Invasive Spartina Alterniflora Research Articles

Effects of land-use on soil C, N, and P stocks and stoichiometry in coastal wetlands dependent on soil depth and latitude

Coastal land-use is increasingly affected by Spartina alterniflora invasion, coastal reclamation, and climate change, which have been suggested to exert vital controls on soil carbon (C), nitrogen (N), and phosphorus (P) cycles, however, the effects on the geographical distributions of these element stocks and stoichiometry remain unclear. We analyzed the depth (0–100 cm) and latitudinal (19–41°N) patterns of soil C, N, and P contents, stocks, and ratios in 120 Chinese coastal wetlands differing in land-use (reclaimed wetlands, natural wetlands, invasive wetlands, and bare mudflats) and climate (tropics, subtropics, and temperate). We observed stable depth patterns of soil contents, stocks, and ratios, owing to the stable depth patterns of soil conditions in each land-use. Top soils (0–30 cm) generally had higher C, N, and P contents and N:P ratio but lower C:N and C:P ratios than deep soils (30–100 cm). We found that soil C content and P stock remained constant, soil C stock, N and P contents and N:P ratio increased significantly, but soil N stock and C:N and C:P ratios decreased significantly with decreasing latitude; however, such correlations tended to be weaker in reclaimed wetlands and invasive wetlands. Land-use changes also significantly altered the magnitude of soil element stoichiometry. The effects of S. alterniflora invasion on soil C, N, and P storages can be shifted from negative in tropics to positive in temperate regions, whereas the negative effects of coastal reclamation in tropics disappeared in temperate regions. Overall, compared with natural wetlands, S. alterniflora invasion slightly decreased, whereas coastal reclamation largely decreased soil C, N, and P contents and stocks. Compared with bare mudflats, S. alterniflora invasion significantly increased C and N contents and N:P ratio but decreased C:N ratio, whereas coastal reclamation significantly decreased P content and stock and C:N ratio but increased N:P ratio, especially in top soils. Our findings will be helpful for understanding and assessing soil C sequestration and nutrient cycling in globally disturbed coastal wetlands.

Differential effects of warming on the complexity and stability of the microbial network in Phragmites australis and Spartina alterniflora wetlands in Yancheng, Jiangsu Province, China.

The impact of climate warming on soil microbial communities can significantly influence the global carbon cycle. Coastal wetlands, in particular, are susceptible to changes in soil microbial community structure due to climate warming and the presence of invasive plant species. However, there is limited knowledge about how native and invasive plant wetland soil microbes differ in their response to warming. In this study, we investigated the temporal dynamics of soil microbes (prokaryotes and fungi) under experimental warming in two coastal wetlands dominated by native Phragmites australis (P. australis) and invasive Spartina alterniflora (S. alterniflora). Our research indicated that short-term warming had minimal effects on microbial abundance, diversity, and composition. However, it did accelerate the succession of soil microbial communities, with potentially greater impacts on fungi than prokaryotes. Furthermore, in the S. alterniflora wetland, experimental warming notably increased the complexity and connectivity of the microbial networks. While in the P. australis wetland, it decreased these factors. Analysis of robustness showed that experimental warming stabilized the co-occurrence network of the microbial community in the P. australis wetland, but destabilized it in the S. alterniflora wetland. Additionally, the functional prediction analysis using the Faprotax and FunGuild databases revealed that the S. alterniflora wetland had a higher proportion of saprotrophic fungi and prokaryotic OTUs involved in carbon degradation (p < 0.05). With warming treatments, there was an increasing trend in the proportion of prokaryotic OTUs involved in carbon degradation, particularly in the S. alterniflora wetland. Therefore, it is crucial to protect native P. australis wetlands from S. alterniflora invasion to mitigate carbon emissions and preserve the health of coastal wetland ecosystems under future climate warming in China.

