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Article

Nitrogen Addition Reduces Negative Plant-Soil Feedback in Invasive Spartina alterniflora: Preliminary Findings from a Mesocosm Experiment

by
Yinhua Wang
1,
Ningning Li
1,
Yuxin Zhang
1,
Changcheng Guo
2,
Lina Xie
1,
Yifan Li
3 and
Hongyu Guo
1,*
1
Tianjin Key Laboratory of Animal and Plant Resistance, College of Life Sciences, Tianjin Normal University, Tianjin 300387, China
2
Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin 300387, China
3
College of Life Sciences, Nankai University, Tianjin 300071, China
*
Author to whom correspondence should be addressed.
Agronomy 2026, 16(1), 86; https://doi.org/10.3390/agronomy16010086 (registering DOI)
Submission received: 21 November 2025 / Revised: 26 December 2025 / Accepted: 27 December 2025 / Published: 28 December 2025
(This article belongs to the Special Issue Weed Biology and Ecology: Importance to Integrated Weed Management)
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Abstract

Spartina alterniflora, an invasive plant species in coastal regions of China, poses significant threats to local biodiversity and has become a pervasive weed in coastal wetlands and agricultural systems. With increasing nitrogen inputs in coastal areas, understanding the impact of nitrogen addition on plant–soil feedback dynamics in S. alterniflora is essential but remains poorly explored. This study aimed to investigate how nitrogen addition affects plant–soil feedback in S. alterniflora and its growth dynamics. We conducted a mesocosm experiment where nitrogen was added at different levels to assess its effects on the plant–soil feedback in S. alterniflora. The results showed that nitrogen addition significantly increased the aboveground biomass of S. alterniflora by approximately 38% to 88%, while decreasing its belowground biomass by about 22% to 41% Nitrogen addition weakened the negative plant–soil feedback, which typically limits the growth of S. alterniflora. This reduction in microbial resistance at higher nitrogen levels contributed to enhanced overall growth of the plant. These findings highlight the critical role of nitrogen inputs in facilitating the growth of invasive S. alterniflora and suggest that excessive nitrogen in coastal ecosystems could accelerate the spread of this invasive species. Future research should focus on exploring strategies to regulate nitrogen levels in coastal wetlands and agricultural systems to mitigate the ecological impact of invasive species.

