Blue carbon drawdown by restored mangrove forests improves with age

Tidal and seasonal effects on sediment methane emissions from three different mangrove species

The anaerobic environment of mangrove sediments due to periodic tides is conducive to methane (CH4) production, but processes and mechanisms of CH4 emission from mangrove sediments are not yet well understood. We used in situ field monitoring and laboratory experiments to investigate the effects of tides and seasons on CH4 emissions from the sediments of Sonneratia apetala (SA), Kandelia obovata (KO), and Avicennia marina (AM), respectively. Methane emissions from the sediments of all mangrove species were significantly higher in summer than in winter, with overall CH4 fluxes being 2.14 times higher during the after-ebb tide compared to the pre-flood tide. Among the mangrove species, AM (16.77 ± 13.73 mg m-2 h-1) exhibited the highest emissions, followed by SA (1.45 ± 0.90 mg m-2 h-1) and KO (0.14 ± 0.16 mg m-2 h-1). CH4 emissions in three mangrove species were mainly driven directly by abiotic factors, including sediment organic carbon (SOC) that could provide substrate for methanogens to generate CH4, and dissolved CH4 concentration in porewater likely served as a carbon source or turnover state for CH4 to eventually enter the atmosphere. Also, sediment CH4 emissions were suppressed by the α-diversity of methanogenic communities. In addition, pH, CH4 flux, SOC, and redox potential significantly shaped structure of the methanogenic communities, potentially regulating sediment CH4 emissions. This study result highlights that abiotic factors can greatly influence CH4 emissions from mangrove sediments, as well as emphasizes the important role of the sediment-porewater-atmosphere pathway on CH4 emissions.

Predicting Climate Mitigation Through Carbon Burial in Blue Carbon Ecosystems-Challenges and Pitfalls

The concept of "blue carbon" is, in this study, critically evaluated with respect to its definitions, measuring approaches, and time scales. Blue carbon deposited in ocean sediments can only counteract anthropogenic greenhouse gas (GHG) emissions if stored on a long-term basis. The focus here is on the coastal blue carbon ecosystems (BCEs), mangrove forests, saltmarshes, and seagrass meadows due to their high primary production and large carbon stocks. Blue carbon sequestration in BCEs is typically estimated using either: 1. sediment carbon inventories combined with accretion rates or 2. carbon mass balance between input to and output from the sediment. The inventory approach is compromised by a lack of accurate accretion estimates over extended time periods. Hence, short-term sedimentation assays cannot be reliably extrapolated to long timescales. The use of long-term tracers like 210Pb, on the other hand, is invalid in most BCEs due to sediment mobility by bioturbation and other physical disturbances. While the mass balance approach provides reasonable short-term (months) estimates, it often fails when extrapolated over longer time periods (> 100 years) due to climatic variations. Furthermore, many published budgets based on mass balance do not include all relevant carbon sources and sinks. Simulations of long-term decomposition of mangrove, saltmarsh (Spartina sp.), and eelgrass (Zostera sp.) litter using a 3-G exponential model indicate that current estimates of carbon sequestration based on the inventory and mass balance approaches are 3-18 times too high. Most published estimates of carbon sequestration in BCEs must therefore be considered overestimates. The climate mitigation potential of blue carbon in BCEs is also challenged by excess emissions of the GHG methane (CH4) and nitrous oxide (N2O) from biogenic structures in mangrove forests and saltmarsh sediments. Thus, in many cases, carbon sequestration into BCE sediments cannot keep pace with the simultaneous GHG emissions in CO2 equivalents.

