Impacts of Shrub Encroachment on Vegetation Community and Soil Characteristics in Coastal Wetlands of the Abandoned Yellow River Course

Abstract

Shrub encroachment in coastal wetlands alters vegetation–soil interactions, yet its impacts on north temperate coastal wetland ecosystems remain poorly quantified. This study investigated the effects of Tamarix chinensis-dominated shrub encroachment in the abandoned Yellow River course wetlands. Encroachment stages (Isolated Tamarix shrub, ITS → Tamarix shrub island, TSI → Tamarix woodland, TWL) were assessed via vegetation surveys and soil sampling (0–60 cm). Encroachment progression significantly increased shrub cover, shrub crown width, and branches per shrub while reducing soil electrical conductivity and soil salt content. Surface soils (0–5 cm) exhibited higher levels of organic carbon (SOC) and elevated total nitrogen (TN) and available nitrogen (AN), while deeper layers (40–60 cm) at the TWL stage exhibited reduced available phosphorus (AP) and total phosphorus (TP). Redundancy analysis (RDA) identified soil bulk density, soil water content, total carbon (TC), and AP as primary drivers of vegetation community restructuring (RDA: 68.68% variance). The average ranges of TC:TN (R_CN), TC:TP (R_CP), and TN:TP (R_NP) were 23.04–92.54, 52.14–92.88, and 0.46–4.29, respectively. T. chinensis encroachment induced nitrogen-limited conditions and reduced deep soil layer phosphorus availability, fundamentally restructuring coastal wetland ecosystems. These findings inform blue carbon ecosystem management in the north temperate zone.

1. Introduction

The expansion of woody shrub into herbaceous ecosystems represents a key shift in community structure and a global ecological regime shift [1,2]. Numerous factors (including climate warming and altered precipitation patterns, fire and grazing regimes, concentrations of atmospheric CO₂, and levels of nitrogen deposition) co-occur and interact to promote or constrain increases in woody dynamics at local scales [3,4]. As species adaptation, land-use history, and climate trends differ markedly among bioclimatic zones [5], no single factor provides dominant influence on woody shrub encroachment. Shrub encroachment research has predominantly focused on arid/semi-arid regions, particularly in grassland and savanna ecosystems, with North America and Europe serving as the main study areas [6]. However, recent evidence confirms shrub growing prevalence in freshwater and saline intertidal settings contemporaneous with the expansion and thickening of woody shrubs in grassland biomes globally [7,8].

Shrub encroachment in coastal wetlands refers to the ecological transition from herbaceous-dominated salt marsh ecosystems to woody shrub-dominated landscapes [9]. These ecosystems exhibit heightened vulnerability to climate-driven stressors, particularly sea-level rise and tidal regime alterations [7,10,11], which may critically influence successional trajectories of shrub encroachment. Soil organic carbon (SOC) in coastal wetlands, recognized as “blue carbon”, constitutes a pivotal component of the global carbon (C) cycle. Notably, wetland ecosystems have significant potential to contribute to long-term C sequestration, due to the higher SOC sequestration rates of sediments compared to terrestrial forests [12], playing a decisive role in regulating regional C dynamics [13]. The synergistic effects of global change drivers and shrub encroachment-induced environmental modifications pose substantial threats to coastal wetland ecosystem stability [14].

