Exotic Spartina Alterniflora Research Articles

Coastal exotic plant serves as a habitat for a notorious wetland pest in unfavorable seasons: A case study of exotic Spartina alterniflora in China

Coastal biological invasions pose a wide-reaching threat to various ecosystems, affecting both vegetation and herbivores in native communities. Although herbivores often exert strong top-down control on vegetation, the impact of invasive species on consumers that strongly regulate native species in invaded ecosystems remains unclear. Therefore, through field surveys and feeding preference experiments, this study examined the effects of the invasive Spartina alterniflora Loisel. (hereafter, Spartina) on the native moth Laelia coenosa Hübner (hereafter, Laelia), a notorious pest that has been documented to cause significant damage to native Phragmites australis (Cav.) Trin. ex Steud. (hereafter, Phragmites) marshes in coastal wetlands of China. Field surveys showed that Laelia larvae were more abundant and had higher grazing rates in Phragmites than in Spartina marshes in summer, but these patterns reversed in autumn. Feeding experiments consistently showed that the dietary preference of Laelia larvae switched from Phragmites in summer to Spartina in autumn, likely because Spartina has a longer growing season and relatively higher nutritional quality than Phragmites in autumn. Thus, by providing shelters (i.e., dietary sources and habitats) during unfavorable seasons, Spartina invasions may facilitate this insect pest Laelia and potentially amplify its damage to native wetland vegetation. Our work reveals a novel, indirect mechanism of coastal invasion impacts and highlights the importance of incorporating seasonal variation in plant-herbivore interactions.

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.

Changes in sediment greenhouse gases production dynamics in an estuarine wetland following invasion by Spartina alterniflora.

Invasive Spartina alterniflora (S. alterniflora) has significant impacts on sediment biogeochemical cycling in the tidal wetlands of estuaries and coasts. However, the impact of exotic Spartina alterniflora invasion on greenhouse gases (GHGs) production dynamics in sediments remain limited. Here, we investigated the dynamics of sediment physicochemical properties, GHGs production rates, and microbial gene abundances in a native Cyperus malacensis habitat and three invasive S. alterniflora habitats (6-, 10-, and 14-year) in the Minjiang River Estuary, China. The methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) production rates varied both spatially and seasonally, while microbial gene abundances (bacterial and fungal gene abundances) and organic matter (TOC and TN) only varied spatially. GHGs production rates were also characterized by higher values in surface sediment (0-10 cm) compared to subsurface sediment (10-20 cm) and by seasonal variations with higher values in summer than in winter. S. alterniflora invasion can significantly increase CH4 and CO2 production rates, organic matter, and microbial gene abundances (p < 0.05). Temperature, organic matter and microbial gene abundances were the most dominating factor controlling the spatio-temporal variations of CH4 and CO2 production rates. Overall, our findings highlighted the significant role of S. alterniflora invasion in regulating GHGs production rates in coastal wetland sediments and provided fundamental data for estimating GHGs emissions and carbon sequestration in the complex tidal wetlands.

2013–2022年福建漳江口互花米草分布无人机遥感数据集

The invasion of exotic Spartina alterniflora into Chinese coastal wetlands poses a threat to biodiversity and ecosystem functions. Thus monitoring the spatio-temporal distribution of the invasion is important for the conservation/restoration and scientific management of coastal wetlands. This dataset includes S. alterniflora spatial data, derived from UAV-based RGB photos over Zhangjiang Estuary, Fujian Province from 2013 to 2022. Using a drone equipped with a visible light camera, we selected low-tide periods during the day, we captured the aerial images in the Zhangjiang Estuary wetland in Fujian Province from 2013 to 2022. Subsequently, we employed the dynamic structure-from-motion (SFM) techniques to reconstruct a three-dimensional model by mosaicking the collected aerial images to obtain a digital orthophoto image covering the study area. This process was repeated annually, resulting in a total of ten orthophoto images. Based on the classified data derived from these orthophoto images, the spatial distribution data of S. alterniflora over ten consecutive years were extracted. The high-quality UAV photos in this dataset are comprehensive and uniformly color-balanced. Located within the Zhangjiang Estuary Mangrove National Nature Reserve with coastal wetland vegetations such as mangroves and S. alterniflora, the study area is one of the most typical mangrove-S. alterniflora ecotones in China. This long-term (over ten consecutive years) and high-resolution (20 cm) image dataset for mangrove-S. alterniflora ecotone can be used in many studies including monitoring coastal wetland vegetation succession, mangrove conservation and restoration, and 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.

