Carbon stocks and greenhouse gas emissions (CH4 and N2O) in mangroves with different vegetation assemblies in the central coastal plain of Veracruz Mexico

Flushing emissions of methane and carbon dioxide from mangrove soils during tidal cycles

Mangroves are transition areas connecting land, freshwater, and the ocean, where a great amount of organic carbon accumulates in the soil, forming a considerable carbon sink. However, the soil might also be a source of greenhouse gas (GHG) emissions. This study hypothesized that measuring GHG emissions solely during low tides can represent diurnal GHG emissions in mangroves. Methane (CH4) and carbon dioxide (CO2) emissions were quantified during tidal cycles using an ultraportable gas analyzer in Kandelia obovata (without pneumatophores) and Avicennia marina (with pneumatophores) mangroves in summer and fall. The results showed that the CH4 fluxes varied greatly during tidal cycles, from -1.25 to 96.24 μmol CH4 m-2 h-1 for K. obovata and from 2.86 to 2662.00 μmol CH4 m-2 h-1 for A. marina. The CO2 fluxes ranged from -4.23 to 20.65 mmol CO2 m-2 h-1 for K. obovata and from 0.09 to 24.69 mmol CO2 m-2 h-1 for A. marina. The diurnal variation in GHG levels in mangroves is predominantly driven by tidal cycles. The peak emissions of CH4 and CO2 were noted at the beginning of the flooding tide, rather than during daytime or nighttime. While the patterns of the CO2 fluxes during tidal cycles were similar between K. obovata and A. marina mangroves, their CH4 flux patterns during the tidal cycles differed. Possibly due to different transport mechanisms, CO2 emissions are primarily influenced by surface soils, whereas CH4 is predominantly emitted from deeper soils, thus being influenced by root structures. To reduce the uncertainty in measuring GHG emissions in mangrove soils during a tidal cycle, it is advisable to increase the number of GHG flux measurements during the period spanning 30 min before and after the beginning of the flooding and ebbing tides.

Mangrove species and site elevation are critical drivers of greenhouse gas fluxes from restored mangrove soils

The combined influences of species selection (Avicennia marina, Kandelia obovata) and site elevation (BSL site, below local mean sea level; ASL site, above local mean sea level) on the greenhouse gas fluxes (nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2)) from restored mangrove soils are investigated in this study. Compared with the A. marina forest, soils in the K. obovata forest at ASL site have higher CO2 fluxes, while higher N2O fluxes in the K. obovata forest are found at BSL site. The highest CH4 fluxes are found at BSL site in the A. marina forest. At each elevation site, the A. marina forest has lower CO2-equivalent fluxes and carbon release in the form of carbon-containing gases. The results suggest that A. marina should be selected for mangrove restoration to minimize carbon release and reduce influence of greenhouse gas fluxes on the global greenhouse effect.

Carbon stock estimation in a Brazilian mangrove using optical satellite data

The research proposes a model to estimate the carbon stock in mangrove forests from multispectral images from Landsat 8 and Sentinel 2B satellites. The Gramame River mangrove, located on the southern coast of Paraíba State, Brazil, was adopted as the study area. Carbon stocks in biomass, below and above ground, were measured from a forest inventory, and vegetation indices were processed on the Google Earth Engine (GEE) platform. To define the fit curves, linear and non-linear regressions were used. The choice of the model considered the highest coefficients of determination (R2), the biomass and carbon stock were estimated from the equations. The biomass carbon stock, calculated from field data, corresponded to 22.27 Gg C, equivalent to 81.75 Gg CO2, with 13.85 Gg C (50.84 Gg CO2) and 8.42 Gg C (30.91 Gg CO2) stored in biomass above and below ground, respectively. Among the models fitted to the indices calculated from Landsat 8 images, NDVI was the one that best explained the spatial distribution of biomass and carbon, with 90.26%. For Sentinel 2B, SAVI was able to explain 80.76%. The total estimated plant carbon stocks corresponded to 26.66 Gg (16.20 Gg C above and 10.36 Gg C below ground) for Landsat 8 and 27.76 Gg C (16.93 Gg C above and 10.83 Gg C below ground) for Sentinel 2B. The proposed work methodology and the suggested mathematical models can be replicated to analyze carbon stocks in other locations, especially in the Americas, because they share the same species.

