Microplastics Alter Growth and Reproduction Strategy of Scirpus mariqueter by Modifying Soil Nutrient Availability

Abstract

Microplastic pollution threatens coastal wetland ecosystems, yet its impacts on the dominant plant species and soil properties remain poorly understood. We investigated the effects of four microplastic types (PP, PE, PS, PET) at three concentrations (0.1%, 0.5%, 1% w/w) on Scirpus mariqueter, a keystone species in the coastal wetlands of China, and the associated soil physicochemical properties. In a controlled pot experiment, microplastics significantly altered the plant biomass, vegetative traits, and reproductive strategies, with type-specific and concentration-dependent responses. PET and PE strongly suppressed the belowground and total biomass (p < 0.05), with reductions in the belowground biomass of 42.87% and 44.13%, respectively, at a 0.1% concentration. PP promoted seed production, particularly increasing the seed number by 25.23% at a 0.1% concentration (p < 0.05). The soil NH₄⁺-N, moisture, and EC were key mediators, with NH₄⁺-N declines linked to biomass reductions via nitrogen limitation. The Spearman correlations confirmed strong associations between the plant traits and soil properties, particularly nitrogen forms. These findings reveal that microplastics disrupt wetland plant performance and soil environments, potentially impairing carbon sequestration and ecosystem stability. Our study underscores the urgent need for microplastic risk assessments in coastal wetlands and highlights soil–microbe–plant interactions as critical mechanisms for future investigation.

1. Introduction

Microplastics, characterized by their small particle size, high mobility [1], and bioavailability [2], have emerged as a major environmental threat to global ecosystems [3,4]. These particles can accumulate in soils, aquatic environments, and biota through various physical, chemical, and biological processes, thereby exerting profound effects on ecosystem functioning and biodiversity [5,6]. In terrestrial systems, microplastics not only alter soil physicochemical properties but also modulate microbial communities and disrupt nutrient cycling, which can indirectly affect plant growth [7,8].

Coastal wetlands, which serve as crucial interfaces between terrestrial and marine ecosystems, play irreplaceable roles in global carbon cycling [9,10], biodiversity maintenance, and shoreline protection [11,12,13]. However, these ecosystems are increasingly exposed to land-based microplastic inputs, raising concerns about escalating pollution risks [14]. Previous studies have shown that microplastics can alter the soil structure, reduce water retention by up to 30% in some soils, and significantly suppress microbial biomass carbon by 20–45%, thereby indirectly influencing plant–soil interactions [15,16]. For instance, de Souza Machado et al. (2019) [15] reported that polyethylene microplastics decreased soil bulk density and water holding capacity, while Zhao et al. (2021) found that microplastics significantly reduced microbial respiration in soils [17]. In wetland systems, the limited experimental evidence suggests that microplastics may also inhibit root elongation by 15–25% and reduce nitrogen uptake efficiency in plants such as Scirpus planiculumis and Phragmites australis [18]. However, most research has focused on simplified settings, with limited attention to the combined effects of microplastic type, concentration, and soil conditions. While previous studies have explored the distribution of microplastics in wetland sediments and their preliminary effects on soil properties [19,20], little is known about how dominant coastal wetland plants—particularly foundation species such as S. mariqueter—respond to microplastic stress at both the physiological and ecological levels.

Most existing research has focused on the impacts of microplastics on terrestrial crops or freshwater macrophytes, such as reduced yields and impaired root development [21,22,23]. In contrast, the synergistic effects of microplastic pollution on plant growth, reproductive strategies, and soil environment dynamics in salt marsh ecosystems remain poorly understood. Moreover, microplastics of different polymer types (e.g., polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyethylene terephthalate (PET)) vary in particle size, density, and surface properties, potentially leading to differential ecological impacts on plant physiology [24,25]. The microplastic types used in this study are among the most commonly produced and discarded polymers globally. They are frequently detected in estuarine and coastal environments due to their extensive use in packaging, textiles, and consumer goods, as well as their persistence and distinct density characteristics that influence their transport and sedimentation dynamics [26,27]. Yet, comparative studies examining these effects are scarce. In particular, the mechanisms through which microplastics influence plant performance through modifications of key soil attributes—such as nitrogen availability, water content, and electrical conductivity—have not been adequately elucidated.

