Invasive Spartina Alterniflora Research Articles

Long-term Spartina alterniflora invasion simplified soil seed bank and regenerated community in a coastal marsh wetland.

The coastal wetland is easily invaded by alien species due to locating in the land and sea transitional area. As a potential driving regeneration force, the soil seed bank is vital to the community restoration and species diversity protection. To reveal the long-term Spartina alterniflora invasion impact on the soil seed banks and regenerated communities, we investigated the seed banks under the different vegetation types (S. alterniflora, Phragmites australis, Scirpus mariqueter, ruderal and unvegetated site) and soil depths (0-5 and 5-10 cm) in the coastal salt marsh wetland, Chongming island, eastern China. The results showed that the soil seed bank richness and species density under different vegetation types were higher than the aboveground vegetation, and those of 0-5 cm seed banks were higher than 5-10 cm, except for the unvegetated site. The species richness and S. alterniflora seed proportion in the seed banks under S. alterniflora communities (S.AS) were lower and larger respectively than those of other sites. The species composition between S.AS and the aboveground communities showed high similarity with aggregation phylogenetic structures in two soil depths. The seed bank variations at 0-5 and 5-10 cm depths were interpreted 3.03% and 2.25% by the aboveground communities, while 4.92% and 5.55% were interpreted by the soil microbial biomass. The SEM model explained 98.1% and 91.8% of the seed banks richness at the 0-5 cm depth and 5-10 cm depth, respectively, and explained 98.8% and 46.1% of the seed banks species density at the 0-5 cm depth and 5-10 cm depth, respectively. The aboveground vegetation biomass and abundance directly affected the 0-5 cm seed banks richness and species density, while its height and biomass only affected the 5-10 cm seed banks species density. The 0-10 cm soil depth microbial biomass indirectly affected the 0-5 cm seed banks richness and species density, while affected the 5-10 cm seed banks richness. Soil physical and chemical properties only indirectly affected the 0-5 cm seed banks species density. The results provided a reference for the ecological evaluation of the impacts of S. alterniflora invasion into the coastal salt marsh wetland of eastern China, and guidance for the protection and restoration of the native plant communities.

Benthic bacterial communities and bacteria–environment interactions after Kandelia obovata introduction and Spartina alterniflora invasion in Yueqing Bay, China

Kandelia obovata and Spartina alterniflora can significantly affect sediment properties and benthic bacterial communities. However, little is known about the different impacts of K. obovata introduction and S. alterniflora invasion on local ecosystems. In this study, sediment properties and bacterial communities were determined at different depths (0–30, 30–60, 60–90 cm) in mudflat, K. obovata and S. alterniflora habitats in Yueqing Bay, Zhejiang Province, China. The results showed that K. obovata sediments had higher soil organic carbon contents and exhibited higher heavy metals, of which Cu posed a moderate contamination according to the geoaccumulation index. K. obovata habitats also presented more distinct bacterial communities, with Gemmatimonadetes abundant and acting as keystone connectors. The shift in bacterial communities after K. obovata introduction was mainly caused by changes in sediment organic carbon, Cd and Zn concentrations. S. alterniflora habitats had higher proportions of Anaerolineae and Sulfurovum and some bacteria that are beneficial to plant growth, including Cyanobacteria and Erythrobacter. Bacteria–environment interactions showed that heavy metals formed a more complex and closer co-occurrence network with Proteobacteria and Gemmatimonadetes after K. obovata introduction, while S. alterniflora invasion simplified sediment bacterial co-occurrence networks and bacteria–environment interactions in native ecosystems. Our results revealed that the impacts of S. alterniflora invasion on local ecosystems were outweighed by K. obovata introduction in Yueqing Bay. The present study could improve the understanding of ecological changes after K. obovata introduction and S. alterniflora invasion.