Soil physicochemical properties (e.g., pH, salinity, bulk density) and nutrient characteristics (total/available nutrients) jointly govern vegetation succession patterns, playing a vital role in the material and energy dynamics of wetland ecosystems [15]. C, nitrogen (N), and phosphorus (P) fundamentally regulate plant growth dynamics through their biogeochemical cycling, with available nutrients serving as sensitive indicators of vegetation–soil interactions. The ecological stoichiometric ratios of soil C:N:P (R_CNP) represent critical biomarkers of ecosystem stability and nutrient equilibrium [16], providing a robust framework for deciphering shrub encroachment impacts on vegetation; soil nutrient cycling. Shrub encroachment significantly alters soil pH, salinity, and C-N cycling processes through mechanisms including modified soil moisture dynamics, organic matter inputs, and root activity [17,18]. Previous studies on coastal shrubs have predominantly focused on organic carbon pools, while overlooking the critical role of soil inorganic carbon (SIC) dynamics in carbonate-rich systems. However, in coastal wetlands dominated by SIC, the dynamic behavior of C:N ratios (R_CN), driven by SIC, provides integrative insights into coupled C-N cycling processes [19,20,21]. Moderate shrub encroachment may enhance vegetation diversity and ecosystem multifunctionality [22,23], whereas excessive proliferation can lead to functional group replacement (e.g., decline of perennial grass) and soil degradation [24]. The encroachment process substantially alters C-N cycling, hydrologic regimes, and ecosystem services by modifying vegetation composition and edaphic conditions [25,26].

Coastal wetlands in the Yellow River Delta, representing the most extensive and intact warm-temperate coastal ecosystem in East Asia, play a vital role in regional C sequestration [27]. Over the past three decades, climate aridification, seawall construction, and petroleum exploitation have driven significant expansion of the native shrub Tamarix chinensis in salt marsh settings [28,29]. Prominent shrub encroachment has been documented in abandoned Yellow River channels (including Shenxiangou and Diaokou courses). Previous regional investigations have primarily documented alterations in soil physicochemical properties, while cross-scale mechanisms governing vegetation–soil interactions remain poorly quantified in this area. Meanwhile, this phenomenon exhibits ecosystem-dependent variability in both progression patterns and ecological impacts [30], significantly modifying regional and global soil environments [31]. It also sheds light on nutrient cycling patterns during vegetation–soil feedback processes in shrub-encroached wetlands, making it a critical research priority.

Here, we aimed to assess the effects of T. chinensis encroachment gradients on vegetation community traits and soil physicochemical characteristics and their potential functions in the abandoned Yellow River course located in the north temperate zone. Through integrated field surveys and laboratory analyses, we aimed to (1) characterize changes in vegetation community structure and soil physicochemical properties across different shrub encroachment stages and (2) quantify stoichiometric constraints on ecosystem material cycling under the influence of T. chinensis shrub encroachment. Our findings provide empirical support for adaptive management of north temperate coastal wetland ecosystems under shrub encroachment.

2. Materials and Methods

2.1. Study Area

The investigation was conducted in the Yellow River Delta National Nature Reserve (118°33′–119°20′ E, 37°35′–38°12′ N), Dongying City, Shandong Province, China (Figure 1). This warm-temperate monsoon climate zone exhibits a mean annual temperature of 12.1 °C, precipitation of 551.6 mm, evaporation of 1962 mm, and a frost-free period of 196 days [32]. Dominant soils include Fluvisols and Solonchaks, with T. chinensis—the native shrub species—thriving due to exceptional salt tolerance, waterlogging resistance, and deep root systems [33].

2.2. Sampling Design and Vegetation Survey

Preliminary field surveys delineated T. chinensis shrub-encroached north temperate coastal wetlands in the abandoned Yellow River course. To investigate vegetation–soil relationships, soil and plant samples were collected in a coastal–inland transect from 23–25 September 2023, according to shrub encroachment succession theory [4,34]. Three different encroachment stages were defined: Isolated Tamarix shrub (ITS), Tamarix shrub island (TSI), and Tamarix woodland (TWL) (Figure 1). Quadrat selection criteria included the following: (1) healthy T. chinensis individuals without pests/diseases or senescence symptoms and (2) minimum 20 m spacing between adjacent quadrats to minimize inter-community interactions, with five 10 × 10 m quadrats per stage.

