Impacts of Saltwater Incursion on Plant Communities, Anaerobic Microbial Metabolism, and Resulting Relationships in a Restored Freshwater Wetland

Modeling strategies and data needs for representing coastal wetland vegetation in land surface models

Vegetated coastal ecosystems sequester carbon rapidly relative to terrestrial ecosystems. Coastal wetlands are poorly represented in land surface models, but work is underway to improve process-based, predictive modeling of these ecosystems. Here, we identify guiding questions, potential simulations, and data needs to make progress in improving representation of vegetation in terrestrial-aquatic interfaces, with a focus on coastal and estuarine ecosystems. We synthesize relevant plant traits and environmental controls on vegetation that influence carbon cycling in coastal ecosystems. We propose that models include separate plant functional types (PFTs) for mangroves, graminoid salt marshes, and succulent salt marshes to adequately represent the variation in aboveground and belowground productivity between common coastal wetland vegetation types. We also discuss the drivers and carbon storage consequences of shifts in dominant PFTs. We suggest several potential approaches to represent the diversity in vegetation tolerance and adaptations to fluctuations in salinity and water level, which drive key gradients in coastal wetland ecosystems. Finally, we discuss data needs for parameterizing and evaluating model implementations of coastal wetland vegetation types and function.

Detecting Coastal Wetland Degradation by Combining Remote Sensing and Hydrologic Modeling

Sea-level rise and climate change stresses pose increasing threats to coastal wetlands that are vital to wildlife habitats, carbon sequestration, water supply, and other ecosystem services with global significance. However, existing studies are limited in individual sites, and large-scale mapping of coastal wetland degradation patterns over a long period is rare. Our study developed a new framework to detect spatial and temporal patterns of coastal wetland degradation by analyzing fine-scale, long-term remotely sensed Normalized Difference Vegetation Index (NDVI) data. Then, this framework was tested to track the degradation of coastal wetlands at the Alligator River National Wildlife Refuge (ARNWR) in North Carolina, United States, during the period from 1995 to 2019. We identified six types of coastal wetland degradation in the study area. Most of the detected degradation was located within 2 km from the shoreline and occurred in the past five years. Further, we used a state-of-the-art coastal hydrologic model, PIHM-Wetland, to investigate key hydrologic processes/variables that control the coastal wetland degradation. The temporal and spatial distributions of simulated coastal flooding and saltwater intrusion confirmed the location and timing of wetland degradation detected by remote sensing. The combined method also quantified the possible critical thresholds of water tables for wetland degradation. The remote sensing–hydrologic model integrated scheme proposed in this study provides a new tool for detecting and understanding coastal wetland degradation mechanisms. Our study approach can also be extended to other coastal wetland regions to understand how climate change and sea-level rise impact wetland transformations.

Desalinization via freshwater restoration highly improved microbial diversity, co-occurrence patterns and functions in coastal wetland soils

Saltwater intrusion has greatly impacted the functions of coastal wetland soils worldwide by increasing the salt stress; desalinization via freshwater restoration has been suggested to recover saline wetland soils and biodiversity, but its effectiveness is debated. To evaluate the desalinization effectiveness, we characterized the microbial communities and activities using high throughput 16S rRNA gene sequencing and ¹⁵N isotopic techniques in freshwater restored (≥10 years) and unrestored wetlands, and then compared the data with reported values of original freshwater wetlands in one of the most dynamic coastal areas, Yellow river estuary (YRE). Our data revealed that freshwater input significantly increased the soil organic carbon (SOC; P < 0.05) after 10 years of restoration, yet it was still 10 times lower than the reported values of original freshwater wetlands. In general, microbial community showed higher diversities and more co-occurrence interactions in the restored than unrestored wetlands. The recovered phylogenetic diversity and the relative abundance of Chloroflexi (9.8-16.3%), Actinobacteria (5.5-10%), Latescibacteria (0.5-1.5%), Nitrospirae (0.9-1.4%) were up to the similar levels of original freshwater wetlands in YRE. Specifically, Gemmatimonadetes_denitrifier clones, as the representatives of denitrifiers, were recovered up to 0.3% with 20 times higher concomitant denitrification rate than anammox rate, significantly contributing to the nitrate removal in restored wetlands; however the rate will be reduced by 80% with a short-term saltwater intrusion. Our study highlighted that freshwater input effectively improved the microbial diversity and their functions and provided a good insight into the desalinization effectiveness via freshwater restoration in coastal wetlands worldwide. ORIGINALITY-SIGNIFICANCE STATEMENT: Salinization is globally spreading with approximately one billion hectares area covered with saline and/or sodic soils on the earth, and the negative effects of salinity on soil microbial communities and their activities have been frequently reported in previous studies all around the world; however, it remains largely unknown about whether the microbial communities and their activities can be recovered or not in soil suffered salinization. Desalinization via freshwater restoration is supposed to offer a good solution to soil salinization in coastal area, but the effectiveness is debated. Here, we are presenting the long-term of field study related to the desalinization effects on microbial diversity, co-occurrence and functions, and find desalinization via freshwater restoration can recover most of microbial communities up to the similar levels of that in original freshwater wetlands, and highly improved microbial diversity and their functions, which sheds a positive light on soil desalinization via freshwater restoration at microenvironments.

