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Article

Coupled Effects of Water Depth, Vegetation, and Soil Properties on Soil Organic Carbon Components in the Huixian Wetland of the Li River Basin

by
Yongkang Wang
1,
Junfeng Dai
2,3,*,
Fan Jiang
4,
Zupeng Wan
2 and
Shuaipu Zhang
2
1
College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541000, China
2
Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541000, China
3
Collaborative Innovation Center for Water Pollution Control and Water Security in Karst Region, Guilin University of Technology, Guilin 541000, China
4
Natural Resources Ecological Restoration Center of Guangxi Zhuang Autonomous Region, Nanning 530000, China
*
Author to whom correspondence should be addressed.
Land 2025, 14(3), 584; https://doi.org/10.3390/land14030584
Submission received: 4 February 2025 / Revised: 7 March 2025 / Accepted: 7 March 2025 / Published: 10 March 2025
(This article belongs to the Section Land, Soil and Water)
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Versions Notes

Abstract

:
Wetland ecosystems are essential to the global carbon cycle, and they contribute significantly to carbon storage and regulation. While existing studies have explored the individual effects of the water depth, vegetation, and soil properties on the soil organic carbon (SOC) components, a comprehensive study of the interactions between these factors is still lacking, particularly regarding their collective impact on the composition of the SOC in wetland soils. This paper focused on the Huixian Wetland in the Li River Basin. The variations in the SOC and its fractions, namely dissolved organic carbon, microbial biomass carbon, light fraction organic carbon, and mineral-associated organic carbon, under different water depths and vegetation conditions were examined. Additionally, the effects of the water depth, vegetation, and soil properties (pH and bulk density, total phosphorus (TP), total nitrogen (TN), ammonium nitrogen (NH4-N), and nitrate nitrogen (NO3-N)) on the changes in the SOC and its components were quantified. Specific water depth–vegetation combinations favor SOC accumulation, with Cladium chinense at a water depth of 20 cm and Phragmites communis at 40 cm exhibiting a higher biomass and higher SOC content. The SOC components were significantly and positively correlated with plant biomass, TP, TN, and NH4-N. The coupling of water depth, vegetation, and soil properties had a significant effect on the SOC components, with the coupling of water depth, vegetation, and soil properties contributing 74.4% of the variation in the SOC fractions. Among them, water depth, plant biomass, and soil properties explained 7.8%, 7.3%, and 6.4% of the changes, respectively, and their interactions explained 25.6% of the changes. The coupling of the three significantly influenced the changes in the SOC components. Optimal water level management and the strategic planting of wetland vegetation can enhance the carbon storage capacity and increase the SOC content. This research offers valuable insights for effectively managing wetland carbon sinks and soil carbon reserves.