1. Introduction

Spartina alterniflora, commonly known as smooth cordgrass, is a perennial halophytic species native to the eastern coast of North America [1]. Due to its ability to thrive in coastal habitats, it was intentionally introduced to various regions, including China, for the purpose of erosion control and habitat restoration [2]. However, the rapid spread and vigorous growth of S. alterniflora have made it a significant invasive species, particularly in coastal areas of China [3]. It outcompetes native vegetation through its fast-growing rhizomes and high tolerance to saline conditions [4], posing a severe threat to coastal wetlands and agricultural systems [5,6,7]. As a result, S. alterniflora is considered a major invasive weed that undermines the stability and sustainability of coastal ecosystems [8]. Consequently, controlling the spread of S. alterniflora has become a key priority for the conservation of coastal wetlands and agricultural systems, especially in China.
As human activities intensify, significant amounts of nitrogen are being introduced into natural ecosystems [9], with human-induced nitrogen emissions now exceeding 60% of the natural nitrogen fixation capacity of ecosystems [10]. Key sources of this excess nitrogen include agricultural practices, industrial emissions, and transportation pollution [11]. In coastal regions, elevated nitrogen levels often lead to eutrophication [12], fueling algal blooms that deplete oxygen and create hypoxic zones [13]. These zones can have a devastating effect on biodiversity and disrupt essential ecosystem functions [14]. Additionally, the influx of nitrogen may alter the biogeochemical cycles in coastal soils, impacting the growth of plant species in these areas [15]. Coastal wetlands, which are vital for the sustainability of coastal agriculture and ecosystem health, are particularly susceptible to these nitrogen-induced changes [16]. Excessive nitrogen can shift plant community composition, reducing species diversity and undermining ecosystem stability [17]. Over time, these disturbances may accelerate coastal degradation, threaten ecosystem health, and endanger the livelihoods of communities dependent on these environments [18].
Plant–soil feedback refers to the reciprocal interactions between plants and their surrounding soil environment [19], where plants influence soil conditions through their root exudates, litter, and interactions with soil organisms, and, in turn, these soil changes can affect plant growth and community composition [20]. These interactions are essential in shaping ecosystem dynamics and can be either positive, negative, or neutral, depending on the specific plant and soil interactions [21]. Positive plant–soil feedback occurs when plants modify the soil in ways that promote their own growth. For example, Crawford and Knight (2017) discovered that the invasive forb Lespedeza cuneata could alter the composition of soil biota, thereby enhancing its growth and creating a positive plant–soil feedback [22]. Duchesneau et al. (2021) found that the invasive plant Alliaria petiolata could modify the community composition of soil nitrogen-cycling bacteria, promoting its own growth and resulting in a positive plant–soil feedback [23]. In contrast, negative plant–soil feedback happens when plants alter the soil in ways that inhibit their growth. For instance, Zhou et al. (2024) revealed that monocot plants increased the proportion of pathogens in the soil of an alpine grassland on the Tibetan Plateau, thereby inhibiting their growth and creating a negative plant–soil feedback [24]. Idbella et al. (2024) found that the legume plant Glycine max could promote soil fungal pathogens, which in turn suppress its growth, resulting in a negative plant–soil feedback [25].
Although considerable research has been conducted on the general effects of nitrogen addition on plant growth [26,27,28], the specific impact of nitrogen enrichment on plant–soil feedbacks in invasive S. alterniflora remains largely unexplored. Understanding how nitrogen influences plant–soil interactions in the context of this invasive species is crucial for maintaining the long-term sustainability of coastal wetlands and agricultural systems. As coastal areas become increasingly exposed to anthropogenic nitrogen inputs [29,30,31], it is essential to investigate how these nutrients may alter the delicate balance between S. alterniflora and the surrounding soil environment [32,33,34]. Such insights are vital for developing effective management strategies to mitigate the ecological and agricultural challenges posed by this invasive species, particularly in regions that are increasingly influenced by anthropogenic nitrogen inputs. This study examined the effects of nitrogen addition on plant–soil feedback in S. alterniflora, focusing on how nitrogen inputs may modify ecological interactions between this invasive species and coastal wetland soils.

2. Materials and Methods

2.1. Plant Material

In November 2024, we collected S. alterniflora seeds from five locations within a coastal wetland (38°39′ N, 117°34′ E) in Tianjin, China, where the invasive species has been expanding, currently covering an area of 570 hm2 [35]. Phragmites australis is the dominant native plant species in the coastal wetland. Additionally, human activities have resulted in a nitrogen input level of approximately 10 g N·m−2·a−1 in the coastal wetland [36], and this level is expected to continue rising in the future. We gathered approximately 300 seeds from each location, and after thoroughly mixing the seeds from all locations, we carefully selected healthy ones, focusing on those within a specific size range around the median to minimize size-related growth variation. These selected seeds were then stored at 4 °C in the laboratory throughout the winter.