Stronger network connectivity with lower diversity of soil fungal community was presented in coastal marshes after sixteen years of freshwater restoration

Freshwater input for salt marsh restoration in the Yellow River Delta induced Phragmites australis expansion and thus may cause shifts of soil fungi from halophilic to desalination-adapted species for increased litter decomposition. In this study, soil fungal communities of restored and natural salt marshes were determined to reveal further details of shift in soil fungal community and its probable prediction for salt marsh restoration. Our results showed a stronger network within Ascomycota (e.g. Sordariales, Aspergillus, Hypocreales and Cladosporium herbarum) in restored marshes, but with a lower diversity of halophilic taxa (e.g. Chytridiomycota and Nematoda) in comparison with natural salt marshes. Contrarily, the occurrence of Chytridiomycota, Ichthyosporea and Discicristoidea in the soil fungal networks of the natural salt marsh emphasized the importance of salt tolerant species at the land-sea transition zone. The Sordariales was dominant and had a strong correlation with other fungal species and aggregate associated soil organic carbon (SOC), which probably contributed to SOC accumulation in restored marshes. But the reduced halophilic species specific to salt marsh elucidated that the formation of monospecific stands of P. australis along with the freshwater input induced desalination to the saline habitats changed the native patterns of vegetation and soil organisms. As the buffer between terrestrial and marine systems, a single habitat type such as dense monocultures of P. australis must be avoided and diverse saltmarsh habitats across a salinity gradient should be reserved. In this way, the diversity and specificity of coastal halophytes and related microorganisms could be maintained and thus might confer benefits in balancing various functions of the salt marsh ecosystem and preserving the system's elasticity and resistance to stress.

Impact of climate change on sustainable livestock production and existence of wildlife and marine species in the South Pacific island countries: a review

Climate change poses a significant threat to humanity by intensifying multiple hazards. South Pacific Island countries (SPICs) are affected and face a dire challenge to survival. Sea level rise is reducing the already limited land for human and animal habitation. Tropical cyclones and droughts are having devastating effects on the lives of humans and animals. Tropical cyclone Winston, for example, destroyed infrastructure for humans and animals in some parts of Fiji, and infectious diseases are spreading to regions where they are not commonly seen following cyclones and floods. Likewise, climate change is making droughts worse. Droughts are destroying crops and pasturelands and making freshwater unavailable for human and animal populations in the Solomon Islands and Tuvalu. There is an urgent need to ascertain the best approaches to tackle the events, which are already happening. Short-term changes can be managed at local levels through public awareness campaigns, understanding the weather patterns to prepare for disasters, reclaiming land, improving livestock breeds, introducing zoos and wildlife sanctuaries and inventing economically feasible technologies to harvest water. Long-term solutions depend on the implementation of international agreements, international aid and collective effort.

A pinch of salt: response of coastal grassland plants to simulated seawater inundation treatments

BACKGROUND AND AIMS

The combination of rising sea levels and increased storm frequency and intensity is predicted to increase the severity of oceanic storm surge events and the impact of flooding on coastal ecosystems globally. Understanding how plant communities respond to this threat necessitates experiments involving plant immersion in saline water, but logistical issues and natural variation in seawater composition mean that pure NaCl solutions or marine aquarium salts (MS) are widely used. Nonetheless, their comparative impact on plant ecophysiology, and thus relevance to understanding real-world flooding scenarios, is unknown.

