in Type III, intermediate in Type II, and lowest in Type I. Overall, Types I and III exhibit higher soil nutrient contents compared to Type II. The main characteristics of the rock fissures in Type II, which predominantly extend vertically with short lengths, frequent turns, high density, and numerous fissures, suggest that such environments are less conducive to nutrient retention. Quantitative comparisons reveal no significant variations in total potassium across the three habitat types. However, significant differences exist for total phosphorus and available phosphorus between Types I and III compared to Type II, as well as for organic carbon, total nitrogen, alkaline hydrolysable nitrogen, and available potassium between Types I and II. This reflects the substantial influence of different rocky fissure network habitats on phosphorus, nitrogen, and potassium levels; except for total potassium, which shows high stability and is less affected by individual factors.

3.1.2. Analysis of Soil Nutrients Involved in the C, N, P, K Cycles in Three Types of Rocky Fissure Network Habitats

Figure 5 demonstrates that the nutrients involved in the carbon (C), nitrogen (N), and phosphorus (P) cycles are most abundant in Type I habitats; with Type III in the middle; and Type II showing the lowest levels. Conversely, potassium (K) cycle nutrients are highest in Type III, intermediate in Type I, and lowest in Type II. This further underscores that rocky fissure network habitats with vertical, frequent turns, and high density are less conducive to the storage of soil nutrients. Habitats with inclined fissures, moderate turns, and high density better facilitate the storage of nutrients involved in the C, N, and P cycles. Meanwhile, habitats with horizontal fissures, fewer turns, and moderate density are more conducive to the storage of nutrients involved in the K cycle. The differences in C and N between Types I, III, and II are significant, as are the differences in P cycle nutrients between Types I, III, and II. However, there are no significant differences in K across the three habitat types, reflecting that the impact of different rocky fissure network habitats on C, N, and P cycles is significant, while their impact on the K cycle is less so. Potassium ions have a strong adsorption capacity on the surface of soil particles, leading to less leaching; hence the influence of rock fissures on soil nutrients involved in the K cycle is limited. These findings underscore the need to consider both biological and physical factors in conservation and management strategies for these unique karst environments.

3.1.3. Stoichiometric Ratios of C, N, P, K in Soil from Three Rocky Fissure Network Habitats

Table 4 shows that the ratios of C/K, N/K, N/P are highest in Type I habitats, intermediate in Type II, and lowest in Type III. Conversely, the C/N ratio is highest in Type II, intermediate in Type III, and lowest in Type I. For P/K, the trend is reversed, while C/P is highest in Type II, intermediate in Type I, and lowest in Type III. This indicates that in Type II habitats, nitrogen and phosphorus are relatively deficient, while potassium content is relatively abundant. In Type I habitats, nitrogen and phosphorus contents are richer, and in Type III habitats, potassium content is comparatively abundant. These findings suggest that the vertical, frequently turning, and densely packed fissure networks in Type II habitats are less conducive to nutrient retention; Type I habitats, likely containing more substantial organic matter deposits, favor the retention of nitrogen and phosphorus; and Type III habitats, with their specific structural advantages, are favorable for potassium retention. This differentiation highlights the influence of both the rocky fissure network structures and the vegetative and organic inputs on the storage and distribution of various soil nutrients.

3.2. Analysis of the Impact of Three Types of Karst Rocky Fissure Network Habitats on Soil Nutrients

3.2.1. Impact of Rocky Fissure Network Structure on Overall Soil Nutrients

Figure 6 demonstrates that the structure of rocky fissure networks significantly explains the variability in soil nutrient content, accounting for 41.1% of the overall variation. This finding underscores the substantial influence of rock fissure characteristics on soil nutrient dynamics within rocky habitats. Detailed analysis reveals that the average trace length, areal density, and integration of the fissure networks are the indicators with the highest explanatory power, explaining 14.1% (significant at p < 0.05), 10.2% (significant at p < 0.05), and 9.1% (significant at p < 0.05) of the variance, respectively. These metrics are identified as the primary structural indicators impacting soil nutrient content, demonstrating how physical formations within the karst systems play a crucial role in nutrient content, while other indicators have lesser impacts. The average trace length shows a positive correlation with total potassium, suggesting that longer fissures facilitate the distant movement and accumulation of potassium; whereas it negatively correlates with other nutrients, likely due to deep water percolation leading to nutrient leaching along these fissures. Areal density shows a negative correlation with all soil nutrients, indicating that in habitats with dense fissure networks, nutrients are more prone to being washed away. Moreover, integration shows a positive correlation with all soil nutrients, indicating that highly integrated fissure networks help in the widespread transport and even distribution of nutrients. These results reveal how different characteristics of rocky fissure networks can influence soil nutrient status in rocky habitats by affecting the movement of water and nutrients and their distribution within the soil.

3.2.2. Impact of Rocky Fissure Network Structure on Soil Nutrients in C, N, P, K Cycles

Figure 7 indicates that the structure of rocky fissure networks can explain a total of 36.2% of the variation in soil nutrients involved in the C, N, P, and K cycles across different rocky habitats, highlighting the significant role of fissure structure in regulating these nutrient cycles. Key factors such as areal density, average trace length, and angle were the main influences, with explanatory powers of 10.9% (p < 0.05), 8.0% (p < 0.05), and 6.5% (p < 0.05), respectively. High areal density reflects accelerated nutrient loss, likely due to dense fissures providing more pathways for water, thus facilitating rapid nutrient leaching. The positive correlation between average trace length and the potassium cycle suggests that longer fissures help maintain potassium in the soil; however, they negatively correlate with other nutrients such as nitrogen and phosphorus, which may be more easily washed away. The impact of angle shows that different fissure angles significantly affect nutrient retention and loss, where a negative correlation might indicate that certain angles enhance nutrient flow into deeper fissure sections or outwards. High potassium levels in Habitat III enhance plant stress tolerance and water efficiency, vital for thriving in rocky terrains. This supports robust plant growth and the development of a unique ecological niche with specialized communities adapted to these conditions.