Vegetation surveys recorded total vegetation cover and quantified community characteristics: species identity, abundance, distribution, and coverage. For T. chinensis, morphological parameters included density, branch number, height, and crown width (E-W and N-S dimensions). Community structure was analyzed using diversity indices [35,36], the Shannon–Wiener diversity index (H′), Pielou evenness index (E) and Simpson diversity index (λ):

$$H^{\prime }=-{\sum }_{i=1}^S{P}_i\ln P_i$$

(1)

$$E=H^{\prime }/\ln S$$

(2)

$$\lambda ={{\sum }_{i=1}^S\left(P_i\right)}^2$$

(3)

where S = observed species number; and P_i = relative abundance of species i.

2.3. Sample Collection and Processing

Following the removal of surface litter, soil samples were collected using a five-point sampling strategy with a stainless-steel slide hammer positioned along quadrat diagonals and central points. At each sampling site, soil profiles were excavated to a 60 cm depth and stratified into five layers (0–5, 5–10, 10–20, 20–40, and 40–60 cm). Homogenized subsamples from identical depths were combined, sealed in polyethylene bags, and processed by removing gravel and roots. Air-dried soils were ground using a mortar, sieved through 80- and 100-mesh screens, and stored for subsequent analyses.

Soil bulk density (BD) was determined via the cutting ring method (100 cm³ core volume). Gravimetric soil water content (SWC) was measured by oven-drying at 105 °C to a constant mass. Soil pH and electrical conductivity (EC) were determined potentiometrically and conductometrically, respectively, using a 1:5 soil-to-water suspension. Soil salt content (SSC) was determined through saturated paste extract evaporation and gravimetric determination. SOC was determined using an analyzer (Elementar vario TOC select, Langenselbold, Germany). Total carbon (TC) and total nitrogen (TN) in soil were measured using an analyzer (Elementar vario MACRO cube, near Frankfurt, Germany). Total phosphorus (TP) in soil was extracted by digestion with a sulfuric–perchloric acid mixture, while available phosphorus (AP) was extracted using NaHCO₃ (pH 8.5) and measured via molybdenum–antimony spectrophotometry. Soil available nitrogen (AN) was extracted with the NaOH–boric acid microdiffusion method [37].

2.4. Data Processing

To quantify the ecological impacts of shrub encroachment on soil nutrient dynamics, we employed the Relative Interaction Index (RII). The index is mathematically defined as

$$\mathrm{R}\mathrm{I}\mathrm{I}=\left({\mathrm{X}}_{\mathrm{C}}-{\mathrm{X}}_{\mathrm{P}}\right)/\left({\mathrm{X}}_{\mathrm{C}}+{\mathrm{X}}_{\mathrm{P}}\right)$$

(4)

where X_C and X_P represent measured parameters at consecutive encroachment stages (X_P: previous stage; X_C: current stage). This dimensionless index spans from −1 to 1, with RII > 0 indicating positive nutrient accumulation effects induced by encroachment progression.

Prior to conducting empirical statistical analyses, the normality of the data was assessed. Data are presented as mean ± standard deviation. Ecological stoichiometric ratios were calculated based on molar concentrations (mmol/kg). Statistical analyses were conducted in SPSS 26, with graphical outputs generated in OriginPro 2021. Differences in vegetation characteristics and soil properties between stages were assessed through one-way ANOVA followed by Duncan’s multiple range test. Vertical soil variations were evaluated using independent Student’s t-tests. Pearson’s correlation analysis was used to explore relationships between vegetation traits and soil parameters across shrub encroachment stages. Redundancy analysis (RDA) implemented in Canoco was used to quantify vegetation–soil interactions.

3. Results

3.1. Vegetation Community Characteristics Across Shrub Encroachment Stages

Total vegetation cover in north temperate coastal wetlands progressively increased with the intensification of T. chinensis shrub encroachment, exhibiting significant differences across stages (ITS: 30.75% → TSI: 80.00% → TWL: 100.00%, p < 0.05,

Table 1. Vegetation community characteristics across T. chinensis shrub encroachment stages. Different lowercase superscript letters indicate significant differences among stages (p < 0.05). Abbreviations: ITS—Isolated Tamarix shrub; TSI—Tamarix shrub island; TWL—Tamarix woodland.

