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Coastal wetlands play an important role in regulating atmospheric carbon dioxide (CO2) concentrations and contribute significantly to climate change mitigation. However, climate change, reclamation, and restoration have been causing substantial changes in coastal wetland areas and carbon exchange in China during recent decades. Here we compiled a carbon flux database consisting of 15 coastal wetland sites to assess the magnitude, patterns, and drivers of carbon fluxes and to compare fluxes among contrasting natural, disturbed, and restored wetlands. The natural coastal wetlands have the average net ecosystem exchange of CO2 (NEE) of −577 g C m−2 year−1, with −821 g C m−2 year−1 for mangrove forests and −430 g C m−2 year−1 for salt marshes. There are pronounced latitudinal patterns for carbon dioxide exchange of natural coastal wetlands: NEE increased whereas gross primary production (GPP) and respiration of ecosystem decreased with increasing latitude. Distinct environmental factors drive annual variations of GPP between mangroves and salt marshes; temperature was the dominant controlling factor in salt marshes, while temperature, precipitation, and solar radiation were co‐dominant in mangroves. Meanwhile, both anthropogenic reclamation and restoration had substantial effects on coastal wetland carbon fluxes, and the effect of the anthropogenic perturbation in mangroves was more extensive than that in salt marshes. Furthermore, from 1980 to 2020, anthropogenic reclamation of China's coastal wetlands caused a carbon loss of ~3720 Gg C, while the mangrove restoration project during the period of 2021–2025 may switch restored coastal wetlands from a carbon source to carbon sink with a net carbon gain of 73 Gg C. The comparison of carbon fluxes among these coastal wetlands can improve our understanding of how anthropogenic perturbation can affect the potentials of coastal blue carbon in China, which has implications for informing conservation and restoration strategies and efforts of coastal wetlands. [ABSTRACT FROM AUTHOR]

Mangrove ecosystems are becoming increasingly important in global climate mitigation. However, large gaps still exist in evaluating mangroves' gross primary productivity (GPP) due to reasons such as the specific influences, for example, temperature and salt stresses are poorly described in Earth System Models (ESMs). This study developed a process‐based biogeochemical model (Mango‐GPP) to improve the GPP simulation in natural and restored mangroves. The model integrates mangrove‐specific physiological processes, including the response to salt and temperature stresses, as well as the light‐use efficiency at different growing stages. Eddy covariance flux measurements at two natural sites and one restored site in China were used to calibrate and validate Mango‐GPP. The model was calibrated by inverse analysis approach based on two cases and independently validated against the other cases. The validation results showed that it was generally capable of simulating the seasonal and interannual GPP variations at different sites. The simulated daily and annual GPPs agreed well with the observations and yielded R2 of 0.67 and 0.96, with model efficiency of 0.64 and 0.93, respectively. In comparison, Mango‐GPP showed better performances than many current satellite‐based GPP products and ESMs. The model was more sensitive to solar radiation, carbon dioxide concentration, and leaf traits. Future improvements should focus on enhancing Mango‐GPP's descriptive power of key processes, and further simulating other carbon fluxes at regional scales. This work provides a model foundation for further simulating carbon exchanges between the atmosphere, mangrove, and ocean for studying the coastal wetland restoration on regional carbon neutrality. Plain Language Summary: Mangroves are becoming increasingly important for climate mitigation, but are greatly under‐represented in current ecosystem models. This study developed a new process‐based model (Mango‐GPP) to specifically estimate carbon taken up by mangrove ecosystems. The model requires few and easily accessible inputs, and showed good performance in both natural and restored mangroves. This work sets the stage for future studies on impacts of mangrove changes (e.g., restoration and reestablishment etc.) on carbon sequestration and climate mitigation. Key Points: We developed a process‐based model simulating daily gross primary productivity (GPP) for mangrovesThe model incorporates the effects of salinity and temperature, as well as light absorption and use efficiency on mangroves' GPPThe model requires only 13 easily accessible inputs and is validated well at both natural and restored mangroves in China [ABSTRACT FROM AUTHOR]