Tracking the Effects of Mangrove Changes and Spartina alterniflora Invasion on Soil Carbon Storage: A Case Study of the Beibu Gulf of Guangxi, China

In order to clarify the long-term changes in mangroves in the Beibu Gulf of Guangxi and the carbon storage changes after the invasion of Spartina alterniflora (S. alterniflora) in the Dandou Sea area, the Continuous Change Detection and Classification (CCDC) algorithm combined with feature indices was first used to track the changes. Subsequently, the random forest algorithm was applied to classify each change segment, and then sampling was conducted based on the distribution of S. alterniflora in different invasion years. The results showed that the Kappa coefficient of the classification result of the latest change segment was 0.78. The rapid expansion of S. alterniflora, aquaculture pond construction, and land reclamation activities have led to changes in mangroves, causing a decrease in the area of the mangrove region. A total of 814.57 hectares of mangroves has been converted into other land-cover types, with most pixels undergoing one to two changes, and many of these changes were expected to continue until 2022. An analysis of the distribution characteristics and influencing factors of soil organic carbon (SOC) and soil organic carbon storage (SOCS) at different invasion stages revealed that SOC and SOCS were mainly influenced by soil bulk density, soil moisture content, and electrical conductivity. It was found that S. alterniflora had higher SOC content compared to the mudflats. With the increase in invasion years, S. alterniflora continuously increased the SOC and SOCS content in coastal wetlands.

Tracking the dynamics of tidal wetlands with time-series satellite images in the Yangtze River Estuary, China

ABSTRACT Tidal wetlands provide a variety of ecosystem services to coastal communities but suffer severe losses due to anthropogenic activities in the Yangtze River Estuary (YRE). However, the detailed dynamics of tidal wetlands have not been well studied with sufficient spatiotemporal resolution. Here, we proposed a rapid classification method that integrates the COntinuous monitoring of Land Disturbance (COLD) algorithm and Median Composite (MC) based on the dense Landsat time series to track the dynamic processes of tidal wetlands in the YRE from 1990 to 2020. The results showed that the COLD-MC demonstrated remarkable effectiveness in detecting the change of tidal wetlands and excellent overall accuracy and kappa coefficient ranging from 90% to 96% and 0.89–0.95, respectively. The overall accuracy of change detection was 97% with an absolute error of 0.4 years. We found that the total area of tidal wetlands experienced a net loss of 59.75 km2 in the YRE, but the gain and loss of the study period were 1556.07 and 1615.82 km2, respectively. Land reclamation, sediment reduction, and Spartina alterniflora invasion pose significant threats to tidal wetlands. Sustainable management could be implemented through the establishment of nature reserves and ecological sediment enhancement engineering projects.

Stocks and Sources of Soil Carbon and Nitrogen in Non-Native Kandelia obovata Afforestation and Spartina alterniflora Invasion: A Case Study on Northern Margin Mangroves in the Subtropical Coastal Wetlands of China

For decades in China, carbon neutrality policies have spurred the establishment of northern margin mangroves as artificial blue carbon ecosystems. However, there has been limited research on the impact of plantation and invasion on the stocks and sources of soil carbon and nitrogen in rehabilitated coastal wetlands. Non-native Kandelia obovata afforestation began on Ximen Island, Zhejiang, China, where Spartina alterniflora invasion had also occurred decades ago. Soil cores were collected from both mangrove and salt marsh habitats with depths from 0 to 50 cm and were analyzed for total carbon (TC), soil organic carbon (SOC), total nitrogen (TN), and the isotope of carbon and nitrogen in sediments. The results indicated that there were no significant differences in the TC, SOC, and C/N ratio between the K. obovata and the S. alterniflora, but there were significant differences in TN, isotope δ13C, and δ15N. The SOC content of both ecosystems in the 0–20 cm layer was significantly higher than that in the 30–50 cm layer. Our study has shown that the main sources of carbon and nitrogen for mangroves and salt marshes are different, especially under the impact of external factors, such as tidal waves and aquaculture. These findings provide insight into the ecological functioning of subtropical coastal wetlands and an understanding of the biogeochemical cycles of northern margin mangrove ecosystems.

How does exotic Spartina alterniflora affect the contribution of iron-bound organic carbon to soil organic carbon in salt marshes?

