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Article

Study on the Spatial Distribution Patterns and Influencing Factors of Soil Organic Carbon Components in Ecological Vegetative Slope Areas

by
Lifei Zheng
1,2,
Xuyuan Zhou
1,2,
Xinlong Zhou
1,2,
Shaoping Huang
1,2,
Zhiying Zhong
1,2 and
Henglin Xiao
1,2,*
1
School of Civil Engineering, Architecture and Environment, Hubei University of Technology, Wuhan 430068, China
2
Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, Ministry of Education, Hubei University of Technology, Wuhan 430068, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(6), 2650; https://doi.org/10.3390/su17062650
Submission received: 17 February 2025 / Revised: 7 March 2025 / Accepted: 13 March 2025 / Published: 17 March 2025
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Abstract

Understanding the spatial distribution patterns of soil organic carbon components in Ecological Vegetative Slopes (EVS) is crucial for optimizing restoration strategies, enhancing soil carbon sequestration capacity, and scientifically evaluating rehabilitation effectiveness. It is an important pathway for promoting sustainable development. This study investigated the effects of slope gradient, slope position, vegetation type, and cement content on the spatial distribution characteristics of soil total organic carbon (TOC) and its components (readily oxidizable organic carbon (ROC) and dissolved organic carbon (DOC)) through slope model experiments and laboratory analyses. The key findings revealed that (1) TOC, ROC, and DOC concentrations decreased with increasing slope gradient and upper slope positions, with steeper slopes amplifying positional effects and exhibiting surface-layer accumulation characteristics. (2) The highest TOC, ROC, and DOC content was achieved in the mixed planting slope with Bermudagrass and Magnolia under a substrate thickness of 3 cm and 3% cement content. Lastly, (3) soil pH increased with the slope gradient and upper positions, showing systematic variations across vegetation types and cement contents. Comparative analysis demonstrated that optimizing vegetation composition and cement dosage effectively enhanced SOC accumulation, while slope gradient and position dominated the spatial heterogeneity of carbon distribution. These results provide theoretical and practical guidance for the precision management of slope ecological restoration. These findings provide robust data support for the ecological sustainability of engineering slope restoration and the enhancement of soil carbon storage.

1. Introduction

In the field of ecological restoration and slope management, constructing EVS has become an important environmental protection measure [1,2,3]. This method not only effectively prevents soil erosion but also promotes the restoration of biodiversity and soil health by increasing soil organic carbon reserves [4,5,6,7]. However, while hydrological changes in current EVS research have received ample attention, studies on soil nutrients—central to ecosystem material cycling—have predominantly focused on natural slope soils. There remains a lack of in-depth exploration into the dynamics of organic carbon components in artificially restored slopes, particularly within specialized EVS systems. This study aims to address this knowledge gap [8,9,10].
TOC, ROC, DOC, and pH are key indicators of soil health and ecosystem function [11,12]. TOC is a primary source of biological activity in soil and a key nutrient, playing a crucial role in maintaining soil structure and enhancing soil moisture retention capacity [13,14]. ROC and DOC are two important components of TOC, reflecting the active portion of the soil carbon pool, and are significant for understanding the dynamic changes in soil carbon pools and the ecosystem’s response to environmental changes [15,16]. Soil pH value is another important factor as it affects the structure and diversity of soil microbial communities and is directly related to the mineralization and stability of organic carbon, thus influencing the forms and contents of TOC, ROC, and DOC [17,18,19]. However, existing studies on EVS systems have predominantly focused on the spatial variation characteristics of single physicochemical properties, particularly TOC as an isolated variable, while neglecting the composite distribution patterns of co-existing soil nutrients critical to soil fertility. This study systematically investigates the impacts of multiple influencing factors on the distribution characteristics of these key parameters, aiming to establish a theoretical foundation for enhancing soil carbon sequestration potential in EVS systems [6,20].
As the main natural topographic factors, slope gradient and slope position directly affect the flow and distribution of water and nutrients, thus affecting the accumulation and decomposition processes of soil organic carbon [20,21,22]. Studies have shown that an increase in slope gradient accelerates soil erosion and reduces the accumulation of organic matter, while different slope positions have varying effects on organic carbon accumulation [23,24,25,26]. Vegetation type has a profound impact on soil organic carbon content through biomass input and root–soil interaction [27]. Different vegetation types not only provide varying amounts and qualities of organic matter input but also alter soil physical properties through root exudates, affecting soil microbial activity and significantly enhancing soil erosion resistance, which benefits vegetation growth and soil organic carbon accumulation [6,28,29,30,31]. For example, research by Yuan et al. [32] found that introducing adaptable vegetation like Bermudagrass is crucial for the ecological restoration of slopes. Xiao et al. also pointed out that vegetation like Magnolia can improve soil organic carbon content and enhance water and nutrient retention on slopes [33,34]. In addition, the effect of cement content in soil ecological substrates on EVS has received increasing attention. As an artificial additive, cement affects the activity of soil microorganisms and the accumulation and stability of organic carbon by changing the structure and air permeability of the soil [35,36,37]. Current research lacks analyses on the effects of topographic factors (slope gradient and slope position), vegetation types, and cement content on soil nutrient indicators in ecological restoration. In response, this study conducts a multifaceted analysis of variations in soil organic carbon components and pH under multiple influencing factors, providing a theoretical foundation for optimizing slope ecological vegetation restoration engineering practices.
Therefore, based on large-scale slope model experiments, this study thoroughly investigates the effects of slope gradient, slope position, vegetation type, and substrate cement content on the content and spatial distribution characteristics of TOC, ROC, and DOC in the EVS. The findings provide a scientific basis for the design and management of ecological vegetation slopes, aiming to improve the soil quality of ecological slopes and enhance carbon sequestration capacity.

