Summer fluxes of atmospheric greenhouse gases N2O, CH4 and CO2 from mangrove soil in South China

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.

Soil greenhouse gas emissions from dead and natural mangrove forests in Southeastern Brazil

Mangroves forests may be important sinks of carbon in coastal areas but upon their death, these forests may become net sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Here we assessed the spatial and temporal variability in soil CO2 and CH4 fluxes from dead mangrove forests and paired intact sites in SE-Brazil. Our findings demonstrated that during warmer and drier conditions, CO2 soil flux was 183 % higher in live mangrove forests when compared to the dead mangrove forests. Soil CH4 emissions in live forests were > 1.4-fold higher than the global mangrove average. During the wet season, soil GHG emissions dropped significantly at all sites. During warmer conditions, mangroves were net sources of GHG, with a potential warming effect (GWP100) of 32.9 ± 10.2 (±SE) Mg CO2e ha-1 y-1. Overall, we found that dead mangroves did not release great amounts of GHG after three years of forest loss.

High-Resolution Estimates of N2O Emissions from Inland Waters and Wetlands in China

Inland waters (rivers, lakes, and reservoirs) and wetlands (marshes and coastal wetlands) represent large and continuous sources of nitrous oxide (N2O) emissions, in view of adequate biomass and anaerobic conditions. Considerable uncertainties remain in quantifying spatially explicit N2O emissions from aquatic systems, attributable to the limitations of models and a lack of comprehensive data sets. Herein, we conducted a synthesis of 1659 observations of N2O emission rates to determine the major environmental drivers across five aquatic systems. A framework for spatially explicit estimates of N2O emissions in China was established, employing a data-driven approach that upscaled from site-specific N2O fluxes to robust multiple-regression models. Results revealed the effectiveness of models incorporating soil organic carbon and water content for marshes and coastal wetlands, as well as water nitrate concentration and dissolved organic carbon for lakes, rivers, and reservoirs for predicting emissions. Total national N2O emissions from inland waters and wetlands were 1.02 × 105 t N2O yr-1, with contributions from marshes (36.33%), rivers (27.77%), lakes (25.27%), reservoirs (6.47%), and coastal wetlands (4.16%). Spatially, larger emissions occurred in the Songliao River Basin and Continental River Basin, primarily due to their substantial terrestrial biomass. This study offers a vital national inventory of N2O emissions from inland waters and wetlands in China, providing paradigms for the inventorying work in other countries and insights to formulate effective mitigation strategies for climate change.

Dredging wastewater discharge from shrimp ponds affects mangrove soil physical-chemical properties and enzyme activities

Dredging wastewater discharge is a significant environmental concern for mariculture near mangrove ecosystems. However, little attention has been paid to its effects on the soil physical-chemical properties and enzyme activities in mangrove habitats. This study compared the soil physical-chemical properties and enzyme activities in the polluted area that received dredging wastewater from a shrimp pond with those in the control area without wastewater to explore the effects of wastewater discharge on the soil physical-chemical properties and enzyme activities. Variations in soil physical-chemical properties and enzyme activities across different tidal flat areas and depths were also examined. The polluted area exhibited lower soil salinity (10.47 ± 0.58 vs. 15.64 ± 0.54) and moisture content (41.85 ± 1.03 % vs. 45.81 ± 1.06 %) than the control area. Wastewater discharge increased soil enzyme activities, (acid phosphatase, protease, and catalase), resulting in higher inorganic nitrogen (13.20 ± 0.00 μg g-1 vs. 11.60 ± 0.03 μg g-1) but lower total nitrogen (0.93 ± 0.01 mg g-1 vs. 1.62 ± 0.11 mg g-1) in the contaminated zone. From the control to polluted area, there was an approximate increase of 0.43 and 0.83 mg g-1 in soil total phosphorus and soluble phosphate, driven by increased acid phosphatase. However, soil humus and organic matter decreased by 0.04 and 1.22 %, respectively, because of wastewater discharge. The impact of wastewater discharge on the soil physical-chemical properties and enzyme activities was most pronounced in the landward and surface soil layers (0-5 cm). The results showed that wastewater discharge altered soil physical-chemical properties and enzyme activities, accumulating soil bioavailable nutrients (inorganic nitrogen and soluble phosphate), but at the cost of reduced soil quality, especially organic matter, further adversely affecting the overall health of mangrove ecosystems. Prioritizing the management of wastewater discharged from mariculture adjacent to mangrove forests is crucial for mangrove conservation.