2.2. Mesocosm Experiment

A mesocosm experiment was carried out in a glasshouse at Tianjin Normal University. The glasshouse was open to the surrounding environment, allowing for natural sunlight, while the temperature was maintained at 25 ± 5 °C to closely simulate outdoor conditions during the study period. The glass roof prevented rainfall from entering the experimental setup.
In early March 2025, we collected rhizosphere soil of Phragmites australis (the dominant native wetland plant species) from the coastal wetland in Tianjin. The collected soil was thoroughly mixed, with a portion reserved as inoculum soil, while the remaining soil was autoclaved twice at 121 °C for 30 min, with a 24 h interval, to prepare sterilized soil, as described in the study by Crawford and Hawkes (2020) [37]. This sterilized soil was then transferred into 100 cultivation pots (20 cm in diameter, 40 cm in height, with drainage holes at the bottom) to a depth of 35 cm, all of which had been surface-sterilized. Half of the pots with sterilized soil were randomly selected for the sterilized soil treatment, while the other half received inoculated soil treatment. For the inoculated soil treatment, the soil in these pots was thoroughly mixed with the previously reserved inoculum soil (constituting 10% (v/v) of the total soil volume), as described in the studies by Crawford and Hawkes (2020) [37] and Zuma et al. (2025) [38]. This approach minimizes potential disturbances to abiotic soil properties, ensuring that the observed plant–soil feedbacks are primarily due to microbial interactions rather than changes in soil chemistry or structure [37,39].
In mid-March 2025, we planted the S. alterniflora seeds into the 100 pots, placing one seed per pot approximately 1 cm below the soil surface. The pots were regularly watered with fresh water to promote the germination of S. alterniflora seeds. Each pot was placed in a water basin, with the water level in the basin maintained at about 3 cm. This setup allowed the soil to absorb water through the drainage holes at the bottom of the pots, thereby keeping the soil moisture level at approximately 35% throughout the experiment. By the end of April 2025, we selected 15 pots with sterilized soil (SS treatment) and 15 pots with inoculated soil (IS treatment), each containing healthy, similarly sized S. alterniflora seedlings (1.5 months old) to minimize variation in initial growth before the experiment began. The 15 pots with sterilized soil and the 15 pots with inoculated soil were each randomly assigned to three nitrogen addition levels (N0: 0 gN·m−2·a−1, N1: 10 gN·m−2·a−1, N2: 20 gN·m−2·a−1), with five replicates per nitrogen level. To reduce shading from adjacent plants, the pots were placed about 0.5 m apart.
Starting on May 5, 2025, we initiated the nitrogen addition treatments for the pots. Each week, NH4NO3 (Solomon Biotechnology Co., Ltd., Tianjin, China) was evenly applied to the surface of the soil, corresponding to the designated nitrogen addition levels. To minimize potential biases from temperature, light, and other environmental factors, the experimental pots were rotated regularly.
At the end of the growing season in October 2025, we measured the plant height (defined as the highest point of the leaves after vertical stretching), basal stem diameter, leaf count, and aboveground and belowground biomass of S. alterniflora. During the collection of belowground biomass, nearly all roots and rhizomes were intact and viable. The belowground biomass was carefully washed with freshwater using a 0.25 mm mesh sieve to remove soil particles. All plant samples were dried at 60 °C until they reached a constant weight, after which they were weighed. We also measured soil properties, including water content, salinity, pH, organic carbon content, total nitrogen content, and available phosphorus content. Soil water content was determined gravimetrically by drying fresh soil samples at 105 °C to a constant weight. Salinity was measured by determining the water content gravimetrically, rehydrating the dried soils in a known volume of distilled water, measuring the salinity of the supernatant, and then back-calculating to obtain the original pore-water volume [40]. Soil pH was assessed using a pH meter (Orion Star A211, Thermo Fisher Scientific Inc., Waltham, USA) in a soil-water suspension (1:2.5 ratio). Organic carbon content was analyzed using the Walkley-Black method, which involves oxidizing organic matter with potassium dichromate and subsequently titrating the excess reagent. Total nitrogen content was measured using the Kjeldahl method, which involves digestion with sulfuric acid followed by distillation and titration. Available phosphorus content was determined using the Olsen method, where phosphorus is extracted with sodium bicarbonate and quantified spectrophotometrically.

2.3. Data Analysis

To evaluate the effects of soil condition (sterilized soil and inoculated soil), nitrogen addition, and their interaction on S. alterniflora growth metrics, we performed ANOVAs with Tukey HSD post hoc tests for multiple comparisons, setting a significance level at 0.05. Data were logarithmically transformed to meet the normality and homogeneity of variance assumptions for ANOVA. We calculated the plant–soil feedback index for S. alterniflora based on various growth metrics across different nitrogen addition levels. The index was determined using the formula: plant–soil feedback index = Ln [(mean growth metric value in inoculated soil) / (mean growth metric value in sterilized soil)] [41,42]. A positive plant–soil feedback index indicates a beneficial effect of the soil microbial community on the growth of invasive S. alterniflora, while a negative value reflects an inhibitory effect. Soil property data were analyzed using Tukey HSD tests to compare differences between treatments. All statistical analyses were conducted using SPSS 21 (IBM Corp., Chicago, IL, USA).

3. Results

3.1. Soil Properties Under Different Experimental Treatments

The soil water content, salinity, and organic carbon content were not significantly different among the treatments (Table 1). Soil pH decreased with increasing nitrogen addition levels, while total nitrogen content and available phosphorus content increased with higher nitrogen addition levels (Table 1).