Coastal blue carbon in China as a nature-based solution toward carbon neutrality

To achieve the Paris Agreement, China pledged to become "Carbon Neutral" by the 2060s. In addition to massive decarbonization, this would require significant changes in ecosystems toward negative CO2 emissions. The ability of coastal blue carbon ecosystems (BCEs), including mangrove, salt marsh, and seagrass meadows, to sequester large amounts of CO2 makes their conservation and restoration an important "nature-based solution (NbS)" for climate adaptation and mitigation. In this review, we examine how BCEs in China can contribute to climate mitigation. On the national scale, the BCEs in China store up to 118 Tg C across a total area of 1,440,377 ha, including over 75% as unvegetated tidal flats. The annual sedimental C burial of these BCEs reaches up to 2.06 Tg C year-1, of which most occurs in salt marshes and tidal flats. The lateral C flux of mangroves and salt marshes contributes to 1.17 Tg C year-1 along the Chinese coastline. Conservation and restoration of BCEs benefit climate change mitigation and provide other ecological services with a value of $32,000 ha-1 year-1. The potential practices and technologies that can be implemented in China to improve BCE C sequestration, including their constraints and feasibility, are also outlined. Future directions are suggested to improve blue carbon estimates on aerial extent, carbon stocks, sequestration, and mitigation potential. Restoring and preserving BCEs would be a cost-effective step to achieve Carbon Neutral by 2060 in China despite various barriers that should be removed.

A novel coupled framework for detecting hotspots of methane emission from the vulnerable Indian Sundarban mangrove ecosystem using data-driven models

Coastal mangroves have been lost to deforestation for anthropogenic activities such as agriculture over the past two decades. The genesis of methane (CH₄), a significant greenhouse gas (GHG) with a high potential for global warming, occurs through these mangrove beds. The mangrove forests in the Indian Sundarban deltaic region were studied for pre-monsoonal and post-monsoonal variations of CH₄ emission. Considering the importance of CH₄ emission, a process-based spatiotemporal (PBS) and an analytical neural network (ANN) model were proposed and used to estimate the amount of CH₄ emission from different land use land cover classes (LULC) of mangroves. The field work was performed in 2020, and gas samples of various LULC were directly collected from the mangrove bed using the enclosed box chamber method. Historical climatic data (1960-1989) were used to predict future climate scenarios and associated CH₄ emissions. The analysis and estimation activities were carried out utilizing satellite images from the pre-monsoonal and post-monsoonal seasons of the same year. The study revealed that pre-monsoonal CH₄ emission was higher in the south-west and northern parts of the deforested mangrove of the Indian Sundarban. A sensitivity study of the anticipated models was conducted using a variety of environmental input parameters and related main field observations. The measured precision area under curve of receiver operating characteristics was 0.753 for PBS and 0.718 for ANN models, respectively. The temperature factor (T_f) was the most crucial variable for CH₄ emissions. Based on the PBS model with coupled model intercomparison project-6 temperature data, a global circulation model was run to predict increasing CH₄ emissions up to 2100. The model revealed that the agricultural lands were the prime emitters of CH₄ in the Sundarban mangrove ecosystem.

Carbon Pool in Mexican Wetland Soils: Importance of the Environmental Service

Mexican wetlands are not included in Earth system models around the world, despite being an important carbon store in the wetland soils in the tropics. In this review, five different types of wetlands were observed (marshes, swamps, flooded grasslands, flooded palms and mangroves) in which their carbon pool/carbon sequestrations in Mexican zones were studied. In addition, it was shown that swamps (forested freshwater wetlands) sequestered more carbon in the soil (86.17 ± 35.9 Kg C m−2) than other types of wetlands (p = 0.011); however, these ecosystems are not taken into consideration by the Mexican laws on protection compared with mangroves (34.1 ± 5.2 Kg C m−2). The carbon pool detected for mangrove was statistically similar (p > 0.05) to data of carbon observed in marshes (34.1 ± 5.2 Kg C m−2) and flooded grassland (28.57 ± 1.04 Kg C m−2) ecosystems. The value of carbon in flooded palms (8.0 ± 4.2 Kg C m−2) was lower compared to the other wetland types, but no significant differences were found compared with flooded grasslands (p = 0.99). Thus, the carbon deposits detected in the different wetland types should be taken into account by policy makers and agents of change when making laws for environmental protection, as systematic data on carbon dynamics in tropical wetlands is needed in order to allow their incorporation into global carbon budgets.