Parameters	Shrub Encroachment Stage
	ITS	TSI	TWL
Total vegetation cover (%)	30.75 ± 24.25 b	80.00 ± 10.00 a	100.00 ± 0.00 a
Herbaceous cover (%)	26.25 ± 23.75 a	61.00 ± 21.00 a	59.00 ± 13.00 a
Shrub cover (%)	7.00 ± 3.00 c	30.00 ± 0.00 b	57.50 ± 7.50 a
Shrub density (clumps/100 m2)	4.50 ± 0.50 b	7.00 ± 0.00 ab	9.50 ± 2.50 a
Shrub height (cm)	121.50 ± 7.50 b	165.29 ± 47.65 a	271.83 ± 33.02 a
Shrub crown width (cm)	125.00 ± 11.81 c	202.14 ± 72.86 b	291.29 ± 82.60 a
Branches per shrub (branches/clump)	1.00 ± 0.00 c	2.00 ± 1.03 b	2.29 ± 0.82 a
Species richness	2.50 ± 0.71 a	4.50 ± 0.71 a	7.50 ± 2.12 a
Shannon-Wiener (H′)	0.55 ± 0.47 a	0.86 ± 0.32 a	1.28 ± 0.16 a
Simpson (λ)	0.56 ± 0.34 a	0.57 ± 0.16 a	0.64 ± 0.01 a
Pielou evenness (E)	0.08 ± 0.11 a	0.44 ± 0.42 a	0.52 ± 0.23 a

). Herbaceous cover remained lowest at the ITS stage (26.25%, p > 0.05), with no significant variation between stages. All shrub architectural parameters—coverage, density, height, crown width, and branches per shrub—demonstrated increases dependent on shrub encroachment. Significant enhancements were observed in shrub cover (ITS: 7.00% → TSI: 30.00% → TWL: 57.50%, p < 0.05), crown width (ITS: 125.00 cm → TSI: 202.14 cm → TWL: 291.29 cm, p < 0.05), and branch number (ITS: 1.00 branches/clump → TSI: 2.00 branches/clump → TWL: 2.29 branches/clump, p < 0.05) across the progression stages. Specifically, shrub height increased by 36.04% and 123.73% at the TSI and the TWL stage, respectively, while crown width expanded 61.71% and 133.03% compared to the ITS baseline.

Herbaceous species richness increased with the progression of shrub encroachment. At the initial ITS stage, the communities were dominated by T. chinensis, Suaeda heteroptera, and Phragmites australis. Transitioning to the TSI stage, T. chinensis became the dominant species, with secondary coverage by S. heteroptera, accompanied by sporadic occurrences of Limonium sinense, Cynodon dactylon, and P. australis. At the TWL stage, maximal coverage of T. chinensis and P. australis was observed alongside a significant increase in companion species including Cirsium setosum, Sonchus brachyotus, Lactuca serriola, Artemisia annua, Euphorbia hypericifolia, Setaria viridis, and Acorus calamus (listed in descending order of average coverage). The shrub encroachment process led to gradual increases in Shannon–Wiener diversity index (H′) and Pielou evenness index (E) scores, while the Simpson diversity index (λ) exhibited non-significant fluctuations across the shrub encroachment stages.