Coupling effects of nitrate reduction and sulfur oxidation in a subtropical marine mangrove ecosystem with Spartina alterniflora invasion

The mangrove ecosystem has a high nitrate reduction capacity, which significantly alleviates severe nitrogen pollution. However, current research on nitrate reduction mechanisms in the mangrove ecosystem is limited. Furthermore, Spartina alterniflora invasion has disrupted the balance of the mangrove ecosystem and the effect of S. alterniflora on nitrate reduction has not yet been fully elucidated. Nitrate reduction was comprehensively investigated in a subtropical mangrove ecosystem in this study, which has been invaded by S. alterniflora for 40 years. Results showed that S. alterniflora significantly increased the relative and absolute abundance of nitrate reduction genes, especially nirS (nitrite reductase), in the mangrove ecosystem. Dissimilatory nitrate reduction to ammonium was the main pathway of nitrate reduction in the mangrove ecosystem. Nitrate reduction was mainly performed by Desulfobacterales and occurred in the shallow layers (0–10 cm) of mangrove sediments. A strong positive correlation was found between nitrate reduction and sulfur oxidation (especially sulfide oxidation), and the sulfide content was significantly positively correlated with the relative abundance of nitrate reduction genes. Moreover, 207 metagenomic assembled genomes (MAGs) were constructed, including 50 MAGs with high numbers (≥ 10) of nitrate reduction genes. This finding indicates that the dominant microbes had strong nitrate reduction potential in mangrove sediments. Our findings highlight the impact of S. alterniflora invasion on nitrate reduction in a subtropical marine mangrove ecosystem. This study provides new insights into our understanding of nitrogen pollution control and contributes to the exploration of new nitrogen-degrading microbes in mangrove ecosystems.

Effects of Spartina alterniflora Invasion on Soil Carbon, Nitrogen and Phosphorus in Yancheng Coastal Wetlands

The rapid invasion of Spartina alterniflora threatens the ability of soils to store carbon (C), nitrogen (N) and phosphorus (P) in coastal wetlands. This study analyzed the temporal and spatial distribution characteristics of soil C, N and P in Spartina alterniflora wetland in the Dafeng Elk Nature Reserve of Yancheng, China, in different invasion stages from 1995 to 2020. The results suggest that: (1) the invasion of Spartina alterniflora increased the content and storage of soil C and N, and decreased the content and storage of soil P; (2) altered soil properties caused by Spartina alterniflora invasion also indirectly affected the accumulation of soil C, N and P in wetland ecosystem. Organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) were positively correlated with soil moisture content (SMC) and electrical conductivity (SEC), and negatively correlated with bulk density (SBD) and pH; (3) Spartina alterniflora invasion increased soil C/P and N/P, and decreased soil C/N. In conclusion, Spartina alterniflora invasion has changed the ecosystem, increased the storage capacity of soil C and N in the invasive ecosystem, but reduced the storage capacity of soil P to a certain extent.

Leaf litter decomposition and its drivers differ between an invasive and a native plant: Management implications.

Litter decomposition is a key process of the carbon cycle in terrestrial and aquatic ecosystems. The dominant conceptual model of litter decomposition assumes that environmental conditions, litter traits, and decomposer composition control litter decomposition in a decreasing order, yet whether this hierarchical model applies to both invasive and native plant species is unknown. Here, by comparing a widespread invasive plant and its native counterpart in Chinese coastal saltmarshes, we aimed to examine whether the hierarchy of factors controlling litter decomposition varies with plant species in the face of plant invasions. Leaf litter of invasive Spartina alterniflora and native Phragmites australis was collected across an 18° latitudinal range to capture wide variation in litter traits. These leaf litter samples were transported to three saltmarsh sites of different latitudes and were incubated in litterbags varying in mesh size (0.1, 2, and 5mm) to manipulate decomposer composition. After 90-day incubation, we found a parallel latitudinal pattern in leaf litter decomposition rate (k) between S. alterniflora and P. australis regardless of saltmarsh site and mesh size. Nonetheless, the k value of S. alterniflora was 2.2-fold higher than that of P. australis. Moreover, there was a shift in the hierarchy of factors controlling k values between S. alterniflora and P. australis: environmental conditions (climate and soil) dominated other factors in P. australis, whereas litter traits contributed more than environmental conditions in S. alterniflora. Overall, our findings show that leaf litter decomposition and its dominant controlling factors across broad geographical ranges can vary with plant invasions, having important implications for managing invasive plants in the context of conserving coastal blue carbon.