2. Materials and Methods

2.1. Overview of the Study Area

The slope model test site is located within the ecological restoration pilot base for fragile ecosystems at Hubei University of Technology. The site is expansive and situated in a subtropical monsoon humid region, characterized by abundant sunlight throughout the year. Rainfall is concentrated in the summer, with short-duration but intense storms; the maximum daily rainfall in the region can reach up to 248 mm, with an annual average precipitation ranging between 1150 mm and 1190 mm.
Representative slope gradients were selected from engineering slopes, including steep slopes, moderate slopes, and gentle slopes. Built as shown in (Figure 1a,b), the model slopes were constructed at gradients of 1:1.5, 1:1.75, and 1:2, including both vegetated slopes and bare soil slopes. The model dimensions were designed with a width of 1.5 m and a height of 2 m. Based on practical engineering requirements, the constraint conditions of rigid support structures such as retaining walls are replicated, the slope models were constructed using brick masonry, with the walls coated with fine sand mortar to create a smooth surface, thereby reducing interface friction effects. The soil samples were air-dried, crushed, and sieved before being used as fill material. The initial moisture content was set to the optimum moisture content, and the slope fill was divided into seven layers, with each layer being roughened to ensure tight inter-layer bonding. During the filling process, a small compactor was used to compact the soil along the direction parallel to the slope surface, and in situ sampling was conducted to test the compaction degree, ensuring the density and homogeneity of the filled soil.
To reduce human interference and achieve the desired experiment results, the test soil samples were collected from a construction site pit in the South Lake area of Wuhan. The samples were undisturbed and contained few impurities, with a plastic limit of 23.0%, a liquid limit of 41.0%, and a natural density of 1.50 g/cm3. The soil’s optimum moisture content was 20.0%, and the natural moisture content was approximately 15.3%. Particle size analysis was conducted using a laser particle size analyzer, and the particle size distribution curve of the soil sample is shown in (Figure 2), indicating good gradation.

2.2. Vegetation Selection and Substrate Formulation

Due to their dual benefits in effective restoration and economic viability, Bermudagrass (grass), Magnolia (shrub), and their mixed planting (grass–shrub)—widely distributed in the Central China region—were selected as slope protection vegetation, with a seeding rate of 30 g/m2. Based on substrate formulation standards, slope characteristics, and prior hydrological studies from our research group, substrate designs were established with cement (P.O42.5 ordinary Portland cement) incorporation rates of 0%, 1%, and 3% (by soil mass) and substrate thicknesses of 1 cm, 3 cm, and 5 cm. After sowing, to maintain 70–80% field capacity while balancing water use efficiency, irrigation was scheduled twice daily (morning and evening) at 24 L per application to establish and maintain the turf. Ecological indicators, including the germination rate and plant height, were monitored during maintenance. Post-cultivation observations confirmed the achievement of targeted growth outcomes (Figure 3a–d).

2.3. Sample Collection

The slope of the model was divided into three zones: upper, middle, and lower slopes, based on the specific length of the slope, as shown in (Figure 3a). Five sampling points per position were randomly arranged in an S-shaped pattern to ensure spatial representativeness. At each point, soil samples were vertically collected from four depth intervals (0–5 cm, 5–10 cm, 10–15 cm, and 15–20 cm) using a 4-cm-diameter geological auger perpendicular to the slope surface, yielding a total of 540 samples collected (108 sampling points × 5 replicates). After sampling, the five soil samples from each layer were thoroughly mixed and placed in plastic-sealed bags, which were then labeled and taken back to the laboratory. The collected soil samples were free of dead branches, leaves, and stones, and were divided into two equal portions. One portion was air-dried in a well-ventilated, cool indoor area, while the other was stored in a sealed bag at 4 °C as fresh samples for subsequent biochemical analyses.

2.4. Physical and Chemical Analysis

Soil pH was measured using the water extraction potentiometric method (water–soil = 2.5:1) [38]. Soil organic carbon was determined using the potassium dichromate oxidation-spectrophotometric method [39]. Readily oxidizable organic carbon in soil was measured using a 333 mmol/L potassium permanganate solution oxidation method [40]. The dissolved organic carbon in the soil was determined using the K2SO4 extraction-spectrophotometric method [41].

2.5. Date Processing

Excel 2016 was used to statistically analyze the data for each indicator and SPSS 20 software was employed for statistical analysis. Correlation analysis was performed using Pearson’s method. The graphical representation of the data was completed using Origin 2018 software.

3. Results

3.1. Spatial Distribution Characteristics of Slope Soil pH Value

Figure 4 shows the spatial distribution of slope soil under different slope gradients, vegetation types, and cement contents. It can be observed that all slope soils exhibit weak alkalinity, with pH values ranging from 7.92 to 8.48.
For the 1:1.5 slope ratio, the pH values of the three slope positions are significantly lower than those of the slopes with the other two ratios, and the pH increases gradually with soil depth. There is no significant difference in pH values between the upper, middle, and lower slope positions. For the 1:1.75 slope ratio, the middle and lower slope positions have pH values higher than those of the 1:1.5 slopes, but lower than those of the 1:2 slopes. However, the upper slope position exhibits a significantly higher pH value than the other two slope ratios. In the 1:2 slope ratio, the pH values of the middle and lower slope positions are higher than those of the other two slopes, while the upper slope position’s pH is higher than that of the 1:1.5 slopes but lower than the 1:2 slopes. Overall, in the footslope and midslope zones, the pH increases as the slope ratio decreases, with the pH in the upper slope position being significantly higher at a 1:1.75 slope ratio than at the 1:2 and 1:1.5 ratios. Additionally, in the footslope and midslope zones of all three slope ratios, the pH increases with increasing soil depth, showing a significant positive correlation, while no significant correlation is observed in the upper slope positions.
For slopes without cement addition (0 cm, 0% cement content), the pH values at all slope positions and soil depths are significantly lower than those of the other three slopes, and the pH increases with soil depth, with no significant correlation with slope position. For slopes with 3 cm 3% cement addition, the pH values at the middle and lower slope positions are significantly lower than those of the 5 cm 3% and 3 cm 5% cement slopes, with the pH increasing with slope position. In slopes with 0 cm 0% and 3 cm 3% cement addition, the pH decreases as the slope position increases, showing a positive correlation. In slopes with 5 cm 3% cement addition, the pH at the 0–5 cm depth is lower than that at the 3 cm 5% cement slope, and the pH value in the 0–5 cm soil layer is higher than that in the 5–10 cm and 10–15 cm soil layers. However, across three consecutive depth intervals (5–10 cm, 10–15 cm, and 15–20 cm), the pH of the 5 cm 3% cement slope is higher than that of the 3 cm 5% cement slope. There is no significant correlation between the pH and slope position in the 5 cm 3% and 3 cm 5% cement slopes.
Overall, the pH values of all slopes are mildly alkaline, and the pH values of the four slopes generally increase with soil depth increase. The overall pH values from low to high are in the following order: grass–shrub slope < Bermudagrass slope < Magnolia slope < bare slope. The pH values of the Bermudagrass slope increase with increases in the slope position, but the change is not significant, with the lower slope position exhibiting a significantly lower pH than the other three vegetation types, and the upper slope position’s average pH also being lower than the other three vegetation types. In the upper slope positions, Magnolia slopes exhibited significantly lower pH values at a 15–20 cm soil depth compared to other vegetation types. Within both upper and middle slope positions, however, the mixed vegetation demonstrated the highest pH levels across three shallow soil layers (0–5 cm, 5–10 cm, and 10–15 cm). No significant correlation is observed between the pH values and slope position for the four vegetation types.