To address these gaps, we conducted a controlled experiment using S. mariqueter, a dominant clonal species in coastal wetlands, as a model organism [28]. S. mariqueter was chosen because it was the dominant native plant species at the sampling site in Hangzhou Bay, forming large natural populations across the area. It served as a foundation species in the local salt marsh ecosystem and primarily competed with the invasive species Spartina alterniflora. We applied four types of microplastics (PP, PE, PS, PET) at varying concentrations (0%, 0.1%, 0.5%, and 1%) to evaluate their effects on plant biomass accumulation, vegetative growth, reproductive traits, and reproductive allocation. We also examined the potential mechanisms through which microplastics modulate plant performance via alterations of soil physicochemical properties. We tested three hypotheses: (1) microplastics of different types and concentrations significantly influence the growth and reproductive performance of S. mariqueter exhibiting both polymer-specific and dose-dependent effects; (2) microplastics indirectly regulate plant traits by altering soil water content and nitrogen availability; and (3) under microplastic stress, S. mariqueter adjusts its reproductive allocation strategy to better cope with environmental changes. This study aims to improve our understanding of the ecological effects of microplastic pollution and uncover plant response mechanisms in coastal wetlands, thereby informing microplastic risk assessment as well as supporting evidence-based conservation and management strategies for wetland ecosystems.

2. Materials and Methods

2.1. Soil Collection

Soil was collected from a typical coastal wetland in Hangzhou Bay, China (30°17′–30°19′ N, 121°05′–121°07′ E). The site was selected due to its representative soil characteristics and dominance of S. mariqueter vegetation. It was a relatively undisturbed natural area with stable hydrology and salinity levels, making it suitable for controlled experiments. In March 2024, surface soil (0–20 cm) was sampled from an area free of industrial or domestic pollution (confirmed by field surveys and historical records, with background microplastic concentration <0.01% w/w) [29]. Plant debris was removed, and the soil was thoroughly mixed. Samples were stored at 4 °C until subsequent experiments. The background concentration of the microplastics in the collected soil (<0.01% w/w) was determined using density separation with saturated NaCl solution, followed by stereomicroscopic examination at the Wetland Ecosystem Research Station laboratory. This approach followed the internal protocols established for the long-term monitoring at the site and was used here to confirm that control soils represented near-natural background conditions.

2.2. Microplastic Preparation

Four types of virgin microplastics were used: polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), and polystyrene (PS), purchased from Mingyuxing Plastic Materials Co., Ltd., Dongguan, China (purity > 99%, free of plasticizers or colorants). The particle size of the microplastics ranged from 53 to 150 μm. Prior to use, the microplastics were cleaned with 0.1 mol·L⁻¹ hydrochloric acid (HCl) to remove potential surface-bound metals. As these microplastics were virgin-grade and free from plasticizers, stabilizers, and dyes, no additional treatment for organic contaminants was deemed necessary.

2.3. Experimental Design

A pot experiment was conducted from March to November 2024 in a greenhouse at wetland ecosystem research station of Hangzhou Bay (temperature 22–28 °C, light intensity 400–600 μmol·m⁻²·s⁻¹, 14 h light/10 h dark). The experiment comprised four microplastic types (PET, PP, PE, PS) at three concentrations (0.1%, 0.5%, 1% w/w), plus a control (0% microplastics, with deionized water added to simulate mixing), totaling 13 treatments, each with three replicates (Figure 1). The selected concentrations (0.1%, 0.5%, and 1% w/w) were chosen to reflect a gradient from low to high microplastic contamination, corresponding to the levels reported in impacted coastal environments and frequently used in experimental studies to simulate realistic exposure scenarios [30]. Microplastics were weighed according to the specified concentration in each treatment and thoroughly mixed with soil. The mixtures were then placed into sterilized plastic pots (55 cm × 45 cm × 35 cm), each containing 15 kg of soil. In each pot, 20 uniform seedlings of S. mariqueter were planted. Throughout the experiment, soil salinity was maintained between 3% and 5%, and no additional nutrients were supplied. Irrigation was applied regularly to keep the soil at saturation. To minimize edge effects, the pots were randomly rearranged and rotated 90° clockwise every week. No fertilizers or pesticides were applied during the experimental period to avoid external interference with soil and plant responses.