3.2. Effects of Soil Condition and Nitrogen Addition on Height of S. alterniflora

The height of S. alterniflora was significantly lower when grown in inoculated soil compared to sterilized soil across all nitrogen addition levels (Figure 1a, Table 2). Nitrogen addition, however, consistently promoted the height of S. alterniflora, regardless of the soil conditions (Figure 1a, Table 2). Additionally, the negative plant–soil feedback index, in terms of S. alterniflora height, reduced as nitrogen addition levels increased (Figure 1b), resulting in a significant interaction between soil condition and nitrogen addition (Table 2).

3.3. Effects of Soil Condition and Nitrogen Addition on Basal Stem Diameter of S. alterniflora

The basal stem diameter of S. alterniflora was lower when grown in inoculated soil compared to sterilized soil across all nitrogen addition levels (Figure 2a, Table 2). However, nitrogen addition consistently enhanced the basal stem diameter of S. alterniflora, irrespective of the soil conditions (Figure 2a, Table 2). Furthermore, the negative plant- soil feedback index, based on basal stem diameter, reduced as nitrogen addition levels increased (Figure 2b), leading to a significant interaction between soil condition and nitrogen addition (Table 2).

3.4. Effects of Soil Condition and Nitrogen Addition on Number of Leaves of S. alterniflora

The number of leaves of S. alterniflora was significantly decreased when grown in inoculated soil compared to sterilized soil across all nitrogen addition levels (Figure 3a, Table 2). In contrast, nitrogen addition consistently increased the number of leaves, regardless of soil condition (Figure 3a, Table 2). Moreover, the negative plant–soil feedback, in terms of the number of leaves, reduced as nitrogen addition levels increased (Figure 3b), resulting in a significant interaction between soil condition and nitrogen addition (Table 2).

3.5. Effects of Soil Condition and Nitrogen Addition on Aboveground Biomass of S. alterniflora

The aboveground biomass of S. alterniflora was significantly lower when grown in inoculated soil compared to sterilized soil at all nitrogen addition levels (Figure 4a, Table 2). Nitrogen addition, however, consistently boosted the aboveground biomass, regardless of soil condition (Figure 4a, Table 2). In addition, the negative plant–soil feedback, based on aboveground biomass, reduced as nitrogen addition levels increased (Figure 4b), resulting in a significant interaction between soil condition and nitrogen addition (Table 2).

3.6. Effects of Soil Condition and Nitrogen Addition on Belowground Biomass of S. alterniflora

The belowground biomass of S. alterniflora was significantly lower in inoculated soil compared to sterilized soil across all nitrogen addition levels (Figure 5a, Table 2). On the other hand, nitrogen addition consistently reduced the belowground biomass of S. alterniflora, regardless of the soil condition (Figure 5a, Table 2). Moreover, the negative plant–soil feedback, in relation to belowground biomass, tended to intensify as nitrogen addition levels increased (Figure 5b).

3.7. Effects of Soil Condition and Nitrogen Addition on Total Biomass of S. alterniflora

The total biomass of S. alterniflora was significantly lower when grown in inoculated soil compared to sterilized soil across all nitrogen addition levels (Figure 6a, Table 2). In contrast, nitrogen addition consistently enhanced the total biomass, irrespective of the soil condition (Figure 6a, Table 2). Furthermore, the negative plant–soil feedback, based on total biomass, reduced as nitrogen addition levels increased (Figure 6b), leading to a significant interaction between soil condition and nitrogen addition (Table 2).