3.2. Spatial Distribution Characteristics of Soil Organic Carbon Components on Slopes

3.2.1. Spatial Distribution Characteristics of Organic Carbon Components Under Different Slope Gradients and Position

Table 1 and Figure 5 show the variations in TOC, ROC, and DOC with soil depth and slope position on vegetated slopes with different gradients. The TOC content ranged from 3.07 to 7.54 g/kg, with an average of 5.0597 ± 0.4057 g/kg. The slope with a gradient of 1:2 exhibited the highest organic carbon content, significantly higher than the other two slope gradients, with increases of 16.34% and 24.51% compared to the 1:1.75 and 1:1.5 slopes, respectively. This suggests that the slope gradient significantly influences the organic carbon content in the substrate of vegetated slopes (p < 0.05). Across all soil layers, TOC content increased as the slope gradient decreased, particularly in the 0–5 cm soil depth of slope soils, where the impact of the slope gradient on TOC content was most significant (p < 0.05). Among the four soil depth intervals analyzed, with the exception of the 5–10 cm stratum, the TOC content of slopes with a gradient of 1:5 is significantly lower than that of slopes with a gradient of 1:2. However, no significant differences were observed in the 5–10 cm soil depth.
The range of ROC content variation is 0.60–2.81 g/kg, with an average of 1.4375 ± 0.1743 g/kg. The slope with a 1:2 gradient has the highest ROC content, being 13.77% and 30.00% higher than the slopes with 1:1.75 and 1:1.5 gradients, respectively. Unlike TOC, although ROC showed a trend of decreasing with increasing slope ratio, no statistically significant differences were detected (p > 0.05). In terms of depth, ROC content decreases as depth increases, following a similar pattern to TOC, but the slope gradient’s effect is most significant in the 15–20 cm depth of soil stratum, while the least significant impact is in the 5–10 cm depth of soil stratum. Overall, TOC content in the 1:2 gradient slope is significantly higher than in the 1:1.5 gradient slope.
The range of DOC content variation is 0.04–0.35 g/kg, with an average of 0.1801 ± 0.0319 g/kg. Similar to TOC and ROC, the DOC content of slopes with a slope gradient of 1:2 was highest, being 59.76% and 63.40% higher than the slopes with 1:1.75 and 1:1.5 gradients, respectively, showing a decrease in TOC content as the slope gradient increases. The difference in DOC content concerning slope gradient is highly significant (p < 0.01). Although the differences in DOC content across soil depths are not significant, they also tend to decrease with increasing depth. The relationship between DOC content and slope gradient is consistent across soil layers, with the highest content in the 1:2 gradient slope, followed by the 1:1.75 and 1:1.5 gradients.
The variation patterns of TOC, ROC, and DOC between different slope positions are consistent and significantly different, with all decreasing as the slope position rises. TOC is the most significantly affected by slope position (p < 0.01). The content of all three organic carbon components increases as the slope gradient decreases. There is no significant change in TOC content across different slope gradients at the lower slope position. The TOC content at the middle slope position of the 1:2 gradient slope is higher than that of the other two gradients, while the TOC content at the upper slope position is higher in the 1:2 and 1:1.75 gradient slopes than in the 1:1.5 gradient slope. The ROC patterns are consistent at the lower and middle slope positions, with significant differences in TOC content between the 1:2 and 1:1.5 gradient slopes. At the upper slope position, the TOC content in the 1:2 gradient slope is significantly higher than in the other two slopes. At the lower, middle, and upper slope positions, the DOC content in the 1:2 gradient slope is significantly higher than in the other two gradients.