The magnitude and variability of mangrove carbon storage are uncertain and still being discussed. In a recent article, Breithaupt and Steinmuller (2022, https://doi.org/10.1029/2022GL100177) completed a literature review and compared mangrove organic carbon burial rates between different coastal environmental settings (CES) that integrate sedimentary supply (terrigenous vs. carbonate) and hydrogeomorphic settings (delta, estuary, lagoon, open coast). They found greater burial rates in terrigenous delta and estuaries while lower rates were reported in lagoons and carbonate settings. Surprisingly, these CES relationships do not strictly match previous mangrove soil carbon stock estimates but were consistent with biomass stocks. The CES approach used by Breithaupt and Steinmuller should be used for other mangrove carbon stocks and fluxes estimates to refine our understanding of mangrove carbon cycling and storage. Plain Language Summary: Mangrove forests are intertidal ecosystems efficient at trapping carbon in their sediments. While this ecological process is well recognized, the quantity of carbon being deposited is still uncertain. Considering the growing number of studies that have collected samples to estimate carbon burial rates, Breithaupt and Steinmuller (2022, https://doi.org/10.1029/2022GL100177) recently published a meta‐analysis to compile and compare carbon burial between mangrove types. Their study reported great variability between mangrove types based on their sediment supply and geomorphological configuration. Their results are interesting because they go against previous estimates of mangrove carbon stocks distribution. Also, the approach used has the potential to be applied to other components of the mangrove carbon budget. Doing so would improve our fundamental understanding of carbon cycling in mangrove ecosystems. It would also help refine carbon storage estimates for future conservation projects interested in generating carbon credits. Key Points: Carbon burial rates between mangrove sedimentary and geomorphic types were analyzed in Breithaupt and Steinmuller (2022, https://doi.org/10.1029/2022GL100177)Greater carbon burial rates were reported in terrigenous deltas and estuaries rather than in lagoons and carbonate settingsComparing carbon burial rates with other stocks and fluxes between mangrove types can help refine the global mangrove carbon budget [ABSTRACT FROM AUTHOR]

Mangrove forests are important carbon sinks, and this is especially true for Indonesia where about 24% of the world's mangroves exist. Unfortunately, vast expanses of these mangroves have been deforested, degraded or converted to other uses resulting in significant greenhouse gas emissions. The objective of this study was to quantify the climate change mitigation potential of mangrove conservation and restoration in Indonesia. We calculated the emission factors from the dominant land uses in mangroves, determined mangrove deforestation rates and quantified the total emissions and the potential emission reductions that could be achieved from mangrove conservation and restoration. Based on our analysis of the carbon stocks and emissions from land use in mangroves we found: (1) Indonesia's mangrove ecosystem carbon stocks are among the highest of any tropical forest type; (2) mangrove deforestation results in greenhouse gas emissions that far exceed that of upland tropical deforestation; (3) in the last decade the rates of deforestation in Indonesian mangroves have remained high; and (4) conservation and restoration of mangroves promise to sequester significant quantities of carbon. While mangroves comprise only ≈2.6% of Indonesia's total forest area, their degradation and deforestation accounted for ≈10% of total greenhouse gas emissions arising from the forestry sector. The large source of greenhouse gas emissions from a relatively small proportion of the forest area underscores the value for inclusion of mangroves as a natural climate solution. Mangrove conservation is far more effective than mangrove restoration in carbon emissions reductions and an efficient pathway to achieve Indonesia's nationally determined contribution (NDC) targets. The potential emission reduction from halting deforestation of primary and secondary mangroves coupled with restoration activities could result in an emission reduction equivalent to 8% of Indonesia's 2030 NDC emission reduction targets from the forestry sector. [ABSTRACT FROM AUTHOR]