1. Introduction

Wetland ecosystems are among the most vital environmental assets for humanity. In the past, our understanding of wetland ecosystem services has been insufficiently comprehensive, often emphasizing the socio-economic benefits of wetlands while neglecting their ecological and environmental value. Increasing attention has been given to immediate gains, often compromising sustainability., which has led to a focus on the development and utilization of wetland resources during socio-economic progress, while little attention or even no attention has been given to the protection of wetland ecosystems [1,2,3,4]. Wetlands play a vital role in water regulation, runoff control, water purification, carbon storage, oxygen release, climate moderation, nutrient cycling, ecological balance maintenance, and habitat provision for diverse species [5,6]. Moreover, wetlands also play a vital role in the global carbon cycle [7]. However, with the continued development of human society and the overexploitation of biological resources, the sizes of wetland areas have been steadily reduced, and their ecological functions have deteriorated. In recent years, as a result of the interplay between natural climate patterns and human activities, the Huixian Wetland has experienced ecological degradation, accelerating the shrinkage, degradation, and structural changes of its ecosystem. This has resulted in serious local water pollution, a sharp reduction in biological habitats, a decline in biodiversity, and a significant reduction in its flood prevention and water retention functions [8,9,10]. Such changes could potentially cause the Huixian Wetland to shift from functioning as a carbon sink to becoming a carbon source [11].
The SOC is a crucial element of the soil carbon pool, and its content and dynamics serve as indicators of the soil quality and also reflect the extent of the soil’s carbon sequestration capacity [12,13,14]. To accurately estimate the response of SOC to environmental changes, it is necessary to subdivide the SOC into its various components. The SOC can be classified into mineral-associated organic carbon (MAOC), the dissolved organic carbon (DOC), the light fraction organic of carbon (LFOC), and microbial biomass carbon (MBC) [15]. Although DOC, MBC, LFOC accounts for a small proportion of the total SOC, its changes are more sensitive and can provide a significant representation of the SOC variations, thus plays an essential role in maintaining the soil carbon pool balance, soil biochemistry, and soil fertility [16]. In contrast, MAOC refers to organic carbon that is bound and fixed to the mineral surfaces of soil minerals, This fraction is more stable, with a lifespan ranging from centuries to millennia [17]. It should be noted that the relationship between labile and mineral-associated organic carbon is not fixed, and they can undergo mutual transformation under specific conditions. For instance, DOC, MBC, LFOC may gradually transform into MAOC, and conversely, MAOC may be decomposed under certain circumstances, contributing to the soil’s carbon cycle. This dynamic interaction leads to complex and changing patterns of the SOC [18]. Consequently, due to its large capacity, the mineral-associated organic carbon pool is generally considered to be an indicator of a “carbon sink”, and plays a key role in carbon sequestration and in mitigating the release of CO₂ via soil respiration [19]. Therefore, changes in the components of the SOC can serve as early indicators of changes in soil carbon storage.
Wetland SOC is primarily influenced by the hydrological conditions, vegetation types, and soil properties. The hydrological conditions affect the soil moisture content. Under wet conditions, organic matter decomposition is slower, leading to greater SOC accumulation [20]. Moderate soil moisture supports organic carbon storage [21], while both excessive dryness and saturation can lead to carbon loss. Variations in soil moisture also alter the chemical stability of soil organic matter, directly influencing the fraction and content of the SOC [22] Furthermore, the recent degradation of wetlands, coupled with falling water levels, has led to alterations in the soil properties, elevated soil temperatures, enhanced aeration, and an increased activity of aerobic microorganisms. These factors accelerate the decomposition of organic matter, resulting in a gradual decrease in the SOC stocks [23]. In contrast, the soil pH is a key factor influencing the composition of the SOC, microbial activity, and plant growth. Changes in plant biomass influence root exudate release and organic carbon accumulation, directly impacting the SOC content [24]. Variations in the soil bulk density also influence the SOC. Both increases and decreases in the bulk density can affect root penetration, water infiltration, and gas exchange, which in turn limit plant growth and microbial activity [25]. Additionally, changes in the N and P contents of wetland soils affect the production of relevant enzymes, which respond to environmental changes, consequently leading to fluctuations in the SOC content [26].
Plant communities under different water depths play an key role in influencing the soil properties and SOC content [27], leading to the potential for significant and multifaceted impacts. These include changes in the soil physical properties [28], nutrient cycling [29], organic matter accumulation [30], soil salinity, and pH [31]. Hydrological variations are closely linked to the ecological response of wetland vegetation, which in turn influences the species composition and distribution. Due to differences in the growth rates, biological availability, and decomposition rates of plant litter, the presence of different plant species tends to result in distinct pathways of carbon transfer and transformation within vegetation communities. As such, changes in the hydrological conditions drive shifts in the vegetation distribution, creating stratified structures along the water depth gradient. Furthermore, with the succession of vegetation communities, the carbon transfer processes in wetlands also evolve. In other words, the hydrological conditions ultimately drive changes in the composition and scale of wetland SOC pools, influencing energy flow in wetland ecosystems and determining key ecological functions such as productivity, community structure, and biodiversity. Even within the same region, variations in the hydrological conditions and plant community types result in differences in the distributions of the SOC and its active fractions [32,33]. However, further research is needed to investigate the interactive effects of various plant species and the water depth.
The Huixian Karst Wetland in Guilin, China, plays a significant role in regional climate regulation, water conservation, and biodiversity protection [34]. Wetland ecosystems are particularly responsive to environmental changes, and human activities have significantly impacted the Huixian Wetland. In recent years, due to climate change and the construction of fish ponds, agricultural land reclamation, and water infrastructure development, the area has been gradually shrinking. The dominant plant communities in the region have also declined, seriously threatening the ecological functions of the wetland.
In this study, we focused on the Cladium chinense and Phragmites communis communities under different water depths in the Huixian Wetland and measured key indicators related to these communities. Our objectives were (1) to evaluate the influence of the water depth and vegetation type on the SOC storage and plant biomass in wetlands, and (2) to analyze how the water depth, plant biomass, and soil physicochemical properties interact to shape the SOC fractions. Although prior research has investigated the individual effects of water depth, plant biomass, and soil properties on wetland carbon storage, there is a lack of systematic research on their interactive effects, especially regarding their impacts on the SOC fractions. This study fills this research gap by systematically analyzing the interactions among the water depth, vegetation biomass, and soil properties in the typical Huixian Wetland ecosystem and identifying the most favorable conditions for vegetation restoration, SOC storage, and carbon sink growth. The findings of this study provide new theoretical insights into wetland carbon storage and cycling, and offer scientific references for wetland ecosystem protection and management.

2. Materials and Methods

2.1. Study Site

The Huixian Karst Wetland, located in Huixian Town, Lingui District, Guilin City, Guangxi Zhuang Autonomous Region, China, (Figure 1) is a karst wetland dominated by herbaceous marshes and lakes, is situated at elevations of 150–160 m, and has a total area of about 120 km2. It is one of the few large-scale karst wetlands at low and medium elevations in China [35]. The soil is predominantly Acrisols and Ferralsols, and it is concentrated in depressions, plains, and gentle slopes. In the mountainous areas, the soil layer is thin, and the bedrock is even exposed. The vegetation in this wetland is dominated by water-supporting vegetation and submerged vegetation, with a large number of plant species and luxuriant growth. The vegetation cover often reaches 80–95%, and the main species are Phragmites communis, Cladium chinense, Ceratophyllum demersumvar, Oryzelorum, and Limnphila sessiliflora.