3.2.2. Distribution Characteristics of Organic Carbon Components Under Different Vegetation Types

Table 2 and Figure 6 show the variations in TOC, ROC, and DOC with soil depth and slope position on slopes with different vegetation types. The TOC content in the substrate of vegetated slopes ranged from 1.05 to 6.27 g/kg, with an average of 2.3639 ± 0.3251 g/kg. The grass–shrub slope has the highest TOC content, significantly higher than the Bermudagrass slope, Magnolia slope, and bare slope by 22.89%, 26.75%, and 51.86%, respectively. The TOC content in the Bermudagrass slope is significantly higher than in the bare slope and slightly higher than in the Magnolia slope. Significant heterogeneity in Total Organic Carbon (TOC) levels is observed between distinct flora types (p < 0.05). Regarding depth, the TOC content in the 15–20 cm soil layer is most affected by vegetation type. Among the four soil layers with the exception of the 15–20 cm, the TOC content in the Bermudagrass slope is higher than in the Magnolia slope but lower than in the grass–shrub slope. The TOC content at the 0–5 cm depth is significantly higher than at other depths, decreasing with increasing depth, showing a surface accumulation phenomenon. Concerning slope position, the TOC content at the middle slope position is most affected by vegetation type (p < 0.05). The grass–shrub slope has the highest TOC content at the lower and middle slope positions, while the Bermudagrass slope has the highest TOC content at the upper slope position. For all vegetation types, TOC content is lowest at the upper slope position and slightly higher at the lower slope position than at the middle slope position, showing an increasing trend in TOC content from the upper to lower slope positions.
The range of ROC content variation is 0.10–0.99 g/kg, with an average of 0.4927 ± 0.0771 g/kg. The grass–shrub slope has the highest ROC content, significantly higher than the Magnolia slope, Bermudagrass slope, and bare slope by 13.38%, 24.96%, and 66.33%, respectively. Unlike TOC, the ROC content in the Bermudagrass slope is slightly lower than in the Magnolia slope. There are highly significant differences in ROC content among different vegetation types (p < 0.05). Regarding depth, ROC content in the 0–5 cm depth is most affected by vegetation type. Unlike TOC content, ROC content is highest in the Magnolia slope at the 10–15 cm and 15–20 cm depths, and ROC content does not show significant changes with depth. Concerning slope position, ROC content at the middle slope position is most affected by vegetation type (p < 0.01), with the grass–shrub slope having the highest ROC content at the upper, middle, and lower slope positions. For all vegetation types, ROC content is lowest at the upper slope position and slightly higher at the lower slope position than at the middle slope position, showing an increasing trend in ROC content from the upper to lower slope positions.
The range of DOC content variation is 0.05–0.32 g/kg, with an average of 0.1526 ± 0.0232 g/kg. The grass–shrub slope has the highest DOC content, higher than the Magnolia slope, Bermudagrass slope, and bare slope by 7.72%, 56.21%, and 72.93%, respectively. A significant increase in DOC levels was observed in Magnolia slopes when contrasted with bare and Bermudagrass slopes, and there are highly significant differences in DOC content among different vegetation types (p < 0.01). Regarding depth, DOC content at different depths is highly significantly correlated with vegetation type (p < 0.01). DOC content is most affected by vegetation type at the 5–10 cm depth. Unlike TOC content, DOC content is highest in the Magnolia slope at the 10–15 cm and 15–20 cm depths. Although DOC content does not show significant changes with depth, DOC content at the 15–20 cm depth is significantly higher than at the other three depths. Concerning slope position, DOC content at different slope positions is highly significantly correlated with vegetation type (p < 0.01). At the lower and middle slope positions, DOC content is most affected by vegetation type (p < 0.01). The grass–shrub slope has the highest DOC content at the lower and middle slope positions, while the Bermudagrass slope has the highest DOC content at the upper slope position. For all vegetation types, DOC content is highest at the lower slope position, followed by the middle and upper slope positions, showing a decreasing trend in DOC content from the lower to upper slope positions.

3.2.3. Distribution Characteristics of Organic Carbon Components Under Different Cement Content in the Substrate

Table 3 and Figure 7 show the variations in TOC, ROC, and DOC contents with soil depth and slope position under different cement dosages in the substrate. Among them, the TOC content in the vegetated slopes ranges from 1.46 to 7.85 g/kg, with an average value of 3.9443 ± 0.4667 g/kg. Among these, the slope with 3 cm 3% cement content has the highest TOC content, 13.27%, 14.62%, and 50.15% higher than the slopes with 3 cm 5%, 5 cm 3%, and 0 cm 0% cement, respectively, indicating a highly significant correlation between different substrate cement contents and TOC content (p < 0.01). Regarding depth, the TOC content at the 15–20 cm depth is most affected by cement content in the substrate. At 5–10 cm, 10–15 cm, and 15–20 cm depths, the TOC content in the slope with 3 cm 3% cement content is higher than in the other three cement content levels. This suggests that in the surface soil, the carbon sequestration capacity of 3 cm 3% cement content is superior to other cement content levels, while at the 0–5 cm depth, the slope with 5 cm 3% cement content has the highest TOC content. There is a significant correlation between depth and substrate TOC content (p < 0.05), with the TOC content at the 0–5 cm depth being significantly higher than at other depths, and it decreases with increasing depth, showing a surface accumulation phenomenon. Concerning slope position, TOC content at the upper slope position is most affected by substrate cement content (p < 0.05), followed by the lower and middle slope segments. At the lower and middle slope positions, the slope with 3 cm 3% cement content has the highest TOC content. Positioned at the upper of the slope, the slope with 3 cm 5% cement content has the highest TOC content. Among all slopes with different cement contents, the upper slope position has the lowest TOC content, with the middle slope position slightly higher than the lower slope position, showing an increasing trend in TOC content from the upper to lower slope positions.
The range of ROC content variation is 0.20 to 2.45 g/kg, with an average of 0.8261 ± 0.1365 g/kg. The slope with 3 cm 3% cement content has the highest ROC content, 50.90%, 55.93%, and 65.50% higher than the slopes with 3 cm 5%, 5 cm 3%, and 0 cm 0% cement, respectively. There are highly significant differences in ROC content among different substrate cement contents (p < 0.01). Regarding depth, ROC content at the 10–15 cm depth is most affected by substrate cement content, followed by the 15–20 cm, 5–10 cm, and 0–5 cm depths. Across all depths, the slope with 3 cm 3% cement content has the highest ROC content. There is a highly significant correlation between soil ROC content and depth (p < 0.01), with ROC content decreasing gradually with increasing depth, and the TOC content at the 0–5 cm soil layer being significantly higher than other depths, showing a surface accumulation phenomenon. Concerning slope position, ROC content at the upper slope position is most affected by substrate cement content (p < 0.01), followed by the lower and middle slope segments. At the lower and middle slope positions, the slope with 3 cm 3% cement content has the highest ROC content, whereas at the upper slope segment, the slope with 3 cm 5% cement content has the highest ROC content. Slope position exhibits a strong statistical association with soil ROC levels (p < 0.01), with ROC content lowest at the upper slope position and slightly higher at the lower slope position than at the middle slope position, showing an increasing trend in ROC content from the upper to lower slope segment.
The range of ROC content variation is 0.07 to 1.10 g/kg, with an average of 0.2300 ± 0.0498 g/kg. The slope with 3 cm 5% cement content has the highest DOC content, 27.31%, 63.74%, and 71.77% higher than the slopes with 5 cm 3%, 3 cm 3%, and 0 cm 0% cement, respectively, showing a highly significant correlation between different substrate cement contents and soil DOC content (p < 0.01). Regarding depth, DOC content at different depths is highly significantly correlated with substrate cement content (p < 0.01). DOC content at the 0–5 cm depth is most affected by substrate cement content. Among all depths, the slope with 3 cm 5% cement content has the highest DOC content, followed by the slopes with 5 cm 3%, 3 cm 3%, and 0 cm 0% cement. Although DOC content does not show significant variation with depth, the DOC concentration peaks in the 5–10 cm stratum, exceeding those at 0–5 cm, 10–15 cm, and 15–20 cm. Concerning slope position, DOC content at different slope positions is highly significantly correlated with substrate cement content (p < 0.01), with DOC content at the middle and upper slope positions being most affected by substrate cement content (p < 0.01). Among all slope positions, the slope with 3 cm 5% cement content has higher DOC content than other cement content levels, followed by the slopes with 5 cm 3%, 3 cm 3%, and 0 cm 0% cement. Among all slopes with different cement contents, the lower slope position has the highest DOC content, followed by the middle and upper slope positions, showing a decreasing trend in DOC content from the lower to upper slope positions. There is a highly significant correlation between soil DOC content and slope position (p < 0.01).