3.2. Analysis of Soil Physicochemical Properties Across Shrub Encroachment Stages

As shown in Figure 2, both EC and SSC exhibited significant decreases (p < 0.05) as T. chinensis shrub encroachment progressed within identical soil depths. Specifically, EC declined from 4.04 mS/cm at the ITS 0–5 cm depth to 1.43 mS/cm at the TWL 0–5 cm depth, while SSC decreased from 3.05 g/kg at the ITS 0–5 cm depth to 0.65 g/kg at the TWL 0–5 cm depth. Distinct alphabetical groupings indicate statistical significance between stages. Soil pH displayed dynamic fluctuations, ranging from 7.06 (TSI 0–5 cm depth) to 8.09 (TWL 5–10 cm depth). The ITS soils maintained neutral–alkaline conditions (7.24–7.79), whereas TWL soils exhibited stronger alkalinity (7.69–8.09), particularly in the subsurface layers. BD showed marked reductions at the TWL stage, decreasing from 1.42 g/cm³ (ITS 0–5 cm depth) to 0.94 g/cm³ (TWL 0–5 cm depth), with significant inter-stage variations (p < 0.05). In contrast, the TSI and TWL stages exhibited significantly lower SWC compared to the ITS stage (p < 0.05), in which SWC exhibited a progressive increase with soil depth.

3.3. Soil Nutrient Contents and Stoichiometric Characteristics

TC content generally decreased with depth, except in the surface layer (0–5 cm), where shrub encroachment led to significant increases (Figure 3). At the TSI stage, the subsoil (5–10 cm, 23.04 g/kg; 10–20 cm, 22.43 g/kg; 20–40 cm, 18.98 g/kg; 40–60 cm, 17.41 g/kg) showed the maximum TC values. TN content exhibited a decreasing vertical trend across the encroachment stages. Furthermore, TN content in each soil layer followed the hierarchical order: TWL > TSI > ITS. The TWL stage consistently showed the highest TN levels across all depths: 0–5 cm (1408.51 mg/kg), 5–10 cm (686.09 mg/kg), 10–20 cm (604.91 mg/kg), 20–40 cm (538.61 mg/kg), and 40–60 cm (444.30 mg/kg). TP content remained stable in the upper 40 cm of soil but decreased significantly in the deeper soils (40–60 cm) as shrub encroachment progressed: ITS (690.21 mg/kg) → TSI (576.52 mg/kg) → TWL (514.84 mg/kg). These trends suggest that shrub encroachment influences both the vertical distribution and concentration of key soil nutrients, with notable increases in TC and TN at surface levels and significant reductions in TP at deeper soil depths.

SOC content exhibited a general decreasing trend with depth across shrub encroachment stages (Figure 3). The TWL stage showed significantly higher (p < 0.05) SOC content (9.72 g/kg) in the 0–5 cm layer compared to both the ITS (4.19 g/kg) and TSI (4.18 g/kg) stage. AN content peaked in the surface soils (0–5 cm) across all shrub encroachment stages, with maximum values of 36.64 mg/kg at the ITS stage, 31.66 mg/kg at the TSI stage, and 81.55 mg/kg at the TWL stage. Vertical AN distribution across shrub encroachment stages consistently followed the order TWL > ITS > TSI. AP content displayed maximum values (52.22 mg/kg) in the surface layer (0–5 cm) of the TSI stage, while deeper layers showed a progressive reduction in AP with increasing shrub encroachment intensity (p < 0.05). No consistent vertical AP content distribution pattern emerged across the shrub encroachment stages. These findings highlight how the intensity of shrub encroachment can influence the vertical distribution and availability of key soil nutrients, particularly SOC, AN, and AP.

Soil total and available nutrient contents exhibited stage-dependent responses to shrub encroachment across depth profiles, with the RII displaying divergent directional shifts during the ITS → TSI and TSI → TWL transitions (Figure 4). TC RII values consistently demonstrated positive effects throughout all soil layers at the ITS → TSI progression but transitioned to predominantly negative effects during the TSI → TWL progression. TN RII values maintained persistent positive effects across all succession phases. TP RII values exhibited contrasting vertical patterns. Positive effects dominated in the surface layer (0–5 cm), while negative effects prevailed in deep soil horizons (40–60 cm), resulting in integrated negative effects across the full profile. SOC and AN RII values shared congruent trends, showing negative effects during the ITS → TSI progression but reversing to positive effects at the TSI → TWL progression. Conversely, AP RII values universally manifested negative effects throughout all encroachment phases.