2.4. Soil and Plant Measurements

In November 2024, soil samples were harvested. For each replicate pot (biological replicate), five soil cores (0–20 cm, ~200 g each) were collected and composited into a single sample to represent that pot. Plant height was measured from the soil surface to the stem apex using a ruler (precision 0.1 cm) and basal diameter with a vernier caliper (0.01 mm), and ramets, seed, and corm numbers were counted manually. Leaf area was determined using a portable leaf area meter (LI-3000C, LI-COR Biosciences, Lincoln, NE, USA) [31]. Plant samples were oven-dried at 70 °C to constant weight to measure above- and belowground biomass and corm dry weight.

Soil pH and electrical conductivity (EC) were measured in a 1:2.5 (w/v) soil:deionized water suspension using a pH meter (FiveEasy Plus, Mettler Toledo, Greifensee, Switzerland) and a conductivity meter (DDSJ-308A, Shanghai Yidian, Shanghai, China) [32]. Soil moisture was determined by oven-drying at 105 °C for 24 h to constant weight [33]. Soil nitrate nitrogen (NO₃⁻-N) and ammonium nitrogen (NH₄⁺-N) were extracted with 2 mol·L⁻¹ KCl (soil:solution 1:10 w/v), shaken for 30 min, filtered, and analyzed using a continuous flow analyzer (AutoAnalyzer 3, SEAL Analytical, Norderstedt, Germany; Mulvaney, 1996) [34]. Available phosphorus (AP) was extracted with 0.5 mol·L⁻¹ NaHCO₃ and measured at 880 nm using a UV–Vis spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) [35]. Soil total carbon (TC) was measured using an elemental analyzer (Vario EL III, Elementar, Langenselbold, Germany) [36].

2.5. Statistical Analysis

Data were tested for normality using the Shapiro–Wilk test and, for homogeneity of variances, using Levene’s test, with non-normal data log-transformed. Two-way ANOVA was performed in R software (v4.4.3; R Core Team, Vienna, Austria) to evaluate the effects of microplastic type and concentration on plant and soil parameters (p < 0.05). Post hoc comparisons were conducted using Tukey’s HSD test. Pearson’s correlation analysis was used to examine relationships between plant height, biomass, and soil factors (pH, EC, NO₃⁻-N, etc.; p < 0.05). Figures were generated using Origin 2024 (OriginLab Corp., Northampton, MA, USA).

3. Results

3.1. Effects of Microplastics on Biomass and Vegetative Traits of S. mariqueter

The microplastic type and concentration affected the belowground, aboveground, and total biomass of S. mariqueter (Figure 2). For belowground biomass (Figure 2a), at 0.1% concentration, the PET, PP, and PE treatments significantly reduced the belowground biomass (p < 0.05), with reductions of 42.87% and 44.13% for PET and PE, respectively, compared to CK; PS slightly increased the belowground biomass, but the effect was not significant (p > 0.05). At 0.5% and 1% concentrations, all microplastic treatments significantly decreased the belowground biomass (p < 0.05), with the lowest values observed for PP and PE at 0.5% and for PET at 1%. The aboveground biomass (Figure 2b) was reduced by all microplastic treatments across concentrations. The magnitude of the reduction varied: PET showed a decreasing inhibitory effect with increasing concentration, whereas PP, PE, and PS exhibited a pattern of an initial decline followed by a partial recovery. The total biomass (Figure 2c) was significantly reduced by all treatments except PS at 0.1% (p < 0.05). At 0.5%, the PS and PET treatments resulted in significantly higher total biomass than PE and PP (p < 0.05). At 1%, PET caused the greatest reduction in the total biomass (31.67%, p < 0.05).

For the vegetative traits, PET significantly suppressed plant height only at 0.1% (p < 0.05), while PE at 0.5% promoted height (Figure 2d, p < 0.05). The leaf area (Figure 2e) increased significantly with PP at 0.1% and PE at 0.5% (p < 0.05), but the other treatments had no significant effect (p > 0.05). The basal diameter (Figure 2f) showed varied responses: at 0.1%, PET and PS significantly increased basal diameter (p < 0.05), with PS having the strongest effect; at 0.5% and 1%, PET consistently promoted basal diameter growth (p < 0.05).