Mechanisms of Seed-To-Seed Interactions between Dominant Species in the Yangtze River Estuary under Saline Condition

Plant community assembly is the central issue in community ecology. As plant traits differ in different life history stages, the form, intensity and mechanism of interspecific interactions may change with the ontogenetic process of plants. However, our understanding of interspecific interaction mechanisms during germination is still limited. Here, we conducted a laboratory germination experiment using five dominant species in Chongming Dongtan (Spartina alterniflora, Scirpus mariqueter, Phragmites australis, Suaeda glauca and Tripolium vulgare) to assess their germination performance in control (monoculture), allelopathy and mixture treatments. The results indicated that seeds could affect germination performance of neighbors through both allelopathy and salinity modification. Salinity of the solution in Petri dishes after seed germination decreased significantly in most species combinations in competition treatment, and was negatively correlated with the number of total germinated seeds. Seed leachate of invasive Spartina alterniflora significantly accelerated the germination of two native halophytes Suaeda glauca and Tripolium vulgare, but not Scirpus mariqueter and Phragmites australis. The salt absorption by Spartina alterniflora seeds had inconsistent effects compared with that of its seed leachate. On the other hand, seed leachate of native Scirpus mariqueter and Phragmites australis significantly slowed down the germination of invasive Spartina alterniflora. The effect of salinity modification of Scirpus mariqueter on Spartina alterniflora was positive, whereas that of other species was neutral. Considering seed-to-seed interactions is an important perspective to understand the underlying mechanisms of community dynamics, species diversity maintenance and invasion of alien species, and can improve the effectiveness in the management of invaded coastal wetlands.

Spartina alterniflora invasion changed enrichment and coupling characteristics of multiple elements in coastal marsh sediment of the Yellow River Delta

Uncertainty surrounds how plant invasion shapes characteristics of multiple-element distribution, which makes it difficult to learn more about the interaction between plants and elemental cycles. In this paper, 14 elements were analyzed in sediments from coastal wetlands where Spartina alterniflora (Sa) and native plants grow in the Yellow River Delta (YRD). Results indicated that mineral elements such as iron (Fe) and manganese (Mn) were highest in wetlands dominated by Phragmites australis (Pa), whereas biogenic elements as carbon (C) and nitrogen (N) were the most abundant in Sa wetlands. Phytogenous processes control elemental variation in Sa wetlands, whereas lithosphere or sedimentation govern elemental changes in Pa wetlands. Sa wetlands had a complicated network with the greatest number of linkages among multiple elements, indicating that S. alterniflora invasion might enhance links among numerous elements. Fe, sulfur (S), Mn, and magnesium (Mg) were cores of element networks in wetlands covered by native plant species, whereas they shifted to Mg in Sa wetlands. The Fe-Mn connection was the most essential link of element networks in wetlands with native plant species, and it switched to the Mg-Mn link in Sa wetlands.

Effects of salinity, temperature, and immersion conditions on seed germination of invasive Spartina alterniflora Loisel (smooth cordgrass) in Japan

Spartina alterniflora is a halophytic perennial grass species that has invaded coastal wetlands worldwide, including tidal flats in Japan. Rapid removal or eradication of invasive plants is desirable because of their severe ecological impact on local ecosystems. Seed germination is the main cause of its distribution and invasion. Understanding germination characteristics of S. alterniflora is critical for the prediction and early detection of its populations in new environments. Therefore, we aimed to investigate the effects of salinity (0, 10 20, 30, and 40 g/L artificial seawater), temperature (15, 20, 25 °C for constant, and 15/25 °C for alternating), and seed immersion (5 ml and 40 ml medium) based on laboratory experiments to evaluate the physicochemical environmental factors that facilitate the germination of S. alterniflora seeds. The cumulative germination rate and mean germination time were significantly affected by temperature and salinity but not by immersion. Increased salinity gradually reduced the cumulative germination rate in all temperature and immersion conditions. The mean germination time tended to be prolonged with the salinity level. A constant temperature of 25 °C promoted higher seed germination compared to constant temperatures of 15 and 20 °C. Notably, alternating temperature between 15 and 25 °C induced a higher seed germination compared to other constant temperatures. In addition, alternating temperatures varied the mean germination time to facilitate sensitivity to salinity, indicating that temperature fluctuation is a key factor that stimulates seed germination of S. alterniflora. These results suggest that S. alterniflora seeds prefer gaps in vegetation canopies and a low depth of burial in soil. Further studies are needed to analyze prevention strategies for the eradication of this invasive species in Japan.