Does Spartina invasion affect the carbohydrate assimilation of polychaetes in mangroves? A case study in the Zhangjiang Estuary Mangrove National Nature Reserve

Carbohydrates, which form a large pool of carbon in plant litter, are a major driver of litter dynamics and benthic food webs in Spartina-invaded mangrove ecosystems. However, there are large gaps in knowledge regarding the effects of exotic leaf litter carbohydrates on benthic carbon assimilation. In this study, the dynamics of carbohydrates, including cellulose, lignin and monosaccharides (glucose, mannose, galactose, xylose and arabinose), during the leaf litter decomposition of the exotic Spartina alterniflora (SA) and the native mangrove plant Kandelia obovata (KO) were analyzed and compared. Meanwhile, the differences in the δ13C values of cellulose and lignin were used to analyze the utilization of leaf litter carbohydrates by polychaetes after Spartina invasion. Our results showed that the decomposition rate of exotic SA leaf litter was lower than that of KO leaf litter. The contents of cellulose, glucose and galactose in SA leaf litter were higher than those in KO leaf litter (P < 0.05). The decomposition rates of cellulose and monosaccharides (arabinose, xylose, mannose and galactose) in KO leaf litter were faster than those in SA leaf litter (P < 0.05). However, there was no significant difference in the decomposition rates of lignin in the leaf litter between exotic SA leaf litter and native KO leaf litter (P > 0.05). In the native KO plot, the carbohydrates of KO leaf litter contributed to >55% (cellulose, 26.91–28.02%; lignin, 28.05–28.64%) of the organic carbon sources of polychaetes. However, in the exotic SA plot, the carbohydrates of exotic SA leaf litter contributed to >65% (cellulose, 36.83–38.30%; lignin, 28.47–30.52%) of the organic carbon sources of polychaetes. The obtained results suggested that Spartina invasion increased the assimilation of cellulose by polychaetes. The high content and low decomposition rate of cellulose in the carbohydrates of exotic SA leaf litter might be important mechanisms for the carbon assimilation changes in the benthos of Spartina-invaded mangrove ecosystems. Given that Spartina has colonized in the Zhangjiang Estuary Mangrove Reserve, this study provides a theoretical basis for the recovery of benthos after ecological restoration. Furthermore, the mechanisms of carbohydrate assimilation by different benthos of different ecosystems should be comprehensively considered in the future.

Exotic Spartina alterniflora invasion enhances sediment N-loss while reducing N retention in mangrove wetland

Wide and rapid Spartina alterniflora invasion has threatened the sustainability of coastal wetlands of China, and has a significant impact on sediment biogeochemical cycle. However, the effects of exotic Spartina alterniflora invasion on sediment dissimilatory nitrates (NOx−) reduction processes and mechanisms underlying these pathways in mangrove wetlands have not been well studied. Here, denitrification (DEN), anaerobic ammonium oxidation (ANA) and dissimilatory nitrate reduction to ammonium (DNRA) rates, contributions (DEN%, ANA%, and DNRA%), associated functional genes (nirS, anammox 16S rRNA and nrfA) and controlling factors were investigated in surface sediments (0–5 cm) in mangrove and Spartina alterniflora communities and ecotone of the Zhangjiang Estuary, China. Significant spatial differences in DEN, DNRA, DEN%, and DNRA% were found among different plant communities (p < 0.05 for all). DEN is the dominant pathway of NOx− reduction processes in Spartina alterniflora community (87–89 %) and ecotone (75–78 %), while DEN (41–51 %) and DNRA (36–51 %) compete with each other in mangrove community. Organic matter was the most dominating factor controlling DEN and DNRA rates. Without accounting for confounding factors, Spartina alterniflora invasion significantly reduced the nitrogen retention index (NIRI), causing a large amount of nitrogen (N) loss in wetlands. We estimated that Spartina alterniflora invasion has increased sediment N-loss (831.09 ± 55.16 t N yr−1) but reduced N retention (783.48 ± 37.73 t N yr−1) in mangrove habitats of China within last 40 years (1980–2020). Thus, Spartina alterniflora invasion enhances sediment N-loss while reducing N retention in mangrove wetlands, which may threaten the mangrove buffer function and wetland N pools protection in estuarine and coastal ecosystems.

