Rich soil carbon and nitrogen but low atmospheric greenhouse gas fluxes from North Sulawesi mangrove swamps in Indonesia

Greenhouse gas fluxes of different land uses in mangrove ecosystem of East Kalimantan, Indonesia

BACKGROUND

Mangrove ecosystems exhibit significant carbon storage and sequestration. Its capacity to store and sequester significant amounts of carbon makes this ecosystem very important for climate change mitigation. Indonesia, owing to the largest mangrove cover in the world, has approximately 3.14 PgC stored in the mangroves, or about 33% of all carbon stored in coastal ecosystems globally. Unfortunately, our comprehensive understanding of carbon flux is hampered by the incomplete repertoire of field measurement data, especially from mangrove ecosystem-rich regions such as Indonesia and Asia Pacific. This study fills the gap in greenhouse gases (GHGs) flux studies in mangrove ecosystems in Indonesia by quantifying the soil CO2 and CH4 fluxes for different land use types in mangrove ecosystems, i.e., secondary mangrove (SM), restored mangrove (RM), pond embankment (PE) and active aquaculture pond (AP). Environmental parameters such as soil pore salinity, soil pore water pH, soil temperature, air temperature, air humidity and rainfall are also measured.

RESULTS

GHG fluxes characteristics varied between land use types and ecological conditions. Secondary mangrove and exposed pond embankment are potential GHG flux sources (68.9 ± 7.0 and 58.5 ± 6.2 MgCO2e ha- 1 yr- 1, respectively). Aquaculture pond exhibits the lowest GHG fluxes among other land use types due to constant inundation that serve as a barrier for the release of GHG fluxes to the atmosphere. We found weak relationships between soil CO2 and CH4 fluxes and environmental parameters.

CONCLUSIONS

The data and information on GHG fluxes from different land use types in the mangrove ecosystem will be of importance to accurately assess the potential of the mangrove ecosystem to sequester and emit GHGs. This will support the GHG emission reduction target and strategy that had been set up by the Indonesian Government in its Nationally Determined Contributions (NDC) and Indonesia's 2030 Forest and Other Land Use (FOLU) Net Sink.

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.

Effects of dredging wastewater input history and aquaculture type on greenhouse gas fluxes from mangrove sediments along the shorelines of the Jiulong River Estuary, China

Dredging wastewater (DW) from aquaculture ponds is a major disturbance factor in mangrove management, and its effects on the greenhouse gas (GHG) fluxes from mangrove sediment remain controversial. In this study, we investigated GHG (N2O, CH4, and CO2) fluxes from mangrove sediment at typical aquaculture pond-mangrove sites that were stimulated by DW discharged for different input histories and from different farm types. The GHG fluxes exhibited differing cumulative effects with increasing periods of DW input. The N2O and CH4 fluxes from mangrove sediment that received DW inputs for 17 y increased by ∼10 and ∼1.5 times, respectively, whereas the CO2 flux from mangrove sediment that received DW inputs for 11 y increased by ∼1 time. The effect of DW from shrimp ponds on the N2O flux was significantly larger than those of DW from fish/crab ponds and razor clam ponds. Moreover, the total global warming potentials (GWPs) at the field sites with DW inputs increased by 29-129% of which the CO2 flux was the main contributor to the GWP (85-96%). N2O as a proportion of CO2-equivalent flux increased from 2% to 12%, indicating that N2O was an important contributor to the increase in GWP. Overall, DW increased the GHG fluxes from mangrove sediments, indicating that the contribution of mangroves to climate warming was enhanced under DW input. It also implies that the carbon sequestration potential of mangrove sediments may be threatened to some extent. Therefore, future assessments of the carbon sequestration capacity of mangroves at regional or global scales should consider this phenomenon.

The Coastal Carbon Library and Atlas: Open source soil data and tools supporting blue carbon research and policy