3.2.3. Impact of Rocky Fissure Network Structure on Soil Nutrient Stoichiometry

Figure 8 reveals that rocky fissure network structures can explain up to 47.9% of the overall variation in soil nutrient stoichiometry ratios across different rocky habitats, demonstrating the substantial impact of fissure structure on soil nutrient cycles. Notably, the angle and lacunarity of the fissure networks stood out, explaining 27.7% (p < 0.05) and 9.5% (p < 0.05), respectively; becoming key factors influencing soil nutrient stoichiometry. The angle mostly shows a positive correlation with most stoichiometric ratios except for a negative correlation with P/K, indicating that fissure angles affect the relative movement and accumulation of phosphorus and potassium in the soil. High lacunarity negatively correlates with P/K, N/K, while positively correlating with C/N, C/P, N/P, and C/K, suggesting that a higher degree of fissure gaps might promote higher carbon to nitrogen ratios, but could be detrimental to the phosphorus to potassium ratio. These findings emphasize the importance of understanding the characteristics of rocky fissure structures for managing and improving soil quality in rocky habitats.

3.3. Comprehensive Evaluation of Soil Nutrient Quality in Three Types of Karst Rocky Fissure Network Habitats

Table 5 shows that after conducting principal component analysis (PCA), two factors were extracted, accounting for a cumulative contribution rate of 77.63%. This indicates that the PCA effectively reflects most of the soil nutrient characteristics across the three types of rocky habitats. The first principal component (PC1) primarily captures the combined variation of total nitrogen, alkaline hydrolysable nitrogen, available phosphorus, and available potassium. The second principal component (PC2) mainly reflects the combined variables of total potassium and total phosphorus, with both components also capturing some aspects of organic carbon variability.

Using the factor score coefficient matrix derived from the principal component analysis, we can calculate the score functions for the two principal components.

$F_1=0.16{OM}_1+0.49{TN}_1+\cdots +0.47{AK}_1$

$F_2=-0.31{OM}_1-0.11{TN}_1+\cdots +0.16{AK}_1$

From Table 6, it is evident that Type I habitats have the highest comprehensive soil nutrient scores, suggesting optimal soil nutrient conditions. The single fissures in Type I predominantly extend along inclined directions, with moderate lengths and moderate turning paths between adjacent fissures. The connectivity between these fissures is relatively low, and the overall density of the fissure network is also low, with fewer surface fissures and a random distribution pattern. This configuration may contribute to better nutrient retention, facilitating the accumulation and stability of soil nutrients. Type III habitats score in the middle range for soil nutrient quality. The fissures in these habitats mostly extend horizontally, are longer, and have fewer turns between them. The overall density of this fissure network is moderate, with higher connectivity between fissures and a moderate number of surface fissures, which are uniformly distributed. This structure may help with better distribution and utilization of soil nutrients. Type II habitats have the lowest soil nutrient scores, indicating the poorest soil nutrient conditions among the three types. Fissures in Type II typically extend vertically, with short lengths and frequent turns, leading to a moderate level of connectivity but a high density of the overall rock fissure network. The large number of patchily distributed surface fissures may promote the loss of water and nutrients, adversely affecting nutrient retention in the soil.

4. Discussion

4.1. Characteristics of Soil Nutrients in Karst Rocky Fissure Network Habitats

The unique fissure structures in carbonate rock landscapes not only lead to significant loss of soil matter, but also make the rock surface a primary attachment point for soil. This structural characteristic causes significant spatial heterogeneity in soil nutrient distribution, affecting vegetation growth and ecosystem functionality. After comparing the existing literature, compared to the loess regions [29,30], alpine meadows [31,32], tropical rainforests [33,34], and desert areas [35], there is a general deficiency of potassium in the soils of the karst region. The C/K ratio in the karst region is also generally lower (10–20) than in other regions (loess regions: 25–70, alpine meadows: 50–150, tropical rainforests: 20–50, desert areas: 50–200), significantly highlighting the limited bioavailability of potassium in the soils of this region. The karst region is predominantly composed of carbonate rocks, which are high in calcium and magnesium content [34]; but natural sources of key nutrients such as phosphorus and potassium are limited. Due to the fissured structure of the karst landscape, soil development is relatively thin, and surface runoff and leakage are easily formed, accelerating the loss of soil nutrients. This characteristic results in relatively low levels of K in the soils of the karst area, further affecting the fertility of the soil and the sustainability of vegetation growth. The unique geological background of the karst area (primarily composed of limestone) and the intense geomorphological erosion restrict the accumulation and cycling of nutrients such as potassium, leading to a widespread deficiency of this element in the region. In summary, due to its unique geomorphological structure, the soil in the karst region shows nutrient ratio differences compared to other geomorphic regions. In future karst ecological restoration and soil management, the unique impact of the karst topography on soil nutrient cycling must be fully considered. Targeted restoration should be conducted based on the differences between various chemical nutrient contents. This general lack of potassium could negatively impact the vegetation growth and soil health in the karst area, suggesting that measures such as potassium supplementation and soil structure improvement should be implemented to enhance soil nutrient levels. The soil element characteristics of the karst area indicate a significant deficiency of potassium as a key issue, while the situations with nitrogen and phosphorus are more complex. Nitrogen is relatively abundant, especially compared to phosphorus, and although the availability of phosphorus is not as severely deficient as in desert areas, it is also not as abundant as in high-altitude meadows or loess regions. This information is helpful in guiding soil management and fertilization strategies in the karst area; particularly the supplementation of potassium, which is likely crucial for improving soil fertility and plant growth.