3.2. Effects of Microplastics on Reproductive Traits and Allocation of S. mariqueter

Microplastic addition significantly influenced the asexual and sexual reproduction of S. mariqueter (Figure 3a–e). For asexual reproduction, all microplastic types significantly reduced ramet numbers across concentrations (p < 0.05, Figure 3a). At 0.1%, the ramet number followed the order CK > PET > PS > PP > PE, with significant differences between PET and PE (p < 0.05). The corm number (Figure 3b) increased under PET at 0.1% but was suppressed by PP, PE, and PS, with PE showing the strongest inhibition (p < 0.05). The corm biomass (Figure 3c) mirrored the trend in the corm number. For sexual reproduction, the seed number (Figure 3d) responded differently to microplastic types. At 0.1%, PP significantly increased the seed number (25.23%, p < 0.05), whereas PE and PS reduced it (>50% decline, p < 0.05). At 0.5% and 1%, PP continued to enhance the seed number, while the differences among other treatments were not significant (p > 0.05). The seed biomass (Figure 3e) was significantly reduced across all treatments (p < 0.05) but remained highest in PP treatments. The reproductive allocation (Figure 3f–h) showed that at 0.1% all microplastic treatments reduced the proportion of seed biomass (Figure 3f); at 0.5% and 1%, PP had the highest seed biomass proportion. The corm biomass proportion (Figure 3g) increased significantly in the PET and PE treatments at 0.5% (p < 0.05), but the differences diminished at 1%. The seed-to-corm biomass ratio (Figure 3h) decreased in the PET, PE, and PS treatments but increased in the PP treatments at higher concentrations.

3.3. Effects of Microplastics on Soil Physicochemical Properties

Microplastic addition had complex effects on the physicochemical properties of the coastal wetland soil (Figure 4). For nitrogen, the NH₄⁺-N content significantly decreased under PET treatments (p < 0.05), with PS showing a concentration-dependent decline, most pronounced at 1% (Figure 4a). Conversely, PE promoted NH₄⁺-N accumulation at higher concentrations. The NO₃⁻-N content (Figure 4b) was highest under low-concentration PS, with a significant increase under PE at 0.5% (p < 0.05). The soil moisture (Figure 4c) varied with concentration, peaking under PE at 0.5%, but showed no consistent concentration-dependent trend (p > 0.05). The soil pH (Figure 4d) exhibited a slight upward trend but was not significantly affected (p > 0.05). The EC and salinity (Figure 4e,f) increased under low-concentration PS but decreased under high-concentration PP. AP (Figure 4g) showed no significant changes (p > 0.05), though some treatments displayed fluctuating patterns (e.g., low-high-low or high-low-high). The TC (Figure 4h) varied minimally, with PP showing a slight increase at certain concentrations.

3.4. Correlation Between Soil Physicochemical Properties and S. mariqueter Traits

Spearman correlation analysis (Figure 5) revealed significant relationships between soil properties and S. mariqueter traits. The belowground biomass positively correlated with the soil moisture (p < 0.01) and salinity (p < 0.05), and the total biomass positively correlated with soil moisture (p < 0.05). Among the vegetative traits, the plant height strongly positively correlated with the NH₄⁺-N content (p < 0.01), and the leaf area was strongly positively correlated with the NO₃⁻-N content (p < 0.01). For the reproductive traits, the seed number and seed biomass positively correlated with the NH₄⁺-N content and negatively correlated with the NO₃⁻-N content (p < 0.05); the seed number also positively correlated with the TC content (p < 0.05). A=the asexual reproductive traits showed no significant correlations with the soil properties (p > 0.05).

4. Discussion

4.1. Microplastics Affect S. mariqueter Biomass and Vegetative Growth

Our findings reveal that microplastic type and concentration significantly influence the biomass and vegetative traits of Scirpus mariqueter, with PET and PE inducing the strongest suppression and PP showing milder effects (Figure 2 and Figure 3). These negative impacts likely stem from both physical root impediments and altered soil nutrient dynamics, particularly reductions in ammonium nitrogen. Such effects are consistent with terrestrial studies demonstrating microplastic-induced soil compaction, reduced aeration, and disrupted microbial function [37,38]. In salt marsh ecosystems, microplastics may alter soil porosity and water retention [39], reduce NH₄⁺-N availability [7] and limit root development [40]. For instance, PET’s high density and rigidity may exacerbate soil compaction, hindering root penetration [41,42], while PE’s flexible structure could disrupt the fine root architecture [43]. Additionally, changes in plant height, leaf area, and basal diameter suggest that microplastics interfere with morphogenetic processes, such as cell elongation and vascular tissue formation, potentially via altered phytohormone signaling [44]. Beyond these physical and physiological effects, microplastics may also indirectly suppress plant growth by altering soil microbial communities and disrupting nutrient cycling. By altering microbial communities and nutrient cycling, microplastics may influence plant nutrient uptake and overall health [45]. Microplastic contamination has been shown to suppress the beneficial microbial taxa involved in nitrogen fixation and mineralization [30,46], further reducing nutrient availability. Given S. mariqueter’s reliance on sediment–root nutrient exchange, such microbial disruptions likely contributed to the observed growth suppression. This pattern, however, is not universally observed across wetland plants. For example, Lemna minor exhibited reduced root length but unaffected shoot traits under PE microbead exposure [47], suggesting localized belowground effects. Phragmites australis and Myriophyllum spicatum, on the other hand, showed no significant changes in biomass or photosynthetic traits under PE or PS treatment in mesocosm settings [48]. These species likely benefit from deeper or more robust root systems or lower dependence on close root–sediment contact. These findings underscore the need to move beyond single-species assessments and consider how functional traits—such as root morphology, sediment dependence, and clonal architecture—influence plant responses to microplastic contamination. As a foundational coastal species, S. mariqueter provides a valuable indicator for assessing broader wetland vulnerability to microplastic pollution. Future studies should adopt comparative, trait-based approaches across wetland taxa to better predict ecosystem-level impacts.