Small Island Developing States (SIDS) face complex socio-economic and environmental threats, making them particularly vulnerable to climate change. Blue Carbon (BC) ecosystems (mangrove forests, tidal marshes, and seagrass meadows) provide climate regulation services by sequestering and storing carbon, presenting an opportunity for SIDS to address climate change and implement Paris Agreement commitments in their Covid-19 recovery. BC habitat decline not only reduces carbon sequestration benefits provided, but can also result in sediment disturbance and the release of previously stored carbon back into the atmosphere. In this work, a scenario analysis informed by a stakeholder workshop and scientific and socio-economic expectations is used to assess the economic importance of Grenada's BC (mangroves and seagrasses) over the next 10, 25 and 50 years. Our findings indicate that sequestration benefits are severely diminished under Business-as-Usual habitat loss, but still marginally outweigh losses from carbon emissions, with overall welfare gains of US$0.5–1.9 million over 50 years. To stimulate economic recovery post-pandemic, stakeholders anticipated a realistic scenario of increased habitat decline resulting in overall losses of US$5.4–19.4 million in the next 50 years. However, if ecosystems are maintained, overall carbon benefits could reach US$10.7 million, while a 20% increase in mangroves over the next 25 years provides benefits reaching US$11.1 million between 2020 and 2070. These results demonstrate a significant increase in value when BC ecosystems are maintained and not disturbed, preventing the release of previously stored carbon and enhancing sequestration capacity. Restoration benefits are marginal, compared to conservation, supporting claims that conservation is more cost effective. • A scenario analysis is used to value Grenada's BC climate mitigation service. • Maintaining BC extents retains sequestration worth up to $10.7million over 50 years. • BAU habitat loss lowers sequestration and releases stored carbon, reducing benefits. • More severe habitat declines result in welfare losses up to US$19.4million by 2070. • Integrating BC into SIDS' Paris Agreement commitments can enrich climate mitigation. [ABSTRACT FROM AUTHOR]

Mangroves have among the highest carbon densities of any tropical forest. These 'blue carbon' ecosystems can store large amounts of carbon for long periods, and their protection reduces greenhouse gas emissions and supports climate change mitigation. Incorporating mangroves into Nationally Determined Contributions to the Paris Agreement and their valuation on carbon markets requires predicting how the management of different land‐uses can prevent future greenhouse gas emissions and increase CO2 sequestration. We integrated comprehensive global datasets for carbon stocks, mangrove distribution, deforestation rates, and land‐use change drivers into a predictive model of mangrove carbon emissions. We project emissions and foregone soil carbon sequestration potential under 'business as usual' rates of mangrove loss. Emissions from mangrove loss could reach 2391 Tg CO2 eq by the end of the century, or 3392 Tg CO2 eq when considering foregone soil carbon sequestration. The highest emissions were predicted in southeast and south Asia (West Coral Triangle, Sunda Shelf, and the Bay of Bengal) due to conversion to aquaculture or agriculture, followed by the Caribbean (Tropical Northwest Atlantic) due to clearing and erosion, and the Andaman coast (West Myanmar) and north Brazil due to erosion. Together, these six regions accounted for 90% of the total potential CO2 eq future emissions. Mangrove loss has been slowing, and global emissions could be more than halved if reduced loss rates remain in the future. Notably, the location of global emission hotspots was consistent with every dataset used to calculate deforestation rates or with alternative assumptions about carbon storage and emissions. Our results indicate the regions in need of policy actions to address emissions arising from mangrove loss and the drivers that could be managed to prevent them. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 fluxes were measured in three subtropical mangroves for 16 site years. • VPD stress inhibits mangrove productivity during both exposed and inundated periods. • Meteorological factors and VPD have interactive effects on mangrove productivity. • Tides have independent effects on mangrove productivity through salinity changes. • The threshold effect range of VPD stress on mangrove productivity is 2.50–2.95 kPa. Subtropical mangroves have great potential to sequester atmospheric carbon dioxide (CO 2) and thus significantly contribute to climate change mitigation. Meanwhile, the carbon cycling processes in subtropical mangroves are vulnerable to the changing climatic conditions, such as the warming-induced vapor pressure deficit (VPD) increases. However, the impacts of VPD on net ecosystem production (NEP) in subtropical mangroves remain poorly understood due to a lack of detailed assessment of NEP responses to VPD among different mangrove forests over a long-term observational period. In this study, we deployed eddy-covariance systems to measure net ecosystem CO 2 exchange at three subtropical mangrove forests for 16 site-years, comprising two natural and one artificial forests. We employed a state-of-the-art data-driven modeling approach (i.e., Shapley additive explanations (SHAP) framework based on extreme gradient boosting model), which enabled us to explore the interactive effects of meteorological and tidal factors (e.g. salinity) with VPD on the mangrove NEP. We revealed that air temperature, global solar radiation (GR) and wind speed have significant interactions on the response of NEP to VPD stress in subtropical mangroves. For instance, when GR was high, the SHAP interaction values of VPD and GR on NEP decreased with increasing VPD, but when GR was low, the trends were the opposite. However, instead of identifying interactive effects between tidal salinity and VPD on mangrove NEP, we came across potential independent influence of salinity on the same. SHAP analysis was also able to disentangle the impact of VPD from other abiotic drivers. Thus, we evaluated the threshold effect of VPD stress on NEP loss in subtropical mangroves and observed a range of 2.50–2.95 kPa. Above this range, VPD stress leveled off. The subtropical mangrove responses to VPD should be therefore considered in the dynamic global vegetation models to increase the accuracy in carbon cycle simulations in the future. [ABSTRACT FROM AUTHOR]

Background: Mangrove forests have gained recognition for their potential role in climate change mitigation due to carbon sequestration in live trees, and carbon storage in the sediments trapped by mangrove tree roots and pneumatophores. Africa hosts about 19% of the world's mangroves, yet relatively few studies have examined the carbon stocks of African mangroves. The available studies report considerable differences among sites and amongst the different pools of carbon stocks. None considered the effects of seaward distance. We present details of AGC and SOC carbon stocks for Lindi in Tanzania, and focus on how these values differ with increasing seaward distance and, how our results compare to those reported elsewhere across Africa. Results: AGC ranged between 11 and 55 Mg C ha−1, but was not significantly affected by seaward distance. SOC for 0–1 m depth ranged from 154 to 484, with a mean of 302 Mg C ha−1. SOC was significantly negatively correlated with seaward distance. Mangrove type (estuarine/oceanic), soil erosion, soil depth may explain these differences We note important methodological differences in previous studies on carbon stocks in mangroves in Africa. Conclusion: This study indicates that seaward distance has an important effect on SOC stocks in the Lindi region of Tanzania. SOC should be fully incorporated into national climate change mitigation policies. Studies should report seaward distance and to describe the type of mangrove stand to make results easily comparable across sites and to assess the true value of Blue Carbon in Africa. We recommend focusing on trees > 10 cm diameter for AGC, and sampling soils to at least 1 m depth for SOC, which would provide a more complete assessment of the potentially considerable mangrove carbon store. [ABSTRACT FROM AUTHOR]