2.2. Sample Collection and Processing

During the peak growing season (May 2024), fixed 1 m × 1 m quadrats (three parallel sample plots were set up for each vegetation type, each with three 1 × 1m sample plots) were established using the transect method based on the different water depths and vegetation types (community types). Within each quadrat, the aboveground biomass (AGB) was collected by cutting all of the aboveground plant material. During the sampling, the root biomass (RB) was also extracted by excavating the soil within the quadrat to retrieve the entire root system. The roots were carefully washed with running water in a tray to remove the adhered soil while minimizing the biomass loss [36,37]. All of the plant samples were then subjected to initial drying at 105 °C for 0.5 h to deactivate the enzymes, followed by drying at a constant temperature of 65 °C until a stable weight was achieved. There was a total of 24 plant samples. To determine the dry weights of both the aboveground and belowground biomass in Lake Mudong, sampling plots were selected based on different water depths (20, 40, and 60 cm), and the weights included the Phragmites communis communities, Cladium chinense communities, and bare soil (no plant areas); there were three parallel sample plots for different vegetation types. Soil samples were collected from four depth intervals (0–20 cm, 20–40 cm, 40–60 cm, and 60–100 cm) with three replicates taken at each depth, resulting in a total of 288 samples. A stainless steel spiral auger was used to determine the SOC content and carbon storage at each depth. Additionally, surface soil (0–20 cm) samples from each plot were homogenized and transported to the laboratory for determination of the pH, TN, TP, soil water content (WC), NH4-N, and NO3-N. The soil fractions, including the LFOC, MAOC, DOC, and MBC, were analyzed. The soil bulk density (BD) was measured using the ring knife method [38], for which undisturbed surface soil samples were collected.

2.3. Sample Analysis and Data Processing Methods

The SOC was quantified using a total organic carbon (TOC) analyzer (Japan SHIMADZU TOC-LSSM5000A). Prior to the SOC measurements, we selected soil samples that had been dried, milled, and sieved (0.15 mm). We then placed 200 mg of the sample in a nickel crucible and added 2 drops of 10% HCl solution. The reaction was allowed to run overnight to remove the inorganic carbon. For better soil dispersion, soil samples were added to deionized water at a ratio of 1 g of soil to 50 mL of water and then processed with an ultrasonic oscillator at a frequency of 20 kHz for 20 min prior to sampling. The WC was determined using the oven-drying method. The TN was analyzed using the micro-Kjeldahl method, whereas the TP was determined using the molybdenum-antimony-scandium colorimetric method. The MBC was measured using the chloroform fumigation-extraction method [39]. The soil pH was determined using a precision pH meter at a soil-to-water ratio of 1:2.5 [36]. The nitrate nitrogen (NO3-N) and ammonium nitrogen (NH4-N) concentrations were determined according to China’s agricultural industry standard (LY/T 1228-2015). The LFOC was quantified using the density fractionation method [16]. The DOC was extracted at a soil-to-water ratio of 1:10 (m/V) [40] and subsequently analyzed using a TOC analyzer. The MAOC was measured using the density fractionation method [41]. The SOC stock was calculated using the method of Yonghui Wang [42].
The SOC stock can be obtained using the following equation:
CSS = D × C × E
where CSS is the SOC stock per unit area (kg/m2); D is the soil bulk density (g/cm3); C is the SOC content (g/kg); and E is the soil thickness (m). Data analysis was performed using SPSS 27, Origin 2024, and R v4.3.0. Pearson correlation analysis was applied to examine the relationships between the components. Variation partitioning analysis (VPA) was conducted using the “varpart” function from the “vegan” package in R to quantitatively assess the contributions of water depth, plant biomass, and soil properties to the variation in the SOC components

3. Results

3.1. Variations in SOC Content Under Different Water Depths and Vegetation Communities

As the soil depth increased, the SOC content of Cladium chinense, Phragmites communis, and no plant sites gradually decreased, with both plant communities exhibiting a higher SOC content than the no plant sites. Firstly, at the 0–20 cm soil depth (Figure 2a), the SOC content of Cladium chinense decreased with increasing water depth, with the SOC content at the 20 cm water depth significantly higher than that at the 40 and 60 cm depths (p < 0.05), being 36.92% and 55.85% higher, respectively. The SOC content of Phragmites communis was highest at the 40 cm water depth, with values 21.58% and 36.05% higher than at the 20 cm and 60 cm water depths, respectively. At the 20–40 cm soil depth (Figure 2b), the SOC content of Cladium chinense was highest at the 20 cm water depth with a value of 30.58 g/kg, while Phragmites communis showed the highest SOC content at the 40 cm water depth with a value of 29.98 g/kg. At the 40–60 cm soil depth (Figure 2c), Cladium chinense exhibited no significant differences in the SOC content across different water depths (p > 0.05), while Phragmites communis had the highest SOC content at the 40 cm water depth. At the 60–100 cm soil depth (Figure 2d), the SOC content of Cladium chinense decreased with increasing water depth, with the SOC content at 20 cm significantly higher than at 40 and 60 cm (p < 0.05). For Phragmites communis, the SOC content was highest at the 40 cm water depth, being 24.34% and 34.35% higher than at the 20 cm and 60 cm water depths, respectively.
Overall, at the same water depth, the surface soil layers had the highest SOC contents. The SOC content gradually decreased with increasing soil depth, but eventually stabilized.

Variations in SOC Stock and Soil Bulk Destiny for Different Water Depths and Vegetation Communities

The changes in the vegetation communities and water depth significantly influenced the SOC storage (SOCs). At the same water depth, the SOCs of the Cladium chinense and Phragmites communis communities differed significantly (p < 0.05; Figure 3). At a water depth of 20 cm, the SOCs of the Cladium chinense community was significantly higher than that of the Phragmites communis community (p < 0.05), being 25.64% higher. However, at water depths of 40 cm and 60 cm, the SOCs of the Cladium chinense community was significantly lower than that of the Phragmites communis community (p < 0.05), being 38.52% and 30.21% lower, respectively.
The water depth also affected the accumulation of SOCs within the same vegetation community, as the SOCs significantly differed among the three water depths (p < 0.05). For the Cladium chinense community, the SOCs was significantly higher at a water depth of 20 cm than at 40 cm and 60 cm (p < 0.05), being 43.98% and 41.28% higher, respectively. For the Phragmites communis community, the SOCs was significantly higher at a water depth of 40 cm than at 20 cm and 60 cm (p < 0.05).