Quantifying carbon fluxes into and out of coastal soils is critical to meeting greenhouse gas reduction and coastal resiliency goals. Numerous 'blue carbon' studies have generated, or benefitted from, synthetic datasets. However, the community those efforts inspired does not have a centralized, standardized database of disaggregated data used to estimate carbon stocks and fluxes. In this paper, we describe a data structure designed to standardize data reporting, maximize reuse, and maintain a chain of credit from synthesis to original source. We introduce version 1.0.0. of the Coastal Carbon Library, a global database of 6723 soil profiles representing blue carbon-storing systems including marshes, mangroves, tidal freshwater forests, and seagrasses. We also present the Coastal Carbon Atlas, an R-shiny application that can be used to visualize, query, and download portions of the Coastal Carbon Library. The majority (4815) of entries in the database can be used for carbon stock assessments without the need for interpolating missing soil variables, 533 are available for estimating carbon burial rate, and 326 are useful for fitting dynamic soil formation models. Organic matter density significantly varied by habitat with tidal freshwater forests having the highest density, and seagrasses having the lowest. Future work could involve expansion of the synthesis to include more deep stock assessments, increasing the representation of data outside of the U.S., and increasing the amount of data available for mangroves and seagrasses, especially carbon burial rate data. We present proposed best practices for blue carbon data including an emphasis on disaggregation, data publication, dataset documentation, and use of standardized vocabulary and templates whenever appropriate. To conclude, the Coastal Carbon Library and Atlas serve as a general example of a grassroots F.A.I.R. (Findable, Accessible, Interoperable, and Reusable) data effort demonstrating how data producers can coordinate to develop tools relevant to policy and decision-making.

Multiple drivers for carbon stocks and fluxes in different types of mangroves

Mangroves are highly efficient in sequestering carbon from the atmosphere and can accumulate carbon in sediments for millennials. However, The fate of mangrove carbon has not been well constrained due to the lack of data on different pools of sediment carbon sinks and sources. This study examined the variation of carbon stocks and fluxes at the water-sediment-air interface in both estuarine mangroves (natural: Mai Po, restored: Gei Wai) and oceanic mangroves (Ting Kok). There are divergent patterns in biogeochemical variables at the sediment-water-air interface, likely due to significant variation within sites. Total sediment carbon stocks (TCs) ranked in the order of restored estuarine mangroves (392.5 ± 8.8 Mg ha-1), natural estuarine mangroves affected by aquaculture (315.2 ± 21.4 Mg ha-1) and oceanic mangroves (229.1 ± 32.3 Mg ha-1). Sediment organic carbon stocks (SOC) and inorganic carbon stocks (SIC) accounted for 84.1-90.2 % and 9.8-15.9 % of TC, respectively. The highest sediment-air CO2 and CH4 fluxes occurred in restored and natural estuarine mangroves affected by aquaculture, respectively. The isotope of CO2 fluxes (δ13C-CO2) indicates higher contributions from the degradation of mangrove-derived organic carbon in restored (-25.94 ‰ ± 3.37 ‰) and natural estuarine mangroves affected by aquaculture (-25.54 ‰ ± 0.96 ‰) than in oceanic mangroves (-21.55 ‰ ± 1.36 ‰). The isotope of CH4 fluxes (δ13C-CH4) indicates CH4 production dominated by acetate fermentation in restored estuarine mangroves but dominated by the reduction of CO2 for other sites. Future studies should better constrain the fate of mangrove carbon by considering local drivers.

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.

Microbial communities in swamps of four mangrove reserves driven by interactions between physicochemical properties and microbe in the North Beibu Gulf, China

Mangroves are distributed in coastal and estuarine regions and are characterized as a sink for terrestrial pollution. It is believed that complex interactions between environmental factors and microbial communities exist in mangrove swamps. However, little is known about environment-microbe interactions. There is a need to clarify some important environmental factors shaping microbial communities and how environmental factors interact with microbial assemblages in mangrove swamps. In the present study, physicochemical and microbial characteristics in four mangrove reserves (named ZZW, Qin, Bei, and GQ) in the North Beibu Gulf were determined. The interactions between environmental factors and microbial assemblages were analyzed with statistical methods in addition to CCA and RDA. Higher concentrations of sulfate (SO₄^2--S) and Fe but lower concentrations of total phosphorus (TP) and NO₃^--N were detected in ZZW and Qin. Nutrient elements (NO₃^--N, NH₄⁺-N, organic matter (OM), SO₄^2--S, Fe, and TP) were more important than heavy metals for determining the microbial assemblages, and NO₃^--N was the most important factor. NO₃^--N, SO₄^2--S, TP, and Fe formed a significant co-occurrence network in conjunction with some bacterial taxa, most of which were Proteobacteria. Notably, comparatively elevated amounts of sulfate-reducing bacteria (Desulfatibacillum, Desulfomonile, and Desulfatiglans) and sulfur-oxidizing bacteria (Thioprofundum and Thiohalophilus) were found in ZZW and Qin. The co-occurrence network suggested that some bacteria involved in sulfate reduction and sulfur oxidation drive the transformation of P and N, resulting in the reduction of P and N in mangrove swamps. Through the additional utilization of multivariate regression tree (MRT) and co-occurrence network analysis, our research provides a new perspective for understanding the interactions between environmental factors and microbial communities in mangroves.