As illustrated in Figure 5, soil stoichiometric ratios in the abandoned Yellow River course wetlands exhibited distinct vertical distributions during T. chinensis shrub encroachment. The TC:TN ratio (R_CN) ranged from 23.04 to 92.54 across different depths and encroachment stages. The highest values were observed at 40–60 cm during the ITS stage (92.54), contrasting with reduced values in the surface layer, where the TWL stage recorded a minimum of 23.04. The TC:TP ratio (R_CP) showed minimal variation, ranging between 52.14 and 92.88. The lowest value occurred in 40–60 cm depth during the ITS stage, with vertical stability observed across different encroachment stages. In contrast, the TN:TP ratio (R_NP) consistently decreased with depth. However, the surface layer (0–5 cm) showed intensification under shrub encroachment, with the following values: ITS (2.38) → TSI (3.28) → TWL (4.29). These results highlight the dynamic changes in soil stoichiometry due to shrub encroachment, with distinct shifts in nutrient relationships at various soil depths.

3.4. Correlation Analysis Between Vegetation Characteristics and Soil Properties

Significant negative correlations were observed between T. chinensis shrub architectural parameters (height, crown width, branch number, and cover) and soil properties such as BD, EC, SSC, and AP (p < 0.01); conversely, positive correlations with pH were observed (p < 0.01) (Figure 6). Herbaceous cover showed a negative correlation with TC (p < 0.01), but positive associations with SOC and pH were found (p < 0.05).

RDA revealed that soil properties explained 68.68% of the variation in vegetation characteristics (Axis 1: 60.87%; Axis 2: 7.81%) (Figure 7 and

Table A1. Significance order and significance level test of soil properties and soil stoichiometric characteristic factors. BD, soil bulk density; SWC, soil water content; TC, total carbon; AP, available phosphorus; SOC, total organic carbon; TN, total nitrogen; SSC, soil salt content; R_CN, TC:TN ratios; R_NP, TN:TP ratios; TP, total phosphorus; R_CP, TC:TP ratios; EC, soil electrical conductivity.

Parameters	Explains %	Contribution %	Pseudo-F	p
BD	41.70	58.40	75.70	0.002
SWC	8.40	11.70	17.60	0.002
TC	5.60	7.90	13.20	0.002
AP	4.20	5.90	10.70	0.002
SOC	3.30	4.60	9.10	0.002
TN	2.80	4.00	8.40	0.004
SSC	1.80	2.50	5.60	–
pH	1.60	2.20	5.10	0.010
RCN	0.60	0.90	2.00	0.130
AN	0.60	0.80	1.80	0.180
RNP	0.50	0.60	1.50	0.218
TP	0.10	0.20	0.40	0.718
RCP	0.20	0.30	0.70	0.518
EC	<0.1	<0.1	<0.1	0.984

). BD showed the strongest explanatory power (41.7%, p = 0.002), followed by SWC (8.4%) and TC (5.6%). The acute angles between vegetation vectors indicated strong positive inter-parameter correlations, particularly between shrub height and cover.

4. Discussion

4.1. Impacts of Shrub Encroachment on Vegetation Community Characteristics

In north temperate coastal wetland ecosystems, herbaceous coverage and species diversity exhibit stage-dependent variations during T. chinensis shrub encroachment. Maximum herbaceous coverage occurs at the TSI stage, which is attributable to competitive exclusion between expanding shrubs and herbaceous plants [3]. Our results demonstrate a progressive increase in total vegetation cover and shrub morphological parameters as the T. chinensis encroachment stage intensifies. This process induces microclimate modification, including temperature regulation and extreme event mitigation [38,39], thereby enhancing soil suitability and facilitating vegetative development. Concurrently, climate warming interacts with these microclimatic changes, accelerating the colonization of T. chinensis [40]. It creates a positive feedback loop (encroachment expansion → microhabitat optimization → enhanced plant adaptability), driving directional succession toward shrub-dominated states in coastal wetland ecosystems.