Variation in mangrove species diversity across gradients of climate-change-induced environmental conditions and hydrological restoration

Increasing drought, elevated temperatures, and salinization are significant challenges to reestablishing species in mangrove restoration areas. In this study, we assessed how the diversity of two key mangrove faunal groups, molluscs and brachyuran crustaceans (hereafter referred to as crabs), varies across a gradient of disturbed, restored, and natural (undisturbed) mangroves. We also explored what are the environmental factors driving these variations in ten sites across the southern Gulf of Mexico, one of the global regions with the largest mangrove coverage. A total of 15 species were recorded (10 mollusks and 5 crabs), with higher abundance in natural (612 individuals) than in restored (554 individuals) or degraded (98 individuals) sites. Community structure analyses revealed that certain species were restricted to specific restoration conditions. For example, the crab Minuca vocator was found only in restored sites, while the mollusc Vitta virginea was exclusive to natural sites. In contrast, species like the crab Minuca rapax were present across all site types. Salinity emerged as the primary environmental factor influencing community structure, with disturbed sites exhibiting significantly higher salinity levels than restored and natural sites. All sites were classified as hypersaline, presenting challenges for species that cannot tolerate such conditions. This study provides a valuable baseline for understanding the ecological conditions that influence on the success of mangrove restoration, offering insights on the effects of environmental factors driving species diversity in this ecosystem.

Stand age-related effects of mangrove on archaeal methanogenesis in sediments: Community assembly and co-occurrence patterns

Mangrove sediment is a key source of methane emissions; however, archaea community structure dynamics and methanogenesis activities during long-term mangrove restoration remain unclear. In this study, microcosm incubations revealed a substantial reduction in microbial-mediated methane production potential from mangrove sediments with increasing stand age; methane production rates decreased from 0.42 ng g-1 d-1 in 6-year-old stands to 0.23 ng g-1 d-1 in 64-year-old stands. High-throughput sequencing revealed a reduction in community diversity because of specific microorganism colonization and species loss, notably a decline in the relative abundance of Bathyarchaeia in sediments of 64-year-old stands. In addition, mangrove sediments, especially those in older stands (20- and 64-year-old), had more complex and stable co-occurrence microbial networks than mudflats. Furthermore, archaea community assembly in older stands was dominated by stochastic processes wherein dispersal limitation was prominent, and that in younger stands (6- and 12-year-old) was driven by deterministic processes. The proportion of dispersal limitation of Bathyarchaeia and traditional methanogens in sediment decreased with an increase in stand age. Quantitative polymerase chain reaction analysis confirmed a decrease in Bathyarchaeia (from 3.50 to 0.54 copies g-1) and mcrA gene (from 3.83 to 0.25 copies g-1) abundance in mangrove sediments with an increase in stand age. These findings demonstrate the critical role of Bathyarchaeia in methanogenesis; the decline in microbial interactions and abundance, and the reduced proportion of dispersal limitation of Bathyarchaeia and traditional methanogens collectively contributed to the mitigation of microbial-mediated methane production potential in older mangrove stands.

Community-based, cost-effective multispecies mangrove restoration innovation to maximize soil blue carbon pool and humic acid and fulvic acid concentrations at Indian Sundarbans

Sundarbans is the world's largest and most diverse contiguous mangrove ecosystem. In this pilot study, three plots (around 1 ha each) were selected, where one site (S1) had 1 year of community involvement, another site (S2) had a community network to support the restoration initiatives for 2 years, while a control site (C) was devoid of any post plantation community protection. Rhizophora mucronata (Rhizophoraceae), Sonneretia caseolaris (Lythraceae) and Avicennia marina (Acanthaceae) were planted at the sites in 2012. After 6 years (in 2017), at S1, the monitoring showed low survival rate for salinity-sensitive species, 2% for R. mucronata and 4% for S. caseolaris. At S2, R. mucronata has high survival rates, i.e. 71%, followed by S. caseolaris with 40%, whereas at C, the survival rate of both species was 0%. At S1 and C, the salinity-tolerant A. marina replaced the planted mangroves partially (S1) or entirely (C). At S2, available soil P increased by 17.5%, in 6 years, and the overall blue carbon pool showed a linear increase from 64.4 to 88.6 Mg C ha-1 (34.3% rise). S1 showed a minimum increment in P and the blue carbon pool (6.9% rise), while site C showed fluctuations in the blue carbon pool with only a 3.1% increase. Humic acid and fulvic acid concentrations in the S2 site indicate positive functional carbon sequestration in the edaphic environment. The community involvement increased the plantation cost (567.70 USD) of S2, in comparison to S1 (342.52 USD) and C (117.34 USD), but it has resulted in better restoration and survival of the mangroves. The study concludes that community participation for at least 2 years can play a significant role in the conservation of mangrove ecosystems and the success of restoration initiatives in tidal, saline wetlands and would aid in compliance with the United Nations Sustainable Development Goal 14 (Life Below Water) targets.