4. Discussion

4.1. Aboveground Growth of S. alterniflora under Different Soil Conditions and Nitrogen Addition Levels

Firstly, we clarify that a limitation of the research is that the experiment was conducted once, and therefore, the results should be interpreted cautiously until validated in repeated trials or under field conditions. Our results demonstrated that nitrogen addition consistently increased the aboveground growth of S. alterniflora, irrespective of whether the soil was inoculated with native microbial communities or sterilized. This finding is in line with previous studies that highlight the positive effect of nitrogen on the growth of invasive species in coastal ecosystems [43,44,45].
Nitrogen is a critical macronutrient for plant growth, and its availability directly influences the physiological processes underlying plant growth, such as photosynthesis, nutrient uptake, and cellular division [46]. In the case of S. alterniflora, the positive correlation between nitrogen addition and increased aboveground growth may be attributed to its ability to assimilate additional nitrogen and other nutrients, such as phosphorus (Table 1) into its tissues, thereby enhancing aboveground biomass accumulation. Li et al. (2021) showed that the invasive S. alterniflora is competitively superior to native plant species in the Yellow River Delta in terms of nitrogen uptake and nitrogen utilization efficiency [34]. This is especially important in coastal wetlands, where nitrogen often limits plant productivity [47]. When nitrogen is added, S. alterniflora appears to benefit from this additional resource, potentially boosting its competitive ability over native species. This enhanced efficiency may be particularly evident in ecosystems that have experienced chronic nitrogen deposition, where S. alterniflora is able to exploit nitrogen more effectively than the resident plant species [43,48].
Moreover, the positive effect of nitrogen addition on the growth of S. alterniflora may be compounded by the plant’s ability to adapt to, or even thrive in environments with elevated nitrogen levels, which is often observed in coastal wetlands due to atmospheric deposition [49]. This adaptation may give S. alterniflora an advantage in ecosystems where nitrogen availability increases, thereby facilitating its invasion and establishment. Xu et al. (2022) demonstrated that under nitrogen enrichment, S. alterniflora exhibited enhanced ecophysiological adaptation in terms of light-use efficiency [50].
Interestingly, the interaction between soil condition and nitrogen addition is significant, suggesting that the impact of nitrogen on S. alterniflora growth is influenced by the underlying soil environment. In sterilized soils, where microbial communities are absent, nitrogen addition likely provides a direct boost to plant growth, with no microbial competition or regulation. In contrast, in inoculated soils, where native microbial communities are present, S. alterniflora may face more complex interactions, including competition for nutrients [51], potential pathogen presence [52], and altered microbial activity [53]. Nonetheless, the fact that nitrogen still consistently promoted growth under both soil conditions suggests that nitrogen’s role in plant growth is a dominant factor, capable of enhancing S. alterniflora aboveground growth even when microbial interactions are at play.

4.2. Belowground Biomass of S. alterniflora under Different Soil Conditions and Nitrogen Addition Levels

In this study, we observed that the belowground biomass of S. alterniflora was significantly lower in inoculated soils compared to sterilized soils across all nitrogen addition levels. This suggests that the soil microbes play a crucial role in regulating root growth and biomass accumulation of S. alterniflora. The microbial community in inoculated soils likely exerts competitive or pathogenic pressure on the roots of S. alterniflora, limiting their growth [54,55,56]. In contrast, sterilized soils, which lack microbial communities, may have provided a more favorable environment for root development of S. alterniflora, resulting in higher belowground biomass in these soils. These findings align with previous studies, which have demonstrated that soil microorganisms may influence root growth, particularly in the context of plant-microbe interactions involving invasive species [57,58,59].
Regarding the effects of nitrogen addition, our results indicated that while nitrogen fertilization consistently increased the aboveground biomass, it had an inhibitory effect on belowground biomass. This phenomenon suggests that S. alterniflora may reallocate resources under nitrogen enrichment, favoring the growth of aboveground structures over root biomass. Nitrogen is a key nutrient for cell division and elongation, particularly in aboveground tissues, such as stems and leaves [60,61]. Therefore, with nitrogen addition, S. alterniflora may prioritize aboveground growth, resulting in a reduced allocation of resources to root biomass. This shift in biomass allocation, driven by nitrogen, is commonly observed in many plant species under nutrient-enriched conditions [62]. For example, Negrini et al. (2020) showed that nitrogen addition increased leaf mass, but decreased root mass in barley [63]. Cheng et al. (2025) demonstrated that nitrogen addition led to a reduction in root biomass, coupled with an increase in shoot biomass of the dominant plant species in sandy ecosystems [64].
Additionally, nitrogen addition may indirectly affect root growth of S. alterniflora by altering the microbial community composition in the soil. Increased nitrogen availability, along with associated phosphorus availability (Table 1), could promote microbial activity [65,66], which in turn may increase competition for resources or exacerbate pathogen pressure on the roots. Such changes in microbial dynamics could limit the plant’s ability to invest in root growth, even in the presence of sufficient nitrogen. These indirect effects highlight the complexity of plant-microbe-nutrient interactions, where the plant’s response to nitrogen is modulated by the soil microbiome [67].
The interaction between soil condition and nitrogen addition further emphasized the complexity of S. alterniflora’s growth responses. In inoculated soils, nitrogen addition exacerbated the reduction in root biomass, suggesting that microbial communities may became more active in the presence of additional nitrogen [68]. This interaction points to the need for a more nuanced understanding of how soil microbes and nitrogen inputs jointly influence plant growth [69], which is especially important for managing S. alterniflora invasions in coastal wetland ecosystems.