Mangroves sequester large quantities of carbon (C) that become significant sources of greenhouse gases when disturbed through land‐use change. Thus, they are of great value to incorporate into climate change adaptation and mitigation strategies. In response, a global network of mangrove plots was established to provide policy‐relevant ecological data relating to interactions of mangrove C stocks with climatic, tidal, plant community, and geomorphic factors. Mangroves from 190 sites were sampled across five continents encompassing large biological, physical, and climatic gradients using consistent methodologies for the quantification of total ecosystem C stocks (TECS). Carbon stock data were collected along with vegetation, physical, and climatic data to explore potential predictive relationships. There was a 28‐fold range in TECS (79–2,208 Mg C/ha) with a mean of 856 ± 32 Mg C/ha. Belowground C comprised an average 85% of the TECS. Mean soil depth was 216 cm, ranging from 22 to >300 cm, with 68 sites (35%) exceeding a depth of 300 cm. TECS were weakly correlated with metrics of forest structure, suggesting that aboveground forest structure alone cannot accurately predict TECS. Similarly, precipitation was not a strong predictor of TECS. Reasonable estimates of TECS were derived via multiple regression analysis using precipitation, soil depth, tree mass, and latitude (R2 = 0.54) as variables. Soil carbon to a 1 m depth averaged 44% of the TECS. Limiting analyses of soil C stocks to the top 1 m of soils result in large underestimates of TECS as well as in the greenhouse gas emissions that would arise from their conversion to other land uses. The current IPCC Tier 1 default TECS value for mangroves is 511 Mg C/ha, which is only 60% of our calculated global mean. This study improves current assessments of mangrove C stocks providing a foundation necessary for C valuation related to climate change mitigation. We estimate mangroves globally store about 11.7 Pg C: an aboveground carbon stock of 1.6 Pg C and a belowground carbon stock of 10.2 Pg C). The differences in the estimates of total ecosystem carbon stocks based on climate, salinity, forest structure, geomorphology, or geopolitical boundaries are not as much of an influence as the choice of soil depth included in the estimate. Choosing to limit soils to a 1 m depth resulted in estimates of <5 Pg whereas those that included the soil profile >1 m depth resulted in global carbon stock estimates that exceeded 11.2 Pg C. [ABSTRACT FROM AUTHOR]

Globally, mangrove forests represents only 0.7% of world's tropical forested area but are highly threatened due to susceptibility to climate change, sea level rise, and increasing pressures from human population growth in coastal regions. Our study was carried out in the Bhitarkanika Conservation Area ( BCA), the second-largest mangrove area in eastern India. We assessed total ecosystem carbon (C) stocks at four land use types representing varying degree of disturbances. Ranked in order of increasing impacts, these sites included dense mangrove forests, scrub mangroves, restored/planted mangroves, and abandoned aquaculture ponds. These impacts include both natural and/or anthropogenic disturbances causing stress, degradation, and destruction of mangroves. Mean vegetation C stocks (including both above- and belowground pools; mean ± standard error) in aquaculture, planted, scrub, and dense mangroves were 0, 7 ± 4, 65 ± 11 and 100 ± 11 Mg C/ha, respectively. Average soil C pools for aquaculture, planted, scrub, and dense mangroves were 61 ± 8, 92 ± 20, 177 ± 14, and 134 ± 17 Mg C/ha, respectively. Mangrove soils constituted largest fraction of total ecosystem C stocks at all sampled sites (aquaculture [100%], planted [90%], scrub [72%], and dense mangrove [57%]). Within BCA, the four studied land use types covered an area of ~167 km2 and the total ecosystem C stocks were 0.07 Tg C for aquaculture (~12 km2), 0.25 Tg C for planted/ restored mangrove (~24 km2), 2.29 teragrams (Tg) Tg C for scrub (~93 km2), and 0.89 Tg C for dense mangroves (~38 km2). Although BCA is protected under Indian wildlife protection and conservation laws, ~150 000 people inhabit this area and are directly or indirectly dependent on mangrove resources for sustenance. Estimates of C stocks of Bhitarkanika mangroves and recognition of their role as a C repository could provide an additional reason to support conservation and restoration of Bhitarkanika mangroves. Harvesting or destructive exploitation of mangroves by local communities for economic gains can potentially be minimized by enabling these communities to avail themselves of carbon offset/conservation payments under approved climate change mitigation strategies and actions. [ABSTRACT FROM AUTHOR]