4. Discussion

4.1. Influence of Slope Gradient, Slope Position, Vegetation Type, and Cement Dosage on the Spatial Distribution of Soil pH in Ecological Vegetative Slopes

The soils of all slopes generally exhibit weak alkalinity. This is primarily due to the increase in pH caused by alkaline substances released from the hydration of cement in the slope model blocks and substrates, which are carried into the slope soil by water. When investigating the effects of cement content and substrate thickness on pH, by comparing the pH values of slopes with 0 cm 0%, 3 cm 3%, 5 cm 3%, and 3 cm 5% cement content, it was found that as the cement content increased, the pH value also increased. This is mainly because cement releases alkaline substances during hydration, significantly raising the soil pH. From a soil depth perspective, the pH value increases with depth. This is primarily because a large amount of plant litter (dead branches and leaves) exists in the surface soil, and microorganisms decompose this organic matter aerobically or anaerobically, producing organic acids that lower the pH of the surface soil. Additionally, microbial activity decreases with soil depth, and these two factors together lead to a lower pH in the surface soil [42,43,44].
In terms of vegetation types, the pH values generally follow the order: grass–shrub slope < Bermudagrass slope < Magnolia slope < bare slope. This is likely because Bermudagrass, as a herbaceous plant, has a well-developed root system that can effectively protect the surface soil, enhance erosion resistance, reduce soil and water loss, and increase organic matter accumulation. The organic matter is further decomposed into organic acids, thereby lowering the pH. Magnolia, as a shrub, has abundant litter that provides organic matter, which decomposes into organic acids and reduces soil pH [45]. The grass–shrub slope combines the advantages of both Bermudagrass and Magnolia, with good erosion resistance and abundant organic matter input, which promotes organic matter accumulation and microbial activity, leading to a further reduction in soil pH [46].
Slope gradient significantly affects soil pH. As the slope gradient decreases, pH increases, which differs from Qi et al.’s study [47]. This phenomenon may occur because, under cement-amended conditions, steeper slopes induce greater soil runoff, thereby washing away more cement. The reduced cement content diminishes alkali production from its hydration reaction, ultimately leading to lower pH levels on steeper slopes. There is a trend of increasing pH values with higher slope positions, primarily observed in the lower and middle slope positions. This may be because, under the influence of gravity, nutrients and plant and animal residues are transported with water flow to lower slope areas, resulting in a relative reduction in acidic substances in the upper slope soil, thereby causing the pH to increase.

4.2. The Impact of Slope Gradient, Slope Position, Vegetation Type, and Cement Content on the Spatial Distribution of Soil Organic Carbon Components in Ecological Vegetative Slopes