Influence of seasonal and environmental variables on the emission of methane from the mangrove sediments of Goa

Mangrove sediments are known sources for methane emission that has a very high global warming potential. The spatio-temporal emission of methane in the mangrove sediments was quantified in the present study using the static closed chamber technique. Besides, the effects of environmental parameters on methane emission were estimated at Betim (mouth), Chorão (midstream), and Volvoi (upstream) stations along the tropical Mandovi estuary. On an average, the methane emission at the upstream estuarine station at Volvoi was maximum (1268.68 ± 176 nM cm^-2 h^-1) compared to the other two stations. Annually, the methane emission was significantly influenced by physicochemical parameters like salinity at Betim and Volvoi and, the redox potential at the midstream station at Chorão. The variation of methane emission between the 3 stations (P < 0.001) is attributed to the variation in methanotrophy (P < 0.05) and methanogenesis (P < 0.05) influenced by differences in the concentration of nutrients (P < 0.05) and organic carbon (P < 0.05). Seasonally, the highest methane emission at Chorão was during the post-monsoon, at Betim was during the monsoon season (1305.34 ± 108.58 nM cm^-2 h^-1), and at the upstream station at Volvoi, the emission of methane was highest during the pre-monsoon season (1514.68 ± 130.94 nM cm^-2 h^-1). The influence of environmental parameters was more prominent on methane emission at the 3 stations during the monsoon season. Spearman's correlation analysis indicated that seasonal changes in methane emission are not only attributed to the influence of seasonal rainfall that leads to the fresh water input, but also to the variation in biogeochemical parameters.

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.

Nitrogen Cycling and Mass Balance in the World’s Mangrove Forests

Nitrogen (N) cycling in mangroves is complex, with rapid turnover of low dissolved N concentrations, but slow turnover of particulate N. Most N is stored in soils. The largest sources of N are nearly equal amounts of mangrove and benthic microalgal primary production. Dissolved N fluxes between the forests and tidal waters show net uptake, indicating N conservation. N2-fixation is underestimated as rapid rates measured on tree stems, aboveground roots and cyanobacterial mats cannot currently be accounted for at the whole-forest scale due to their extreme patchiness and the inability to extrapolate beyond a localized area. Net immobilization of NH4+ is the largest ecosystem flux, indicating N retention. Denitrification is the largest loss of N, equating to 35% of total N input. Burial equates to about 29% of total inputs and is the second largest loss of N. Total inputs slightly exceed total outputs, currently suggesting net N balance in mangroves. Mangrove PON export equates to ≈95% of PON export from the world’s tropical rivers, but only 1.5% of the entire world’s river discharge. Mangrove N2O emissions, denitrification, and burial contribute 0.4%, 0.5–2.0% and 6%, respectively, to the global coastal ocean, which are disproportionate to their small worldwide area.

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

The aim of this study was to quantify carbon stocks and the emission of the greenhouse gases (N₂O and CH₄) in mangrove forests with different vegetation assemblies in coastal lagoons of Veracruz Mexico. The vegetation included: black mangrove BM, dominated by Avicennia germinans, white mangrove WM, dominated by Laguncularia. racemose, red mangrove RM, dominated by Rhizophora mangle and mixed mangrove MM, dominated by the three species. Soil C stocks ranged 187-671 Mg C ha ¹ without significant (p = 0.149) differences among the mangroves with different vegetation. Significantly (p = 0.049) higher tree biomass C stock was observed in RM (127 Mg ha^-1) than in MM (24.23 Mg ha^-1). Methane emissions in RM (0.58-6.03 mg m^-2 min^-1) were significantly higher (p < 0.05) than in MM. (0.0035-0.07 mg m^-2 min^-1), in WM (-0.0026-0.029 mg m^-2 min^-1) and in BM (0.0054-0.0097 mg m^-2 min^-1),during rainy, windy and dry season.RM had the longest period of inundation, the highest soil carbon concentration, and the lowest salinity. CH₄ emissions showed a significantly positive correlation with soils carbon concentration, water level and water pH and, negative correlation with water salinity and Cl^-1 concentration in soil and water. Emissions of N₂O (0.04-3.25 μg m^-2 min^-1) were not significantly different among the mangroves with different vegetation, but they showed seasonal variations, with higher emissions during windy and dry seasons. N₂O emissions showed significantly positive correlations with soil nitrate concentration and soil temperature. Results of this research are useful for mangrove conservation and restoration strategies to maximize carbon storage and mitigate greenhouse gas emissions.