4.3. Plant–Soil Feedback in Invasive Spartina alterniflora Across Nitrogen Addition Levels

As nitrogen input levels increased, we observed a consistent reduction in the negative plant–soil feedback for aboveground growth metrics, such as plant height, basal stem diameter, number of leaves, and aboveground biomass. These findings suggest that nitrogen enrichment weakens the inhibitory effects of soil microbes, which would suppress the growth of invasive S. alterniflora. This phenomenon is likely due to the shift in microbial community composition that favors nitrogen-tolerant microbes, which may enhance nutrient availability (Table 1) to the plant. Dong et al. (2025) showed that nitrogen enrichment enhanced soil enzyme activities, which may promote the decomposition of organic matter and the release of nutrients in grassland soils [70]. Weng et al. (2022) demonstrated that moderate nitrogen addition promoted the utilization rates of carbohydrates, amino acids, and esters by soil microorganisms [71]. As a result, invasive S. alterniflora may benefit from increased nitrogen input, enabling it to grow taller, develop a thicker stem base, produce more leaves, and accumulate more aboveground biomass, all of which would enhance its competitive ability in coastal wetlands and agricultural systems.
However, the dynamics observed for the belowground biomass of S. alterniflora are notably different. While nitrogen addition consistently reduced belowground biomass across different soil conditions, the negative plant–soil feedback based on belowground biomass intensified with increased nitrogen levels. This suggests that although nitrogen promotes the aboveground growth of S. alterniflora, it also leads to a redistribution of the plant’s resources away from root development. This trade-off between aboveground and belowground growth is also observed in other plants under nutrient-enriched conditions, where resources are often directed toward maximizing aboveground growth rather than reinforcing the root systems [72]. For instance, Gong et al. (2015) demonstrated that nitrogen addition significantly increased aboveground production but had no effect on belowground production in a grassland in Inner Mongolia [73]. Zhang et al. (2020) showed that nitrogen addition reduced the root-to-shoot ratio of plant species in a semiarid sandy grassland [74]. The intensification of negative feedback for belowground biomass may be related to changes in soil microbial interactions that inhibit root expansion, possibly due to nitrogen-induced shifts in microbial communities that promote antagonistic interactions with the plant’s root system.
The weakening of overall negative plant–soil feedback with nitrogen addition is particularly significant for understanding the mechanisms behind S. alterniflora’s invasion success in coastal wetlands and coastal agricultural systems. As nitrogen inputs increase, a key factor influencing the invasive plant’s competitive advantage is the shift from a system where soil microbes regulate the growth of the plant to one where microbial communities become more conducive to the establishment of the invasion [75]. In the context of coastal wetlands and agricultural systems, which are often subjected to elevated nitrogen inputs from atmospheric deposition [76], this phenomenon could exacerbate the spread of invasive S. alterniflora [77]. The weakening of negative plant–soil feedback under higher nitrogen levels may remove a critical natural limitation on the growth of invasive S. alterniflora, promoting its encroachment into native coastal wetlands and coastal agricultural systems, where it may further displace native vegetation and alter ecosystem functions [78,79,80].
These findings have significant implications for the management of coastal wetland ecosystems and sustainable agriculture practices. By demonstrating that nitrogen addition weakens the overall negative plant–soil feedback in invasive S. alterniflora, this study highlights the need to manage nutrient inputs carefully, in order to control invasive species in coastal habitats. Strategies that reduce nitrogen pollution could help restore the natural plant–soil feedback mechanisms, thereby mitigating the impact of invasive S. alterniflora on native plant communities and preserving the ecological integrity of coastal wetlands and coastal agricultural systems. Furthermore, understanding how nitrogen affects plant–soil feedback in invasive species offers valuable insights for developing integrated management approaches that combine nutrient regulation with invasive species control to maintain the health of these critical coastal ecosystems.