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.

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.

Salinity, mineralogy, porosity, and hydrodynamics as drivers of carbon burial in urban mangroves from a megacity

Mangrove ecosystems are an important blue carbon store but exhibit considerable variation in soil carbon stocks globally. Unravelling the conditions controlling carbon stock is critical for assessing current and future carbon budgets. Mangrove soil biogeochemical cycles can strongly influence carbon storage capacities. We thus investigated carbon sequestration and the environmental parameters shaping variability in biogeochemical cycling and carbon storage in sediment samples from four mangrove sites along an estuarine-to-marine gradient in Hong Kong, a megacity. Our results showed that organic matter in Hong Kong mangroves is sourced principally from autochthonous mangrove plants. Total nitrogen was higher in the freshwater-influenced sites and supplied from different sources. Marine-influenced sites had larger sulfur fractionations, reflecting higher marine-sourced sulfate concentrations and indicating a relatively open sulfate system. We estimated an average organic carbon stock of 115 ± 8 Mg C ha-1 in the upper 100 cm soil layer placing Hong Kong mangroves at the lower end of the global spectrum of the soil carbon stock. Carbon accumulation was found to be driven by a combination of higher total organic matter inputs, soil fluxes, and porosity. Notably, despite having the highest mass-specific soil organic carbon contents, Mai Po had the lowest integrated soil organic carbon storage (77 ± 3 Mg C ha-1). This was primarily due to lower sediment density and higher tidal pumping leading to a decrease in carbon retention. Total organic matter input, sediment characteristics, and hydrodynamics were the main factors influencing soil organic carbon storage. Overall, our results suggest that (1) while multiple parameters can enhance soil organic carbon content and increase carbon storage capacities, (2) hydrodynamics and sediment characteristics can increase the potential for leakage of carbon, and (3) high carbon content does not always equal high carbon sequestration and stock.

Psychrobacter species enrichment as potential microplastic degrader and the putative biodegradation mechanism in Shenzhen Bay sediment, China

Microplastic (MP) pollution has emerged as a pressing environmental concern due to its ubiquity and longevity. Biodegradation of MPs has garnered significant attention in combatting global MP contamination. This study focused on MPs within sediments near the sewage outlet of Shenzhen Bay. The objective was to elucidate the microbial communities in sediments with varying MPs, particularly those with high MP loads, and to identify microorganisms associated with MP degradation. The results revealed varying MP abundance, ranging from 211 to 4140 items kg-1 dry weight (d. w.), with the highest concentration observed near the outfall. Metagenomic analysis confirmed the enrichment of Psychrobacter species in sediments with high MP content. Psychrobacter accounted for ∼16.71% of the total bacterial community and 41.71% of hydrocarbon degrading bacteria at the S3 site, exhibiting a higher abundance than at other sampling sites. Psychrobacter contributed significantly to bacterial function at S3, as evidenced by the Kyoto Encyclopedia of Genes and Genomes pathway and enzyme analysis. Notably, 28 enzymes involved in MP biodegradation were identified, predominantly comprising oxidoreductases, hydrolases, transferases, ligases, lyases, and isomerases. We propose a putative mechanism for MP biodegradation, involving the breakdown of long-chain plastic polymers and subsequent oxidation of short-chain oligomers, ultimately leading to thorough mineralization.

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.

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.

China's conservation and restoration of coastal wetlands offset much of the reclamation-induced blue carbon losses

China's coastal wetlands have experienced large losses and gains with rapid coastal reclamation and restoration since the end of the 20th century. However, owing to the difficulties in mapping soil organic carbon (SOC) in blue carbon stocks of coastal wetlands on a national scale, little is known about the spatial pattern of SOC stock in China's coastal wetlands and the loss and gain of SOC stock following coastal reclamation, conservation, and restoration over the past decades. Here, we developed a SOC stock map in China's coastal wetlands at 30 m spatial resolution, analyzed the spatial variability and driving factors of SOC stocks, and finally estimated SOC losses and gains due to coastal reclamation and wetland management from 1990 to 2020. We found that the total SOC stocks in China's coastal wetlands were 77.8 Tg C by 2020 with 3.6 Tg C in mangroves, 8.8 Tg C in salt marshes, and 65.4 Tg C in mudflats. Temperature, rainfall, and seawater salinity exerted the highest relative contributions to SOC spatial variability. The spatial trend of SOC density gradually decreased from south to north except for Liaoning province, with the lowest density in Shandong province. About 24.9% (19.4 Tg C) of SOC stocks in China's coastal wetlands were lost due to high-intensity reclamation, but SOC stock gained from conservation and restoration offset the reclamation-induced losses by 58.2% (11.3 Tg C) over the past three decades. These findings demonstrated the great potential of conservation and restoration of coastal wetlands in reversing the loss trend of blue carbon and contributing to the mitigation of climate change toward carbon neutrality. Our study provides significant spatial insights into the stocks, sequestration, and recovery capacity of blue carbon following rapid urbanization and management actions, which benefit the progress of global blue carbon management.