METHODS

In the first of two experiments, we examined how six ecophysiological responses in white clover (Trifolium repens) varied when plants were subjected to five different inundation treatments: deionized water, natural seawater, an MS solution and two NaCl solutions. In a second experiment, we examined how immersion in deionized water, MS solution and natural seawater affected six European perennial herb species, three native to Spanish sand dunes, and three from British coastal grasslands.

RESULTS

The two NaCl solutions induced exceptional Trifolium mortality, but responses varied little between MS and seawater treatments. In the second experiment, although leaf tissue necrosis and proline concentrations increased, and growth decreased compared with untreated controls, only one response in one species varied between MS and seawater treatments. Chemical speciation modelling revealed major variation in free Na+ and Cl- between NaCl solutions and seawater, but minor differences between MS and seawater.

CONCLUSIONS

We show that NaCl solutions are unsuitable surrogates to investigate plant response to elevated environmental salinity. Although responses to natural seawater and MS were consistent within species, there was notable between-species variation. Consequently, the first steps to elucidating how these species-specific responses influence coastal plant community recovery following storm surge can likely be achieved using commercial marine aquarium salts as substitutes for natural seawater.

Cumulative Sulfate Loads Shift Porewater to Sulfidic Conditions in Freshwater Wetland Sediment

It is well established that sulfide can be toxic to rooted aquatic plants. However, a detailed description of the effects of cumulative sulfate loads on sulfide and iron (Fe) porewater geochemistry, plant exposure, and ecological response is lacking. Over 4 yr, we experimentally manipulated sulfate loads to self-perpetuating wild rice (Zizania palustris) populations and monitored increases in the ratio of sulfur (S) to Fe in sediment across a range of sulfide loading rates driven by overlying water sulfate. Because natural settings are complicated by ongoing Fe and S loads from surface and groundwater, this experimental setting provides a tractable system to describe the impacts of increased S loading on Fe-S porewater geochemistry. In the experimental mesocosms, the rate of sulfide accumulation in bulk sediment increased linearly with overlying water sulfate concentration up to 300 µg-SO4 cm-3 . Seedling survival at the beginning of the annual life cycle and seed mass and maturation at the end of the annual life cycle all decreased at porewater sulfide concentrations between 0.4 and 0.7 µg cm-3 . Changes to porewater sulfide, plant emergence, and plant nutrient uptake during seed production were closely related to the ratio of S to Fe in sediment. A mass balance analysis showed that porewater sulfide remained a small and relatively transient phase compared to sulfate in the overlying water and Fe in the sediment solid phase. The results illuminate the evolution of the geochemical setting and timescales over which 4 yr of cumulative sulfate loading resulted in a wholesale shift from Fe-dominated to sulfide-dominated porewater chemistry. This shift was accompanied by detrimental effects to, and eventual extirpation of, self-perpetuating wild rice populations. Environ Toxicol Chem 2019;38:1231-1244. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Response of Soil Microbes to Vegetation Restoration in Coal Mining Subsidence Areas at Huaibei Coal Mine, China

Vegetation restoration is an available way to ameliorate degraded lands. In order to study the response of soil microbes to vegetation restoration in coal mining subsidence areas, the composition and distribution of soil microbes were discussed through three plots: unsubsided area (CA), new subsided area (NSA), and old subsided area (OSA) with different vegetation restoration time in Huabei coal mine. Meanwhile, changes in soil catalase and urease activity were explored and the correlation between soil bacteria, fungi, and environmental factors was analysed. The results demonstrated that Nitrospira was the dominant bacteria in all areas sampled. Microorganisms in the 0–20 cm and 40–60 cm soil layers of OSA had the highest Simpson index, whereas the index in NSA was lowest (at all soil depths). The catalase activity in NSA was significantly higher than that in CA, and there was no significant difference in catalase activity with soil depth, while the urease activity declined gradually with increasing soil depth. The urease activity in the 20–60 cm soil layer of NSA and OSA was significantly higher than that of CA. Furthermore, the distribution of bacteria was mainly affected by soil organic matter, available potassium, available phosphorus, and alkali-hydrolyzable nitrogen, whereas pH and catalase activity mainly affected fungal distribution. These results implied that soil catalase activity in NSA and urease activity in the 20–40 cm soil layer of NSA and OSA were significantly enhanced after vegetation restoration, and that long-term plant restoration could improve soil fertility and soil microbial community diversity in coal mining areas.