Phenological heterogeneities of invasive Spartina alterniflora salt marshes revealed by high-spatial-resolution satellite imagery

The rapid expansion of invasive Spartina alterniflora in China’s coastal wetlands has seriously threatened regional ecosystem health. Characterizing phenology of S. alterniflora salt marshes supports the studies on plant invasion mechanism and carbon fluxes in coastal wetlands. To date, little is known about the capacity of high-spatial-resolution imagery in revealing local scale heterogeneities and temporal variations in the phenology; in addition, the driving factors of such heterogeneities are not well understood. This study took Yellow River Estuary (YRE) as the study site where S. alterniflora area has expanded since 2008. We retrieved start of season (SOS), end of season (EOS), length of growing season (LOS), and maximum Normalized Difference Vegetation Index (NDVImax) from time series Sentinel-2 imagery (10-meter resolution) during 2017 ∼ 2019 with assist of a rule-based data noise removal approach. Results showed that phenological metrics differ with invasion years. The differences in the median NDVImax reached 0.13, in the median SOS reached 26 days, and in the median LOS reached 51 days. S. alterniflora with 5-year invasion had the highest NDVImax, and younger S. alterniflora marshes had earlier SOS and longer LOS. Both SOS and EOS showed negative relationships with NDVImax, indicating the marshes with greater green vegetation cover had earlier SOS and EOS. More spatial details can be revealed from higher-resolution PlanetScope imagery (3-meter resolution), showing the influence of tidal creeks on phenology. The spatial heterogeneity in phenology was associated with environmental factors such as tidal inundation, elevation, micro-climate, nutrients, and canopy structure. S. alterniflora showed markedly higher NDVImax and earlier EOS in 2018 compared to 2017 and 2019, associated with larger river water discharge and sediment load brought by Water Sediment Regulation Scheme, greater precipitation in the wettest season and lower preseason temperature in 2018. Large phenological variations at both spatial and temporal scales suggest that S. alterniflora marshes are sensitive to the local environment and climate change. This study has the potential to provide references for the management of invasive plants threatening coastal wetlands in the world.

Invasive Spartina alterniflora accelerates soil gross nitrogen transformations to optimize its nitrogen acquisition in an estuarine and coastal wetland of China

Saltmarsh plants are important components of estuarine and coastal wetlands because they regulate ecosystem nitrogen (N) dynamics. However, complex interactions between the N uptake of saltmarsh plants and soil N transformation remain unclear. Here, we conducted a series of 15N tracing experiments with native Phragmites australis, invasive Spartina alterniflora, and bulk sediment without plants to explore the effect of plants on soil N cycling. The results showed that the NH4+ and NO3− uptake rates by the saltmarsh plants were 4.62–5.38 mg N kg⁻1 d⁻1 and 1.29–2.90 mg N kg⁻1 d⁻1, respectively, and the invasive S. alterniflora had a higher N uptake than the native P. australis. The presence of saltmarsh plants promoted N mineralization and dissimilatory NO3− reduction to NH4+, increasing the available NH4+ supply for the plants. Conversely, NH4+ immobilization and autotrophic nitrification rates were drastically reduced in the presence of the saltmarsh plants, indicating that the plants were able to outcompete soil microorganisms in NH4+ acquisition. Meanwhile, heterotrophic nitrification (organic N oxidation), which accounted for 66–82% of the total nitrification, was stimulated by the saltmarsh plants. Increased heterotrophic nitrification in the saltmarsh plants helped to provide NO3− substrates to meet the needs of the soil microorganisms and the plants. The regulatory effect of the invasive S. alterniflora on soil gross N transformation was more pronounced than that of the native P. australis due to the higher N requirements of the former. Microbial carbon sources and energy sources, relevant gene abundances and exoenzyme activities were the main factors by which the saltmarsh plants regulated gross N transformations. Overall, our results show that there are various interactions between soil microorganisms and saltmarsh plants and that S. alterniflora accelerates gross N transformations in the soil to meet its large demand for N. These findings provide valuable insights into the ecological management of invasive plants in estuarine and coastal ecosystems.