Microbial diversity and keystone species drive soil nutrient cycling and multifunctionality following mangrove restoration

Vegetation restoration exerts transformative effects on nutrient cycling, microbial communities, and ecosystem functions. While extensive research has been conducted on the significance of mangroves and their restoration efforts, the effectiveness of mangrove restoration in enhancing soil multifunctionality in degraded coastal wetlands remains unclear. Herein, we carried out a field experiment to explore the impacts of mangrove restoration and its chronosequence on soil microbial communities, keystone species, and soil multifunctionality, using unrestored aquaculture ponds as controls. The results revealed that mangrove restoration enhanced soil multifunctionality, with these positive effects progressively amplifying over the restoration chronosequence. Furthermore, mangrove restoration led to a substantial increase in microbial diversity and a reshaping of microbial community composition, increasing the relative abundance of dominant phyla such as Nitrospirae, Deferribacteres, and Fusobacteria. Soil multifunctionality exhibited positive correlations with microbial diversity, suggesting a link between variations in microbial diversity and soil multifunctionality. Metagenomic screening demonstrated that mangrove restoration resulted in a simultaneous increase in the abundance of nitrogen (N) related genes, such as N fixation (nirD/H/K), nitrification (pmoA-amoA/B/C), and denitrification (nirK, norB/C, narG/H, napA/B), as well as phosphorus (P)-related genes, including organic P mineralization (phnX/W, phoA/D/G, phnJ/N/P), inorganic P solubilization (gcd, ppx-gppA), and transporters (phnC/D/E, pstA/B/C/S)). The relationship between the abundance of keystone species (such as phnC/D/E) and restoration-induced changes in soil multifunctionality indicates that mangrove restoration enhances soil multifunctionality through an increase in the abundance of keystone species associated with N and P cycles. Additionally, it was observed that changes in microbial community and multifunctionality were largely associated with shifts in soil salinity. These findings demonstrate that mangrove restoration positively influences soil multifunctionality and shapes nutrient dynamics, microbial communities, and overall ecosystem resilience. As global efforts continue to focus on ecosystem restoration, understanding the complexity of mangrove-soil interactions is critical for effective nutrient management and mangrove conservation.

Replacing Spartina alterniflora with northward-afforested mangroves has the potential to acquire extra blue carbon

Climate change provides an opportunity for the northward expansion of mangroves, and thus, the afforestation of mangroves at higher latitude areas presents an achievable way for coastal restoration, especially where invasive species S. alterniflora needs to be clipped. However, it is unclear whether replacing S. alterniflora with northward-afforested mangroves would benefit carbon sequestration. In the study, we examined the key CO2 and CH4 exchange processes in a young (3 yr) northward-afforested wetland dominated by K. obovata. We also collected soil cores from various ages (3, 15, 30, and 60 years) to analyze the carbon storage characteristics of mangrove stands using a space-for-time substitution approach. Our findings revealed that the young northward mangroves exhibited obvious seasonal variations in net ecosystem CO2 exchange (NEE) and functioned as a moderate carbon sink, with an average annual NEE of -107.9 g C m-2 yr-1. Additionally, the CH4 emissions from the northward mangroves were lower in comparison to natural mangroves, with the primary source being the soil. Furthermore, when comparing the vertical distribution of soil carbon, it became evident that both S. alterniflora and mangroves contributed to organic carbon accumulation in the upper soil layers. Our study also identified a clear correlation that the biomass and carbon stocks of mangroves increased logarithmically with age (R2 = 0.69, p < 0.001). Notably, both vegetation and soil carbon stocks (especially in the deeper layers) of the 15 yr northward mangroves, were markedly higher than those of S. alterniflora. This suggests that replacing S. alterniflora with northward-afforested mangroves is an effective long-term strategy for future coasts to enhance blue carbon sequestration.