Research on karst rock fractures has traditionally focused on geological and geological engineering aspects, such as their impact on groundwater movement, rock stability, and construction [36,37]. Studies typically examine the formation, growth, and distribution of these fractures and their response in geological settings, providing essential insights for understanding and managing karst terrains. However, the ecological roles of these fractures—such as their impact on soil formation, water storage, nutrient cycling, and plant root development—are less explored, though they significantly influence soil thickness and quality, which is especially crucial in karst areas with typically thin soil layers. These systems also support unique habitats and microbial communities, which contribute to the biogeological shaping of the landscape. Therefore, expanding research into the ecological functions of karst fractures is critical—especially given the increasing impacts of climate change and human activities—to guide future conservation and restoration efforts effectively.

4.2. Impact Characteristics and Insights of Different Rocky Fissure Network Structures on Soil Nutrients

In the three types of rocky habitats, soil nutrients such as organic carbon, total nitrogen, alkaline hydrolysable nitrogen, available phosphorus, and available potassium were found to be highest in Type I habitats, intermediate in Type III, and lowest in Type II. Total phosphorus content was highest in Type III, intermediate in Type I, and lowest in Type II, while total potassium content followed the same pattern. These findings indicate that different rocky fissure network structures significantly influence nutrients essential for plant growth, with lesser impact on organic carbon and total potassium. The findings are consistent with those of Wang et al. [6], who also noted the impact of different rock fissure structures on soil properties. Overall, this study found that average trace length, plane density, and integration are the main factors affecting soil nutrients. These factors influence the soil nutrients in the habitat by affecting litter retention, soil storage, and plant growth space in rocky habitats. Specifically, average trace length, areal density, and integration were found to be the primary influencers on soil nutrients. The longer average trace lengths are beneficial for intercepting organic matter such as leaf litter [38], which can enhance nutrient retention. Integration and areal density—which represent the connectivity of rock fissure networks—facilitate better conditions for the rhizosphere effects of plants [39,40], thus accelerating nutrient decomposition. The orientation of rock fissures also plays a significant role, affecting the movement and retention of soil and organic matter [41], with smaller angles helping to distribute nutrients more evenly and promoting effective cycling.

For the impact of individual soil nutrients, organic carbon is relatively stable and is not significantly influenced by any single factor [42]. The influence of different rocky fissure networks on organic carbon is minimal because its accumulation predominantly depends on the type and amount of plant-derived organic matter present. Total nitrogen content is largely influenced by soil organic matter content and nitrogen sources [43], which correlates with the structural characteristics of rocky fissures that facilitate organic matter accumulation and decomposition, thereby affecting nitrogen availability. Alkaline hydrolysable nitrogen content is associated with organic matter decomposition, and better connectivity of rock fissures enhances conditions for microbial activity, which in turn promotes nitrogen mineralization. Available phosphorus and available potassium are related to soil water retention and nutrient release capacity [44,45]. The configuration and orientation of fissures influence water movement and retention, impacting the availability of these nutrients. The content of total phosphorus is typically influenced by the overall phosphorus stock in the soil [46,47]. Therefore, total phosphorus, available phosphorus, total nitrogen, alkaline hydrolysable nitrogen, and available potassium in the soil are influenced by the type of rocky habitat to varying degrees. The angle of fissures significantly affects nutrient cycles by altering soil mobility and organic matter retention, thereby influencing nutrient distribution and accumulation. Smaller angles promote more uniform nutrient distribution and efficient cycling. Lacunarity and angle not only affect the physical pore structure of the soil but also modify the microenvironmental conditions crucial for nutrient retention and cycling, significantly impacting the stoichiometric ratios of various soil nutrients. The comprehensive soil nutrient evaluation ranks Type I habitats as the best, Type III as moderate, and Type II as the worst. The rocky fissure network in Type I habitats optimally balances humus retention and nutrient cycling efficiency, leading to higher soil nutrient content. Type III habitats provide ample soil humus as the primary nutrient source, but the intermediate connectivity of the fissure network moderately affects material cycling, leading to nutrient content slightly lower than in Type I. Type II habitats retain less litter, leading to greater nutrient loss and consequently lower soil nutrient content.

In summary, the characteristics of rocky fissures can serve as indicators of ecological quality in rocky habitats. The findings suggest that habitats with longer average trace lengths, higher areal density, and better integration might have higher soil nutrient contents, reflecting better material cycling and ecological stability. The variability in angle can reveal differences in nutrient availability and distribution across different element cycles. In developing ecological restoration strategies for karst regions, it is crucial to consider these geological structural features and their impact on soil nutrient dynamics to devise more effective restoration plans. For Type I habitats, maintaining existing ecological functions while enhancing litter retention is recommended to ensure continuous nutrient supply and long-term ecosystem stability. For Type II habitats, enhancing litter interception and soil nutrient storage measures is crucial to improve ecological functions. For Type III habitats, maintaining soil and litter storage while enhancing soil aeration and plant root interaction conditions should be prioritized to promote more efficient nutrient cycling and richer soil nutrient supply.