3.2. Variations in Soil Properties Under Different Water Depths and Vegetation Communities

Within the same vegetation community, the soil pH did not significantly differ among the three water depths (p > 0.05; Figure 4a). However, at water depths of 20 cm and 40 cm, the soil pH was significantly higher in the Cladium chinense community and unvegetated plots than in the Phragmites communis community (p < 0.05). At a water depth of 60 cm, there was no significant difference in the soil pH between the two vegetation communities (p > 0.05). The WC of the Phragmites communis community was not significantly different among the three water depths (p > 0.05; Figure 4b). However, for the Cladium chinense community, the WC was significantly higher at 20 cm and 40 cm than in the deeper water zone (p < 0.05). For the Cladium chinense community, the soil TP content varied significantly among the three water depths (p < 0.05; Figure 4c). The lowest TP content occurred at a depth of 60 cm. The WC at 60 cm was 33.33% and 26.33% lower than those at 20 cm and 40 cm, respectively. Similarly, for the Phragmites communis community, the TP content was significantly higher at 20 cm and 40 cm than in the deeper water zone (p < 0.05). For both vegetation communities, the soil TP content decreased as the water depth increased, and the vegetated plots had significantly higher TP contents than the unvegetated plots (p < 0.05, Figure 4d). The TN contents for the Cladium chinense community and unvegetated plots decreased with increasing water depth. Moreover, the TN contents of the Cladium chinense and Phragmites communis communities were significantly higher than that of the unvegetated plots (p < 0.05). At all of the sampling sites, the NH4-N content decreased as the water depth increased (Figure 4e). Additionally, the vegetated plots had significantly higher NH4-N contents than the unvegetated plots. The NO3-N content of the unvegetated plots was significantly higher than those of the Cladium chinense and Phragmites communis communities (p < 0.05; Figure 4f). Moreover, at all of the sampling sites, the NO3-N contents increased with increasing water depth.

3.3. Variations in Soil Organic Carbon Components Under Different Water Depths and Vegetation Communities

The SOC fractions (DOC, MBC, MAOC, and LFOC) were significantly affected by the different vegetation communities and water depths (Figure 5). The DOC contents of the Cladium chinense community and Phragmites communis community were significantly higher than that of the unvegetated sample site at different water depths (p < 0.05) (Figure 5a). The DOC content of the Cladium chinense community was significantly higher than that of the Phragmites communis community at water depths of 20 cm and 40 cm (p < 0.05). The MBC contents of both the Cladium chinense community and unvegetated sample sites decreased with increasing water depth (Figure 5b), but the difference in the MBC contents of the Phragmites communis community among the three water depths was not statistically significant. The LFOC and MAOC contents of the Cladium chinense community and Phragmites communis community were significantly higher than those of the unvegetated site at the different water depths (p < 0.05). For the same vegetation community, there were no significant differences (p > 0.05) in the LFOC and MAOC contents at the different water depths (Figure 5c,d).

3.4. Variations in Biomass of Cladium chinense and Phragmites communis Communities Under Different Water Depths

The biomasses of the two vegetation communities exhibited some differences at the three water depths (Figure 6). The AGB and RB biomass of the Cladium chinense community were both highest at the 20 cm water depth, with 2462.15 g/m2 and 3369.43 g/m2, respectively. The AGB and RB were significantly higher at 20 cm than at 40 cm and 60 cm (p < 0.05). The AGB and RB of the Phragmites communis community were highest at 40 cm, with values of 1588.87 g/m2 and 1844.78 g/m2, respectively. The AGB and RB of the Phragmites communis community were the highest at a depth of 40 cm, with values of 1588.87 g/m2 and 1844.78 g/m2, respectively, and the AGB and RB were significantly higher at a depth of 40 cm than at depths of 20 cm and 40 cm, respectively (p < 0.05).

3.5. Relationships Between Plant Biomass, Soil Properties and Soil Organic Carbon Components

The plant biomass exhibited strong correlations with the soil properties (Figure 7). The AGB exhibited a negative correlation with the soil pH (p < 0.05) and significant positive correlations with the WC, TP, NH4-N, and NO3-N (p < 0.05). Similarly, the RB was negatively correlated with the soil pH (p < 0.05), was strongly significantly positively correlated with the water content (WC) (p < 0.01), and was significantly positively correlated with the TP, TN, and NO3-N (p < 0.05).
The plant biomass and soil properties jointly influenced the SOC. The SOC exhibited a significant positive correlation with both the AGB and RB (p < 0.05). Additionally, the SOC was significantly correlated with the various soil parameters: positively correlated with the WC (p < 0.05), strongly positively correlated with the TP (p < 0.01), and strongly positively correlated with the TN, NO3-N, and NH4-N (p < 0.05). However, there was no strong correlation between the SOC and soil pH (p > 0.05).
Furthermore, the plant biomass and soil properties jointly influenced the SOC fractions. The soil pH exhibited a significant negative correlation with all of the measured SOC fractions including the DOC, MAOC, LFOC, and MBC (p < 0.05). The DOC exhibited significant positive correlations with the AGB, RB, WC, TP, and NH4-N (p < 0.05) but did not exhibit significant correlations with the NO3-N and TN (p > 0.05). The MBC was significantly negatively correlated with the NO3−-N and TN (p < 0.05) and positively correlated with the AGB, RB, and WC (p < 0.05). The LFOC exhibited significant positive correlations with the RB, WC, TP, and NH4-N (p < 0.05), but was not significantly correlated with the NO3-N or TN (p > 0.05). Notably, the MAOC exhibited a strong significant and positive correlation with the WC (p < 0.01).