Iron-bound organic carbon (OC-FeR) is important for the stability of soil organic carbon (SOC) in salt marshes, and the Spartina alterniflora invasion reshaped local salt marshes and changed the SOC pool. To evaluate the effects of S. alterniflora invasion on the contribution of OC-FeR to SOC, we determined the OC-FeR content and soil characteristics in the 0–50 cm soil profile along the vegetation sequence, including mudflats (MF), S. alterniflora marshes established in 2003 (SA03) and 1989 (SA89), the ecotone of S. alterniflora and Phragmites australis (SE), S. salsa marsh (SS), and P. australis marsh (PA). The SOC content was 6.55–17.5 mg g−1 in the S. alterniflora marshes. Reactive iron oxides (Fed, Feo, Fep) accumulated significantly in the S. alterniflora and P. australis salt marshes. PA and S. alterniflora marshes had higher DOC contents of 0.28–0.77 mg g−1. The OC-FeR content in the 0–50 cm soil profile in these ecosystems ranged from 0.3 to 3.29 mg g−1, with a contribution to the SOC content (fOC-FeR) of approximately 11 %, which was highest in SA03 (16.3 % ~ 18.8 %), followed by SA89, SE, and PA. In addition, the molar ratios of OC-FeR to Fed were <1, indicating that the iron oxides were associated with SOC through sorption more than coprecipitation. According to the structural equation model, SOC, DOC and iron oxides were the direct driving factors of OC-FeR formation, while the vegetation zone indirectly functioned by regulating organic C inputs, iron oxide formation, and pH. This study suggested that S. alterniflora invasion promotes iron-bound organic carbon accumulation by increasing organic C inputs and regulating iron oxide formation in salt marshes, but such promotion will degenerate with development duration.

Dynamics of benthic microeukaryotic communities in a mangrove wetland invaded by Spartina alterniflora: Effects of vegetation, seasonality, and sediment depth

Benthic microeukaryotes are crucial mediators of biogeochemical cycles in coastal wetland ecosystems, yet their spatial and temporal variability remains poorly understood. This study delineates the diversity patterns of benthic microeukaryotes in a Spartina alterniflora-invaded mangrove ecosystem in Fujian, China. Using high-throughput sequencing of 18S rRNA gene transcripts, we identified the influences of vegetation, seasonality, and sediment depth on microeukaryotic communities. We discovered that vegetation cover significantly affects community composition, primarily driven by nutrient concentrations and pH. The community structure of microeukaryotes varied seasonally and vertically, correlating with changes in sediment temperature, pH, salinity, and fucoxanthin concentration. Notably, invasive Spartina alterniflora habitats showed enhanced heterotrophic interactions, suggesting that invasive species can reshape benthic microeukaryotic co-occurrence patterns. Seasonal co-occurrence patterns revealed dominant Bacillariophyta assemblages exhibited distinct network modules enriched in the cold (spring) and warm (summer and fall) seasons, respectively, which indicated potential ecological niche differentiation. Our findings reveal the complex relationships between environmental factors and benthic microeukaryotic diversity, offering insights into microbial responses to natural and invasive vegetation influences.

Latitudinal responses of wetland soil nitrogen pools to plant invasion and subsequent aquaculture reclamation along the southeastern coast of China

The impact of invasive species and land use change on soil nitrogen pools in coastal wetlands has been reported at local scale, but uncertainty persists for regional pattern due to geographical variability and limited field data. This study measured the top soil (upper 20 cm) organic nitrogen (SON), inorganic nitrogen (SIN) and total nitrogen (STN) concentrations and stocks across 21 coastal wetland sites in China (20°42′N-31°51′ N) that had undergone the same sequence of transformation from mudflats (MFs) to invasive Spartina alterniflora marshes (SAs) then to earthen aquaculture ponds (APs). Results showed that the conversion of MF to SA significantly increased SON and SIN concentrations and stocks by 37.7–86.1%, but subsequent conversion to APs significantly decreased them by 13.5–34.6%. SON/SIN ratio decreased upon invasion by S. alterniflora and it had a negative effect on STN accumulation, whereas conversion of SAs to APs showed the opposite trends. The change rates of SON, SIN and STN stocks showed clear decreasing trends with increasing latitude in the MF-to-SA conversion scenario, reflecting the strong influence of environmental temperatures, but weaker or insignificant trends were observed in the SA-to-AP conversion scenario, likely because of mitigating anthropogenic activities in aquaculture ponds. Our findings can be used to inform strategies to control invasive species and reduce the greenhouse gas nitrous oxide (N2O) emissions, and support global N model for climate change in response to habitat modifications in coastal wetlands.