Seasonality and assembly of soil microbial communities in coastal salt marshes invaded by a perennial grass

Plant invasion profoundly changes the microbial-driven processes in the ecosystem; however, the seasonality of soil microbial communities and their assembly under plant invasion is poorly understood. In this study, coastal salt marshes with native Suaeda salsa (L.) Pall. and exotic Spartina alterniflora Loisel. in the Yellow River Estuary, North China, were selected, and soil bacterial and fungal communities and their seasonal variance were characterized by metabarcoding sequencing of the 16S rRNA gene and ITS2 regions, respectively. The importance of deterministic and stochastic processes in shaping bacterial and fungal seasonal assembly was explored by the null model. Results showed that soil microbes exhibited the lowest diversities in spring, while their diversity significantly improved in summer and autumn with the increase in organic carbon and nitrogen content in soils. Strong seasonal variances in microbial communities were observed, but plant invasion reduced the seasonal variation strength of soil bacteria. For the microbial assembly, the seasonal variability of soil bacterial community was mainly controlled by homogeneous selection, whereas soil fungal community was dominantly structured by stochastic processes. Among the selected variables, soil pH was the key abiotic factor driving the seasonal changes in bacteria and fungi. The microbial function annotation derived from taxonomy-based inference suggested that carbon metabolism was relatively stronger in spring, but nitrogen and sulfur metabolism increased evidently in summer and autumn, and the proportion of saprophytic fungi increased substantially after plant invasion. The seasonal turnover of bacterial and fungal groups were tightly associated with the seasonal variation in soil carbon and nitrogen contents. Collectively, these findings reveal the strong seasonal variability of different soil microbial constituents in plant-invaded coastal salt marshes and suggest the linkage between microbial community assembly and microbial-mediated functions in the context of plant invasions.

Plant invasion reshapes the latitudinal pattern of soil microbial necromass and its contribution to soil organic carbon in coastal wetlands

An emerging consensus is reached that microbial necromass carbon (MNC) significantly contributes to the formation and stabilization of soil organic carbon (SOC). Yet less is known about how plant invasion regulates MNC accumulation in coastal “Blue Carbon” ecosystems, and the environmental and microbial controls on MNC distribution across large scales are not well revealed. Here, we evaluated the contribution of MNC to SOC across a broad scale in coastal habitats including salt marshes and mangroves before and after exotic Spartina alterniflora Loisel. invasion. Specifically, we explored the influence of biotic and abiotic factors on soil MNC accumulation in paired native and invaded soils. The MNC pool was quantitatively inferred by analyzing soil microbial amino sugar content. We found that soil MNC contents significantly increased from temperate salt marshes to tropical mangroves, and the average MNC in mangrove soils was 1.3–5.0 times that in salt marshes. In native soils, the estimated contribution of MNC to SOC decreased along the climatic gradient, ranging from 13.4% to 61.2%. However, the invasion homogenized the latitudinal variation of MNC/SOC as the invasion reduced the contribution of soil MNC to SOC in salt marshes but increased the accumulation of microbial residues in mangrove habitats. Furthermore, MNC derived from fungi was 3.6–19.0 times that of bacteria and the contribution of fungal MNC to total MNC decreased against the increasing mean annual temperatures. The soil carbon-to-nitrogen ratio and soil texture had significant negative effects on MNC/SOC, indicating that soil nutrient status and physical protection are vital abiotic controls in regulating MNC accumulation in coastal wetlands. In summary, these findings revealed that MNC considerably contributes to soil carbon sequestration in coastal ecosystems, and emphasized that plant invasion can change the latitudinal pattern of MNC accumulation by altering environmental and microbial attributes and thus influencing soil carbon stabilization.