4.2. Microplastics Modulate Reproductive Strategies of S. mariqueter

Microplastic exposure significantly altered the reproductive traits and allocation strategies of S. mariqueter. Treatments with PET, PE, and PS suppressed both sexual (seed) and asexual (corm) reproduction, while PP exposure increased seed number and sexual allocation at specific concentrations (Figure 3b–e). Treatments with PET and PE led to increased investment in corms and reduced seed production (Figure 3f–h), suggesting a shift toward clonal propagation—favoring individual persistence over population expansion under stress [49,50,51]. This shift mirrors the typical reproductive responses of wetland plants to salinity or heavy metal stress. The contrasting promotion of sexual reproduction under PP exposure highlights a type-specific regulatory effect, likely influenced by microplastic properties such as particle size and surface chemistry [52].

Similar reproductive shifts under microplastic exposure have been reported for other species. Lozano et al. found that film- and fiber-shaped microplastics delayed seed germination in Daucus carota while increasing synchrony, implying that particle shape and surface area influence reproductive timing [53]. In Pisum sativum, exposure to polystyrene microplastics at higher concentrations (up to 1.0%) and smaller particles (≤50 μm) significantly reduced germination rate, root elongation, and seedling biomass, indicating that both microplastic concentration and particle size critically affect early reproductive development [54]. Bosker et al. further showed that PS beads inhibited seed germination in Lepidium sativum by obstructing seed coat pores as well as impairing water and enzyme uptake [55]. Given its shallow, fine roots and clonal growth form, S. mariqueter may be particularly sensitive to microplastic-induced soil structure disruptions. PET fragments, being angular and rigid, likely interfere more with nutrient uptake and reproductive resource allocation, while PP’s smoother morphology imposes less mechanical and microbial stress [56]. In contrast, recent reviews suggest that plant responses to microplastic stress vary widely depending on root system architecture and particle characteristics. Blanco et al. reported that microplastics can obstruct seed or root pores and impair water and nutrient uptake, but the severity of these effects is strongly influenced by particle size, shape, and plant root morphology [57]. Liu et al. further noted that although microplastics can disrupt the root functions and sediment properties in wetland systems, certain floating or deep-rooted species may maintain growth and reproductive capacity under similar exposures [58]. These findings support the idea that S. mariqueter, with its shallow, fine roots, may be more vulnerable to microplastic-induced reproductive suppression. Taken together, our findings indicate that microplastic-induced shifts in plant reproduction result from a combination of polymer traits, stress intensity, and plant-specific functional strategies. As a dominant species in estuarine wetlands, S. mariqueter’s reproductive plasticity under microplastic stress may signal broader ecological impacts, underscoring the need for cross-species, trait-based studies in future research.