The impact of slope gradient on soil nutrients is mainly realized indirectly by influencing water flow and infiltration, vegetation, and soil resistance to erosion. The results of this study indicate that as the slope gradient decreases, the contents of TOC, ROC, and DOC significantly increase, consistent with related research findings [25]. The observed phenomenon may be attributed to intensified soil nutrient loss under steeper slopes, driven by enhanced rainfall-induced surface runoff erosion and gravitational leaching [21,48]. Additionally, plants, as a crucial component of ecological restoration slopes, also influence soil resistance to erosion. When the slope gradient increases, vegetation cover decreases, resulting in more severe soil erosion and nutrient loss [49,50]. This study shows that the TOC content in slopes with a gradient ratio of 1:1.5 and 1:1.75 decreases with increasing depth, but in slopes with a gradient ratio of 1:2, the TOC content in the 10–15 cm soil layer is the highest. The variation in DOC in the 1:2 slopes also shows little change with depth, which is consistent with the patterns observed in hydrological studies [21]. On vegetated slopes, a smaller gradient promotes greater rainfall infiltration, less slope surface erosion, and reduced accumulated runoff. The increase in slope ratio led to disparities in organic carbon content between slope positions, indicating that steeper slopes resulted in greater variations in TOC, ROC, and DOC across different slope segments. This phenomenon might be attributed to the intensified rainfall-runoff erosion on slope soils caused by steeper gradients, which amplified the differences in organic carbon loss. This suggests that as the slope gradient increases, the differences in TOC, ROC, and DOC across slope positions become more significant, likely due to increased erosion of slope soil by rainfall runoff, which exacerbates the loss of organic carbon.
Slope position is a key factor influencing the distribution of soil organic carbon components, primarily due to the redistribution of erosion and deposition caused by slope rainfall [21,26]. The TOC, ROC, and DOC contents in bare, Bermudagrass, Magnolia, and Grass-Shrub slopes follow the pattern lower slope > middle slope > upper slope. This is because the upper slope position has less vegetation cover, shallower soil, higher sand content, and more severe soil erosion, leading to nutrient accumulation at the lower slope position due to rainfall runoff and the interception effect of plant branches and leaves. The good vegetation cover and mature soil development in the lower slope position result in higher soil organic carbon content [51,52]. The TOC, ROC, and DOC contents in bare slopes are lower than in vegetated slopes at all positions, indicating that vegetation better conserves soil nutrients and has better erosion resistance, mainly because plant stems, leaves, and litter can intercept part of the rainfall, reduce effective rainfall intensity, and decrease the impact of raindrops on the slope, inhibiting hydraulic erosion caused by runoff [53]. The TOC, ROC, and DOC contents in the grass–shrub slopes are higher than in the other three slopes at all positions, suggesting superior carbon sequestration performance. This may be because grass–shrub slopes combine the high coverage of Bermudagrass with the deep and broad root system of Magnolia, enhancing erosion resistance [34]. Bermudagrass’ high vegetation cover and dense stems and leaves result in more nutrient accumulation at the lower slope position, making the TOC content at the lower and middle of slope positions higher than in Magnolia. Studies have shown that lower slopes have a higher rainfall infiltration rate, which this study also confirms from the perspective of DOC, with Magnolia and grass–shrub slopes having significantly higher DOC content at the upper slope than Bermudagrass and bare slopes [54]. Due to Magnolia’s deeper root system, DOC content is also higher in the lower and middle slope positions than in Bermudagrass and bare slopes [34,55,56].
Bermudagrass and Magnolia are the most widely used herbaceous and shrub plants in central China, influencing organic carbon components in different soil depths primarily by reducing rain impact through stems and leaves and forming root–soil complexes that enhance soil erosion resistance [36,57,58]. The results show that TOC, ROC, and DOC contents in Bermudagrass, Magnolia, and grass–shrub follow the pattern 0–5 cm > 5–10 cm > 10–15 cm > 15–20 cm, closely related to the concentration of plant litter in the surface soil (0–5 cm). The favorable temperature and aeration conditions, along with higher organic matter decomposition and microbial activity, lead to higher TOC, ROC, and DOC contents in the 0–5 cm soil layer than in other layers [59,60,61]. In contrast, bare slopes exhibit the opposite pattern, as rain and runoff erosion on bare slopes lead to nutrient loss without organic carbon input from decomposing litter, resulting in a pattern opposite to that of vegetated slopes. The TOC and DOC contents in the 0–5 cm soil layer of Bermudagrass slopes are higher than in Magnolia but lower than in grass–shrub slopes, due to Bermudagrass’ higher vegetation cover and more developed root systems, providing better erosion resistance [62]. In all soil layers, the TOC content in the grass–shrub is significantly higher than in the other three slopes, indicating better carbon sequestration capability. The DOC content in Magnolia slopes is significantly higher than in Bermudagrass slopes at all depths and higher than in grass–shrub slopes at the 10–15 cm and 15–20 cm depths. This is mainly because Magnolia has longer and thicker roots, allowing water and DOC to penetrate deeper soil layers, resulting in higher DOC content [56].
The addition of cement as a binder in ecological substrates not only enhances slope stability but also integrates soil and vegetation, improving erosion resistance [35,63]. The TOC and ROC contents in slopes with 3 cm 3% cement content are higher than in the other three cement content levels at the 5–10 cm, 10–15 cm, and 15–20 cm depths, indicating that the 3 cm 3% cement content has a stronger ability to conserve soil nutrients. This may be because the addition of 3 cm 3% cement and certain greening additives promotes the growth of Bermudagrass, increasing vegetation cover, root length, and microbial activity, while also enhancing slope erosion resistance [64,65,66]. At the lower and middle slope positions, the organic carbon content is higher in slopes with 3 cm 3% cement content, possibly due to the lower pH at these positions, leading to higher vegetation cover. At the upper slope position, slopes with 3 cm 5% and 5 cm 3% cement content have higher organic carbon content, possibly because the higher cement content enhances erosion resistance at the upper slope position and, with lower pH, promotes vegetation cover [35]. The ROC content is higher in slopes with 3 cm 3% cement content. This study demonstrated that adding 3% cement significantly increased soil organic carbon content and its components in restored slopes, enhancing carbon sequestration capacity and creating the potential for carbon credit trading in ecological restoration projects. DOC content is higher in slopes with 5 cm 3% and 3 cm 5% cement content, consistent with pH patterns, indicating a positive correlation between DOC and pH. This may be because pH influences the content of DOC in soil solution through processes like adsorption, dissolution, and complexation [67].