Blue carbon effects of mangrove restoration in subtropics where Spartina alterniflora invaded

Mangrove swamps and salt marshes are noted for their large carbon sequestration and storage capacity. In southeastern China, mangrove transplantation is conducted as ecological restoration where exotic Spartina alterniflora (hereafter Spartina) is removed. This study aimed to evaluate the effectiveness of mangrove transplantation on carbon sink potential. We analyzed chronological sediment accretion, soil carbon sequestration, and carbon storage of mangroves and Spartina on Maoyan and Ximen islands. Results showed that on both islands mangroves and invasive Spartina stored 12.7%–31.4% more carbon than bare mudflats over top meter soils, with Spartina outperforming restored mangroves. Both restored mangroves and invasive Spartina enhanced sediment accretion, but this enhancement was minimal in high sediment supply areas like Maoyan. Spartina invasion improved carbon sequestration capacity compared to bare mudflats in Maoyan, but mangrove restoration failed. In low sediment supply areas like Ximen, restored mangroves have outperformed Spartina invasion on carbon sequestration enhancement since >5 years of planting. Thus, carbon sink potential of mangrove restoration depended on sediment supply and time, with sediment supply being more important. We suggested that it was preferable to conduct mangrove restoration with Spartina eradication in low sediment supply areas in subtropics.

Effects of Spartina alterniflora invasion on the benthic invertebrate community in intertidal wetlands

AbstractExotic species invasions have profound effects on native ecosystems, with the ecological effects of non‐native plants being especially pervasive. We investigated the impacts of a plant invasion on benthic invertebrate assemblages in intertidal wetlands of the Yellow River Delta, comparing benthic communities associated with exotic Spartina alterniflora against those associated with native plants (Tamarix chinensis, Suaeda salsa, and mudflat). Our study surprisingly found that the density, diversity, and richness of benthic invertebrates associated with S. alterniflora were higher than with native habitats. Multivariate analysis also revealed that the structure of benthic invertebrates in S. alterniflora habitats was different than in T. chinensis and mudflat habitats. The structure of invertebrates associated with the invasive S. alterniflora and native S. salsa was more similar, although the dominant species differed. Indicator species analyses indicated that each habitat was characterized by unique taxa. S. alterniflora invasion increased the beta diversity of the community, with the species turnover component being of most importance. Mollusks were most likely to be influenced by Spartina invasions. Given the extent of the S. alterniflora invasion in the Yellow River Delta, altered benthic invertebrate communities will likely have important implications for the ecology of the intertidal wetland ecosystems and deserve increased attention.

Significant but short time assimilation of organic matter from decomposed exotic Spartina alterniflora leaf litter by mangrove polychaetes

Leaf litter is an important nutrient source for benthic communities within mangrove ecosystems and in adjacent areas. Exotic Spartina alterniflora invasion to the eastern coastlines of China has been shown to alter trophic relationship of benthos in mangrove wetlands, but little is known about potential effect of exotic litter decomposition on food sources of benthic macrofauna. We took the advantage of significant differences in the δ13C values of exotic S. alterniflora (C4 plant) from a native mangrove species Kandelia obovata (C3 plant) to investigate leaf carbon assimilation by polychaetes during litter decomposition. We found that exotic S. alterniflora leaf litter decomposed more slowly than that of native mangrove species at the mangrove site (P = 0.003), but there was no significant difference between them at the salt marsh site (P = 0.270). The polychaetes assimilated major organic matter from K. obvata and the particulate organic matter (> 90%) at the mangrove site. But the polychaetes assimilated major organic matter from S. alterniflora (> 60%) at the salt marsh site. However, after adding S. alterniflora leaf litter at the mangrove site, polychaetes assimilated more carbon from S. alterniflora (> 45%) than that from K. obovata (< 25%). Furthermore, the significant carbon assimilation only occurred within a short period time (1–2 months) during decomposition. Our results suggested that Spartina invasion can change food sources of local mangrove polychaetes through utilizing the organic matter released from litter decomposition, but such assimilation of exotic foods is of short time nature.

