Blue carbon potential of coastal wetland restoration varies with inundation and rainfall

Planning hydrological restoration of coastal wetlands: Key model considerations and solutions

The hydrological restoration of coastal wetlands is an emerging approach for mitigating and adapting to climate change and enhancing ecosystem services such as improved water quality and biodiversity. This paper synthesises current knowledge on selecting appropriate modelling approaches for hydrological restoration projects. The selection of a modelling approach is based on project-specific factors, such as costs, risks, and uncertainties, and aligns with the overall project objectives. We provide guidance on model selection, emphasising the use of simpler and less expensive modelling approaches when appropriate, and identifying situations when models may not be required for project managers to make informed decisions. This paper recognises and supports the widespread use of hydrological restoration in coastal wetlands by bridging the gap between hydrological science and restoration practices. It underscores the significance of project objectives, budget, and available data and offers decision-making frameworks, such as decision trees, to aid in matching modelling methods with specific project outcomes.

Warming influences CO2 emissions from China's coastal saltmarsh wetlands more than changes in precipitation

Coastal wetlands are an important carbon sink but are sensitive to climate changes. The response of CO₂ emissions to these changes differs under different hydroclimatic conditions. Here, this article used meta-analysis to synthesize data from Chinese coastal salt marshes, to analyze sensitivities for CO₂ emissions, and then to assess the relative contributions of air temperature (T_a) and precipitation (Pre). This article used the ratio between potential evaporation (E_p) and Pre to divide Chinese coastal saltmarshes into water- (E_p/Pre > 1) and energy-limited regions (E_p/Pre ≤ 1). Results show that emissions are more sensitive to both Pre and T_a in water-limited regions (E¯ = 0.60 eV, slope = 0.37) than in energy-limited regions (E¯ = 0.23 eV, slope = 0.04). Comparing the relative effects of changes in T_a (△CO₂ = 21.86 mg m^-2 h^-1) and Pre (△CO₂ = 7.19 mg m^-2 h^-1) on CO₂ emissions shows that warming contributes more to changes in CO₂ emissions. The response of emissions to changes in Pre is asymmetric and shows that warmer and drier may have antagonistic effects, while warmer and wetter may have synergistic effects. There was a 2.15 mg m^-2 h^-1 change in emissions in energy-limited regions when Pre increased by 139.69 mm, and a decrease of -0.15 mg m^-2 h^-1 in emissions when Pre decreased by 1.28 mm in water-limited regions. Climate change has the greatest impact on Phragmites australis in CO₂ emissions, especially under warmer and wetter conditions in energy-limited regions. This indicates that warming drives CO₂ emissions, while changes in Pre (resulting in wetter or dryer conditions) can mitigate or strengthen CO₂ emissions from coastal wetlands in China. This article offers a new perspective and suggests that differences in hydroclimatic conditions should be considered when discussing carbon emissions from coastal wetlands.

Productive wetlands restored for carbon sequestration quickly become net CO2 sinks with site-level factors driving uptake variability

Inundated wetlands can potentially sequester substantial amounts of soil carbon (C) over the long-term because of slow decomposition and high primary productivity, particularly in climates with long growing seasons. Restoring such wetlands may provide one of several effective negative emission technologies to remove atmospheric CO2 and mitigate climate change. However, there remains considerable uncertainty whether these heterogeneous ecotones are consistent net C sinks and to what degree restoration and management methods affect C sequestration. Since wetland C dynamics are largely driven by climate, it is difficult to draw comparisons across regions. With many restored wetlands having different functional outcomes, we need to better understand the importance of site-specific conditions and how they change over time. We report on 21 site-years of C fluxes using eddy covariance measurements from five restored fresh to brackish wetlands in a Mediterranean climate. The wetlands ranged from 3 to 23 years after restoration and showed that several factors related to restoration methods and site conditions altered the magnitude of C sequestration by affecting vegetation cover and structure. Vegetation established within two years of re-flooding but followed different trajectories depending on design aspects, such as bathymetry-determined water levels, planting methods, and soil nutrients. A minimum of 55% vegetation cover was needed to become a net C sink, which most wetlands achieved once vegetation was established. Established wetlands had a high C sequestration efficiency (i.e. the ratio of net to gross ecosystem productivity) comparable to upland ecosystems but varied between years undergoing boom-bust growth cycles and C uptake strength was susceptible to disturbance events. We highlight the large C sequestration potential of productive inundated marshes, aided by restoration design and management targeted to maximise vegetation extent and minimise disturbance. These findings have important implications for wetland restoration, policy, and management practitioners.