Carbon Cycling in the World’s Mangrove Ecosystems Revisited: Significance of Non-Steady State Diagenesis and Subsurface Linkages between the Forest Floor and the Coastal Ocean

Carbon cycling within the deep mangrove forest floor is unique compared to other marine ecosystems with organic carbon input, mineralization, burial, and advective and groundwater export pathways being in non-steady-state, often oscillating in synchrony with tides, plant uptake, and release/uptake via roots and other edaphic factors in a highly dynamic and harsh environment. Rates of soil organic carbon (CORG) mineralization and belowground CORG stocks are high, with rapid diagenesis throughout the deep (>1 m) soil horizon. Pocketed with cracks, fissures, extensive roots, burrows, tubes, and drainage channels through which tidal waters percolate and drain, the forest floor sustains non-steady-state diagenesis of the soil CORG, in which decomposition processes at the soil surface are distinct from those in deeper soils. Aerobic respiration occurs within the upper 2 mm of the soil surface and within biogenic structures. On average, carbon respiration across the surface soil-air/water interface (104 mmol C m−2 d−1) equates to only 25% of the total carbon mineralized within the entire soil horizon, as nearly all respired carbon (569 mmol C m−2 d−1) is released in a dissolved form via advective porewater exchange and/or lateral transport and subsurface tidal pumping to adjacent tidal waters. A carbon budget for the world’s mangrove ecosystems indicates that subsurface respiration is the second-largest respiratory flux after canopy respiration. Dissolved carbon release is sufficient to oversaturate water-column pCO2, causing tropical coastal waters to be a source of CO2 to the atmosphere. Mangrove dissolved inorganic carbon (DIC) discharge contributes nearly 60% of DIC and 27% of dissolved organic carbon (DOC) discharge from the world’s low latitude rivers to the tropical coastal ocean. Mangroves inhabit only 0.3% of the global coastal ocean area but contribute 55% of air-sea exchange, 14% of CORG burial, 28% of DIC export, and 13% of DOC + particulate organic matter (POC) export from the world’s coastal wetlands and estuaries to the atmosphere and global coastal ocean.

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.

Biogeochemical Processes of C and N in the Soil of Mangrove Forest Ecosystems

The mangrove forest provides various ecosystem services in tropical and subtropical regions. Many of these services are driven by the biogeochemical cycles of C and N, and soil is the major reservoir for these chemical elements. These cycles may be influenced by the changing climate. The high plant biomass in mangrove forests makes these forests an important sink for blue C storage. However, anaerobic soil conditions may also turn mangrove forests into an environmentally detrimental producer of greenhouse gases (such as CH4 and N2O), especially as air temperatures increase. In addition, the changing environmental factors associated with climate change may also influence the N cycles and change the patterns of N2 fixation, dissimilatory nitrate reduction to ammonium, and denitrification processes. This review summarizes the biogeochemical processes of C and N cycles in mangrove forest soils based on recently published studies, and how these processes may respond to climate change, with the aim of predicting the impacts of climate change on the mangrove forest ecosystem.

Methane Emissions from Subtropical and Tropical Mangrove Ecosystems in Taiwan

Mangroves are one of the blue carbon ecosystems. However, greenhouse gas emissions from mangrove soils may reduce the capacity of carbon storage in these systems. In this study, methane (CH4) fluxes and soil properties of the top 10 cm layer were determined in subtropical (Kandelia obovata) and tropical (Avicennia marina) mangrove ecosystems of Taiwan for a complete seasonal cycle. Our results demonstrate that CH4 emissions in mangroves cannot be neglected when constructing the carbon budgets and estimating the carbon storage capacity. CH4 fluxes were significantly higher in summer than in winter in the Avicennia mangroves. However, no seasonal variation in CH4 flux was observed in the Kandelia mangroves. CH4 fluxes were significantly higher in the mangrove soils of Avicennia than in the adjoining mudflats; this trend, however, was not necessarily recapitulated at Kandelia. The results of multiple regression analyses show that soil water and organic matter content were the main factors regulating the CH4 fluxes in the Kandelia mangroves. However, none of the soil parameters assessed show a significant influence on the CH4 fluxes in the Avicennia mangroves. Since pneumatophores can transport CH4 from anaerobic deep soils, this study suggests that the pneumatophores of Avicennia marina played a more important role than soil properties in affecting soil CH4 fluxes. Our results show that different mangrove tree species and related root structures may affect greenhouse gas emissions from the soils.