Variations of activity and community structure of nitrite-driven anaerobic methanotrophs in soils between native and invasive species in China's coastal wetlands

Nitrite-driven anaerobic oxidation of methane (nitrite-driven AOM), mediated by ‘Candidatus Methylomirabilis oxyfera’ (M. oxyfera)-related bacteria, is a newly-discovered CH4 consumption process in coastal wetlands. Although Spartina alterniflora invasion significantly affects CH4 emissions from coastal wetlands, its impact on the nitrite-driven AOM process and the underlying mechanisms remain unknown. Here, we examined nitrite-driven AOM activity and M. oxyfera-related bacterial community in four coastal wetlands along the southeastern coast of China, under invasive Spartina alterniflora and native plants, including Kandelia candel, Avicennia marina or Phragmites australis. Linear mixed-effects models indicated that the Spartina alterniflora invasion stimulated the overall nitrite-driven AOM activity by an average of 61.5% in coastal wetlands (p < 0.05), but had no impact on the M. oxyfera-related bacterial abundance (p > 0.05). The nitrite-driven AOM activity was 7.1 times higher under Spartina alterniflora than under native species in Yueqing Bay (p < 0.05), and was 34.7%, 8.9% and 15.1% higher under Spartina alterniflora than under native species in Hengsha Island, Jiulong River and Zhanjiang, respectively (p > 0.05). Spartina alterniflora invasion increased the bacterial abundance in Yueqing Bay and Jiulong River Estuary by 6.8 and 7.6 times, respectively, while decreased the abundance by 34.4% and 51.4%, respectively, in Hengsha Island and Zhanjiang (p > 0.05). The partial least squares path model indicated an indirect impact of Spartina alterniflora invasion on the nitrite-driven AOM activity through its effect on soil properties, primarily including dissolved organic carbon content and nitrate content. The Spartina alterniflora invasion did not greatly alter M. oxyfera-related bacterial community. Overall, we shed new light on the potential impact of Spartina alterniflora invasion on CH4 cycling in coastal wetlands.

Monitoring Wetland Changes and Analyzing the Spartina alterniflora Invasion in the Yellow River Delta Over the Past 30 Years Based on Google Earth Engine

Monitoring Wetland Changes and Analyzing the Spartina alterniflora Invasion in the Yellow River Delta Over the Past 30 Years Based on Google Earth Engine

Source and stability of soil organic carbon jointly regulate soil carbon pool, but source alteration is more effective in mangrove ecosystem following Spartina alterniflora invasion

Spartina alterniflora invasion seriously affects blue carbon function in mangrove ecosystems. However, the mechanisms by which S. alterniflora invasion affects the sources and stability of soil organic carbon (SOC) in mangrove ecosystems have not been fully elucidated. In this study, stable isotopes and nuclear magnetic resonance spectroscopy techniques were used to investigate the impacts of S. alterniflora invasion on SOC sources, decomposition characteristics, and SOC pool stability in the Zhangjiang Estuary, Fujian Province, China. The results showed that SOC content in the mangrove was 8.25–51.46 g kg−1, which was 21.3 % and 93.4 % higher than those in the S. alterniflora and tidal flat, respectively. Additionally, the SOC storage at depths of 0–30 cm was 26.0 % and 96.9 % higher in mangrove than in S. alterniflora and tidal flats, respectively. The values of soil δ13C and δ15N were lower, whereas the C/N ratio was higher in mangrove than in S. alterniflora. A Bayesian stable isotope mixing model revealed that mangrove plant tissues were the highest contributor to SOC (40.8 ± 18.7 %) in mangrove topsoil because of litter effects. However, in the S. alterniflora and tidal flat ecosystems, SOC was mainly derived from allochthonous sources with particulate organic matter (POM) proportions of 72.1 ± 3.3 % and 56.4 ± 5.3 %, respectively. Alkyl C (16.3–41.4 %) was the dominant functional group in all soils, followed by O-alkyl C and aromatic C. Alkyl C, A/O-A, and hydrophobicity index (HI) were significantly lower in S. alterniflora soil than in mangrove and tidal flat soils. Therefore, the SOC sources and stability, including fractions and chemical composition, were substantially altered in mangrove ecosystems after S. alterniflora invasion, leading to decreases in SOC pools. Nevertheless, source alteration was more effective in mangrove wetland invaded by S. alterniflora. These findings may provide new perspectives regarding blue carbon sources, decomposition, stabilization, and sequestration in mangrove wetlands invaded by S. alterniflora.