Salinity stress changed the biogeochemical controls on CH4 and N2O emissions of estuarine and intertidal sediments

Elevated salinity is expected to drive changes in biogeochemical cycling and microbial communities in estuarine and intertidal wetlands. However, limited information regarding the role of salinity in shaping biogeochemical controls and mediating greenhouse gas emissions is currently available. In this study, we used incubation experiment across salinity gradients of the estuarine and intertidal sediments to reveal the underlying interconnections of CH₄ and N₂O emissions, biogeochemical controls and salinity gradients. Our results indicated that sediment biogeochemical properties were significantly affected by the increasing salinity, which were attributed to the accelerated sediment enzyme activities. The increasing salinity promoted CH₄ and N₂O emission rates by stimulating organic carbon decomposition and nitrogen transformation rates. In addition, the copy number of mcrA, nirS and nirK genes increased along with the salinity gradients, which strongly mediated the CH₄ and N₂O emission rates. Stepwise regression analysis suggested that labile organic carbon and denitrification were the most crucial determinants of CH₄ and N₂O emission rates, respectively. Overall, salinity could enhance CH₄ and N₂O emission mainly by altering sediment geochemical variables, microbial activity and functional gene abundance in estuarine and intertidal environments. Furthermore, increasing salinity could enhance the carbon and nitrogen export, which may pose a threat to the ecological function of estuarine and intertidal ecosystems. This study may contribute to the knowledge about the importance of biogeochemical controls induced by salinity in mediating greenhouse gas emissions.

Causal mechanisms of soil organic matter decomposition: deconstructing salinity and flooding impacts in coastal wetlands

Coastal wetlands significantly contribute to global carbon storage potential. Sea-level rise and other climate-change-induced disturbances threaten coastal wetland sustainability and carbon storage capacity. It is critical that we understand the mechanisms controlling wetland carbon loss so that we can predict and manage these resources in anticipation of climate change. However, our current understanding of the mechanisms that control soil organic matter decomposition, in particular the impacts of elevated salinity, are limited, and literature reports are contradictory. In an attempt to improve our understanding of these complex processes, we measured root and rhizome decomposition and developed a causal model to identify and quantify the mechanisms that influence soil organic matter decomposition in coastal wetlands that are impacted by sea-level rise. We identified three causal pathways: (1) a direct pathway representing the effects of flooding on soil moisture, (2) a direct pathway representing the effects of salinity on decomposer microbial communities and soil biogeochemistry, and (3) an indirect pathway representing the effects of salinity on litter quality through changes in plant community composition over time. We used this model to test the effects of alternate scenarios on the response of tidal freshwater forested wetlands and oligohaline marshes to short- and long-term climate-induced disturbances of flooding and salinity. In tidal freshwater forested wetlands, the model predicted less decomposition in response to drought, hurricane salinity pulsing, and long-term sea-level rise. In contrast, in the oligohaline marsh, the model predicted no change in response to drought and sea-level rise, and increased decomposition following a hurricane salinity pulse. Our results show that it is critical to consider the temporal scale of disturbance and the magnitude of exposure when assessing the effects of salinity intrusion on carbon mineralization in coastal wetlands. Here, we identify three causal mechanisms that can reconcile disparities between long-term and short-term salinity impacts on organic matter decomposition.

Evolutionary history influences the salinity preference of bacterial taxa in wetland soils

Salinity is a major driver of bacterial community composition across the globe. Despite growing recognition that different bacterial species are present or active at different salinities, the mechanisms by which salinity structures community composition remain unclear. We tested the hypothesis that these patterns reflect ecological coherence in the salinity preferences of phylogenetic groups using a reciprocal transplant experiment of fresh- and saltwater wetland soils. The salinity of both the origin and host environments affected community composition (16S rRNA gene sequences) and activity (CO2 and CH4 production, and extracellular enzyme activity). These changes in community composition and activity rates were strongly correlated, which suggests the effect of environment on function could be mediated, at least in part, by microbial community composition. Based on their distribution across treatments, each phylotype was categorized as having a salinity preference (freshwater, saltwater, or none) and phylogenetic analyses revealed a significant influence of evolutionary history on these groupings. This finding was corroborated by examining the salinity preferences of high-level taxonomic groups. For instance, we found that the majority of α- and γ-proteobacteria in these wetland soils preferred saltwater, while many β-proteobacteria prefer freshwater. Overall, our results indicate the effect of salinity on bacterial community composition results from phylogenetically-clustered salinity preferences.