3.6. Contributions of Water Depth, Vegetation, and Soil Properties to Changes in the SOC Components

The results showed that the contributions of the water depth, vegetation, and soil properties to the variations in the SOC components were 7.8%, 7.3%, and 6.4%, respectively (Figure 8). The interactions between the water depth and soil properties accounted for 13.5% of the variations in the SOC components, while the interaction between the water depth and vegetation accounted for 12.6%. The interaction between the soil properties and vegetation contributed 6.5% of the variation. Notably, the combined influence of the water depth, vegetation, and soil properties explained 25.6% of the variations in the SOC components.

4. Discussion

4.1. Effects of Water Depth and Vegetation on Soil Organic Carbon Stocks

As a critical component of the carbon cycle in wetland ecosystems, the SOC is influenced by multiple factors that affect its accumulation and stability [43]. Notably, the water depth and vegetation biomass play pivotal roles in the formation and accumulation of the SOC in wetland soils [44,45]. The SOC content in the Phragmites communis community was highest at a water depth of 40 cm, which correlated with the peak values of both the aboveground and belowground biomass components in this community at this specific water depth. This highlights the adaptability of Phragmites communis to particular water levels, optimizing growth and biomass accumulation. The water depth, as a central factor in wetland environments, directly influences the plant growth and soil oxygen conditions [46], thereby impacting the accumulation of SOC. The SOC reserves in the Cladium chinense and no plant sites decreased with increasing water depth; this was primarily due to the inhibition of plant growth and the organic carbon input in the deeper water zones. In shallower water regions, plant growth tends to be more vigorous [47], particularly for wetland plants, which fix carbon dioxide through photosynthesis and input organic carbon into the soil via their roots [48]. This process contributes to increasing the SOC content. However, in deeper water regions, due to the occurrence of water saturation and limited plant growth, the SOC reserves are lower. Vegetation biomass is also a key source of organic carbon in wetlands [49]. Root systems and organic matter such as fallen branches and leaves are input into the soil, and their decomposition further promotes the accumulation of the SOC [50]. In shallow water areas, plant growth is less constrained, leading to higher biomass, thus providing more organic carbon inputs into the soil [51].
The SOC density is a key indicator of the soil’s carbon storage capacity. It is not only associated with the total amount of SOC, but is also closely linked to the BD, depth, and structure [52]. In this study, we found that the SOC density was significantly higher in the shallow water areas compared with the deeper water regions, mainly due to the higher vegetation biomass and lower BD in the shallow zones, which facilitated organic carbon accumulation. In contrast, the deeper water areas tended to have looser soil and limited plant growth, leading to a lower SOC density.

4.2. Effects of Water Depth and Vegetation on Soil Properties

4.2.1. Soil pH

Soil pH is a key environmental factor in wetland ecosystems, and it influences the soil chemical processes, nutrient availability, and microbial community structure. In wetland ecosystems, the water depth and vegetation type significantly affect the soil pH variations [53]. The water depth influences the soil pH by regulating the redox state of the soil, the decomposition rate of organic matter, and nutrient cycling processes [54]. In general, soils in shallow water zones tend to have higher oxygen contents, which promotes the aerobic decomposition of organic matter and microbial activity. This in turn facilitates the release of hydrogen ions, resulting in a decrease in the soil pH [55]. The results of this study align with these findings (i.e., the soil pH decreased with increasing water depth in the Cladium chinense and Phragmites communis communities). Additionally, the soil pH was significantly higher in the Cladium chinense community than in the Phragmites communis community. This can be attributed to the fact that certain plant species, such as Phragmites communis, excrete root exudates that are typically rich in organic acids [56], which lower the rhizosphere pH, while other plants, such as Cladium chinense, increase the soil pH by absorbing ammonium ions (NH4+) and releasing hydroxide ions (OH−). In contrast, deep water zones are characterized by prolonged flooding, which leads to a reducing redox state in the soil. The reduction of iron and manganese oxides consumes hydrogen ions, causing the soil pH to increase significantly. Furthermore, the lower organic matter decomposition rates in deep water zones result in less accumulation of acidic substances, which may contribute to the higher soil pH [57].
The vegetation type also plays a crucial role in influencing nutrient inputs and the microbial community composition, thereby regulating the soil pH. Different plant communities affect the soil pH through processes such as root exudates, litter decomposition, and nutrient absorption [58]. The soil pH was generally higher in the no plant sites than in the vegetated sites, which can be attributed to the fact that plant root growth promotes the proliferation of rhizosphere microbes. During the process of decomposing organic matter, these microbes produce organic acids such as lactic acid and acetic acid, which contribute to soil acidification [59].