Improving soil fertility by driving microbial community changes in saline soils of Yellow River Delta under petroleum pollution

It is promising to use indigenous microorganisms for fertility improvement in petroleum-contaminated coastal soil. As a result, the microbial community and physicochemical property are the base for the restoration. For the detailed information, the Phragmites Communis (P), Chinese Tamarisk (C), Suaeda salsa (S), and new Bare Land (B) soil of Yellow River Delta was 90 g in 100 mL sterile bottles simulated at 25 °C with soil: petroleum = 10:1 in the incubator for four months. The samples were detected at 60 and 120 days along with untreated soil and aged Oil Sludge (O) as control. The results showed that all the samples were alkaline (pH 7.99-8.83), which the salinity and NO₃^- content of incubate soil followed the in situ samples as P (1.09-1.72‰, 8.02-8.17 mg kg^-1), C (10.61-13.79‰, 5.99-6.07 mg kg^-1), S (10.19-12.43‰, 3.64-4.22 mg kg^-1), B (31.85-32.45‰, 3.56-3.72 mg kg^-1) and O (31.61-34.30‰, 0.89-0.90 mg kg^-1). NO₃^- and organic carbon decreased after incubation, which the polluted samples (86.63-92.63 g kg^-1) still had higher organic carbon than untreated ones with more NH₄⁺ consumption. The high-throughput sequence results showed that the Gammaproteobacteria and Alphaproteobacteria were dominant in all samples, while sulfate reducting bacteria Alphaproteobacteria decreased at 120 days. Meanwhile, the electroactive Gammaproteobacteria might symbiosis with Methanosaetaceae and Methanosarcinaceae, degrading petroleum after electron receptors depletion. Nitrososphaeraceae and Nitrosopumilaceae oxidise NH₄⁺ to NO₂^- for intra-aerobic anaerobes and denitrifying bacteria producing oxygen for biodegradation in polluted Phragmites Communis soil. The halotolerant Halomicrobiaceae and Haloferacaceae predominated in saline Chinese Tamarisk, Suaeda Salsa and Bare Land, which were potential electroactive degradater. As the ageing sludge formed, the hydrogen trophic methanogens Methanothermobacteraceae (73.90-92.72%) was prevalent with the petroleum pollution. In conclusion, petroleum initiated two-phase in the sludge forming progress: electron acceptor consumption and electron transfer between degradater and methanogens. Based on the results, the domestic sewage N, P removal coupling and electron transport will be the basement for polluted soils fertility improvement.

Effect of degradation of a black mangrove forest on seasonal greenhouse gas emissions

Mangroves play an essential role in the global carbon cycle. However, they are highly vulnerable to degradation with little-known effects on greenhouse gas (GHG) emissions. This study compared seasonal soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes from a black mangrove (Avicennia germinans) forest in the Tampamachoco coastal lagoon, Veracruz, Mexico, in areas subjected to different degrees of environmental degradation (full canopy, transitional and dead mangrove), caused by hydrological perturbation. Furthermore, we aimed at determining the environmental factors driving seasonal fluxes. There was a combined effect of seasonality and degradation on CH₄ fluxes, highest during the rainy season in the dead mangrove (0.93 ± 0.18 mg CH₄ m^-2 h^-1). CO₂ fluxes were highest during the dry season (220 ± 23 mg CO₂ m^-2 h^-1), with no significant differences among degradation levels. N₂O fluxes did not vary among seasons or degradation levels (- 3.8 to 2.9 mg N₂O m^-2 h^-1). The overall CO₂-eq emission rate was 15.3 ± 2.7 Mg CO₂-eq ha^-1 year^-1, with CO₂ as the main gas contributing to total emissions. The main factors controlling CH₄ fluxes were seasonal porewater salinity and the availability of NO₂^-, NO₃^-, and SO₄^-2 in the soil, favored by high water level and temperature in the absence of pneumatophores. The main determining factors controlling CO₂ fluxes were water level, porewater redox potential, and soil Cl^- and SO₄^-2 concentration. Finally, N₂O fluxes were related to NO₂^-, NO₃^-, and SO₄^-2 soil concentrations. This study contributes to improving the knowledge of soil GHG fluxes dynamics in mangroves and the effect of degradation of these ecosystems on the coastal biogeochemical cycles, which may bring important insights for assessing accurate ways to mitigate climate change protecting and restoring these ecosystems.

Restoration impacts on rates of denitrification and greenhouse gas fluxes from tropical coastal wetlands