4.3. Microplastics Indirectly Affect Plant Growth via Soil Physicochemical Properties

Microplastics significantly altered soil nitrogen forms, moisture, EC, and salinity, indirectly influencing S. mariqueter growth and reproduction. Spearman correlation analysis revealed strong associations between belowground biomass, total biomass, and reproductive traits (seed number and biomass) with soil moisture and nitrogen forms (Figure 5). The decline in NH₄⁺-N content closely corresponded to the reductions in above- and belowground biomass, suggesting that limited nitrogen availability is a key mechanism driving growth suppression. This aligns with terrestrial studies where microplastics reduced available nitrogen and impair plant growth [59,60]. Additionally, the microplastic-induced fluctuations in soil moisture and EC likely exacerbated plant growth stress by disrupting soil–plant water relations and the ion balance. Irregular soil moisture can hinder root water uptake, while elevated EC increases osmotic stress, limits nutrient absorption, and disrupts electrochemical gradients at the root–soil interface, negatively affecting plant metabolism and development [61,62]. In our study, soil moisture peaked under PE at 0.5% but did not show a consistent concentration-dependent trend (p > 0.05, Figure 4c), indicating that the effects of microplastics on moisture dynamics depend on microplastic type and concentration. Soil pH showed a slight increase, with no significant changes (p > 0.05, Figure 4d), and EC and salinity increased under low-concentration PS but decreased under high-concentration PP (Figure 4e,f). These results are consistent with previous studies, although our findings suggest greater variability, indicating that factors such as microplastic type and concentration influence soil properties differently. In addition to directly affecting plant growth through root development disruption, microplastics also alter soil microenvironmental processes. S. mariqueter may influence microplastic fate by slowing water flow, enhancing sediment stability, and promoting particle deposition, which could increase microplastic accumulation in the rhizosphere. These results indicate that microplastics exert both direct physical effects (e.g., impeding root development) and indirect effects through changes in soil microenvironmental processes [63], resulting in compounded negative impacts. In addition to directly affecting plant growth through root development disruption, microplastics also alter soil microenvironmental processes. S. mariqueter may influence microplastic fate by slowing water flow, enhancing sediment stability, and promoting particle deposition, which could increase the microplastic accumulation in the rhizosphere. Recent studies suggest that vegetation can actively mediate microplastic transport and persistence through such physical and biogeochemical interactions [64]. Future research should further explore this bidirectional relationship in coastal wetland systems.

4.4. Limitations and Future Directions

This study focused primarily on the responses of plant biomass and reproductive allocation to microplastic exposure, using a single size range of unaged microplastics (53–150 μm). While this controlled design allowed us to isolate plant-level responses, it limited our ability to assess size-dependent effects or the ecological impacts of aged microplastics. Environmental weathering processes—such as photodegradation, oxidation, and biofilm colonization—can substantially alter the surface properties of microplastics, affecting their interaction with soil, nutrients, and co-contaminants [65,66]. Future studies should include aged particles to better reflect field conditions and improve the environmental relevance of laboratory findings. In addition, although changes in soil nitrogen and moisture were measured in this study, microbial community composition and function—key mediators of nutrient cycling—were not examined. This omission reflects this study’s primary focus on plant physiological responses, but we acknowledge that soil microbiota likely play a crucial role in modulating the plant–microplastic interface. Future research should incorporate metagenomic, transcriptomic, or enzyme activity analyses to characterize microbial responses and link them to observed plant-level effects [67]. Moreover, this experiment was conducted under short-term greenhouse conditions. Long-term field studies that integrate variable environmental drivers (e.g., tidal dynamics, seasonal shifts) and multi-trophic interactions (e.g., plant–soil–microbe feedback) will be critical for evaluating how microplastics influence broader ecosystem functions such as carbon storage, nutrient retention, and wetland biodiversity. Addressing these knowledge gaps will help develop more holistic frameworks for predicting and managing microplastic risks in coastal wetland ecosystems.

5. Conclusions

This study revealed that microplastic pollution poses a significant threat to S. mariqueter, a keystone species in coastal wetlands, with type-specific and concentration-dependent effects on growth, reproduction, and physicochemical soil properties. PET and PE strongly suppressed belowground biomass, total biomass, and seed production, while PP promoted certain growth and reproductive traits, accompanied by alterations in soil moisture, NH4⁺-N, and EC. Our results suggest that these effects are linked to changes in soil conditions and potential soil–microbe interactions. While our study identified these patterns, more in-depth experimental studies are needed to clarify the underlying mechanisms and confirm the causal pathways involved. By impairing S. mariqueter performance, microplastics may destabilize wetland carbon sinks, biodiversity, and ecosystem services, underscoring the urgent need for risk assessments in coastal ecosystems. Future research should build upon these findings by employing long-term field studies, multi-factorial pollution scenarios, and integrated analyses of plant–soil–microbe dynamics to deepen our understanding of microplastics’ impacts and support effective wetland management strategies.