4.2.2. Soil Water Content

In wetland ecosystems, the water depth and vegetation are key factors influencing the soil moisture content [60]. In this study, we found that at water depths of 20 cm and 40 cm, the soil moisture content was higher in the Cladium chinense community than that in the Phragmites communis community and no plant sampling points. For both the Cladium chinense and Phragmites communis communities, the soil moisture content was highest at a water depth of 40 cm. This can be attributed to the ability of plant roots to modify the soil’s physical structure, increasing the soil porosity and thus enhancing its water retention capacity [61]. The water depth directly determines the saturation level of the soil, while vegetation indirectly regulates the soil moisture conditions through its physiological and ecological functions [62]. The soil moisture content is a critical component of ecosystem functions and directly affects plant growth, microbial activity, nutrient cycling, and carbon sequestration processes [63]. Changes in the soil moisture content can significantly impact the stability and sustainability of ecosystems in wetlands, agricultural lands, and forested areas [64]. Additionally, vegetation transpiration processes depletes the soil moisture, affecting the moisture balance in the soil [65]. The high plant biomass of the Cladium chinense and Phragmites communis communities at water depths of 20 cm and 40 cm effectively explains this phenomenon. However, for both plant communities, the biomass was lowest at a depth of 60 cm, and the reduced vegetation cover at this depth exacerbated the evaporation of the soil moisture, leading to a decrease in the soil moisture content. The consistently lower soil moisture contents at all water depths in the no plant sampling sites compared with the vegetated areas further supports this conclusion.

4.2.3. Soil Bulk Density

There is a complex interaction between the water depth and vegetation, which jointly influence the BD. The optimal water depth promotes plant growth, which in turn improves the soil structure through root activity and organic matter input, increasing the porosity and reducing the compaction and ultimately lowering the BD [66,67]. Our experimental results indicate that for all three vegetation types, the BD increased with increasing water depth. Moreover, the BD was higher in the Phragmites communis sampling sites than in the vegetated sites. However, excessively high or low water levels may restrict plant growth, reducing the organic matter input and causing deterioration of the soil structure, which increases the bulk density [68]. The impact of different vegetation types on the BD varies. For example, herbaceous plants, which have shallower root systems, can still improve the surface BD by increasing the surface organic matter and microbial activity. In contrast, woody plants, which have deeper root systems, have a more pronounced effect on the deep BD by increasing the soil porosity and reducing soil compaction [69,70]. Through root activity and the input of organic carbon from plant litter, the Cladium chinense and Phragmites communis communities increased the soil porosity and loosened the soil, thereby reducing the BD.

4.2.4. Soil TP, TN, NH4-N, and NO3-N

N and P are two key nutrients for plant growth [68]. The concentrations and bioavailability of the TN, T, NH4-N, and NO3-N directly influence the plant growth rate, biomass accumulation, and overall stability of ecosystems [71]. The experimental results of this study showed that the soil TN, TP, and NH4-N concentrations were higher in the Cladium chinense and Phragmites communis communities than in the no plant sites across all three water depths. This can be attributed to the fact that the primary source of TP and TN in soil is the decomposition of plant litter and roots [72]. There were significant positive correlations between the plant biomass and the TP, NH4-N, and TN (p < 0.05; Figure 6). In contrast, in no plant zones, soil evaporation, water loss, and severe weathering exacerbate nitrogen and phosphorus leaching [73], which is consistent with the findings of this study.
Therefore, in wetland management, it is essential to consider the interplay between the water depth and vegetation in order to maintain the balance of soil nutrients and promote the health and stability of wetland ecosystems. Notably, the NO3-N content was significantly higher in the no plant sites than in the vegetated sites. This is because plant roots actively absorb nitrate nitrogen from the soil for growth and metabolism. In vegetated zones, the continuous absorption by plant roots leads to a reduction in the soil nitrate nitrogen concentration [74]. Moreover, in vegetated areas, root-zone microbes (such as nitrifying and denitrifying bacteria) are more active, accelerating the transformation and consumption of nitrate nitrogen [75].

4.3. Effects of Water Depth on Plant Biomass

The water depth is a key environmental factor influencing the growth and biomass distribution of wetland plants as it affects plant growth and metabolism by regulating the oxygen availability, nutrient accessibility, and other factors [76,77]. The results of our study indicate that the aboveground and belowground biomass of the Cladium chinense and Phragmites communis communities respond differently to changes in the water depth. The Cladium chinense community had the highest biomass at a water depth of 20 cm, and the biomass gradually decreased as the water depth increased. In contrast, the aboveground and belowground biomass of the Phragmites communis community was highest at a water depth of 40 cm, and both the lower and higher water depths led to a reduction in the biomass. This suggests that the Cladium chinense community thrives and has the greatest adaptability at shallower water depths. In contrast, the biomass of the Phragmites communis community initially increased and then decreased with increasing water depth, indicating its limited adaptability to extreme aquatic environments. This highlights the influence of the interaction between the water depth and plant communities [78]. Therefore, it can be concluded that Phragmites communis performs better in terms of maintaining stable growth and reproductive capacity under both lower and higher water depths.
Water depth is a key environmental factor influencing the growth and biomass distribution of wetland plants as it affects plant growth and metabolism by regulating oxygen availability, nutrient accessibility, and other factors [76,77]. Our study indicates that the aboveground and belowground biomass of Cladium chinense and Phragmites communis communities respond differently to changes in water depth. The Cladium chinense community exhibited the highest biomass at a water depth of 20 cm, with biomass gradually decreasing as the water depth increased. In contrast, the Phragmites communis community reached its maximum aboveground and belowground biomass at a water depth of 40 cm, with both lower and higher water depths leading to a reduction in biomass. This suggests that the Cladium chinense community thrives and shows the greatest adaptability at shallower water depths. On the other hand, while the biomass of the Phragmites communis community initially increased with increasing water depth, it began to decline as the water depth continued to rise, indicating its limited adaptability to extreme aquatic environments. This highlights the interactive influence of water depth and plant communities [78]. Therefore, Phragmites communis performs better in maintaining stable growth and reproductive capacity under both lower and higher water depths.