A comparative study of coastal wetland soil bacterial and fungal colonies following Spartina alterniflora invasion

The current body of research on the impact of plant invasion on bacterial and fungal communities in coastal wetlands remains limited. This study focuses on the analysis of soil samples collected from coastal wetlands at three distinct stages: before (BI), during (DI), and after the invasion of Spartina alterniflora (AI). Overall, the total organic carbon (TOC), soil moisture, available phosphorus (AP), and available potassium (AK) changed significantly after plant invasion, with these changes significantly explaining the variation within bacterial colonies. Nevertheless, the impact of these factors on fungal colonies had little statistical significance. Additionally, the composition and functional diversity of bacterial colonies exhibited more significant changes before and after invasion. Specifically, significant changes were observed within Tenericutes, Actinobacteria, Fusobacteria, and Patescibacteria, while a significant enrichment of the genus Prevotellaceae_UCG_001 was noted after plant invasion. However, only a significant p value reduction in AI compared to BI was observed in Dothideomycete at the order level within the fungi. For the bacterial colonies, the abundance of substances involved within glycosphingolipid synthesis and other metabolic pathways at the KEGG Level 3 also decreased significantly, while their ecological niche width increased significantly. Conversely, the fungal colonies exhibited a somewhat changed shape and functional abundance, but they reacted to the plant invasion with reduced sensitivity. The neutral community model, which showed higher rates of migration for both fungi and bacteria, validated these outcomes. The dynamic changes that occur in soil ecosystems as a result of plant invasion are highlighted in the current work.

Distribution of mudsnail Bullacta caurina along smooth cordgrass Spartina alterniflora invasion stages on a coast of the Yellow Sea, China

On a Chinese coast of the Yellow Sea, a 15-year Spartina alterniflora invasion sequence was classified into five stages: no invasion, initial invasion, immature invasion, mature invasion, and senescing invasion. The effects of invasion on Bullacta caurina distribution were studied. The stem density and vegetation coverage, and sediment organic matter content increased after S. alterniflora invaded, whereas chlorophyll a concentration and porewater salinity decreased. The stem density and vegetation coverage, and porewater salinity were the dominant factors explaining habitat variations. The invasion stages, seasons and their interaction had significant effects on B. caurina density, and the density decreased after initial invasion stage of S. alterniflora. Here, a clumped spatial distribution pattern was detected on B. caurina population. Organic matter content and chlorophyll a concentration were distinguished for predicting B. caurina density. The hydrologic condition, food resources, temperature, and predation risk comprehensively affected B. caurina distribution after S. alterniflora invasion.

Mapping trade-offs among key ecosystem functions in tidal marsh to inform spatial management policy for exotic Spartina alterniflora

Invasive Spartina alterniflora has become a global management challenge in coastal wetlands. China has decided to eradicate it completely, but the high costs and its provision of beneficial ecosystem functions (EF, in the form of blue carbon and coastal protection) have raised concerns about its removal. Here, using the Yangtze Estuary as a case study, we explore a reasonable pathway of S. alterniflora management that balanced control of invasive species and EF. We simulated the spatial patterns of two key EF – blue carbon storage and wave attenuation – and identified appropriate zones for eradicating S. alterniflora based on their trade-offs. We observed contrasting patterns along the land-sea gradient for S. alterniflora community, with a decrease in blue carbon storage and an increase in wave attenuation. Notably, pioneer S. alterniflora near the foreshore displayed a high cluster of blue carbon storage (63.61 ± 7.33 Mg C ha−1) and dissipated nearly 70% of wave energy by a width of 163 m. The trade-offs between the two EF indicated that the eradication project should be implemented along the seawall rather than the foreshore. Even in the scenario of prioritized shore defense with the largest eradication zone, S. alterniflora still stored 43.1% more carbon (10.67 Gg C) compared to complete eradication and dissipated over 70% of wave energy in extreme events. Our study innovatively integrates eradication and reservation in S. alterniflora management, providing a sustainable and flexible spatial strategy that meets the needs of stakeholders.