Forested coastal wetlands are globally important systems sequestering carbon and intercepting nitrogen pollution from nutrient-rich river systems. Coastal wetlands that have suffered extensive disturbance are the target of comprehensive restoration efforts. Accurate assessment of restoration success requires detailed mechanistic understanding of wetland soil biogeochemical functioning across restoration chrono-sequences, which remains poorly understood for these sparsely investigated systems. This study investigated denitrification and greenhouse gas fluxes in mangrove and Melaleuca forest soils of Vietnam, using the ¹⁵N-Gas flux method. Denitrification-derived N₂O was significantly higher from Melaleuca than mangrove forest soils, despite higher potential rates of total denitrification in the mangrove forest soils (8.1 ng N g^-1 h^-1) than the Melaleuca soils (6.8 ng N g^-1 h^-1). Potential N₂O and CO₂ emissions were significantly higher from the Melaleuca soils than from the mangrove soils. Disturbance and subsequent recovery had no significant effect on N biogeochemistry except with respect to the denitrification product ratio in the mangrove sites, which was highest from the youngest mangrove site. Potential CO₂ and CH₄ fluxes were significantly affected by restoration in the mangrove soils. The lowest potential CO₂ emissions were observed in the mid-age plantation and potential CH₄ fluxes decreased in the older forests. The mangrove system, therefore, may remove excess N and improve water quality with low greenhouse gas emissions, whereas in Melaleucas, increased N₂O and CO₂ emissions also occur. These emissions are likely balanced by higher carbon stocks observed in the Melaleuca soils. These mechanistic insights highlight the importance of ecosystem restoration for pollution attenuation and reduction of greenhouse gas emissions from coastal wetlands. Restoration efforts should continue to focus on increasing wetland area and function, which will benefit local communities with improved water quality and potential for income generation under future carbon trading.

Source or sink? A study on the methane flux from mangroves stems in Zhangjiang estuary, southeast coast of China

Mangrove ecosystems are an important component of "blue carbon". However, it is not clear whether the stems play roles in the CH₄ budget of mangrove ecosystems. This study investigated the CH₄ emission from mangrove stems and its potential driving factors. We set up six sample plots in the Zhangjiang Estuary National Mangrove Nature Reserve, where Kandelia obovata, Avicennia marina and Aegiceras corniculata are the main mangrove tree species. Soil properties such as total carbon content, redox potential and salinity were determined in each plot. The dynamic chamber method was used to measure mangrove stems and soil CH₄ fluxes. Combined field survey results with Principal Component Analysis (PCA) of soil properties, we divided the six plots into two sites (S1 and S2) to perform statistical analyses of stem CH₄ fluxes. Then the CH₄ fluxes from mangrove tree stems and soil were further scaled up to the ecosystem level through the mapping model. Under different backgrounds of soil properties, salinity and microbial biomass carbon were the main factors modified soil CH₄ fluxes in the two sites, and further affected the stem CH₄ fluxes of mangroves. The soil of both sites are sources of CH₄, and the soil CH₄ emission of S2 was about twice higher than that of S1. Results of upscaling model showed that mangrove stems in S1 were CH₄ sinks with -105.65 g d^-1. But stems in S2 were CH₄ sources around 1448.24 g d^-1. Taken together, our results suggested that CH₄ emission from mangrove soils closely depends on soils properties. And mangrove stems were found to act as both CH₄ sources and CH₄ sinks depend on soil CH₄ production. Therefore, when calculating the CH₄ budget of the mangrove ecosystem, the contribution of mangrove plant stems cannot be ignored.

Carbon Fluxes and Stocks by Mexican Tropical Forested Wetland Soils: A Critical Review of Its Role for Climate Change Mitigation

Wetland soils are important stores of soil carbon (C) in the biosphere, and play an important role in global carbon cycles in the response strategy to climate change. However, there areknowledge gaps in our understanding of the quantity and distribution in tropical regions. Specifically, Mexican wetlands have not been considered in global carbon budgets or carbon balances for a number of reasons, such as: (1) the lack of data, (2) Spanish publications have not been selected, or (3) because such balances are mainly made in the English language. This study analyzes the literature regarding carbon stocks, sequestration and fluxes in Mexican forested wetlands (Forest-W). Soil carbon stocks of 8, 24.5 and 40.1 kg cm^-2 were detected for flooded palms, mangroves, and freshwater or swamps (FW) wetland soils, respectively, indicating that FW soils are the Forest-W with more potential for carbon sinks (p = 0.023), compared to mangroves and flooded palm soils. While these assessments of carbon sequestration were ranged from 36 to 920 g-C m^-2 year^-1, C emitted as methane was also tabulated (0.6-196 g-C m^-2 year^-1). Subtracting the C emitted of the C sequestered, 318.2 g-C m^-2 year^-1 were obtained. Such data revealed that Forest-W function is mainly as carbon sink, and not C source. This review can help to inform practitioners in future decisions regarding sustainable projects, restoration, conservation or creation of wetlands. Finally, it is concluded that Forest-W could be key ecosystems in strategies addressing the mitigation of climate change through carbon storage. However, new studies in this research line and public policies that protect these essential carbon sinks are necessary in order to, hopefully, elaborate global models to make more accurate predictions about future climate.