4.4. Effect of Water Depth, Vegetation and Soil Properties on Soil Organic Carbon Components

Water depth is a crucial factor influencing the differences in the contents of the SOC components in wetland ecosystems. Studies have shown that under varying water depths, the distributions and contents of SOC and its components (DOC, MBC, LFOC, and MAOC) exhibit significant differences. Our experimental results demonstrated that the content of each SOC component decreased with increasing water depth across the three sampling types. This phenomenon can be attributed to several factors. (1) In shallow water areas, the environment typically promotes plant growth, increasing the root exudates and organic matter input, which in turn increase the DOC content. Additionally, the aerobic conditions in shallow environments facilitate the microbial decomposition of organic matter, leading to higher DOC concentrations [79]. The increased availability of oxygen in these areas supports the growth of aerobic microorganisms, which further increases the MBC content. Shallow environments also accelerate the decomposition of soil organic matter, causing easily decomposable organic carbon (such as DOC and LFOC) to rapidly transform, thus reducing the relative proportion of MAOC [43]. Shallow water environments may also receive more sunlight, promoting photosynthesis and the growth of primary producers (e.g., phytoplankton), which subsequently increases LFOC production [80]. (2) In deeper water areas, the soil experiences anaerobic conditions, reducing the microbial degradation efficiency and consequently lowering the release of DOC. As the water depth increases, the microbial community structure changes, and the metabolic efficiency of anaerobic microorganisms is generally lower than that of aerobic microorganisms. Furthermore, the reduced light availability in deeper waters hampers plant growth, which results in an increase in the LFOC content.
Vegetation contributes to SOC accumulation through the combined effect of litter, root exudates, and microbial activity, which introduce substantial amounts of organic matter into the soil. A high biomass typically correlates with higher DOC and MBC contents because microorganisms in the rhizosphere extensively utilize root exudates for metabolism [81]. Additionally, a high biomass and rapidly decomposing root exudates provide a rich carbon source for soil microbes [82], which is consistent with our experimental results (Figure 5a). Herbaceous vegetation, such as Cladium chinense and Phragmites communis, tends to increase the MBC content due to the faster decomposition of their litter, which is quickly assimilated by soil microbes [83]. The higher soil moisture content under the Cladium chinense community (Figure 4b) led to microbial hypoxia, resulting in a decrease in microbial activity and a slower organic carbon decomposition rate, which facilitated the accumulation of ROC.
In our study, DOC, LFOC, and MAOC showed significant positive correlations with NH4-N, while MBC exhibited a significant negative correlation with NO3-N. Additionally, DOC and LFOC were significantly negatively correlated with TP (Figure 6). This can be explained by the fact that soil properties are key factors influencing plant growth, microbial activity, and changes in the organic carbon components. Ammonium nitrogen (NH4-N) is one of the primary sources of plant nutrition [84]. After absorbing NH4-N, plants convert a portion of it into organic matter and exude it into the soil via their roots. Enhanced root activity, in turn, facilitates the input of more organic carbon into the soil. When nitrate nitrogen (NO3-N) concentrations are high in the soil, microorganisms exhibit lower efficiency in utilizing it, leading to a reduction in microbial biomass. This explains the inhibitory effect of NO3-N on MBC [85]. TP may affect the metabolic processes of microbial communities in the soil, particularly those responsible for decomposing organic carbon. Under excess phosphorus conditions, the rate of organic carbon transformation may slow down, resulting in decreased accumulation of DOC and LFOC [86].
In summary, the variations in the SOC components are influenced by the water depth, vegetation, and soil properties. The interactions between these factors and their combined influence enhance the contribution to changes in the SOC components. Different water depths significantly affect vegetation growth and biomass, and these changes in water depth directly impact the properties of the soil such as the soil water content, bulk density, and soil pH. These properties in turn influence the organic carbon storage and turnover rates in the soil.

4.5. How Can We Effectively Strengthen Wetland Management and Enhance the Carbon Sequestration Capacity of the Huixian Wetland?

Effective wetland management plays a pivotal role in optimizing carbon sequestration and maintaining ecosystem stability. Our findings highlight the importance of rational water level regulation and targeted vegetation restoration in enhancing the SOC accumulation. The Huixian Karst Wetland is affected by water diversion and transfer from upstream reservoirs and water conservancy facilities, and its hydrological rhythms show cyclical and seasonal fluctuations as well as approximate steady state changes between years [87]. Specifically, maintaining appropriate water depths that align with the growth preferences of wetland plant species can significantly improve carbon storage. For instance, the Cladium chinense community at a water depth of 20 cm and the Phragmites communis community at a water depth of 40 cm demonstrated superior SOC sequestration potential. Therefore, adjusting the water levels to support these species can be a practical approach to increasing the soil carbon stocks. Additionally, soil pH regulation through vegetation selection is a critical consideration in wetland management. Our results indicate that excessively high soil pH can hinder SOC accumulation, while planting specific species, such as Phragmites communis, can effectively lower the pH and improve the soil conditions. This suggests that integrating vegetation management with water level control can create a more favorable environment for long-term carbon sequestration.
Future wetland management efforts should prioritize the combined effects of hydrological conditions, vegetation selection, and soil physicochemical properties. Implementing adaptive water management strategies and promoting the establishment of diverse plant communities can enhance wetland resilience and maximize the carbon sequestration potential. Moreover, long-term monitoring and targeted ecological interventions will be essential for sustaining wetland carbon sink functions in the face of climate change and anthropogenic disturbances.