Current extent and future opportunities for living shorelines in Australia

Living shorelines aim to enhance the resilience of coastlines to hazards while simultaneously delivering co-benefits such as carbon sequestration. Despite the potential ecological and socio-economic benefits of living shorelines over conventional engineered coastal protection structures, application is limited globally. Australia has a long and diverse coastline that provides prime opportunities for living shorelines using beaches and dunes, vegetation, and biogenic reefs, which may be either natural ('soft' approach) or with an engineered structural component ('hybrid' approach). Published scientific studies, however, have indicated limited use of living shorelines for coastal protection in Australia. In response, we combined a national survey and interviews of coastal practitioners and a grey and peer-reviewed literature search to (1) identify barriers to living shoreline implementation; and (2) create a database of living shoreline projects in Australia based on sources other than scientific literature. Projects included were those that had either a primary or secondary goal of protection of coastal assets from erosion and/or flooding. We identified 138 living shoreline projects in Australia through the means sampled starting in 1970; with the number of projects increasing through time particularly since 2000. Over half of the total projects (59 %) were considered to be successful according to their initial stated objective (i.e., reducing hazard risk) and 18 % of projects could not be assessed for their success based on the information available. Seventy percent of projects received formal or informal monitoring. Even in the absence of peer-reviewed support for living shoreline construction in Australia, we discovered local and regional increases in their use. This suggests that coastal practitioners are learning on-the-ground, however more generally it was stated that few examples of living shorelines are being made available, suggesting a barrier in information sharing among agencies at a broader scale. A database of living shoreline projects can increase knowledge among practitioners globally to develop best practice that informs technical guidelines for different approaches and helps focus attention on areas for further research.

Monospecific mangrove reforestation changes relationship between benthic mollusc diversity and biomass: Implication for coastal wetland management

Anthropogenic causes are overtaking natural factors to reshape patterns of biodiversity and ecosystem functioning. Mangrove reforestation aimed at reversing losses of mangroves has been conducted worldwide for several decades. However, how reforestation influences the link between ecological processes that shape community diversity and the consequent effects on ecosystem functions such as biomass production is less well known. Here we used data collected before and after mangrove planting to examine the effects of reforestation on molluscan species richness and biomass production by testing the changes in species richness, compositional similarities, distance-decay effects (community similarity decreases with increasing geographical distance) in metacommunity across a regional scale of 480 km (23-27 °N) in southeast Chinese coasts. Additionally, we further detected the impact of landscape configuration caused by different intensities of reforestation on the mollusc community. After the mangrove reforestation, mollusc species richness and biomass increased significantly. The increases in species richness and biomass of mollusc community were mediated by reducing distance-decay effect, indicating an increase in relationship strength between species richness and biomass might be associated with a decrease in distance-decay effect with rising mangrove habitat. We highlight the importance of considering the effects of anthropogenic changes on the relationship between biodiversity and ecosystem functioning. Quantifying the distance-decay effect of these influences enables management decisions about coastal restoration to be based upon ecological mechanisms rather than wishful thinking or superficial appearance.