5. Conclusions

Based on ecological vegetative slope model experiments, a comparative analysis was conducted on the spatial distribution patterns of soil pH, TOC, ROC, and DOC under different slope gradients, vegetation types, and substrate cement dosages. The following conclusions and insights were obtained:
(1)
The soil pH of all slopes is weakly alkaline and exhibits a significant correlation with the slope gradient; as the slope gradient decreases, the pH increases. Grass–shrub slopes, which combine the advantages of Magnolia and Bermudagrass, have the lowest pH, followed by Bermudagrass, Magnolia, and bare slopes. The amount of cement added to the soil substrate significantly affects the variation in soil pH on slopes—the higher the cement dosage, the higher the pH value. Additionally, pH increases with soil depth, with the surface soil having the lowest pH. Slope position has a minimal impact on soil pH, primarily in the following order: lower slope < middle slope < upper slope.
(2)
In this experiment, the TOC, ROC, and DOC contents of slope soils were at moderate to low levels, with their contents decreasing as soil depth and slope gradient increased. The TOC, ROC, and DOC contents on Bermudagrass slopes significantly increased as the slope gradient decreased. An increase in slope gradient exacerbates the variation in the contents of different organic carbon components across slope positions. These findings provide actionable guidelines for the design and management of eco-restored slopes.
(3)
The content of each organic carbon component follows the order of grass–shrub slope > Bermudagrass > Magnolia > bare slope. On vegetated slopes, the contents of TOC, ROC, and DOC decrease with increasing slope position and soil depth, showing a surface accumulation phenomenon. However, on bare slopes, the trend is the opposite, with organic carbon content increasing with depth. Planting Bermudagrass is suitable for the stabilization and protection of surface soil, while Magnolia is more suitable for the protection of deeper soil layers. Grass–shrub slopes, by effectively combining the advantages of Bermudagrass and Magnolia, promote the accumulation of organic carbon components in the soil. Mixed planting of Bermudagrass and Magnolia floribunda significantly promoted SOC accumulation in slope soils.
(4)
The addition of cement significantly increases soil pH. Slopes with a 3 cm 3% cement dosage have higher contents of organic carbon components than slopes with 0 cm 0%, 5 cm 3%, and 3 cm 5% cement dosages. However, in the 0–5 cm soil layer, slopes with a 3 cm 3% cement dosage have slightly lower organic carbon content than those with 5 cm 3% and 3 cm 5% cement dosages. DOC content decreases as cement dosage increases. The appropriate amount of cement dosage can increase organic carbon content on slopes, prevent soil erosion, positively influence vegetation growth on slopes, and ensure the long-term stability of the slope. Optimal cement incorporation not only improved slope stability but also elevated SOC content, strengthened carbon sequestration, mitigated soil erosion, and supported vegetation growth, ensuring long-term slope sustainability.

Author Contributions

Methodology, X.Z. (Xinlong Zhou).; investigation, L.Z.; data curation, X.Z. (Xuyuan Zhou). and L.Z.; writing—original draft, S.H.; funding acquisition, H.X. writing—review and editing Z.Z. and X.Z. (Xuyuan Zhou). All authors have read and agreed to the published version of the manuscript.

Funding

Financial support for this research came from the Joint Funds of the National Nature Science Foundation of China (U22A20232), the Youth Science and Technology Research program of Hubei Education Department (Q20231408), and the Open Project Funding of Key Laboratory of Intelligent Health Perception and Ecological Restoration of Rivers and Lakes, Ministry of Education, Hubei University of Technology (HGKFYB10).