Carbon sources and trophic levels in the food web of the largest mangrove reserve of China

Zhanjiang Mangrove National Nature Reserve (ZMNNR) is the largest mangrove natural reserve in China. We conducted the first comprehensive isotope study on the ZMNNR. Primary producers, including mangroves, grasses, microphytobenthos (MPB), etc. formed the lowest trophic level. Gastropods and bivalves, crustaceans and carnivorous fish were on the 2nd, 3rd and 4th trophic levels, respectively. In Zhanjiang tidal estuary, mangroves contributed about 1/3 of the carbon sources for consumers, substantially smaller than those in swamps and lagoons, but higher than those in river delta mangroves. The major carbon sources to the food web shifted from particulate organic matter (POM), consisting of detritus of mangrove leaves and phytoplankton, perennial grasses and mangrove leaves in the high tide zone to invasive Spartina alterniflora, POM and mangrove leaves in the low tide mudflat. Insignificant contributions of MPB were due to restricted light penetration by the dense mangroves in the high tide zone, and prolonged submersion and sediment resuspension in the low tide mudflat. Aquaculture effluents caused substantially higher δ15N values of all primary producers and consumers in the ZMNNR. In addition, high nutrient content and N/P ratios in aquatic effluents might allow cyanobacteria in MPB to be outcompeted by other fast growing microorganisms. Aquaculture effluents might also contribute to shortened trophic levels and promotion of the dominance of resistant species in the ZMNNR. We call for a sustainable development strategy to conserve the valuable ecosystem in the ZMNNR.

Effects of methyl halide flux characteristics following Spartina alterniflora invasion in a seaward direction in a temperate salt marsh, China

In this study, we explored the source-sink characteristics of methyl halide (CH3X; X = Cl, Br, I) in coastal wetlands located in temperate regions, and identified key factors affecting the spatio-temporal variation of CH3X during the invasion of Spartina alterniflora. We used static chamber-gas chromatography to monitor CH3X fluxes in the S. alterniflora area and bare flat area of the Jiaozhou Bay salt marsh for a long time from August 2015 to May 2017. Our results indicated that CH3X emissions showed obvious seasonal and diurnal variations. The S. alterniflora area was a source of CH3X, with higher fluxes in the spring and autumn seasons. CH3X fluxes were higher during the daytime than at night, and the diurnal difference in CH3Br was the most significant (4.51 times). The bare flat area was mainly a sink for CH3X, and the maximum absorption flux occurred in summer. At this time, the microbial activity was greater, and the consumption rate during the day was higher than that at night. Extreme linear correlations existed between the fluxes of CH3Cl, CH3Br, and CH3I (P < 0.01), indicating that the production and consumption of the three gases were likely to have similar mechanisms and were affected by the same factors. S. alterniflora invasion increased CH3X emissions and shifted the original bare flat area from a sink to a source of CH3X. The biomass of S. alterniflora, especially the leaf, significantly affects CH3X fluxes. Additionally, S. alterniflora increased the content of total organic carbon, total sulfur, available sulfur, and iron (III) in the soil, which were the main factors promoting the source-sink transformation of CH3X. Based on the current invasive area of S. alterniflora in China, we estimated that the annual emissions of CH3Cl, CH3Br, and CH3I from S. alterniflora into the troposphere were 9.04 × 106, 2.42 × 105 and 2.06 × 105 mol, respectively.

Unraveling the Effects of Cold Stratification and Temperature on the Seed Germination of Invasive Spartina alterniflora Across Latitude.

Seed germination is critical to the life history of plants, playing an important role in the successful recruitment, colonization, and even invasion of new individuals within and outside population distribution ranges. Cold stratification and temperature are the key factors affecting seed germination traits. Studying how these two factors drive geographical variation in seed germination is essential to analyze and predict the geographical distribution range of alien plants in novel habitats. Spartina alterniflora, native to the United States, was introduced into China in 1979 and has spread over 20° of latitude along the eastern coast of China. Germination plays a crucial role in S. alterniflora’s large-scale invasion and diffusion across latitude. To evaluate the effects of cold stratification and temperature on seed germination of S. alterniflora across latitude, we collected seeds at seven locations across latitude in China. We exposed these provenances to cold stratification at 4°C (0, 1, 3, and 5 months) and germination temperature (5°C, 15°C, 25°C, and 35°C) treatments in growth chambers. Seed germination was observed for 98 days, and we calculated germination rate, germination index, and germination time. Results indicated that longer cold stratification significantly promoted germination rate and germination index, but decreased germination time. Similarly, higher germination temperature significantly promoted germination rate and germination index, but decreased germination time. Moreover, there were significant interactive effects on germination traits between cold stratification and temperature. Seed germination traits showed linear relationships with latitude, indicating that S. alterniflora seeds from different provenances germinated at different times and adopted different germination strategies. The stratification and temperature are the most important factors regulating the dormancy and germination seeds, so they can be important drivers of this variation along latitude. Under scenarios of warmer regional temperature, seeds at higher latitudes could germinate earlier and have higher germination rate, which would favor a potential northern expansion of this invasive plant.