Both freshwater floodplain (várzeas and igapós) forests and brackish-saline mangroves are abundant and well-described ecosystems in Brazil. 1 However, an interesting and unique wetland forest exists in the Amazon Delta where extensive mangroves occur in essentially freshwater tidal environments. Unlike the floodplain forests found upriver, the hydrology of these ecosystems is driven largely by large macro-tides of 4–8 m coupled with the significant freshwater discharge from the Amazon River. We explored these mangroves on the Amazon Delta (00°52ʹ N to 01°41ʹ N) and found surface water salinity to be consistently <5; soil pore water salinity in these mangrove forests ranged from 0 nearest the Amazon mouth to only 5–11 at the coastal margins to the north (01°41ʹ N, 49°55′ W). We also recorded a unique mix of mangrove-obligate (Avicennia sp., Rhizophora mangle) and facultative-wetland species (Mauritia flexuosa , Pterocarpus sp.) dominating these forests. This unique mix of plant species and soil porewater chemistry exists even along the coastal strands and active coastlines of the Atlantic Ocean. Part of these unique mangroves have escaped current global satellite mapping efforts, and we estimate that they may add over 180 km2 (20% increase in mangrove area) within the Amazon Delta. Despite having a unique structure and function, these freshwater-brackish ecosystems likely provide similar ecosystem services to most mangroves worldwide, such as sequestering large quantities of organic carbon, protection of shoreline ecosystems from erosion, and habitats to many terrestrial and aquatic species (monkeys, birds, crabs, and fish). • We describe a new forest structure for mangroves in the Amazon Delta • Amazon Delta mangroves thrive in hyposaline and acidic soils • We identified over 180 km2 of previously unmapped forests in the Amazon Delta • These mangroves may hold large coastal carbon stocks Bernardino et al. survey the Amazon Delta and find an unexpected structure and composition of mangroves, where they coexist with trees that are typical of freshwater wetlands. These mangroves were previously unmapped and will have an unprecedented value to climate mitigation and other ecosystem services on this region of the coast. [ABSTRACT FROM AUTHOR]

Intertidal habitats provide numerous ecosystem services, including the sequestration and storage of carbon, a topic of great recent interest owing to land-cover transitions and climate change. Mangrove forests and seagrass meadows form a continuum of intertidal habitats, alongside unvegetated mudflats and sandbars, however, studies that consider carbon stocks across these spatially-linked, threatened ecosystems are limited world-wide. This paper presents the results of a field-based carbon stock assessment of aboveground, belowground and sediment organic carbon stock to a depth of 1 m at Chek Jawa, Singapore. It is the first study of ecosystem carbon stocks of both vegetated and unvegetated intertidal habitats in the tropics. Ecosystem carbon stocks were 497 Mg C ha-1 in the mangrove forest and 138 Mg C ha-1 in the seagrass meadow. Sediment organic carbon stock dominated the total storage in both habitats, constituting 62% and >99% in the mangrove forest and seagrass meadow, respectively. In the adjacent mudflat and sandbars, which had no vegetative components, sediment organic carbon stock ranged from 124-143 Mg C ha-1, suggesting that unvegetated habitats have a carbon storage role on the same order of importance as seagrass meadows. This study reinforces the importance of sediment in carbon storage within the intertidal ecosystem, and demonstrates the need to consider unvegetated habitats in intertidal 'blue carbon' stock assessments. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Keywords: blue carbon; mangrove forests; national governments; saltmarsh; seagrass; tidal wetlands; tragedy of the commons EN blue carbon mangrove forests national governments saltmarsh seagrass tidal wetlands tragedy of the commons 477 479 3 07/10/20 20131001 NES 131001 The benefits of coastal wetlands to coastal communities have been known for decades. Through the guidance for carbon accounting in coastal wetlands provided in the upcoming IPCC supplement, the responsibilities and benefits of improved coastal wetland management for national governments are clearer. The Wetland Supplement is due to be delivered at the end of 2013, thereby providing guidance to national governments on reporting carbon emissions and sequestration for land-use transitions in coastal wetlands. Overall, successful wetland conservation is a product of the property rights and other institutions that define access to wetland resources [13]. [Extracted from the article]