Pyrolysis and steam gasification properties of mangroves

The rising atmospheric CO2 emissions from the burning of fossil fuels remains a global concern. Mangrove forests, which are important for global biodiversity, are known for their high carbon fixation capacity. However, the characteristics of the pyrolysis and gasification of mangroves remain unclear. Thus, this study focused on mangroves' basic pyrolysis and steam gasification properties for use as a gasification fuel. Three mangrove species: Rhizophora mucronata, Bruguiera cylindrica, and Avicennia marina, were used as experimental samples. In addition, three species of land wood were used for comparison: Eucalyptus, Japanese cedar, and Japanese cypress. In addition to the raw sample, a demineralized sample was used for each sample to account for the influence of the alkali and alkaline earth metals (AAEMs) on pyrolysis and steam gasification. Thermogravimetric analysis was performed to obtain thermogravimetric curves of mangroves and land wood. A laboratory-scale instrument for pyrolysis and gasification using a batch-type horizontal electric furnace was also used at 800 °C in an inert and steam atmosphere. The char yield of raw mangroves was high and independent of the Klason lignin content, suggesting that AAEMs influence char formation during the initial pyrolysis of the mangroves. The results of pyrolysis and gasification under steam atmosphere showed that the H2 production ratio (Steam/Inert) from mangroves was 2.52-5.33, compared to 1.76-2.35 for land woods, the addition of steam significantly enhanced the steam gasification of mangroves. Mangroves contain relatively large amounts of AAEMs, which indicates their potential as a gasification feedstock.

Soil organic carbon stocks increased across the tide-induced salinity transect in restored mangrove region

Blue carbon in mangrove ecosystems contributes significantly to the global carbon cycle. However, large uncertainties maintain in the soil organic carbon (SOC) storage throughout the tide-induced salinity and alkalinity transect in the mangrove restoration region in Southern China. Total 125 soil samples were obtained to detect the SOC content and physicochemical properties. The mean SOC content of each layer ranged from 6.82 to 7.86 g kg-1, while the SOC density ranged from 2.99 to 11.41 kg m-2, increasing with soil depths. From different land covers in the study region, the SOC content varied from 4.63 to 9.71 g kg-1, increasing across the salinity and alkalinity transect, while the SOC density fluctuated from 3.01 kg m-2 in mudflats to 10.05 kg m-2 in mangrove forests. SOC concentration was favorably linked with total nitrogen (r = 0.95), and total phosphorus (r = 0.74), and negatively correlated with Cl- (r = - 0.95), electrical conductivity (r = - 0.24), and total dissolved solids (r = - 0.08). There were significant logarithmic relationships between SOC content and the concentrations of clay (r = 0.76), fine silt (r = 0.81), medium silt (r = - 0.82), and coarse silt (r = - 0.78). The spatial patterns of SOC concentration were notably affected by soil texture, physicochemical properties, and land-cover type, providing essential reference for future investigations of blue carbon budget in restored mangrove forests.

Nature-based solutions on floodplain restoration with coupled propagule dispersal simulation and stepping-stone approach to predict mangrove encroachment in an estuary

The mangrove ecosystem is significantly affected by human activities, climate change, and rising sea level. The propagules of mangroves dispersal with tide and river currents that extend upstream habitats are why mangroves are the dominant species in the tidal area. Bridging critical knowledge gaps can help to create restoration plans for mangrove extension. However, studies on the hydrodynamic and propagation trajectory model (PTM) simulation of propagule long-distance dispersal (LDD) and mangrove growth potential are scarce. By combining various numerical methods and empirical formulas and verifying them with the data obtained through field surveys, this study established a comprehensive model to assess the dispersal and growth of the propagules of Kandelia ovobata. The stepping-stone approach (SSA) and habitat suitability index (HSI) model were also employed to determine the location of the appropriate new habitats through iterative simulation in propagule dispersal. Dike removal was proposed as a nature-based solution and modeled to evaluate the benefits of ecological conservation and flood prevention. The PTM simulations indicated that the deterministic process of horizontal advection accounted for >80 %, and that the remaining variability in the model could be explained by stochastic processes in predicting mangrove propagules pathways. The integrated model of the PTM and SSA proved that propagules have LDD in an estuary. There were few matches in the regions for mangrove growth when comparing the suitability of habitat distribution and the probability of propagule movement. We suggested that the mangrove spread model incorporating the SSA and HSI models predict the potential for mangrove dispersal into new habitats. In addition, the removal of levees aids floodplain regeneration and allows propagules to disperse across the floodplain at high tide and establishment at low tide. The Guandu floodplain restoration with dike removal supplied a cobenefits on ecological demands and flood risk reduction. Future research could thus utilize the adaptation and mitigation strategies presented in this study by incorporating socioeconomic considerations to enhance practical feasibility.