Crab Bioturbation and Seasonality Control Nitrous Oxide Emissions in Semiarid Mangrove Forests (Ceará, Brazil)

Seasonality and crab activity affects the nutrients and physicochemical parameters in mangrove soils, thus, affecting the emissions of greenhouse gases, such as nitrous oxide (N2O). Climate change may intensify rainfall and/or enhance droughts, affecting mangroves and associated biota. Crabs are natural soil bioturbators responsible for soil aeration and turnover. We evaluated the effect of Ucides cordatus crab on N2O emissions from mangrove soils under a semiarid climate in Northeastern Brazil. Soil and gas samples were collected over the rainy and dry seasons in crab-naturally-bioturbated and crab-exclusion mangrove plots. We measured the soil’s pH, redox potential, and the total contents of carbon, nitrogen, and sulfur. We found higher N2O emissions in the crab-exclusion sites compared to the bioturbated sites, as well as higher N2O emissions in the rainy season compared to the dry season. The fluxes of N2O (µg m−2 h−1) were 47.3 ± 9.7 and 8.9 ± 0.5 for the crab-exclusion sites, and 36.5 ± 7.8 and 4.5 ± 2.1 for the bioturbated sites (wet and dry seasons, respectively). The soil turning over by macrofauna led to lower N2O fluxes in natural crab-bioturbated areas, and seasonality was the environmental factor that contributed the most to the changes in N2O emissions. Broadly, anthropic activities and seasonality influence nitrogen fate, N2O emissions, and ecological services in coastal ecosystems.

Sensitivity of mangrove soil organic matter decay to warming and sea level change

Mangroves are among the world's most carbon-dense ecosystems, but they are threatened by rapid climate change and rising sea levels. The accumulation and decomposition of soil organic matter (SOM) are closely tied to mangroves' carbon sink functions and resistance to rising sea levels. However, few studies have investigated the response of mangrove SOM dynamics to likely future environmental conditions. We quantified how mangrove SOM decay is affected by predicted global warming (+4°C), sea level changes (simulated by altering of the inundation duration to 0, 2, and 6 hr/day), and their interaction. Whilst changes in inundation duration between 2 and 6 hr/day did not affect SOM decay, the treatment without inundation led to a 60% increase. A warming of 4°C caused SOM decay to increase by 21%, but longer inundation moderated this temperature-driven increase. Our results indicate that (a) sea level rise is unlikely to decrease the SOM decay rate, suggesting that previous mangrove elevation gain, which has allowed mangroves to persist in areas of sea level rise, might result from changes in root production and/or mineral sedimentation; (b) sea level fall events, predicted to double in frequency and area, will cause periods of intensified SOM decay; (c) changing tidal regimes in mangroves due to sea level rise might attenuate increases in SOM decay caused by global warming. Our results have important implications for forecasting mangrove carbon dynamics and the persistence of mangroves and other coastal wetlands under future scenarios of climate change.

Diurnal and seasonal patterns of soil CO2 efflux from the Pichavaram mangroves, India

The diurnal and seasonal variation of soil carbon dioxide (CO₂) flux was measured in the Pichavaram mangrove forest, the Southeast coast of India from February 2016 to October 2016 using an automated soil CO₂ flux chamber system. Maximum soil CO₂ efflux reached at 14:00 h and minimum at 00:00 h. The surface soil CO₂ concentration ranged from 375 to 532 ppm with the mean 405 ± 18 ppm. The daily soil CO₂ flux varied from near zero to about 7 μmol m^-2 s^-1 with a mean value of 2.4 ± 1.3 μmol m^-2 s^-1. The highest seasonal CO₂ efflux from soil was during the summer and premonsoon seasons, whereas low flux values were recorded during the monsoon season. Soil CO₂ efflux values were highly correlated with soil temperature. Tidal inundation during monsoon season, extreme drought condition in summer, and unusual precipitation are the major factors controlling the soil CO₂ flux.

Biofilm and temperature controls on greenhouse gas (CO2 and CH4) emissions from a Rhizophora mangrove soil (New Caledonia)