5. Conclusions

Overall, significant interactions between the water depth, vegetation, and soil physicochemical properties were observed in the wetland SOC cycle. Rational water level management and vegetation restoration measures are crucial for optimizing the carbon storage capacity and improving the carbon sequestration efficiency in wetlands. Specific plant community–water depth combinations significantly enhance the sequestration of SOC and its components. For instance, the Cladium chinense community at a water depth of 20 cm and the Phragmites communis community at a water depth of 40 cm notably increased the contents of the SOC and its components. Our findings emphasize the critical role of plant species–water depth combinations in promoting soil quality restoration and carbon sequestration. Notably, excessively high soil pH values can reduce the contents of the SOC and its components. This can be mitigated by planting specific vegetation, such as the Phragmites communis community, for which the soil pH was significantly lower at all three water depths compared with the unvegetated sites. This study not only deepens our understanding of wetland carbon cycling, but also provides a scientific basis for the protection and management of wetland ecosystems. Future research should further explore the effects of specific water depth–vegetation combinations on wetland carbon sink functions to more comprehensively assess the role of wetlands in the global carbon cycle.

Author Contributions

Conceptualization, Y.W.; software, Y.W.; validation, Y.W. and J.D.; formal analysis, F.J.; investigation, Y.W., Z.W. and S.Z.; resources, J.D.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W. and J.D.; visualization, J.D.; supervision, Z.W.; project administration, S.Z. and F.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded the by the Science and Technology Planning Project of Guangxi, China (No. AB 23026045), the National Natural Science Foundation of China (No. 52269010), and the Science and Technology Plan Project of Guilin (No. 20220114-2).

Data Availability Statement

Data are contained within the article.

Acknowledgments

We thank LetPub (www.letpub.com.cn, accessed on 3 February 2025) for its linguistic assistance during the preparation of this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Maps showing the location of the study area.
Figure 1. Maps showing the location of the study area.
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Figure 2. Soil organic carbon content at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) 0–20 cm soil depth; (b) 20–40 cm soil depth; (c) 40–60 cm soil depth; (d) 60–100 cm soil depth.
Figure 2. Soil organic carbon content at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) 0–20 cm soil depth; (b) 20–40 cm soil depth; (c) 40–60 cm soil depth; (d) 60–100 cm soil depth.
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Figure 3. Soil organic carbon stocks and soil bulk density at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) SOC stocks; (b) soil bulk density.
Figure 3. Soil organic carbon stocks and soil bulk density at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) SOC stocks; (b) soil bulk density.
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Figure 4. Soil properties at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) pH; (b) WC; (c) TP; (d) TN; (e) NO3-N; and (f) NH4-N.
Figure 4. Soil properties at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) pH; (b) WC; (c) TP; (d) TN; (e) NO3-N; and (f) NH4-N.
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Figure 5. Soil organic carbon components at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) DOC; (b) MBC; (c) LFOC; and (d) ROC.
Figure 5. Soil organic carbon components at three water depths (20, 40, and 60 cm) with Cladium chinense, Phragmites communis, and no plants. (a) DOC; (b) MBC; (c) LFOC; and (d) ROC.
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Figure 6. Plant biomass at different water depths. (a) AGB; (b) RB.
Figure 6. Plant biomass at different water depths. (a) AGB; (b) RB.
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Figure 7. Pearson’s correlation analysis between the plant biomass, soil properties, and SOC component. The symbols * and ** indicate significant correlations at the 0.05 and 0.01 levels, respectively.
Figure 7. Pearson’s correlation analysis between the plant biomass, soil properties, and SOC component. The symbols * and ** indicate significant correlations at the 0.05 and 0.01 levels, respectively.
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Figure 8. Venn diagram showing variation partitioning in the plant biomass, soil properties, and water depth activities on the SOC component changes.
Figure 8. Venn diagram showing variation partitioning in the plant biomass, soil properties, and water depth activities on the SOC component changes.
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MDPI and ACS Style

Wang, Y.; Dai, J.; Jiang, F.; Wan, Z.; Zhang, S. Coupled Effects of Water Depth, Vegetation, and Soil Properties on Soil Organic Carbon Components in the Huixian Wetland of the Li River Basin. Land 2025, 14, 584. https://doi.org/10.3390/land14030584

AMA Style

Wang Y, Dai J, Jiang F, Wan Z, Zhang S. Coupled Effects of Water Depth, Vegetation, and Soil Properties on Soil Organic Carbon Components in the Huixian Wetland of the Li River Basin. Land. 2025; 14(3):584. https://doi.org/10.3390/land14030584

Chicago/Turabian Style

Wang, Yongkang, Junfeng Dai, Fan Jiang, Zupeng Wan, and Shuaipu Zhang. 2025. "Coupled Effects of Water Depth, Vegetation, and Soil Properties on Soil Organic Carbon Components in the Huixian Wetland of the Li River Basin" Land 14, no. 3: 584. https://doi.org/10.3390/land14030584

APA Style

Wang, Y., Dai, J., Jiang, F., Wan, Z., & Zhang, S. (2025). Coupled Effects of Water Depth, Vegetation, and Soil Properties on Soil Organic Carbon Components in the Huixian Wetland of the Li River Basin. Land, 14(3), 584. https://doi.org/10.3390/land14030584

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