4.3. Significance and Prospects of Soil Research in Karst Rocky Fissure Network Stone Habitats

Research on the spatial network structure of rocky fissures in karst areas has often been overlooked in the past, yet it plays a critical role in maintaining plant and soil quality. This study, by analyzing the soil nutrients within the rocky fissure network structure, proposes effective management measures for specific habitat soils. In the practical application of regulating and supporting ecosystem services, the initial step involves dividing ecological functional areas or classifying green spaces based on the morphological structure of rocky fissures, thereby enhancing the precision of land use efficiency and ecological protection, and selecting suitable plant species to optimize vegetation configuration. Further, the impact of fissures on plant growth and soil at the microscale of rocks is explored, which provides a more refined scientific basis for ecosystem management compared to traditional macro-ecological service research methods. Additionally, incorporating rocky fissure network characteristic indicators into the division of ecological functional areas and forest site conditions, as compared to traditional divisions based on biotic communities and topography, directly reflects the specific impact of the rock environment’s physical structure on the ecosystem. These assessments based on ecological structure not only provide new practical directions for ensuring the health and sustainable development of forest ecosystems but also offer theoretical support for fundamental ecological functions such as material cycling and soil formation.

Moreover, this study not only enhances understanding of soil nutrients in karst areas, but also provides new research perspectives and practical application paths for formulating effective land management policies and ecological protection measures based on the theory of rocky fissure networks; particularly significant for policymakers and environmental scientists among other broad readerships. For example, when delineating forest sites, planners should consider incorporating characteristics of the rocky fissure network into the evaluation of soil and topographical conditions, including the impact of rocky fissures on soil fertility, preventing soil erosion, and the development of plant root systems. Furthermore, the use of locally available organic materials as soil amendments in soil management strategies and restoration measures, through simple operational steps and cost-effective solutions, facilitates adoption and implementation by farmers. Regular monitoring of soil and moisture changes in the rocky fissure stone habitats and adjusting fertilization strategies help improve soil management in karst areas, especially in increasing agricultural yields and enhancing the ecological environment. The results of this study are not only significant for ecological restoration in karst areas but also provide a theoretical basis and practical guidance for managing and restoring global karst ecosystems, thereby offering new perspectives on achieving ecosystem service functions.

This study conducted a detailed analysis of soil nutrients in the Maolan area and explored the impact of the rocky fissure network on the soil nutrient characteristics of the karst stone habitat. Although this research provides preliminary insights into soil management and ecological restoration in karst areas, it has certain limitations. During the preliminary survey, the team covered 46 sample sites across the typical karst regions of Guizhou Province, known for its well-developed karst topography. However, this study primarily focused on sample sites in the Maolan area and did not comprehensively include all the sites from the preliminary survey. Therefore, future research should consider expanding the survey area to these regions, or even to the entire southern karst region of China, to further argue the generalizability and universality of the theory from the perspective of the rocky fissure network. Future studies should also explore the practical aspects of karst soil management, particularly through in-depth research at other unsurveyed sites, to verify and refine the current management strategies and restoration measures based on the rocky fissure network.

5. Conclusions

The distribution of soil nutrients varies significantly across different rocky habitats. Type I habitats exhibit the highest levels of organic carbon, total nitrogen, alkaline hydrolysable nitrogen, available phosphorus, and available potassium. Type III ranks in the middle for these nutrients, while Type II shows the lowest levels. Conversely, total phosphorus is highest in Type III, with Type I in the middle and Type II having the least. Similarly, total potassium is highest in Type III, followed by Type II, and is lowest in Type I. The impact of different rocky fissure network habitats is significant on total phosphorus, available phosphorus, total nitrogen, alkaline hydrolysable nitrogen, and available potassium; but less so on organic carbon and total potassium. Nutrient cycles of C, N, and P are most robust in Type I habitats, with Type III in the middle and Type II the least. In the case of potassium, the cycle is most prominent in Type III, followed by Type I, and is least in Type II. Stoichiometric ratios such as C/K, N/K, N/P are highest in Type I, with Type II in the middle and Type III the lowest. Conversely, the P/K ratio is highest in Type II, Type III is intermediate, and Type I is the lowest; while C/P, is the opposite. The comprehensive soil nutrient evaluation ranks Type I as having the best soil quality, followed by Type III in the middle, and Type II as the poorest. Key factors such as average trace length, areal density, and integration significantly influence soil nutrients by impacting humus accumulation, soil storage, and the growth space for plant roots within rocky habitats. Additionally, the orientation of fissures primarily impacts nutrient cycling, while both the angles and lacunarity significantly affect the stoichiometric ratios of nutrients.

The study suggests tailored strategies for different habitat types to bolster their ecological sustainability. For Type I habitats, maintaining ecological functions and enhancing litter retention are crucial for continuous nutrient supply and stability. Type II habitats benefit from improved litter interception and soil nutrient storage to enhance ecological functions. For Type III habitats, focusing on maintaining soil and litter storage while enhancing soil aeration and plant root interactions is essential for efficient nutrient cycling and enriched soil nutrients. These strategies are key to supporting the ecological health of diverse habitats. The research results not only provide new practical directions for ensuring the health and sustainable development of forest ecosystems but also offer theoretical support for fundamental ecological functions such as material cycling and soil formation. These findings are of significant importance for ecological restoration in karst areas and also provide a theoretical basis and practical guidance for managing and restoring global karst ecosystems, thereby offering new perspectives on achieving ecosystem service functions.