Seasonal variations of CO₂ and CH₄ fluxes were investigated in a Rhizophora mangrove forest that develops under a semi-arid climate, in New Caledonia. Fluxes were measured using closed incubation chambers connected to a CRDS analyzer. They were performed during low tide at light, in the dark, and in the dark after having removed the top 1-2 mm of soil, which may contain biofilm. CO₂ and CH₄ fluxes ranged from 31.34 to 187.48 mmol m^-2 day^-1 and from 39.36 to 428.09 μmol m^-2 day^-1, respectively. Both CO₂ and CH₄ emissions showed a strong seasonal variability with higher fluxes measured during the warm season, due to an enhanced production of these two gases within the soil. Furthermore, CO₂ fluxes were higher in the dark than at light, evidencing photosynthetic processes at the soil surface and thus the role of biofilm in the regulation of greenhouse gas emissions from mangrove soils. The mean δ¹³C-CO₂ value of the CO₂ fluxes measured was -19.76 ± 1.19‰, which was depleted compared to the one emitted by root respiration (-22.32 ± 1.06‰), leaf litter decomposition (-21.43 ± 1.89‰) and organic matter degradation (-22.33 ± 1.82‰). This result confirmed the use of the CO₂ produced within the soil by the biofilm developing at its surface. After removing the top 1-2 mm of soil, both CO₂ and CH₄ fluxes increased. Enhancement of CH₄ fluxes suggests that biofilm may act as a physical barrier to the transfer of GHG from the soil to the atmosphere. However, the δ¹³C-CO₂ became more enriched, evidencing that the biofilm was not integrally removed, and that its partial removal resulted in physical disturbance that stimulated CO₂ production. Therefore, this study provides useful information to understand the global implication of mangroves in climate change mitigation.

Methane Emission from Mangrove Wetland Soils Is Marginal but Can Be Stimulated Significantly by Anthropogenic Activities

Mangrove wetland soils have been considered as important sources for atmospheric CH4, but the magnitude of CH4 efflux in mangrove wetlands and its relative contribution to climate warming compared to CO2 efflux remains controversial. In this study, we measured both CH4 and CO2 effluxes from mangrove soils during low or no tide periods at three tidal zones of two mangrove ecosystems in Southeastern China and collected CH4 efflux data from literature for 24 sites of mangrove wetlands worldwide. The CH4 efflux was highly variable among our field sites due to the heterogeneity of mangrove soil environments. On average, undisturbed mangrove sites have very low CH4 efflux rates (ranging from 0.65 to 14.18 μmol m−2 h−1; median 2.57 μmol m−2 h−1), often less than 10% of the global warming potentials (GWP) caused by the soil CO2 efflux from the same sites (ranging from 0.94 to 9.50 mmol m−2 h−1; median 3.67 mmol m−2 h−1), even after considering that CH4 has 28 times more GWP over CO2. Plant species, study site, tidal position, sampling time, and soil characteristics all had no significant effect on mangrove soil CH4 efflux. Combining our field measurement results and literature data, we demonstrated that the CH4 efflux from undisturbed mangrove soils was marginal in comparison with the CO2 efflux in most cases, but nutrient inputs from anthropogenic activities including nutrient run-off and aquaculture activities significantly increased CH4 efflux from mangrove soils. Therefore, CH4 efflux from mangrove wetlands is strongly influenced by anthropogenic activities, and future inventories of CH4 efflux from mangrove wetlands on a regional or global scale should consider this phenomenon.

Exotic Spartina alterniflora invasion increases CH4 while reduces CO2 emissions from mangrove wetland soils in southeastern China

Mangroves are critical in global carbon budget while vulnerable to exotic plant invasion. Spartina alterniflora, one of typical salt marsh plant grows forcefully along the coast of China, has invaded the native mangrove habitats in Zhangjiang Estuary. However, the effects of S. alterniflora invasion on soil carbon gases (CH₄ and CO₂) emission from mangroves are not fully understood. Accordingly, we conducted a field experiment to investigate the soil CH₄ and CO₂ emission during growing seasons in 2016 and 2017 at four adjacent wetlands, namely bare mudflat (Mud), Kandelia obovata (KO), Avicennia marina (AM) and S. alterniflora (SA). Potential methane production (PMP), potential methane oxidation (PMO), functional microbial abundance and soil biogeochemical properties were measured simultaneously. Our results indicate that S. alterniflora invasion could dramatically increase soil CH₄ emissions mainly due to the enhancement in PMP which facilitated by soil EC, MBC, TOC and mcrA gene abundance. Additionally, S. alterniflora invasion decreases soil CO₂ emission. Both heterotrophic microbial respiration (16S rRNA) and methane oxidation (pmoA and ANME-pmoA) are responsible for CO₂ emission reduction. Furthermore, S. alterniflora invasion greatly increases GWP by stimulating CH₄ emissions. Thus, comparing with mangroves, invasive S. alterniflora significantly (p < 0.001) increases CH₄ emission while reduces CO₂ emission.