5. Conclusions

This study examined the effects of nitrogen addition on plant–soil feedback dynamics in invasive S. alterniflora under controlled pot conditions. Our findings suggest that nitrogen addition weakens the negative plant–soil feedback typically limiting the growth of S. alterniflora, thereby promoting its aboveground growth. Specifically, nitrogen enrichment enhanced aboveground biomass while consistently reducing belowground biomass, indicating a shift in resource allocation. The results also suggest that the reduction in negative plant–soil feedback, driven by nitrogen inputs, could facilitate the growth and competitive advantage of S. alterniflora in coastal ecosystems. Although the pot-based design limits the direct application of these findings to field conditions, the study provides valuable insights into the potential mechanisms behind S. alterniflora invasion success in nitrogen-enriched environments. These findings underscore the importance of understanding nutrient dynamics in controlling invasive species. However, further studies, including more in-depth investigations into changes in soil microbial communities and field-based research, are needed to confirm the underlying mechanisms and broader ecological implications of nitrogen inputs on plant–soil feedback in coastal ecosystems.

Author Contributions

Conceptualization, Y.W. and H.G.; funding acquisition, Y.W. and H.G.; methodology, Y.W. and H.G.; investigation, N.L., Y.Z., C.G., L.X. and Y.L.; formal analysis, Y.W., N.L., Y.Z., C.G., L.X., Y.L. and H.G.; writing—original draft preparation, Y.W., N.L., Y.Z., C.G., L.X., Y.L. and H.G.; writing—review and editing, Y.W., N.L., Y.Z., C.G., L.X., Y.L. and H.G. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by National Natural Science Foundation of China (31300357, 32001179) and Natural Science Foundation of Tianjin City (20JCZDJC00220).