Plant biomass and rates of carbon dioxide uptake are enhanced by successful restoration of tidal connectivity in salt marshes

Salt marshes, due to their capability to bury soil carbon (C), are potentially important regional C sinks. Efforts to restore tidal flow to former salt marshes have increased in recent decades in New England (USA), as well as in some other parts of the world. In this study, we investigated plant biomass and carbon dioxide (CO₂) fluxes at four sites where restoration of tidal flow occurred five to ten years prior to the study. Site elevation, aboveground biomass, CO₂ flux, and porewater chemistry were measured in 2015 and 2016 in both restored marshes and adjacent marshes where tidal flow had never been restricted. We found that the elevation in restored marsh sites was 2-16 cm lower than their natural references. Restored marshes where porewater chemistry was similar to the natural reference had greater plant aboveground biomass, gross ecosystem production, ecosystem respiration, as well as net ecosystem CO₂ exchange (NEE) than the natural reference, even though they had the same plant species. We also compared respiration rates in aboveground biomass (AR) and soil (BR) during July 2016, and found that restored marshes had higher AR and BR fluxes than natural references. Our findings indicated that well-restored salt marshes can result in greater plant biomass and NEP, which has the potential to enhance rates of C sequestration at 10-yrs post restoration. Those differences were likely due to lower elevation and greater flooding frequency in the recently restored marshes than the natural marsh. The inverse relationship between elevation and productivity further suggests that, where sea-level rise rate does not surpass the threshold of plant survival, the restoration of these salt marshes may lead to enhanced organic and mineral sedimentation, extending marsh survival under increased sea level, and recouping carbon stocks that were lost during decades of tidal restriction.

A synthesis of methane emissions from shallow vegetated coastal ecosystems

Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10× more C than temperate forests. Methane (CH₄ ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH₄ emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH₄ fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH₄ flux in these systems. CH₄ fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, -67.33 to 72,867.83; salt marsh: 224.44, -92.60 to 94,129.68; seagrass: 64.80, 1.25-401.50 µmol CH₄ m^-2 day^-1 ). Together CH₄ emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33-0.39 Tmol CH₄ -C/year-an addition that increases the current global marine CH₄ budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH₄ fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH₄ flux, they were not significant predictors of CH₄ flux when data were combined across studies. Salinity was negatively correlated with CH₄ emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH₄ emissions in vegetated coastal systems. Finally, we examine stressor effects on CH₄ emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH₄ emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH₄ budget.

Seasonal and diurnal variations of greenhouse gas emissions from a saline mangrove constructed wetland by using an in situ continuous GHG monitoring system