Data Availability Statement

The data are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Slope model diagram. (a) Slope model parameters. (b) Real map of slope.
Figure 1. Slope model diagram. (a) Slope model parameters. (b) Real map of slope.
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Figure 2. Grading curve.
Figure 2. Grading curve.
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Figure 3. Slopes of different plant species. (a) Bare slope; (b) Bermudagrass slope; (c) Magnolia slope; (d) grass–shrub slope.
Figure 3. Slopes of different plant species. (a) Bare slope; (b) Bermudagrass slope; (c) Magnolia slope; (d) grass–shrub slope.
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Figure 4. Variation characteristics of soil pH on slopes with different gradients, vegetation types, and cement content base materials. (a) Soil pH at lower of the slope positions under different slope gradients; (b) soil pH at middle of the slope positions under different slope gradients; (c) soil pH at upper of the slope positions under different slope gradients; (d) soil pH at lower of the slope positions under different cement contents in base materials; (e) soil pH at middle of the slope positions under different cement contents in base materials; (f) soil pH at upper of the slope positions under different cement contents in base materials; (g) soil pH at lower of the slope positions under different vegetation types on slopes; (h) soil pH at middle of the slope positions under different vegetation types on slopes; (i) soil pH at upper of the slope positions under different vegetation types on slopes.
Figure 4. Variation characteristics of soil pH on slopes with different gradients, vegetation types, and cement content base materials. (a) Soil pH at lower of the slope positions under different slope gradients; (b) soil pH at middle of the slope positions under different slope gradients; (c) soil pH at upper of the slope positions under different slope gradients; (d) soil pH at lower of the slope positions under different cement contents in base materials; (e) soil pH at middle of the slope positions under different cement contents in base materials; (f) soil pH at upper of the slope positions under different cement contents in base materials; (g) soil pH at lower of the slope positions under different vegetation types on slopes; (h) soil pH at middle of the slope positions under different vegetation types on slopes; (i) soil pH at upper of the slope positions under different vegetation types on slopes.
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Figure 5. The TOC, ROC, and DOC of Bermudagrass slope with different slopes and 3 cm 3% cement content changes with depth and slope position. Uppercase letters are assigned to denote statistically significant variations (p < 0.05) between soil layers or topographic positions. Lowercase letters distinguish significant differences (p < 0.05) across varying slope gradients. (a) Vertical distribution of TOC content of different slope gradients. (b) Vertical distribution of ROC content of different slope gradients. (c) Vertical distribution of DOC content of different slope gradients. (d) Spatial variation of TOC across slope positions under varying gradient conditions. (e) Spatial variation of ROC across slope positions under varying gradient conditions. (f) Spatial variation of DOC across slope positions under varying gradient conditions.
Figure 5. The TOC, ROC, and DOC of Bermudagrass slope with different slopes and 3 cm 3% cement content changes with depth and slope position. Uppercase letters are assigned to denote statistically significant variations (p < 0.05) between soil layers or topographic positions. Lowercase letters distinguish significant differences (p < 0.05) across varying slope gradients. (a) Vertical distribution of TOC content of different slope gradients. (b) Vertical distribution of ROC content of different slope gradients. (c) Vertical distribution of DOC content of different slope gradients. (d) Spatial variation of TOC across slope positions under varying gradient conditions. (e) Spatial variation of ROC across slope positions under varying gradient conditions. (f) Spatial variation of DOC across slope positions under varying gradient conditions.
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Figure 6. Different vegetation types with 0 cm 0% cement content and a slope ratio of 1:1.75 for TOC, ROC, and DOC with depth and slope position changes. Uppercase letters denote significant variations (p < 0.05) between soil horizons or slope positions. Lowercase letters represent statistically distinct values (p < 0.05) across vegetation communities. (a) Vertical distribution of TOC content of different vegetation types. (b) Vertical distribution of ROC content of different vegetation types. (c) Vertical distribution of DOC content of different vegetation types. (d) Spatial variation of TOC along slope positions across vegetation types. (e) Spatial variation of ROC along slope positions across vegetation types. (f) Spatial variation of DOC along slope positions across vegetation types.
Figure 6. Different vegetation types with 0 cm 0% cement content and a slope ratio of 1:1.75 for TOC, ROC, and DOC with depth and slope position changes. Uppercase letters denote significant variations (p < 0.05) between soil horizons or slope positions. Lowercase letters represent statistically distinct values (p < 0.05) across vegetation communities. (a) Vertical distribution of TOC content of different vegetation types. (b) Vertical distribution of ROC content of different vegetation types. (c) Vertical distribution of DOC content of different vegetation types. (d) Spatial variation of TOC along slope positions across vegetation types. (e) Spatial variation of ROC along slope positions across vegetation types. (f) Spatial variation of DOC along slope positions across vegetation types.
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Figure 7. Different cement content slope ratios of 1:1.75 Bermudagrass slope to TOC, ROC, and DOC with depth and slope position changes. Uppercase letters denote statistically distinct values (p < 0.05) between soil horizons or topographic positions. Lowercase letters signify significant variations (p < 0.05) in cement content levels. (a) Vertical distribution of TOC content of different cement contents. (b) Vertical distribution of ROC content of different cement contents. (c) Vertical distribution of DOC content of different cement contents. (d) Spatial variation of TOC across slope positions with varying cement content levels. (e) Spatial variation of ROC across slope positions with varying cement content levels. (f) Spatial variation of DOC across slope positions with varying cement content levels.
Figure 7. Different cement content slope ratios of 1:1.75 Bermudagrass slope to TOC, ROC, and DOC with depth and slope position changes. Uppercase letters denote statistically distinct values (p < 0.05) between soil horizons or topographic positions. Lowercase letters signify significant variations (p < 0.05) in cement content levels. (a) Vertical distribution of TOC content of different cement contents. (b) Vertical distribution of ROC content of different cement contents. (c) Vertical distribution of DOC content of different cement contents. (d) Spatial variation of TOC across slope positions with varying cement content levels. (e) Spatial variation of ROC across slope positions with varying cement content levels. (f) Spatial variation of DOC across slope positions with varying cement content levels.
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Table 1. Basic situation of different slope plots.
Table 1. Basic situation of different slope plots.
Vegetation TypeSubstrate Depth (cm)Cement Content (%)Slope GradientTOC
(g/kg)		ROC
(g/kg)		DOC
(g/kg)
Bermudagrass331:1.54.4217 ± 1.35171.1782 ± 0.3567 0.1108 ± 0.0302
Bermudagrass331:1.754.9004 ± 1.2004 1.4514 ± 0.2726 0.1218 ± 0.0072
Bermudagrass331:25.8572 ± 1.6375 1.6831 ± 0.4199 0.3027 ± 0.0189
Table 2. Basic situation of different plant types of sample plots.
Table 2. Basic situation of different plant types of sample plots.
Vegetation TypeSubstrate Depth (cm)Cement Content (%)Slope GradientTOC
(g/kg)		ROC
(g/kg)		DOC
(g/kg)
Bare001:1.751.5250 ± 0.19890.2247 ± 0.0774 0.0628 ± 0.0060
Bermudagrass001:1.752.4427 ± 0.6244 0.5008 ± 0.1572 0.1016 ± 0.0188
Magnolia001:1.752.3202 ± 0.5007 0.5781 ± 0.1488 0.2141 ± 0.0261
Grass-Shrub001:1.753.1677 ± 0.9401 0.6674 ± 0.1437 0.2320 ± 0.0280
Table 3. Basic situation of sample plots with different cement contents.
Table 3. Basic situation of sample plots with different cement contents.
Vegetation TypeSubstrate Depth (cm)Cement Content (%)Slope GradientTOC
(g/kg)		ROC
(g/kg)		DOC
(g/kg)
Bermudagrass001:1.752.4427 ± 0.6244 0.5008 ± 0.1572 0.1016 ± 0.0188
Bermudagrass331:1.754.9004 ± 0.6111 1.4514 ± 0.2726 0.1305 ± 0.0205
Bermudagrass531:1.754.1839 ± 1.0005 0.6397 ± 0.1492 0.2616 ± 0.0266
Bermudagrass351:1.754.2503 ± 1.1089 0.7127 ± 0.1419 0.3599 ± 0.0455
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MDPI and ACS Style

Zheng, L.; Zhou, X.; Zhou, X.; Huang, S.; Zhong, Z.; Xiao, H. Study on the Spatial Distribution Patterns and Influencing Factors of Soil Organic Carbon Components in Ecological Vegetative Slope Areas. Sustainability 2025, 17, 2650. https://doi.org/10.3390/su17062650

AMA Style

Zheng L, Zhou X, Zhou X, Huang S, Zhong Z, Xiao H. Study on the Spatial Distribution Patterns and Influencing Factors of Soil Organic Carbon Components in Ecological Vegetative Slope Areas. Sustainability. 2025; 17(6):2650. https://doi.org/10.3390/su17062650

Chicago/Turabian Style

Zheng, Lifei, Xuyuan Zhou, Xinlong Zhou, Shaoping Huang, Zhiying Zhong, and Henglin Xiao. 2025. "Study on the Spatial Distribution Patterns and Influencing Factors of Soil Organic Carbon Components in Ecological Vegetative Slope Areas" Sustainability 17, no. 6: 2650. https://doi.org/10.3390/su17062650

APA Style

Zheng, L., Zhou, X., Zhou, X., Huang, S., Zhong, Z., & Xiao, H. (2025). Study on the Spatial Distribution Patterns and Influencing Factors of Soil Organic Carbon Components in Ecological Vegetative Slope Areas. Sustainability, 17(6), 2650. https://doi.org/10.3390/su17062650

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