Author Contributions

Z.L.: writing—original draft, software, methodology, investigation, formal analysis, conceptualization; Z.H.: writing—review and editing, supervision, project administration, investigation, funding acquisition, conceptualization; M.W.: writing—review and editing, supervision, funding acquisition, conceptualization; H.X.: writing—review and editing, supervision, investigation, conceptualization; S.L.: writing—review and editing, software, resources, data curation; Y.C.: writing—review and editing, resources. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are not fully available because the research is not yet complete. However, they are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Shaowei Lu is employed by PowerChina Guiyang Engineering Co., Ltd., his employer’s company was not involved in this study, and there is no relevance between this research and their company.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Overview of the study area. (a) Location of the study area. (b) Map of natural resource distribution in Guizhou Province. (c) Boundary map of the Maolan National Nature Reserve. (d) Karst peak-cluster depressions in Maolan. (e) Plants and soil: current situation in Maolan. (f) Current status of the rocky habitats in the study area.

Figure 2. Planar Network Models of Rock Fracture Structures for Three Types of Rocky Habitats. ((A): Schematic diagram of the rock fissure network in the field, (B): Digital image of the rock fissure network after digitization).

Figure 3. Data processing flowchart.

Figure 4. Soil nutrient content across three types of rocky fissure network habitats. (Note: Different lowercase letters indicate significant differences between habitat types at p [less than] 0.05.).

Figure 5. Normalized soil nutrient content values in C, N, P, K cycles across three types of rocky fissure network habitats. (Note: Different lowercase letters indicate significant differences between habitat types at p [less than] 0.05).

Figure 6. Overall soil nutrients and rocky habitat indicators; RDA analysis results (* indicates p [less than] 0.05).

Figure 7. RDA analysis results of nutrient cycles in soil and rocky habitat indicators (* indicates p [less than] 0.05).

Figure 8. RDA analysis results of soil nutrient stoichiometry ratios and rocky habitat indicators (* indicates p [less than] 0.05).

Table 1

Indicators of rocky habitat types.

Habitat Types	Ads	ATL	Fra	Lac	Dag	Int	Con
Type I	0.167	4.331	1.211	1.422	24.504	0.561	1.634
Type II	0.263	3.820	1.395	1.784	51.550	0.678	1.844
Type III	0.173	5.196	1.316	1.441	12.973	0.488	1.877

Note: Ads = Areal density, ATL = Average Trace Length, Fra = Fractal dimension, Lac = Lacunarity, Dag = Dip angle, Int = Integration, Con = Connectivity (as hereinafter).

Table 2

Basic information of sample plots.

Rock Habitat Type		I			II			III
Plot ID		1	2	3	4	5	6	7	8	9
Elevation (m)		602	684	610	673	656	629	598	600	620
Slope direction		WS			WS			WS
Dominant species	Tree layer	Bauhinia purpurea; Cyclobalanopsis delavayi; Rauvolfia verticillata			Staphylea forrestii; Clerodendrum mandarinorum; Photinia bodinieri			Bauhinia purpurea; Eurycorymbus cava-leriei; Pinus massoni-ana
	Shrub layer	Mallotus philippensis; Tirpitzia sinensis			Lindera com-munis; Zanthox-ylum armatum			Lindera com-munis; Rhus chinensis; Zan-thoxylum simulans
	Herb layer	Nephrolepis cordifolia; Pilea peploides; Selaginella uncinata			Elatostema oblongifo-lium; Microstegium vimineum; Nephro-lepis cordifolia			Selaginella uncinata; Scleria levis Retz; Pilea peploides
Average height (m)	Tree layer	5.62	6.10	5.45	5.85	6.90	6.62	6.32	6.52	6.14
	Shrub layer	1.30	1.16	1.21	1.43	1.08	1.11	1.25	1.17	1.42
Average canopy cover	Herb layer	72%	85%	80%	70%	74%	80%	78%	75%	73%
Average diameter at Breast height (cm)	Tree layer	27.07	25.07	27.05	19.05	23.31	22.30	26.96	32.18	32.89
Number	Tree layer	95	107	105	74	78	85	105	87	102

Table 3

Soil nutrient indicators and their testing methods.

Soil Nutrient	Main Function	Flame Photometry Method
Organic carbon	Carbon cycle participation	Dichromate sulfuric acid external heating method [23]
Total nitrogen	Nitrogen cycle participation	Kjeldahl method [24]
Alkaline hydrolysable nitrogen		Diffusion method [25]
Total phosphorus	Phosphorus cycle participation	Alkaline fusion method [26]
Available phosphorus		Colorimetric method [27]
Total potassium	Potassium cycle participation	Alkaline fusion method [28]
Available potassium		Flame photometry method [28]

Table 4

Stoichiometric characteristics of soil nutrients in three types of rocky fissure network habitats.

	C/N	C/K	C/P	N/K	N/P	P/K
Type I	13.05 ± 1.18 a	15.85 ± 2.18 a	91.88 ± 9.09 b	1.91 ± 0.72 a	8.50 ± 1.53 a	0.18 ± 0.02 a
Type II	17.19 ± 1.41 a	15.74 ± 2.81 a	144.85 ± 25.50 a	0.88 ± 0.13 a	7.93 ± 1.09 a	0.11 ± 0.01 ab
Type III	14.25 ± 1.59 a	10.48 ± 0.67 b	86.74 ± 10.08 b	0.81 ± 0.07 a	6.02 ± 0.47 a	0.14 ± 0.02 b

Note: Different lowercase letters indicate significant differences between habitat types at p < 0.05.

Table 5

Results of the principal component analysis of soil nutrients.