Contrary to reported decreases in species richness during grassland encroachment in Eurasian steppes and alpine meadows [23,41,42], coastal wetland systems in the abandoned Yellow River course demonstrated progressive herbaceous diversification. Initial ITS communities dominated by T. chinensis, S. heteroptera, and P. australis transitioned to the TWL stage, with an increase in annual/biennial species (e.g., C. setosum, S. brachyotus, A. annua, L. serriola, A. annua, S. viridis and E. hypericifolia). These species most likely exploit the microhabitat changes induced by T. chinensis encroachment, accumulating biomass rapidly and reflecting enhanced niche complementarity and functional diversity that stabilize community structure [43,44].

4.2. Alterations in Soil Physicochemical Properties

Relationships between vegetation communities and environmental factors are complex, as changes in soil moisture, salinity, and nutrient availability often occur simultaneously [45]. During shrub encroachment, previous studies have confirmed key soil physicochemical properties, such as BD, SWC, nutrient content, and pH, which are closely related to vegetation biomass, coverage, height, and diversity indices [23,46,47]. RDA indicated that BD, SWC, TC, and AP were critical factors influencing vegetation community characteristics (p = 0.002), indicating a strong connection between shrub encroachment, soil water retention, and nutrient dynamics. As plant roots expand and organic matter accumulates during shrub encroachment, soil BD decreases, with soil porosity and permeability increasing. The reduction in BD further facilitates shrub growth and water retention.

Eldridge et al. [48] analyzed 244 global shrub encroachment cases and found a decline in soil pH post encroachment, suggesting soil acidification. In contrast, our study observed soil pH values ranging from 7.06 to 8.09 (weakly alkaline), which deviates from these global trends. This discrepancy may be attributed to the combined effects of waterlogging and salt stress in the north temperate coastal wetlands or the “salt island” effect of T. chinensis. Notably, in 0–20 cm soil layers, pH was significantly higher in the TWL stage than in the ITS or TSI stages (p < 0.05). This phenomenon could stem from bicarbonate (HCO_3-) production via CO₂ assimilation by T. chinensis salt glands, which elevates the surface soil pH [49], also observed by Han et al. [17] in the Yellow River Delta. Our results demonstrate that T. chinensis shrub encroachment significantly enhances spatial heterogeneity of topsoil pH, with diminished effects at greater depths. The extent of this impact depends largely on T. chinensis shrub encroachment stages. Additionally, EC and SSC decreased markedly across all soil depths with advancing encroachment (p < 0.05). These trends may correlate with proximity to the sea at sampling sites or a weakened “salt island” effect as shrub encroachment progresses.

4.3. Stoichiometric Shifts in Soil C-N-P Cycling

Soil total and available nutrient contents exhibited distinct variations across shrub encroachment stages. The maximum values of SOC, TN, and AN occurred at the TWL stage, and TN and AN reached their maximum accumulation effect during the TSI → TWL progression, likely due to peak shrub density, canopy coverage, and litter input. The high C and N reserves found in plant litter and roots suggest that the dynamics of SOC and TN during encroachment are driven by litter decomposition, root turnover, and rhizodeposition [50]. These findings align with global shrub encroachment patterns reported by Du et al. [51]. While TP in 0–40 cm soil layers did not show a clear trend, TP in deeper soils (40–60 cm) decreased with shrub encroachment, and the negative effect on nutrient accumulation also reached its maximum. The roots of T. chinensis exhibited horizontal expansion throughout all developmental stages [52], and deeper root proliferation during advanced encroachment increased nutrient foraging volume [53]. Herbaceous plants primarily acquire nutrients from surface soils, whereas shrubs may extract P from deeper layers [54], altering subsoil P availability. Gao et al. [18] observed that shrubs in Tibetan meadows enhance P uplift from subsoil to upper profiles, accelerating P cycling and exacerbating P depletion in alpine soils. However, the mechanisms underlying the reduction of TP in north temperate coastal wetlands likely differ due to habitat-specific factors (e.g., salinity, species traits), warranting further investigation. Our results suggest that T. chinensis shrub encroachment in coastal wetlands triggers divergent response mechanisms for total and available nutrients.