Mangrove ecosystems store large amounts of 'Blue Carbon', in particular in the sediment. Research in the past decade has emphasized the quantitative significance of carbon storage in mangrove forests in climate change mitigation, mainly by determining carbon stocks and calculating potential CO 2 emissions caused by mangrove degradation. However, while this approach focuses on the total amount of carbon that can be lost to degradation, it fails to capture the amount that is sequestered annually. Therefore, carbon accumulation in mangrove sediments also needs to be taken into account. This study (i) explains the differences between carbon stocks and carbon accumulation rates (CAR), (ii) it addresses the geographical variation of carbon storage and underlying factors and (iii) it assesses the global relevance of 'Blue Carbon' sequestration in mangrove sediments. Results indicate that reducing uncertainties in carbon storage estimates of individual systems requires a representative set of data that covers within-system variability. An example from Indonesia illustrates that a mangrove ecosystem with a high C stock can have a low CAR and vice versa. It is therefore conceivable that coastal environmental settings with high allochthonous supply of mineral sediment, organic matter and nutrients mostly have low carbon stocks, but high CARs. As these settings represent >80% of the global mangrove area they are most important in terms of long-term carbon storage. While a C stock is a measure of the "vulnerability potential" in the case of ecosystem degradation or total loss, a CAR is rather a measure of the "mitigation potential" of carbon storage in mangrove ecosystems. The global carbon storage in mangrove sediments of 32 Tg yr−1 estimated from CARs in this study is at the upper end of the range of global budgets (14.6–31.1 Tg yr−1, mean 22.9 Tg yr−1). It highlights that the mangrove carbon sink may be larger than previously thought, but the high variation in the global average CAR of 233 ± 280 g C m−2 yr−1 also indicates the need for further data. • Variation of C stocks and accumulation rates within one mangrove system can be high. • C stocks are a measure of the "vulnerability potential" upon mangrove degradation. • C accumulation rates are a measure of the "mitigation potential" of mangroves. • The mangrove carbon sink may be larger than previously thought. [ABSTRACT FROM AUTHOR]

Mangrove ecosystems store large amounts of 'Blue Carbon', in particular in the sediment. Research in the past decade has emphasized the quantitative significance of carbon storage in mangrove forests in climate change mitigation, mainly by determining carbon stocks and calculating potential CO 2 emissions caused by mangrove degradation. However, while this approach focuses on the total amount of carbon that can be lost to degradation, it fails to capture the amount that is sequestered annually. Therefore, carbon accumulation in mangrove sediments also needs to be taken into account. This study (i) explains the differences between carbon stocks and carbon accumulation rates (CAR), (ii) it addresses the geographical variation of carbon storage and underlying factors and (iii) it assesses the global relevance of 'Blue Carbon' sequestration in mangrove sediments. Results indicate that reducing uncertainties in carbon storage estimates of individual systems requires a representative set of data that covers within-system variability. An example from Indonesia illustrates that a mangrove ecosystem with a high C stock can have a low CAR and vice versa. It is therefore conceivable that coastal environmental settings with high allochthonous supply of mineral sediment, organic matter and nutrients mostly have low carbon stocks, but high CARs. As these settings represent >80% of the global mangrove area they are most important in terms of long-term carbon storage. While a C stock is a measure of the "vulnerability potential" in the case of ecosystem degradation or total loss, a CAR is rather a measure of the "mitigation potential" of carbon storage in mangrove ecosystems. The global carbon storage in mangrove sediments of 32 Tg yr−1 estimated from CARs in this study is at the upper end of the range of global budgets (14.6–31.1 Tg yr−1, mean 22.9 Tg yr−1). It highlights that the mangrove carbon sink may be larger than previously thought, but the high variation in the global average CAR of 233 ± 280 g C m−2 yr−1 also indicates the need for further data. • Variation of C stocks and accumulation rates within one mangrove system can be high. • C stocks are a measure of the "vulnerability potential" upon mangrove degradation. • C accumulation rates are a measure of the "mitigation potential" of mangroves. • The mangrove carbon sink may be larger than previously thought. [ABSTRACT FROM AUTHOR]