Consistent pattern of higher lability of leaves from high latitudes for both native Phragmites australis and exotic Spartina alterniflora

Abstract Global variation in litter decomposition rates is driven by climate, decomposer taxa and litter quality. Most large‐scale studies of litter quality have made comparisons across species, rather than within a species sourced from different latitudes. Here, we aim to explore latitudinal variation in leaf litter decomposition rate and litter quality within a species. We investigated whether the typical interspecific pattern of increased litter lability at higher latitudes also holds within plant species, by comparing leaf litter decomposition rates and litter traits from stands of native Phragmites australis and exotic Spartina alterniflora in Chinese coastal wetlands spanning 20° of latitude using parallel greenhouse and field transplant experiments. We found that leaf litter from high‐latitude plants decomposed 46.6 ± 5.1% faster than that from low‐latitude conspecifics, with similar patterns in both plant species in both decomposition experiments. Higher lability of leaves from high latitudes was associated with lower levels of plant defensive compounds (phenolics and tannins) and higher levels of nutrients (Ca, Mg and Na). Litter from the exotic S. alterniflora decomposed faster than litter from the native P. australis, being consistent with lower defences and higher nutrient concentrations in S. alterniflora litter. Our results show that individual species follow the same pattern of increasing litter lability at higher latitudes previously reported in cross‐species geographical comparisons. Moreover, this pattern can develop rapidly (&lt;4 decades) in an introduced species, raising the question of whether it is caused by phenotypic plasticity or adaptation. A free Plain Language Summary can be found within the Supporting Information of this article.

Archaeal and bacterial communities assembly and co-occurrence networks in subtropical mangrove sediments under Spartina alterniflora invasion

BackgroundMangrove ecosystems are vulnerable due to the exotic Spartina alterniflora (S. alterniflora) invasion in China. However, little is known about mangrove sediment microbial community assembly processes and interactions under S. alterniflora invasion. Here, we investigated the assembly processes and co-occurrence networks of the archaeal and bacterial communities under S. alterniflora invasion along the coastlines of Fujian province, southeast China.ResultsAssembly of overall archaeal and bacterial communities was driven predominantly by stochastic processes, and the relative role of stochasticity was stronger for bacteria than archaea. Co-occurrence network analyses showed that the network structure of bacteria was more complex than that of the archaea. The keystone taxa often had low relative abundances (conditionally rare taxa), suggesting low abundance taxa may significantly contribute to network stability. Moreover, S. alterniflora invasion increased bacterial and archaeal drift process (part of stochastic processes), and improved archaeal network complexity and stability, but decreased the network complexity and stability of bacteria. This could be attributed to S. alterniflora invasion influenced microbial communities diversity and dispersal ability, as well as soil environmental conditions.ConclusionsThis study fills a gap in the community assembly and co-occurrence patterns of both archaea and bacteria in mangrove ecosystem under S. alterniflora invasion. Thereby provides new insights of the plant invasion on mangrove microbial biogeographic distribution and co-occurrence network patterns.

Coupling Scirpus recruitment with Spartina control guarantees recolonization of native sedges in coastal wetlands

Recruiting native coastal vegetation and preventing the reinvasion of exotic Spartina alterniflora is a major challenge in China, due to harsh tidal environment for vegetation establishment and rapid Spartina colonization. This paper first reviewed the revegetation course of native Scirpus mariqueter in a coastal wetland in the Yangtze Estuary, where the adjacent Spartina was effectively eliminated. From 2014 to 2019, the area of planted Scirpus increased by 26.7 times, and the plant density and biomass growth amounted to the levels of mature sedge community. The establishment of Scirpus promoted the sedimentary accretion with an annual sedimentary rate of 9.2 cm yr−1, which was significantly higher than that in the mudflat. Six years of monitoring revealed that the revegetation approach with belowground propagules (corms) was effective for Scirpus establishment under strong hydrodynamic disturbances in the tidal mudflat because the morphology and germination rate of corms benefited the anchoring function of seedlings. Both recruited Scirpus and the original community expanded constantly due to positive feedback between plant progradation and sedimentary accretion. Then, we implemented a model simulation under the scenario of Spartina retention. The prediction showed that the high-speed invasion of Spartina would colonize and replace the whole Scirpus community in the study area during the period of 2014–2019. Hence, our results highlighted the importance of coupling Scirpus recruitment with Spartina eradication in guaranteeing recolonization of native coastal sedges. As a long-lasting management strategy, unremitting field monitoring and assistant decision-making tools (such as the forecasting models) on Spartina reinvasion risk are highly suggested for preserving the restoration of native marshes.