Soil carbon storage in mangroves is primarily controlled by soil properties: A study at Dongzhai Bay, China

Coastal wetlands are well known for their considerable capacity to store carbon (C). However, the spatial patterns and major controls of soil C concentration and C density in coastal wetlands remain poorly known. We measured soil total C concentration up to one meter depth and assessed environmental and biological factors influencing soil C input and decomposition processes across various geomorphologic settings and mangrove forest types at Dongzhai Bay, China. Structural equation modeling (SEM) was used to determine the causal pathways of influencing factors on soil C concentration. We found that the variation pattern of soil C concentration across geomorphologic settings and forest types was mirrored by soil properties. From 68 to 94% (varying with soil depth) variations of soil C concentration were explained by the inter-related influencing factors included in SEM. In the upper 60cm soil layers, soil moisture was the most important factor affecting soil C concentration. In the 60-100cm subsoil zone, the proportion of finer soil particles was the primary control of soil C concentration variation. In contrast, aboveground biomass and nearness of sampling site to the open water, which affect autochthonous and allochthonous C inputs, had relatively weak effects on soil C concentration compared to soil properties, which affect C decomposition. Soil C concentration was a good predictor of soil C density at all soil depths. The results suggest that top- and subsoil C concentrations in mangroves are subjected to different environmental controls, but taken together, mangrove soil C storage may be primarily controlled by soil property-mediated C decomposition rate. Subsoil C deserves more attention since it may respond differently to environmental changes than the better-known topsoil C.

Quantification of N2O and NO emissions from a small-scale pond-ditch circulation system for rural polluted water treatment

The pond-ditch circulation system (PDCS) is an efficient and economical solution for the restoration of degraded rural water environments. However, little is known about nitrous oxide (N₂O) and nitric oxide (NO) emissions in the microbial removal process of nitrogen in PDCSs, and their contribution to nitrogen removal. The aim of this study was to quantify N₂O and NO emissions from the PDCS, evaluate their capacities, and elucidate the key environmental factors controlling them. The results showed that N₂O and NO fluxes were in the ranges 1.1-2055.1μgNm^-2h^-1 and 0.1-6.8μgNm^-2h^-1 for the PDCS, respectively. Meanwhile, the N₂O and NO fluxes from the two ponds in the PDCS were significantly higher than those in the static system. Moreover, the amount of N₂O and NO emissions in the PDCS accounted for 0.17-4.32% of the total nitrogen (TN) removal. According to the partial least squares (PLS) approach and Pearson's correlation coefficients, nitrate nitrogen in water (W-NO₃^--N), dissolved oxygen in water (W-DO), dissolved oxygen in sediment (DO), pH in water (W-pH), pH in sediment (pH), total kjeldahl nitrogen (TKN), and soil organic carbon (SOC) significantly affected the N₂O flux (p<0.05), whereas W-NO₃^--N, DO, and nitrite nitrogen in sediment (NO₂^--N) significantly affected the NO emission (p<0.05).

High salinity leads to accumulation of soil organic carbon in mangrove soil

Although mangrove forests are one of the most well-known soil organic carbon (SOC) sinks, the mechanism underlying SOC accumulation is relatively unknown. High net primary production (NPP) along with the typical bottom-heavy biomass allocation and low soil respiration (SR) have been considered to be responsible for SOC accumulation. However, an emerging paradigm postulates that SR is severely underestimated because of the leakage of dissolved inorganic carbon (DIC) in groundwater. Here we propose a simple yet unique mechanism for SOC accumulation in mangrove soils. We conducted sequential extraction of water extractable organic matter (WEOM) from mangrove soils using ultrapure water and artificial seawater, respectively. A sharp increase in humic substances (HS) concentration was observed only in the case of ultrapure water, along with a decline in salinity. Extracted WEOM was colloidal, and ≤70% of it re-precipitated by the addition of artificial seawater. These results strongly suggest that HS is selectively flocculated and maintained in the mangrove soils because of high salinity. Because sea salts are a characteristic of any mangrove forest, high salinity may be one of mechanisms underlying SOC accumulation in mangrove soils.

Mangroves as a major source of soil carbon storage in adjacent seagrass meadows

Mangrove forests have the potential to export carbon to adjacent ecosystems but whether mangrove-derived organic carbon (OC) would enhance the soil OC storage in seagrass meadows adjacent to mangroves is unclear. In this study we examine the potential for the contribution of mangrove OC to seagrass soils on the coast of North Sulawesi, Indonesia. We found that seagrass meadows adjacent to mangroves had significantly higher soil OC concentrations, soil OC with lower δ ¹³C, and lower bulk density than those at the non-mangrove adjacent meadows. Soil OC storage to 30 cm depth ranged from 3.21 to 6.82 kg C m⁻², and was also significantly higher at the mangrove adjacent meadows than those non-adjacent meadows. δ¹³C analyses revealed that mangrove OC contributed 34 to 83% to soil OC at the mangrove adjacent meadows. The δ¹³C value of seagrass plants was also different between the seagrasses adjacent to mangroves and those which were not, with lower values measured at the seagrasses adjacent to mangroves. Moreover, we found significant spatial variation in both soil OC concentration and storage, with values decreasing toward sea, and the contribution of mangrove-derived carbon also reduced with distance from the forest.