This study demonstrated that shrub encroachment increased TC and TN contents, while reduced R_CN may be linked to interactions between SIC and SOC in saline soils. Organic acids secreted by T. chinensis roots promote the dissolution of SIC (CaCO₃ + 2H⁺ → Ca²⁺ + CO₂↑ + H₂O), releasing Ca²⁺ that stabilizes organic matter via ionic bridging, thereby enhancing N sequestration efficiency [26,55]. This mechanism drives concurrent TN accumulation and R_CN decline. This suggests that TC:TN (R_CN) dynamics holistically reflect indirect regulation of C-N cycling mediated by SIC, whereas SOC:TN ratios may underestimate the contribution of SIC. Notably, the increase in TN and decrease in R_CN across soil depths suggest that shrub encroachment in coastal wetlands represents ecological progression rather than degradation, in contrast to grassland ecosystems. Soil R_NP < 14 in the study area indicates N limitation during shrub encroachment, consistent with regional nutrient assessments [56]. The rise in AN and decline in R_CN with shrub encroachment implies a partial alleviation of N limitation. Significant increases in R_CP and R_NP and the decline in R_CN in the 40–60 cm layer (p < 0.05) reflect the reduction in P availability in deeper soils. While Du et al. [51] found stronger stoichiometric shifts in the surface soils post encroachment, our R_NP trends across depths align with global patterns. Overall, this study confirms that shrub encroachment intensifies nutrient limitation through modifications to soil R_CNP stoichiometric ratios, thereby compromising wetland ecosystem stability.

Prolonged human activities (e.g., land reclamation, agriculture) may replace zonal vegetation with subclimax communities, complicating the progression of encroachment succession [57]. Qu et al. [56] found higher TC and lower R_CN in reclaimed P. australis wetlands compared to new-born P. australis coastal wetlands, highlighting the anthropogenic impacts on soil nutrients in the Yellow River Delta. Meng et al. [58] further demonstrated that a longer cultivation history in older estuarine regions elevated TN (0–5 cm) and reduced R_CN (0–10 cm), attributable to consistent tillage and fertilization practices in the estuarine region. These findings emphasize the necessity of integrating anthropogenic factors (e.g., land-use history, management practices) into future studies on shrub encroachment in both new-born and mature estuarine zones. In the context of global coastal wetland degradation, shrub encroachment may paradoxically serve as a potential restoration tool by enhancing structural complexity and C sequestration capacity, with woody plants demonstrating promise for rehabilitating degraded coastal ecosystems.

5. Conclusions

T. chinensis shrub encroachment in coastal wetlands of the abandoned Yellow River course drives significant vegetation community restructuring, enhancing structural complexity through niche complementarity and improved resource-use efficiency. EC and SSC exhibited progressive declines with encroachment intensity, while BD showed marked reductions in the TWL stage. The encroachment significantly elevated surface soil SOC and AN, though AP declined substantially. TN accumulation paralleled the notable TP reduction in deeper soil layers. BD, SWC, TC, and AP, as critical edaphic drivers of vegetation community dynamics, demonstrated tight coupling between plant community assembly and soil hydrologic-nutrient regimes during encroachment. Ecological stoichiometric ratios revealed stoichiometric constraints on ecosystem material cycling across soil profiles: the coastal wetland soils exhibited persistent N limitation, with shrub encroachment exacerbating P scarcity in deep soil horizons through reduced P availability. In short, shrub encroachment occurring in north temperate coastal wetlands will trigger profound vegetation–soil feedback, driving stoichiometric constraints in nutrient cycling and highlighting the urgency for adaptive management strategies.