Changes to the structure and function of microbial communities in Spartina alterniflora and Kandelia obovata sediments as a factor of stand age

The effects of Kandelia obovata introduction and Spartina alterniflora invasion on the microorganisms are very important for coastal ecology and restoration. MiSeq sequencing of the 16S rRNA gene and Tax4Fun predictive functional profiles were used to investigate the diversity estimators, community structures and potential metabolic functions of benthic bacteria in surface sediments of coastal wetlands invaded by S. alterniflora and artificially planted with K. obovata at stand ages of 2-, 8-, 11-, 16- and 60-years on Ximen Island, Yueqing Bay, China. The results showed that the sediment bacterial richness was significantly decreased in 60-year K. obovata stands compared with younger K. obovata stands and S. alterniflora stands. The age of K. obovata and specie of exotic plants formed distinct bacterial communities and functional structures in sediments, respectively. With increasing plantation of K. obovata, the bacterial communities shifted from the class Anaerolineae, the genus Sulfurovum, and bacteria involved in sulfate reduction as abundant taxa to higher proportions of bacteria involved in degradation of plant polysaccharide and nitrate reduction. The shift in bacterial community structures was mainly driven by changes in sediment total organic matter, total nitrogen, total phosphorus, ammonium concentrations, pH and temperature. The community functions also changed from nitrogen fixation to more nitrate reduction and denitrification processes. Compared with 60-year K. obovata, S. alterniflora was occupied by higher proportions of the phylum Bacteroidetes, the orders Rhodobacterales, Flavobacteriales and Desulfuromonadales, and total nitrogen, total phosphorus concentrations and sediment temperature were major environmental factors affecting the variation. Among the major sulfur cycling processes examined, higher potential of dissimilatory sulfate reduction was observed in S. alterniflora. Our results indicated that K. obovata introduction has a greater effect on the bacterial community diversity and structure than S. alterniflora invasion. This study could improve the understanding of microbial processes and potential functions of K. obovata introduction and S. alterniflora invasion and provide reference in the restoration and management of the coastal wetlands.

Responses of Soil Microbiota to Different Control Methods of the Spartina alterniflora in the Yellow River Delta.

Spartina alterniflora invasion has negative effects on the structure and functioning of coastal wetland ecosystems. Therefore, many methods for controlling S. alterniflora invasion have been developed. S. alterniflora control methods can affect plant community, which results in changes in microbial communities and subsequent changes in soil ecological processes. However, the effects of controlling S. alterniflora on soil microbial communities remain poorly understood. We aimed to examine the responses of bacterial and fungal communities to invasion control methods (cutting plus tilling treatment: CT; mechanical rolling treatment: MR). Soil bacterial and fungal community diversity and composition structure were assessed using high-throughput sequencing technology. The findings of the study showed that bacterial diversity and richness in the CT treatment reduced substantially, but fungal diversity and richness did not show any remarkable change. Bacterial and fungal diversity and richness in the MR treatment were not affected considerably. In addition, the two control methods significantly changed the soil microbial community structure. The relative abundance of bacteria negatively associated with nutrient cycling increased considerably in the CT treatment. The considerable increases in the relative abundance of certain bacterial taxa in the MR treatment may promote soil nutrient cycling. Compared with mechanical rolling, soil bacterial community diversity and structure were more sensitive to cutting plus tilling.