In-situ measurement of greenhouse gas emissions from a coastal estuarine wetland using a novel continuous monitoring technology: Comparison of indigenous and exotic plant species

This study investigated in-situ the seasonal and diurnal variation of emissions of greenhouse gases (GHGs) from both indigenous and exotic plant species and different environments in the Kaomei Estuary Wetland in central Taiwan with a self-designed non-dispersive infrared monitoring system. This study computed CO2 equivalent (CO2-e) emissions to identify their contribution to global warming. The net primary production and carbon sequestration were then estimated to determine the carbon budget of the coastal estuarine wetland. It concluded that the Kaomei Estuary Wetland functioned as a GHG source and a carbon sink. A significant diurnal variation of GHG emissions was observed, with generally lower daytime CO2 emissions than those at nighttime, while an opposite trend was observed for CH4 and N2O emissions. High solar radiation in the daytime enhanced the CO2 uptake by plant species via photosynthesis, and also accelerated the microbial activities in waters and soil/mud, both resulting in the decrease in atmospheric CO2 concentration. The highest GHG emissions were observed in summer, followed by fall, spring, and winter. Although the concentrations of GHG emissions from the coastal estuarine wetland were in the order as CO2>CH4>N2O, N2O has the highest impact on global warming. Biomass debris played an important role in carbon sequestration, which is stored in soils and muds and stimulated methanogenic bacteria to emit CH4. Tidal fluctuation and sewage discharge brought nitrogen-containing organics to the coastal estuarine wetland, resulting in high emission of N2O from nitrification and denitrification processes. Two vascular plants, Spartina alterniflora, and Phragmites australis emitted more GHGs than the other two plant species. However, the highest GHG emissions from the Kaomei Estuary Wetland was attributed to Bolboschoenus planiculmis due to its largest coverage area. The annual net primary production (NPP) varied mainly with vegetation coverage and season. The exotic Spartina alterniflora had the highest annual NPP compared to the indigenous plant species because of its high nutrient uptake from the soil/mud by its thriving roots.

Contrasting effects of the aboveground litter of native Phragmites australis and invasive Spartina alterniflora on nitrification and denitrification

Aboveground litter inputs from plants are among the most important pathways for carbon and nutrient fluxes to the soil. Previous studies on the effects of aboveground litter from invasive plants on ecosystem processes have primarily focused on biogeochemical cycling processes such as C and N mineralization, whereas the effects of aboveground litter from invasive plants on nitrogen removal processes are not well understood. In this study, the effects of the aboveground litter of native Phragmites australis and exotic Spartina alterniflora on soil nitrification and denitrification were compared. Results showed that the removal of the aboveground litter of both species had no effect on nitrification or denitrification in the early growth phase. However, after aboveground litter removal in the late growth phase, nitrification and denitrification in the P. australis stands decreased by 41.18% and 25.11%, respectively, whereas no such changes were observed in the S. alterniflora stands. These results indicate that the impacts of aboveground litter on nitrification and denitrification are species-specific. The aboveground litter from indigenous P. australis affected the SOC content and then indirectly affected nitrification or denitrification, and these effects were clearer in the late growth phase. Although other studies have reported that the invasive S. alterniflora have strong impacts on nitrogen removal processes, our study showed that the aboveground litter from S. alterniflora did not alter nitrification or denitrification, which indicates that other pathways may play important roles in nitrogen removal processes than its aboveground litter does.