Data Availability Statement

All data supporting the findings of this study were included within the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Height of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on height of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 1. Height of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on height of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Figure 2. Basal stem diameter of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on basal stem diameter of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 2. Basal stem diameter of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on basal stem diameter of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Figure 3. Number of leaves of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on number of leaves of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 3. Number of leaves of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on number of leaves of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Figure 4. Aboveground biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on aboveground biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 4. Aboveground biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on aboveground biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Figure 5. Belowground biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on belowground biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 5. Belowground biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on belowground biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Figure 6. Total biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on total biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
Figure 6. Total biomass of S. alterniflora under different soil conditions (SS: sterilized soil; IS: inoculated soil) and nitrogen addition levels (a), and the plant–soil feedback index based on total biomass of S. alterniflora, across different nitrogen addition levels (b). N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Panel (a) displays data as means + SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level. Data were logarithmically transformed before statistical analysis.
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Table 1. Soil properties under different experimental treatments. N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Data are means ± SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level.
Table 1. Soil properties under different experimental treatments. N0: 0 gN·m−2·a−1; N1: 10 gN·m−2·a−1; N2: 20 gN·m−2·a−1. Data are means ± SE (n = 5). Means with shared letters are not significantly different from each other, based on Tukey’s HSD tests at the 0.05 significance level.
Soil PropertiesSterilized Soil (SS)Inoculated Soil (IS)
N0N1N2N0N1N2
Water content (%)35.37 ± 0.07 a35.52 ± 0.06 a35.53 ± 0.07 a35.51 ± 0.03 a35.39 ± 0.11 a35.53 ± 0.04 a
Salinity (PSU)20.10 ± 0.12 a20.22 ± 0.21 a20.15 ± 0.23 a20.08 ± 0.18 a20.29 ± 0.19 a20.22 ± 0.03 a
pH7.53 ± 0.02 a7.45 ± 0.03 b7.39 ± 0.02 c7.54 ± 0.02 a7.45 ± 0.02 b7.36 ± 0.02 c
Organic carbon
content (g/kg)		15.37 ± 0.14 a15.38 ± 0.13 a15.33 ± 0.05 a15.40 ± 0.14 a15.50 ± 0.16 a15.46 ± 0.18 a
Total nitrogen
content (mg/kg)		61.51 ± 0.09 c70.52 ± 0.18 b79.57 ± 0.25 a61.34 ± 0.22 c70.60 ± 0.23 b79.58 ± 0.21 a
Available phosphorus
content (mg/kg)		15.56 ± 0.23 c16.68 ± 0.17 b17.61 ± 0.29 a15.80 ± 0.13 c16.72 ± 0.19 b17.81 ± 0.14 a
Table 2. Summary of ANOVAs examining the effects of soil condition (sterilized soil and inoculated soil), nitrogen addition (N0, N1 and N2), and the interaction between them on height, basal stem diameter, number of leaves, aboveground biomass, belowground biomass, and total biomass (all Ln-transformed) of invasive S. alterniflora.
Table 2. Summary of ANOVAs examining the effects of soil condition (sterilized soil and inoculated soil), nitrogen addition (N0, N1 and N2), and the interaction between them on height, basal stem diameter, number of leaves, aboveground biomass, belowground biomass, and total biomass (all Ln-transformed) of invasive S. alterniflora.
Source of VariancedfFp
Height of S. alterniflora
  Soil condition1, 2469.993<0.001 ***
  Nitrogen addition2, 24151.828<0.001 ***
  Soil condition × Nitrogen addition2, 244.8600.017 *
Basal stem diameter of S. alterniflora
  Soil condition1, 2443.835<0.001 ***
  Nitrogen addition2, 24130.672<0.001 ***
  Soil condition × Nitrogen addition2, 243.5890.043 *
Number of leaves of S. alterniflora
  Soil condition1, 2434.426<0.001 ***
  Nitrogen addition2, 2455.181<0.001 ***
  Soil condition × Nitrogen addition2, 243.8350.036 *
Aboveground biomass of S. alterniflora
  Soil condition1, 2432.053<0.001 ***
  Nitrogen addition2, 2425.816<0.001 ***
  Soil condition × Nitrogen addition2, 248.7630.002 **
Belowground biomass of S. alterniflora
  Soil condition1, 247.7910.010 *
  Nitrogen addition2, 2412.765<0.001 ***
  Soil condition × Nitrogen addition2, 241.0970.350
Total biomass of S. alterniflora
  Soil condition1, 2422.548<0.001 ***
  Nitrogen addition2, 2410.232<0.001 ***
  Soil condition × Nitrogen addition2, 243.6180.042 *
Note: *, 0.01 < p < 0.05; **, p < 0.01; ***, p < 0.001.
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MDPI and ACS Style

Wang, Y.; Li, N.; Zhang, Y.; Guo, C.; Xie, L.; Li, Y.; Guo, H. Nitrogen Addition Reduces Negative Plant-Soil Feedback in Invasive Spartina alterniflora: Preliminary Findings from a Mesocosm Experiment. Agronomy 2026, 16, 86. https://doi.org/10.3390/agronomy16010086

AMA Style

Wang Y, Li N, Zhang Y, Guo C, Xie L, Li Y, Guo H. Nitrogen Addition Reduces Negative Plant-Soil Feedback in Invasive Spartina alterniflora: Preliminary Findings from a Mesocosm Experiment. Agronomy. 2026; 16(1):86. https://doi.org/10.3390/agronomy16010086

Chicago/Turabian Style

Wang, Yinhua, Ningning Li, Yuxin Zhang, Changcheng Guo, Lina Xie, Yifan Li, and Hongyu Guo. 2026. "Nitrogen Addition Reduces Negative Plant-Soil Feedback in Invasive Spartina alterniflora: Preliminary Findings from a Mesocosm Experiment" Agronomy 16, no. 1: 86. https://doi.org/10.3390/agronomy16010086

APA Style

Wang, Y., Li, N., Zhang, Y., Guo, C., Xie, L., Li, Y., & Guo, H. (2026). Nitrogen Addition Reduces Negative Plant-Soil Feedback in Invasive Spartina alterniflora: Preliminary Findings from a Mesocosm Experiment. Agronomy, 16(1), 86. https://doi.org/10.3390/agronomy16010086

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