	OM	TN	TK	TP	AN	AP	AK	Eigenvalues	Contribution Rate	Cumulative Contribution Rate
PC1	0.31	0.96	0.03	0.6	0.91	0.87	0.92	3.76	53.76%	53.76%
PC2	−0.39	−0.14	0.94	0.64	−0.08	−0.33	0.21	1.67	23.87%	77.63%

Table 6

Comprehensive evaluation of soil nutrient quality in three types of rocky habitats.

Rock Habitat Type	Comprehensive Soil Nutrient Score	Rank
Type I	0.59	1
Type II	−0.80	3
Type III	0.21	2

References

1. Wang, S. Discussion on the concept of karst desertification and its scientific connotation. Chin. Karst; 2002; 21, pp. 31-35.

2. Long, J.; Jiang, X.; Deng, Q.; Qiong, L. Study on the essential characteristics of soil desertification in karst areas of Guizhou. Acta Pedol. Sin.; 2005; 42, pp. 419-427.

3. He, X.J.; Wang, L.; Ke, B.; Ming, Q.Y.; Wang, K.L.; Cao, J.H.; Xiong, K.N. Progress in research on karst ecology protection and restoration in China. Acta Ecol. Sin.; 2019; 39, pp. 6577-6585.

4. Yi, X.; Zhang, Y.; He, J.; Wang, Y.; Dai, Q.; Hu, Z.; Zhou, H.; Lu, Y. Characteristics and influencing factors of farmland abandonment in the karst rocky desertification area of Southwest China. Ecol. Indic.; 2024; 160, 111802. [DOI: https://dx.doi.org/10.1016/j.ecolind.2024.111802]

5. Zhu, S.Q. Karst Forest Ecology Research III; Guizhou Science and Technology Press: Guiyang, China, 2003; pp. 286-295.

6. Wang, M.Q.; Guan, Q.W.; Sheng, H.Z.; Zhao, J.H.; Liu, Z.J.; Zhang, H.; Bao, X.W.Q.; Wang, L.; Ye, Y.Q. Morphological characteristics of rock fissure networks and the main factors affecting their soil nutrients and enzyme activities in Guizhou Province, China. J. Mt. Sci.-Engl.; 2022; 19, pp. 2587-2600. [DOI: https://dx.doi.org/10.1007/s11629-022-7345-2]

7. Zhang, Z.H.; Hu, G.; Zhu, J.D.; Ni, J. Spatial heterogeneity of soil nutrients in karst forests and its effects on tree species distribution. J. Plant Ecol.; 2011; 35, pp. 1038-1049.

8. Wang, D.L.; Zhu, S.Q.; Huang, B.L. Evaluation of karst desertification types and degrees in Guizhou. Acta Ecol. Sin.; 2005; 25, pp. 1057-1063.

9. Fu, Y.H.; Peng, Q.; Li, A.D.; Huang, Z.S. Soil Quality of Underground Habitats in Karst Limestone Areas. J. For. Environ.; 2017; 37, pp. 353-359. [DOI: https://dx.doi.org/10.13324/j.cnki.jfcf.2017.03.018]

10. Fu, Y.H.; Yu, L.F.; Huang, Z.S.; Li, H.; Hou, D.H. Study on Root Zone Habitats and Their Types in Typical Karst Rocky Desertification Areas. Res. Soil Water Conserv.; 2012; 10, pp. 66-72. (In Chinese)

11. Zhang, J.; Fu, Z.; Nie, Y.; Lian, J.; Luo, Z.; Wang, F.; Chen, H. Microclimate stability on the critical zone of a karst hillslope in southwest China: Insights from continuous temperature observations at the air oil pikarst interface. J. Environ. Manage; 2023; 336, 117656. [DOI: https://dx.doi.org/10.1016/j.jenvman.2023.117656]

12. He, J.; Dai, Q.; Yi, X.; Wang, Y.; Peng, X.; Yan, Y. Effects of soil and rock microhabitats on soil organic carbon stability in a karst peak-cluster depression region of Southwestern China. Geoderma Reg.; 2023; 32, e00623. [DOI: https://dx.doi.org/10.1016/j.geodrs.2023.e00623]

13. Jiang, C.; Zhu, B.; Zeng, H. Soil extracellular enzyme stoichiometry reflects the unique habitat of karst tiankeng and helps to alleviate the P-limitation of soil microbes. Ecol. Indic.; 2022; 144, 109552. [DOI: https://dx.doi.org/10.1016/j.ecolind.2022.109552]

14. Wang, H.; Huang, B.; Zhao, H.; Dai, X.; Chen, M.; Ding, F.; Wu, P.; Hao, L.; Yang, R.; Yuan, C. Unraveling plant adaptation to nitrogen limitation from enzyme stoichiometry aspect in Karst soils: A case study of Rhododendron Pudingense. Front. Plant Sci.; 2023; 14, 1267759. [DOI: https://dx.doi.org/10.3389/fpls.2023.1267759] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38098793]

15. Tang, S.; Liu, J.; Lambers, H.; Zhang, L.; Liu, Z.; Lin, Y.; Kuang, Y. Increase in leaf organic acids to enhance adaptability of dominant plant species in karst habitats. Ecol. Evol.; 2021; 11, pp. 10277-10289. [DOI: https://dx.doi.org/10.1002/ece3.7832]

16. Wang, M.M.; Chen, H.S.; Fu, T.G.; Zhang, W.; Wang, K.L. Differences in soil nutrients among different vegetation types in a typical karst small watershed and their spatial prediction methods. Chin. J. Appl. Ecol.; 2016; 27, pp. 1759-1766. [DOI: https://dx.doi.org/10.13287/j.1001-9332.201606.033]

17. Fan, F.J.; Song, T.Q.; Huang, G.Q.; Zeng, F.P.; Peng, W.X.; Du, H.; Lu, S.Y.; Shi, W.W.; Tan, Q.J. Spatial variability of soil nutrients in canyon-type karst slopes in Southwest China. Chin. J. Appl. Ecol.; 2014; 25, pp. 92-98. [DOI: https://dx.doi.org/10.13287/j.1001-9332.2014.01.013]

18. Zhou, T.; Dai, Q.H.; Wu, X.Q.; Dong, Y.H.; Deng, Y.H. Soil nutrient effects and evaluation of converting cropland to forest in karst mountain areas. Res. Soil Water Conserv.; 2011; 18, pp. 71-74.