Effect of human reclamation and Spartina alterniflora invasion on C-N-P stoichiometry in plant organs across coastal wetlands over China

Effect of human reclamation and Spartina alterniflora invasion on C-N-P stoichiometry in plant organs across coastal wetlands over China

Plant invasion strengthens the linkages between dissolved organic matter composition and the microbial community in coastal wetland soils

Exotic plant invasion dramatically alters soil carbon transformation and shapes the structure and diversity of the microbial community in coastal wetlands. However, it remains unclear how the dissolved organic matter (DOM) optical components and chemical composition and their linkages with soil microorganisms change with increasing time since invasion. Here, we explored the impacts of Spartina alterniflora (S. alterniflora) invasion on DOM compositions and relationships with microbial communities along a short-term chronosequence (i.e., 3-, 5-,10-, and 15-year) in the Yellow River Delta. The results showed an increase in the soil water content, soil organic carbon, dissolved organic carbon, and C/N ratio along the invasion chronosequence, whereas bulk density showed the opposite trend. Moreover, invaded wetland soils had a higher abundance of total phospholipid fatty acids, gram-positive bacteria (G + ), gram-negative bacteria (G-), and fungi than tidal flat soils. The DOM in invaded wetland soils was mainly composed of endogenous protein-like components, and the proportion of autochthonous DOM increased over invasion time. The DOM was characterized by low molecular weight, low aromatic content, and low hydrophobic compounds in S. alterniflora wetland soils, whereas in tidal flat soils, the humification degree and DOM molecular weight were higher. The soil microbial communities exhibited stronger linkages with various environmental variables and DOM fluorescence components in invaded wetlands based on redundancy analysis (RDA) and correlation analysis using Mantel tests. Structural Equation Model (SEM) revealed that the soil microbial abundance and community structure induced by increasing S. alterniflora invasion time were the most important drivers of DOM content and chemical composition. The findings of this work provide a better understanding of labile carbon pool turnover in coastal wetlands invaded by S. alterniflora.

Soil organic nitrogen content and composition in different wetland habitat types along the south-east coast of China

Soil organic nitrogen (SON) turnover regulates soil nitrogen (N) storage and availability. The coastal mudflats (MFs) in China have undergone drastic transformation due to invasive Spartina alterniflora (SAs) and subsequent reclamation of Spartina marshes to create aquaculture ponds (APs), but the impact on the amounts and compositions of soil nitrogen remains unclear. This study measured the topsoil total nitrogen (STN) and organic nitrogen (SON) compositions in 21 coastal wetlands in southeastern China. Results show that conversion of MFs to SAs increased STN by 38.5%, whereas subsequent conversion to APs decreased it by 16.4%, and the effect was consistent across the broad geographic and climate gradients. Most of the change occurred in the non-acid-hydrolysable fraction of SON, which accounted for 32–42% of STN. Within the acid-hydrolysable fraction, amino acid N, ammonia N and amino sugar N together accounted for about 57%, with the remaining 43% unidentified chemically. Our results suggest that invasion by S. alterniflora was the overwhelming driver to increase bioavailability of nitrogen and related biogeochemical processes in coastal soil, and the effects were partly reversed in subsequent reclamation of Spartina marshes to create aquaculture ponds.

Invasive Spartina alterniflora accelerates the increase in microbial nitrogen fixation over nitrogen removal in coastal wetlands of China

Salt marsh plants play a vital role in mediating nitrogen (N) biogeochemical cycle in estuarine and coastal ecosystems. However, the effects of invasive Spartina alterniflora on N fixation and removal, as well as how these two processes balance to determine the N budget, remain unclear. Here, simultaneous quantifications of N fixation and removal via 15N tracing experiment with native Phragmites australis, invasive S. alterniflora, and bare flats as well as corresponding functional gene abundance by qPCR were carried out to explore the response of N dynamics to S. alterniflora invasion. Our results showed that N fixation and removal rates ranged from 0.77 ± 0.08 to 16.12 ± 1.13 nmol/(g·h) and from 1.42 ± 0.14 to 16.35 ± 1.10 nmol/(g·h), respectively, and invasive S. alterniflora generally facilitated the two processes rates. Based on the difference between N removal and fixation rates, net N2 fluxes were estimated in the range of −0.39 ± 0.14 to 8.24 ± 2.23 nmol/(g·h). Estimated net N2 fluxes in S. alterniflora stands were lower than those in bare flats and P. australis stands, indicating that the increase in N removal caused by S. alterniflora invasion may be more than offset by N fixation process. Random forest analysis revealed that functional microorganisms were the most important factor associated with the corresponding N transformation process. Overall, our results highlight the importance of N fixation in evaluating N budget of estuarine and coastal wetlands, providing valuable insights into the ecological effect of S. alterniflora invasion.