Dramatic change of bacterial assembly process and co-occurrence pattern in Spartina alterniflora salt marsh along an inundation frequency gradient

Exotic Spartina alterniflora has become widely distributed along most of the coastlines in China in a wide range of inundation frequencies. However, the assembly processes and co-occurrence patterns of the bacterial community in S. alterniflora wetlands under different inundation frequencies remain elusive. In this study, an in-situ mesocosm was established to investigate the changes in soil bacterial community. We found that soil water content was the most decisive factor in influencing the bacterial community. Balanced variation, rather than abundance gradients, accounted for the major shifts in bacterial communities and was significantly and positively correlated with the changes in water content, suggesting that species substitution was facilitated by the increased water content. Deterministic processes were dominant in community assembly, and a large degree of change in water content increased variable selection. Co-occurrence network revealed that increasing water content significantly decreased the average degree and the relative abundance of keystone species, resulting in a network with less complexity. Structural equation modelling suggests that increasing inundation frequency has strong impacts on bacterial community, primarily by altering water content, network degree, and the relative abundance of keystone species. Overall, our results illustrate that increasing inundation frequency significantly influences the bacterial community assembly processes and co-occurrence patterns.

Geographic Linkages of Root Traits to Salt Marsh Productivity

Root traits are fundamental characteristics of belowground ecosystems that regulate plant growth and drive ecosystem functioning. Nevertheless, the way root traits respond to environmental factors and consequently influence productivity remains unexplored on large geographic scales. We examined the root traits of exotic Spartina alterniflora and native Phragmites australis across China’s coastal salt marshes. Using structural equation models (SEMs), we quantified the direct and indirect effects of mean annual temperature, soil nutrients (e.g., soil dissolved inorganic nitrogen and phosphorus), and root traits on aboveground net primary productivity. Our results showed that root traits of S. alterniflora were more sensitive to changing soil nutrient availability than those of P. australis. The SEMs indicated that soil nutrient availability increased S. alterniflora productivity by increasing root nitrogen concentration and root length density. In P. australis, temperature could increase productivity by both increasing root length density and soil nutrient-mediated root nitrogen concentration. The studied root architectural trait (root length density) and nutrient trait (root nitrogen concentration) were effective in predicting productivity, whereas none of the root morphological traits (i.e., specific root length, root tissue density, and root diameter) significantly affected productivity. We provide the first empirical evidence that root trait-based responses modulate the effects of climate and soil nutrients on geographic variation in vegetation productivity, but these effects are species specific.

Effects of exotic Spartina alterniflora on saltmarsh nitrogen removal in the Yangtze River Estuary, China

Salt marshes play an important role in removing excess nitrogen through microbial processes such as denitrification and anaerobic ammonia oxidation (anammox), and one of the major factors known to influence the nitrogen purification capacity of salt marshes worldwide is the invasion of alien species. Previous studies on the effects of invasive plants on nitrogen removal have tended to focus on the upper soils, whereas only a few have sought to examine the effects of exotic species on nitrogen removal in deep soils. This disparity may thus have led to unrepresentative evaluations when attempting to make integrated assessments of the ecosystem services provided by exotic species. Here, using the 15N tracer technique, we investigated the processes of denitrification and anammox in sediments of four habitat types (bare mudflat and marshlands dominated by the native plant species Scirpus mariqueter and Phragmites australis and the invasive plant Spartina alterniflora) on Chongming Island, located in the Yangtze River Estuary, China. Measurements were taken in May and October 2018 from six different depth ranges (0–5, 5–10, 10–20, 20–30, 30–50, and 50–100 cm) within the soil profile. With respect to the entire soil profile, we found that S. alterniflora did not facilitate effective nitrogen removal compared with the native plants S. mariqueter and P. australis. However, S. alterniflora did significantly increase nitrogen removal in the upper 50 cm of soil, whereas a slight reduction in nitrogen removal was observed in the deeper 50–100 cm soil layer. These results indicate that nitrogen removal was significantly increased only in the upper soils subsequent to S. alterniflora invasion, which contributes to our existing knowledge regarding the effects of invasive species on the nitrogen cycle in salt marshes. Furthermore, we established that in the upper soil layers between 0 and 50 cm, denitrification is primarily regulated by NO3− and the C/N ratio, whereas anammox is predominantly regulated by bulk density. Contrastingly, in the deeper soil layers between 50 and 100 cm, both these processes are largely regulated by redox potential and pH. These results serve to highlight that when evaluating the ecosystem services provided by S. alterniflora in salt marshes, it is necessary to estimate nitrogen removal in both the upper and deeper soil layers.