19. Wei, X.; Fu, T.; He, G.; Zhong, Z.; Yang, M.; Fe, L.; He, T. Characteristics of rhizosphere and bulk soil microbial community of Chinese cabbage (Brassica campestris) grown in Karst area. Front. Microbiol.; 2023; 14, 1241436. [DOI: https://dx.doi.org/10.3389/fmicb.2023.1241436]

20. Zeng, Z.X.; Wang, K.L.; Liu, X.L.; Zeng, F.P.; Song, T.Q.; Peng, W.X.; Zhang, H.; Du, H. Ecological stoichiometry characteristics of plants-litter-soil in northwestern Guangxi karst forests. J. Plant Ecol.; 2015; 39, pp. 682-693.

21. Wu, P.; Zhou, H.; Ying, C.C.; Wen, J.Z.; Yi, J.H.; Zhu, J.; Fang, J.D. Stoichiometric Characteristics of Leaf Nutrients in Karst Plant Species During Natural Restoration in Maolan National Nature Reserve, Guizhou, China. J. Sustain. Forest.; 2023; 42, pp. 95-119. [DOI: https://dx.doi.org/10.1080/10549811.2021.1948868]

22. Wu, P.; Zhou, H.; Cui, Y.C.; Zhao, W.J.; Hou, Y.J.; Tan, C.J.; Yang, G.N.; Ding, F.J. Stoichiometric Characteristics of Leaf, Litter and Soil during Vegetation Succession in Maolan National Nature Reserve, Guizhou, China. Sustainability; 2022; 14, 16517. [DOI: https://dx.doi.org/10.3390/su142416517]

23. Shuang, L.; Ni, S.N.; Du, J.; Teng, Z.Y.; Shi, C.G. Determination of organic carbon content in geochemical soil samples by potassium dichromate oxidation-external heating method. Anhui Chem. Ind.; 2016; 42, pp. 110-112.

24. Quirino, D.F.; Lima, N.S.A.; Palma, M.N.N.; Franco, M.; Detmann, E. Variations of the Kjeldahl method for assessing nitrogen concentration in tropical forages. Grass Forage Sci.; 2023; 78, pp. 648-654. [DOI: https://dx.doi.org/10.1111/gfs.12641]

25. Roberts, T.L.; Norman, R.J.; Slaton, N.A.; Wilson, C.E., Jr. Changes in Alkaline Hydrolyzable Nitrogen Distribution with Soil Depth: Fertilizer Correlation and Calibration Implications. Soil Sci. Soc. Am. J.; 2009; 73, pp. 2151-2158. [DOI: https://dx.doi.org/10.2136/sssaj2009.0089]

26. Bei, M.R.; Luo, X.H.; Yang, H.Z.; Cha, Z.Z.; Lin, Q.H. Rapid determination of total phosphorus in soils by alkaline melting method using a continuous flow analyzer. J. Trop. Crops; 2018; 39, pp. 2290-2295.

27. Matula, J. Differences in available phosphorus evaluated by soil tests in relation to detection by colorimetric and ICP-AES techniques. Plant Soil Environ.; 2010; 56, pp. 297-304. [DOI: https://dx.doi.org/10.17221/23/2010-PSE]

28. Gao, C.; Li, C.; Zhang, L.; Guo, H.; Li, Q.; Kou, Z.; Li, Y. The influence of soil depth and tree age on soil enzyme activities and stoichiometry in apple orchards. Appl. Soil Ecol.; 2024; 202, 105600. [DOI: https://dx.doi.org/10.1016/j.apsoil.2024.105600]

29. Zeng, Q.C.; Li, X.; Dong, Y.H.; Li, Y.Y.; Cheng, M.; An, S.S. Latitude variation characteristics of soil properties and ecological stoichiometry in the Loess Plateau of Northern Shanxi. J. Nat. Resour.; 2015; 30, pp. 870-879.

30. Jiao, F.; Wen, Z.; An, S.; Yuan, Z. Successional changes in soil stoichiometry after land abandonment in Loess Plateau, China. Ecol. Eng.; 2013; 58, pp. 249-254. [DOI: https://dx.doi.org/10.1016/j.ecoleng.2013.06.036]

31. Wang, J.J.; Huang, G.; Lu, K.; Su, Y.G. Soil nutrient and chemical stoichiometry characteristics of different vegetation types in tropical-subtropical forests. J. Appl. Environ. Biol.; 2023; 29, pp. 423-431. [DOI: https://dx.doi.org/10.19675/j.cnki.1006-687x.2021.11055]

32. Ma, Y.; Li, L.Z.; Zhang, D.G.; Xiao, H.L.; Chen, J.G. Response of rhizosphere soil chemical stoichiometry to grassland degradation in alpine meadows. Chin. J. Appl. Ecol.; 2019; 30, pp. 3039-3048. [DOI: https://dx.doi.org/10.13287/j.1001-9332.201909.004]