• Organic carbon burial rates and sources were assessed across Papuan mangroves. • Organic carbon burial rates ranged between 0.21 and 1.19 Mg C ha−1 yr−1. • Soil organic carbon stocks in the top 50 cm varied between 62 and 179 Mg C ha−1. • Soil organic carbon stocks are sourced from autoch- and alloch-thonous sources. Mangrove organic carbon is primarily stored in soils, which contain more than two-thirds of total mangrove ecosystem carbon stocks. Despite increasing recognition of the critical role of mangrove ecosystems for climate change mitigation, there is limited understanding of soil organic carbon sequestration mechanisms in undisturbed low-latitude mangroves, specifically on organic carbon burial rates and sources. This study assessed soil organic carbon burial rates, sources and stocks across an undisturbed coastal mudflat and mangrove hydrogeomorphological catena (fringe mangrove and interior mangrove) in Bintuni Bay, West Papua Province, Indonesia. 210Pb radionuclide sediment dating, and mixing model of natural stable isotope signatures (δ 13C and δ15N) and C/N ratio were used to estimate organic carbon burial rates and to quantify proportions of allochthonous (i.e., upland terrestrial forest) and autochthonous (i.e., on-site mangrove forest) organic carbon in the top 50 cm of the soil. Burial rates were in the range of 0.21–1.19 Mg C ha−1 yr−1. Compared to the fringe mangroves, organic carbon burial rates in interior mangroves were almost twice as high. Primary productivity of C 3 upland forest vegetation and mangroves induced soil organic carbon burial in interior mangroves and this was consistent with the formation of the largest organic carbon stocks (179 ± 82 Mg C ha−1). By contrast, organic carbon stored in the fringe mangrove (68 ± 11 Mg C ha−1) and mudflat (62 ± 10 Mg C ha−1) soils mainly originated from upland forests (allochthonous origin). These findings clearly indicate that carbon sequestered and cycling in mangrove and terrestrial forest ecosystems are closely linked, and at least a part of carbon losses (e.g., erosion) from terrestrial forests is buried in mangrove ecosystems. [ABSTRACT FROM AUTHOR]

Macroalgae drive the largest CO2 flux fixed globally by marine macrophytes. Most of the resulting biomass is exported through the coastal ocean as detritus and yet almost no field measurements have verified its potential net sequestration in marine sediments. This gap limits the scope for the inclusion of macroalgae within blue carbon schemes that support ocean carbon sequestration globally, and the understanding of the role their carbon plays within distal food webs. Here, we pursued three lines of evidence (eDNA sequencing, Bayesian Stable Isotope Mixing Modeling, and benthic‐pelagic process measurements) to generate needed, novel data addressing this gap. To this end, a 13‐month study was undertaken at a deep coastal sedimentary site in the English Channel, and the surrounding shoreline of Plymouth, UK. The eDNA sequencing indicated that detritus from most macroalgae in surrounding shores occurs within deep, coastal sediments, with detritus supply reflecting the seasonal ecology of individual species. Bayesian stable isotope mixing modeling [C and N] highlighted its vital role in supporting the deep coastal benthic food web (22–36% of diets), especially when other resources are seasonally low. The magnitude of detritus uptake within the food web and sediments varies seasonally, with an average net sedimentary organic macroalgal carbon sequestration of 8.75 g C·m−2·yr−1. The average net sequestration of particulate organic carbon in sediments is 58.74 g C·m−2·yr−1, the two rates corresponding to 4–5% and 26–37% of those associated with mangroves, salt marshes, and seagrass beds, systems more readily identified as blue carbon habitats. These novel data provide important first estimates that help to contextualize the importance of macroalgal‐sedimentary connectivity for deep coastal food webs, and measured fluxes help constrain its role within global blue carbon that can support policy development. At a time when climate change mitigation is at the foreground of environmental policy development, embracing the full potential of the ocean in supporting climate regulation via CO2 sequestration is a necessity. [ABSTRACT FROM AUTHOR]