Exogenous nitrogen from riverine exports promotes soil methane production in saltmarshes in China

Methane emissions from saltmarshes can potentially promote climate warming. Soil methane production is positively correlated with methane emissions from saltmarshes. Understanding the factors influencing soil methane production will improve the prediction of methane emissions, but an investigation of these factors has not been conducted in saltmarshes in China. We collected soils from native Phragmites australis and invasive Spartina alterniflora saltmarshes along the coast of China; the soil potential methane production (PMP) was determined by incubation experiments. The large-scale investigation results showed that the ratios of methanogens relative to sulfate-reducing bacteria (RMRS) and total organic carbon (TOC) were positively correlated with soil PMP for both species. Dissolved inorganic nitrogen (DIN) was positively correlated with the soil PMP of P. australis saltmarshes, and plant biomass was positively correlated with the soil PMP of S. alterniflora saltmarshes. Our results showed that exogenous nitrogen from riverine exports was positively correlated with DIN and plant biomass in both P. australis and S. alterniflora saltmarshes. In addition, exogenous nitrogen was also positively correlated with TOC in S. alterniflora saltmarshes. Consequently, exogenous nitrogen indirectly promoted soil methane production in P. australis saltmarshes by increasing the DIN and promoted soil methane production in S. alterniflora saltmarshes by enhancing the TOC and plant biomass. Moreover, we found that the promoting effect of DIN on the soil PMP of P. australis saltmarshes increased when the incubation temperature increased from 15 °C to 25 °C. Thus, the promoting effect of exogenous nitrogen on the soil methane production in P. australis saltmarshes might be strengthened in the peak of growing season. Our findings are the first to confirm that exogenous nitrogen inputs from rivers indirectly promote soil methane production in P. australis and S. alterniflora saltmarshes and provide new insights into the factors responsible for soil methane production in saltmarshes.

Effects of Spartina invasion on the soil organic carbon content in salt marsh and mangrove ecosystems in China

Abstract Coastal wetlands are large reservoirs of soil carbon (C) and have been invaded globally by the exotic species Spartina alterniflora. However, the effects of these invasions on soil C content remain unclear. We performed a meta‐analysis of 2479 soil organic C (SOC) content observations collected from 91 field studies conducted in coastal China, the world's largest introduced range for S. alterniflora. Spartina alterniflora invasions had no significant effect on the SOC content in vegetated native wetlands in coastal China. S. alterniflora increased the SOC content in the top 30 cm of salt marshes after invading only one of the nine marsh types, which was dominated by a dwarf succulent species, Suaeda salsa in the northern subtropics and decreased the SOC content in the top 60 cm of mangroves dominated by Kandelia obovata and mixed communities after invasion in the southern subtropics. S. alterniflora invasions did not significantly affect the SOC content in the other salt marsh or mangrove ecosystems. Moreover, the SOC content in S. alterniflora‐invaded ecosystems increased only on a decadal scale and then decreased, rather than increasing monotonically. Synthesis and applications. Our findings reveal that the SOC content of S. alterniflora‐invaded ecosystems is similar to or lower than that of most native ecosystems. As soil bulk density is often reduced by S. alterniflora invasions, the C storage capabilities of vegetated native ecosystems might be substantially reduced after S. alterniflora invasions. Moreover, the SOC content in these invaded ecosystems gradually decreased after long‐term S. alterniflora invasions (&gt;20 years). Therefore, the displacement of native salt marsh or mangrove species by S. alterniflora may not be considered a natural climate solution to increase the C sink in coastal China. Future efforts are required to control S. alterniflora preferentially in coastal wetlands where S. alterniflora invasions do not increase the SOC content, and to conserve and restore native coastal wetlands, which are the core areas of the World Natural Heritage site serving as China's Migratory Bird Sanctuaries that have been severely threatened by S. alterniflora invasions, to increase ecological integrity and C sinks and to mitigate climate change.

Spartina alterniflora Invasion Enhances Dissimilatory Nitrate Reduction to Ammonium (DNRA) Rates in the Yangtze River Estuary, China