Edaphic factors controlling summer (rainy season) greenhouse gas emissions (CO2 and CH4) from semiarid mangrove soils (NE-Brazil)

The soil attributes controlling the CO2, and CH4 emissions were assessed in semiarid mangrove soils (NE-Brazil) under different anthropogenic activities. Soil samples were collected from different mangroves under different anthropogenic impacts, e.g., shrimp farming (Jaguaribe River); urban wastes (Cocó River) and a control site (Timonha River). The sites were characterized according to the sand content; physicochemical parameters (Eh and pH); total organic C; soil C stock (SCS) and equivalent SCS (SCSEQV); total P and N; dissolved organic C (DOC); and the degree of pyritization (DOP). The CO2 and CH4 fluxes from the soils were assessed using static closed chambers. Higher DOC and SCS and the lowest DOP promote greater CO2 emission. The CH4 flux was only observed at Jaguaribe which presented higher DOP, compared to that found in mangroves from humid tropical climates. Semiarid mangrove soils cannot be characterized as important greenhouse gas sources, compared to humid tropical mangroves.

Temporal and spatial variations of greenhouse gas fluxes from a tidal mangrove wetland in Southeast China

Tidal mangrove wetlands are a source of methane (CH4) and nitrous oxide (N2O); but considering the high productivity of mangroves, they represent a significant sink for carbon dioxide (CO2). An exotic plant Spartina alterniflora has invaded east China over the last few decades, threatening these coastal mangrove ecosystems. However, the atmospheric gas fluxes in mangroves are poorly characterized and the impact of biological invasion on greenhouse gas (GHG) fluxes in the wetland remains unclear. In this study, the temporal and spatial dynamics of key GHG fluxes (CO2, CH4, and N2O) at an unvegetated mudflat, cordgrass (S. alterniflora), and mangrove (Kandelia obovata) sites along an estuary of the Jiulong River in Southeast China were investigated over a 2-year period. The CO2 and CH4 fluxes demonstrated a seasonal and vegetation-dependent variation while N2O fluxes showed no such dependent pattern. Air temperature was the main factor influencing CO2 and CH4 fluxes. Cumulative global warming potential (GWP) ranked in the order of mangrove > cordgrass > mudflat and summer > spring > autumn > winter. Moreover, CH4 accounted for the largest proportion (68%) of GWP, indicating its dominant contribution to the warming potential in mangroves. Notwithstanding the lack of information on plant coverage, cordgrass invasion exhibited a minor influence on GHG emissions. These findings support the notion that mangrove forests are net accumulation sites for GHGs. As vegetation showed considerable effects on fluxes, more information about the significance of vegetation type with a special emphasis on the effects of invasive plants is crucial.

Temporal variability of CO₂ fluxes at the sediment-air interface in mangroves (New Caledonia)

Carbon budgets in mangrove forests are uncertain mainly due to the lack of data concerning carbon export in dissolved and gaseous forms. Temporal variability of in situ CO2 fluxes was investigated at the sediment-air interface in different seasons in different mangrove stands in a semi-arid climate. Fluxes were measured using dynamic closed incubation chambers (transparent and opaque) connected to an infra-red gas analyzer. Microclimatic conditions and chl-a contents of surface sediments were determined. Over all mangrove stands, CO2 fluxes on intact sediments were relatively low, ranging from -3.93 to 8.85 mmolCO₂·m(-2)·h(-1) in the light and in the dark, respectively. Changes in the fluxes over time appeared to depend to a great extent on the development of the biofilm at the sediment surface. We suggest that in intact sediments and in the dark, CO2 fluxes measured at the sediment-air interface rather reflect the metabolism of benthic organisms than sediment respiration (heterotrophic and autotrophic). However, without the biofilm, sediment water content and air temperature were main drivers of seasonal differences in CO2 fluxes, and their influence differed depending on the intertidal location of the stand. After removal of the biofilm, Q10 values in the Avicennia and the Rhizophora stands were 1.84 and 2.1, respectively, revealing the sensitivity of mangrove sediments to an increase in temperature. This study provides evidence that, if the influence of the biofilm is not taken into account, the in situ CO2 emission data currently used to calculate the budget will lead to underestimation of CO2 production linked to heterotrophic respiration fueled by organic matter detritus from the mangrove.