Wetland systems play important roles in the issues of global warming. That is because wetlands can not only intake carbon and nitrogen in the plants and sediments, but could also release carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) to the atmosphere during the microbial decomposition processes in the water. In the past, greenhouse gases (GHGs) were measured mostly by using the manual sampling technique; however, it is difficult to measure the diurnal variation of greenhouse gas emissions. In this study, a floating chamber used to collect GHGs, which were then in-site monitored with a continuous GHG analyzer, through a Teflon tube connected to the top of the chamber was designed. The data for seasonal and diurnal variations of GHGs, emission fluxes of GHGs, and equivalent carbon dioxide emissions (CO₂-e) were measured to explore the extent of a saline mangrove constructed wetland contributing to global warming. In addition, the correlation and regression analysis among greenhouse gas emissions, water quality, and other environmental factors were analyzed statistically. The results of continuous monitoring of GHGs showed that the concentrations of CO₂ emitted from a saline mangrove constructed wetland ranged from 383.5 ± 25.9 to 476.8 ± 24.2 ppm. The diurnal variation of GHG concentrations was significant, which showed that the GHG concentrations in the daytime were generally lower than those at nighttime. The emissions of methane (CH₄) from the wetland were monitored in a range between 3.7 ± 1.4 and 28.6 ± 7.6 ppm, while the concentrations of nitrous oxide (N₂O) emitted from the wetlands ranged from 3.1 ± 2.6 to 0.9 ± 0.6 ppm. Hence, the diurnal variations of CH₄ and N₂O concentrations showed higher values in the daytime than those at nighttime. The correlation analytical results among GHG emissions, water quality, and other environmental factors presented that the emission fluxes of CO₂ and BOD₅ were positively correlated, but a negative correlation was shown for global solar radiation (GSR). Moreover, the concentrations of CH₄ and N₂O had moderately positive correlation with both air temperature and water temperature.

Impacts of land management practices on blue carbon stocks and greenhouse gas fluxes in coastal ecosystems-A meta-analysis

Global recognition of climate change and its predicted consequences has created the need for practical management strategies for increasing the ability of natural ecosystems to capture and store atmospheric carbon. Mangrove forests, saltmarshes and seagrass meadows, referred to as blue carbon ecosystems (BCEs), are hotspots of atmospheric CO₂ storage due to their capacity to sequester carbon at a far higher rate than terrestrial forests. Despite increased effort to understand the mechanisms underpinning blue carbon fluxes, there has been little synthesis of how management activities influence carbon stocks and greenhouse gas (GHG) fluxes in BCEs. Here, we present a global meta-analysis of 111 studies that measured how carbon stocks and GHG fluxes in BCEs respond to various coastal management strategies. Research effort has focused mainly on restoration approaches, which resulted in significant increases in blue carbon after 4 years compared to degraded sites, and the potential to reach parity with natural sites after 7-17 years. Lesser studied management alternatives, such as sediment manipulation and altered hydrology, showed only increases in biomass and weaker responses for soil carbon stocks and sequestration. The response of GHG emissions to management was complex, with managed sites emitting less than natural reference sites but emitting more compared to degraded sites. Individual GHGs also differed in their responses to management. To date, blue carbon management studies are underrepresented in the southern hemisphere and are usually limited in duration (61% of studies <3 years duration). Our meta-analysis describes the current state of blue carbon management from the available data and highlights recommendations for prioritizing conservation management, extending monitoring time frames of BCE carbon stocks, improving our understanding of GHG fluxes in open coastal systems and redistributing management and research effort into understudied, high-risk areas.

Temporal variations of greenhouse gas emissions and carbon sequestration and stock from a tidal constructed mangrove wetland

This study used a self-developed a NDIR monitoring system to continuously monitor GHG emissions from a tidal constructed mangrove wetland at three typical habitats (mudflat, mangrove, and water surface) in four seasons. The NDIR monitoring system is able to explore the diurnal and seasonal variation of GHG emissions from the tidal constructed mangrove wetland and to estimate more precisely for the GHG emission based carbon budget of the wetland. The continuous monitoring technique is feasible and valuable for assessing the temporal variation of GHG uptake/emission to/from the wetland. Daytime CO₂ emissions were always lower than those at nighttime due to photosynthesis process, while an opposite trend was observed for CH₄ and N₂O emissions. Seasonal variation of GHGs showed that the highest GHG emissions was observed in summer, and followed by fall, spring, and winter. For three typical habitats, mangrove emitted more amounts of GHGs than mudflat and water surface.