Dissimilatory nitrate reduction to ammonium (DNRA) can save N by converting nitrate into ammonium and avoiding nitrate leaching and runoff in saltmarshes. However, little is known about the effects of invasive plants on DNRA in the upper and deeper soil layers in salt marshes. Here, we investigated DNRA rates in the soils of six different depth layers (0–5, 5–10, 10–20, 20–30, 30–50, and 50–100 cm) from the invasive Spartina alterniflora marshland, two native plants Scirpus mariqueter and Phragmites australis marshlands, and bare mudflat on Chongming Island, located in the Yangtze River Estuary, China. Our results show that S. alterniflora significantly increased DNRA rates in both the upper 50 cm soil and deeper 50–100 cm soil layers. With respect to the entire soil profile, the NO3− reduction content calculated from DNRA in S. alterniflora marshland was 502.84 g N m−2 yr−1, increased by 47.10%, 49.42%, and 38.57% compared to bare mudflat, S. mariquete, and P. australis, respectively. Moreover, NO3− reduction content from the 50–100 cm soil layers was almost identical to that in the upper 50 cm of the soil. In the month of May, DNRA is primarily regulated by SO42− and pH in the upper and deeper soil layers, respectively, whereas, in the month of October, soil pH accounted for the most variables of DNRA in both the upper and deeper soil layers. Altogether, these results from a new perspective confirm that S. alterniflora invasion increases soil N pool and may further push its invasion in salt marshes, and the importance of deeper soil in nitrogen cycling cannot be ignored.

Spartina alterniflora invasion and mangrove restoration alter diversity and composition of sediment diazotrophic community

Ecological restoration using native mangrove species (i.e., Kandelia obovata) is a practical approach for controlling Spartina alterniflora invasion in the coastal wetlands. Diazotrophs play a critical role in enhancing productivity of both S. alterniflora and mangrove plant, via providing new nitrogen to the coastal wetlands ecosystems. However, response of the diazotrophic community to S. alterniflora invasion and subsequent mangrove restoration remains unclear. The present study monitored sediment physicochemical properties and diazotrophic communities across a chronosequence of restored mangrove wetland (bare mudflat, invasive S. alterniflora stands, 3-year and 10-year K. obovata restoration areas, and mature K. obovata forests over 30 years). The results showed an orderly succession in the sediment properties and diazotrophic community composition after S. alterniflora invasion and along mangrove restoration chronosequences. Almost all sediment nutrient contents (e.g., TC, TN, and C/N) were significantly (P < 0.05) increased by S. alterniflora invasion, then they decreased sharply in the newly restored mangrove and increased gradually with restoration ages, with the highest values in the mature mangrove. The diazotrophs demonstrated distinct community structure in different seasons. Diazotrophic alpha diversity increased with S. alterniflora invasion and mangrove restoration age in winter. Sulfate-reducing bacteria (SRB) was the dominant diazotrophic group, especially Desulfuromonadales. Moreover, network analyses revealed that SRB groups were the major keystone taxa. The relative abundance of Desulfuromonadales decreased, while the abundance of Chromatiales increased gradually with the S. alterniflora invasion and mangrove restoration age. Redundancy analysis revealed that TC, TN, and TS were the significant (P < 0.05) environmental factors in altering sediment diazotrophic communities in both seasons. These findings expand the current understanding of succession patterns of diazotrophic communities in mangrove restoration and provide new perspectives on the sustainable management of mangrove ecosystems.

Comparison of macrobenthic communities between the invasive Spartina alterniflora and native Suaeda glauca habitats in the Yellow River Delta

Salt marsh habitats in estuaries play important roles in species compositions and macrobenthos abundances. Here, the macrobenthic communities and environmental conditions in two habitats, which are dominated by the invasive species Spartina alterniflora (SA) and native species Suaeda glauca (SG), in the Yellow River Delta (YRD) were studied to assess habitat function in four months of one year. The species diversity and abundance of macrobenthos in the SA habitat were much higher than those in the SG habitat. The taxonomic groups in the former showed significant changes over time and shifted from polychaeta (68%) in spring to mollusca (97%) in autumn, but in the latter, it was dominated by crustacea (63 ~ 86%), mollusca (1 ~ 25%) and polychaeta (9 ~ 13%), and only mollusca exhibited obvious month changes. The sediments in the SA habitat contained richer organic matter contents and exhibited higher Chl-a concentrations than those in the SG habitat, although the grain sizes were coarser in the SA habitat. Macrobenthos in the SA habitat displayed significant negative correlations with salinity and organic matter in four months. The results indicated that the taxonomic groups of macrobenthos in the SA habitat were simpler and more sensitive to environmental changes than those in the SG habitat. The effects of invasive SA on macrobenthos need to be further observed and explored, if it continues to expand in the YRD.