Spartina alterniflora invasion differentially alters microbial residues and their contribution to soil organic C in coastal marsh and mangrove wetlands

Coastal wetlands play a critical role in global carbon (C) cycling while they are frequently challenged by exotic plant invasion. Microbial residues are increasingly recognized as the key constituent of stable soil organic C (SOC) but their responses to plant invasion in coastal wetlands remain largely unknown. Here, we investigated the effect of Spartina alterniflora invasion on microbial residues and their contribution to SOC in coastal wetland ecosystems with distinct native species (Phragmites australis (marsh), Kandelia candel or Avicennia marina). S. alterniflora invasion differentially altered microbial residue C accumulation in three coastal wetlands. Overall, the invasion of S. alterniflora significantly increased microbial residue C accumulation (17.8%) in marsh with native P. australis, while it showed negative or minor impacts on mangroves with K. candel and A. marine communities, despite nuanced site-specific differences. The divergent responses of microbial residues could be related to changes in edaphic conditions and plant inputs along with plant invasion process. Invasion of mangrove and marsh communities by S. alterniflora triggered divergent fungal and bacterial residue responses, leading to contrasting fungal to bacterial C ratios in invaded marsh and mangrove wetlands. The proportion of microbial residues in SOC pool was enhanced to some extent, with an increase by 14.4%, 33.0% and 1.9% in P. australis, K. candel and A. marine wetlands, respectively, despite inconsistent variations in total SOC with S. alterniflora invasion. This has an implication for an elevated preservation of microbial residue C relative to the other organic C fractions in the invaded coastal wetlands. These findings highlight the importance of anabolic microbial C sequestration process for the maintenance of wetland soil C, which may impact C cycling and balance in coastal wetland ecosystem subjected to S. alterniflora invasion. Our work provides new insights to capture the properties and mechanisms of microbial control over C sequestration as well as SOC source configuration in coastal wetlands undergoing S. alterniflora invasion.

Effect of Spartina alterniflora Invasion on Soil C:N:P Stoichiometry in Coastal Wetland of Hangzhou Bay

The invasion of Spartina alterniflora poses a great threat to coastal wetland ecosystems. In this study, the stoichiometric characteristics of soil carbon, nitrogen, and phosphorus under a Spartina alterniflora invasion were explored using ANOVA in a coastal wetland in Hangzhou Bay, and the driving coupling relationship between soil environmental factors and soil C:N:P stoichiometric characteristics of the coastal wetland were further explored based on the redundancy analysis (RDA), boosted regression tree (BRT), and partial least squares-structural equation (PLS-SEM) model. The results showed that:① after the invasion of Spartina alterniflora, soil N:P and total nitrogen (TN) in the wetland increased significantly, and with the increase in invasion time, TN and N:P decreased significantly, whereas soil organic carbon (SOC), C:N, and C:P increased significantly. ② The RDA model revealed that the main factors affecting the stoichiometric characteristics of topsoil C:N:P were SOC>electrical conductivity (EC)>TN in winter and SOC>bulk density (BD)>TN in summer. ③ The BRT model showed that under the invasion of Spartina alterniflora, TN was the key factor affecting soil C:N and N:P, and SOC was the key factor affecting C:P. ④ The PLS-SEM model showed that clay and water content directly affected SOC, thus affecting C:N and C:P; the clay and EC directly affected total phosphorus (TP), thus affecting N:P and C:P; and the EC directly affected TN, thus affecting C:N and N:P. In conclusion, the invasion of Spartina alterniflora had a significant impact on soil C:N:P stoichiometric characteristics in the study area. Soil physical properties and nutrient content directly or indirectly affected soil C:N:P stoichiometric characteristics to varying degrees.