33. Wang, X.; Li, Y.; Wang, L.; Duan, Y.; Yao, B.; Chen, Y.; Cao, W. Soil extracellular enzyme stoichiometry reflects microbial metabolic limitations in different desert types of northwestern China. Sci. Total Environ.; 2023; 874, 162504. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2023.162504] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36863586]

34. Chen, B.; Chen, L.; Jiang, L.; Zhu, J.; Chen, J.; Huang, Q.; Liu, J.; Xu, D.; He, Z. C:N:P Stoichiometry of Plant, Litter and Soil along an Elevational Gradient in Subtropical Forests of China. Forests; 2022; 13, 372. [DOI: https://dx.doi.org/10.3390/f13030372]

35. Fang, Z.; Yu, H.; Li, C.; Wang, B.; Huang, J. Soil microbial biomass C:N:P stoichiometry is driven more by climate, soil properties and plant traits than by N enrichment in a desert steppe. Catena; 2022; 216, 106402. [DOI: https://dx.doi.org/10.1016/j.catena.2022.106402]

36. de Castro, D.L.; Bezerra, F.H.R. Characterization of a ghost-rock karst system controlled by fracture network and bedding planes in a semiarid region (NE Brazil) using ground penetrating radar (GPR). Environ. Earth Sci.; 2023; 82, 336. [DOI: https://dx.doi.org/10.1007/s12665-023-11052-5]

37. Méndez, J.N.; Jin, Q.; González, M.; Zhang, X.; Lobo, C.; Boateng, C.D.; Zambrano, M. Fracture characterization and modeling of karsted carbonate reservoirs: A case study in Tahe oilfield, Tarim Basin (western China). Mar. Petrol. Geol.; 2020; 112, 104104. [DOI: https://dx.doi.org/10.1016/j.marpetgeo.2019.104104]

38. Liu, P.W.; Tan, Y.B.; Mo, H.T.; Zhou, Z.H.; Meng, W.M.; Huang, Q.X.; Ma, J.M. Effects of changes in litter and root input on soil nutrients and extracellular enzymes in karst regions. J. Guangxi Norm. Univ. (Nat. Sci. Ed.); 2023; 41, pp. 179-191. [DOI: https://dx.doi.org/10.16088/j.issn.1001-6600.2023031303]

39. Zhang, Y.; Liu, C.; Song, A.; Jin, Z.J.; Li, Q. Study on the relationship between soil physicochemical properties and soil enzyme activity in the Huixian karst wetland based on canonical correspondence analysis. China Karst; 2016; 35, pp. 11-18. (In Chinese)

40. Xu, Z.W.; Yu, G.R.; Zhang, X.Y.; Ge, J.P.; He, N.P.; Wang, Q.F.; Wang, D. The variations in soil microbial communities, enzyme activities and their relationships with soil organic matter decomposition along the northern slope of Changbai Mountain. Appl. Soil Ecol.; 2015; 86, pp. 19-29. [DOI: https://dx.doi.org/10.1016/j.apsoil.2014.09.015]

41. Hayano, K.; Watanabe, K.; Asakawa, S. Activity of protease extracted from rice-rhizosphere soils under double cropping of rice and wheat. Soil Sci. Plant Nutr.; 2012; 41, pp. 597-603. [DOI: https://dx.doi.org/10.1080/00380768.1995.10419621]

42. Xin, Y.; Zhang, D.; Qi, Q.; Zhang, Z.; Zhang, M.; Tong, S.; Xing, X. Disturbance alters soil organic carbon content and stability in Carex tussock wetland, Northeast China. Sci. Total Environ.; 2024; 951, 175417. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2024.175417] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/39153622]

43. Yan, Y.; Yang, Y.; Dai, Q. Effects of preferential flow on soil nutrient transport in karst slopes after recultivation. Environ. Res. Lett.; 2023; 18, 034012. [DOI: https://dx.doi.org/10.1088/1748-9326/acb8cc]

44. Zhang, H.; Jiang, N.; Wang, H.; Zhang, S.; Zhao, J.; Liu, H.; Zhang, H.; Yang, D. Importance of plant community composition and aboveground biomass in shaping microbial communities following long-term nitrogen and phosphorus addition in a temperate steppe ecosystem. Plant Soil; 2024; pp. 1-18. [DOI: https://dx.doi.org/10.1007/s11104-024-06881-7]

45. Seid, H.; Kessy, J.; Asfaw, Z.; Dahlin, A.S. Soil Physicochemical Properties under Selected Avocado Cultivars in Ethiopian Smallholder Agroforestry. J. Soil Sci. Plant Nut.; 2024; 24, pp. 5552-5564. [DOI: https://dx.doi.org/10.1007/s42729-024-01925-4]

46. Li, Y.; Zhang, K.; Li, Y.; Wan, P.; Zhou, Z.; Zhao, W.; Zhang, N.; Chai, N.; Li, Z.; Huang, Y. et al. Temperature sensitivity of soil respiration to elevated temperature and nitrogen availability. Soil Till Res.; 2024; 244, 106267. [DOI: https://dx.doi.org/10.1016/j.still.2024.106267]

47. Zhao, Y.; Han, Z.; Zhang, G.; Chen, D.; Zang, L.; Liu, Q.; Guo, Y.; Xie, P.; Chen, H.; He, Y. Variability of soil enzyme activities and nutrients with forest gap renewal interacting with soil depths in degraded karst forests. Ecol. Indic.; 2024; 166, 112332. [DOI: https://dx.doi.org/10.1016/j.ecolind.2024.112332]