Marine estuaries act as better sink for greenhouse gases during winter in undisturbed mangrove than degraded ones in Sundarban, India

The estuaries provide the key pathway for travelling carbon across the land-ocean interfaces and behave as both source and sink of greenhouse gases (GHGs) in water-atmosphere systems. The sink-source characteristics of estuaries for GHGs vary spatially. The primary driving factors are adjacent ecologies (agriculture, aquaculture, etc.) and proximities to the sea. To study the sink-source characteristics of estuaries for GHGs (methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2)), the water samples were collected from 53 different locations in the estuaries for estimation of dissolved GHGs concentration and air-water GHGs exchanges. The locations represent five zones (Zone I, II, III, IV and V) based on the type and degradation status of mangroves (degraded and undisturbed), anthropogenic activities, and distance from the sea. Zone I, III, V represents to the degraded mangroves far from sea, whereas, Zone II, IV surrounded by undisturbed mangroves and nearer to sea. The average dissolved CH4 concentrations were higher in the estuaries which were adjacent to degraded mangroves (154.4 nmol L-1) than undisturbed mangroves (81.7 nmol L-1). Further, the average dissolved N2O concentrations were 48% higher in the estuaries nearer to degraded mangroves than that of undisturbed ones. Among the degraded mangrove sites, the dissolved CO2 concentrations were higher at Zone I (30.1 μmol L-1) followed by Zone III and IV, whereas in undisturbed sites, it was higher in Zone IV (22.3 μmol L-1) than Zone II (17.6 μmol L-1). Among the 53 locations, 36, 51 and 33 locations acted as a sink (negative value of exchanges) for CH4, N2O and CO2, respectively. The higher sink potential for CH4 was recorded to those estuaries adjacent to undisturbed mangroves (-791.69 μmol m-2 d-1) than the degraded ones (-23.18 μmol m-2 d-1). Similarly, the average air-water N2O and CO2 exchanges were more negative in the estuaries which were nearer to undisturbed mangroves indicating higher sink potential. The pH, and salinity of the estuary water were negatively correlated with air-water CH4 and N2O exchanges, whereas those were positively correlated with CO2 exchanges. Significantly lower dissolved GHGs and air-water GHGs exchange was observed in the estuaries adjacent to the undisturbed mangrove as compared to the degraded mangrove. The reason behind higher sink behaviours of estuaries nearer to undisturbed mangroves are higher intrusion of seawater, less nutrient availability, higher salinity, low carbon contents and alkaline pH compared to estuaries adjacent to degraded mangroves and far from sea.

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.

Factors affecting N2O fluxes from heavy metal-contaminated mangrove soils in a subtropical estuary

A 1-year field monitoring program was carried out to observe seasonal variation in N₂O fluxes at two typical mangrove wetlands in a subtropical estuary. The soils in the island-type mangrove wetland had a higher level of heavy metal(loid) contamination and a lower level of salinity compared to the small bay-type mangrove wetland. While there was a high level of similarity in the seasonal variation pattern of N₂O fluxes between the two investigated sites with both being significantly higher in summer than in other seasons, the average of N₂O fluxes in the island-type mangrove wetland was 7.19 μg·m^-2·h^-1， which tended to be lower compared to the small bay-type mangrove wetland (15.63 μg·m^-2·h^-1). Overall, N₂O flux was closely related to soil-borne heavy metal(loid)s, showing a trend to decrease with increasing concentration of these heavy metal(loid)s. The N₂O fluxes increased with decreasing abundance of either denitrifiers or nitrifiers. But the opposite was observed for the anammox bacteria present in the soils. The anammox bacteria were more sensitive to heavy metal(loid) stress but more tolerated high salinity encountered in the investigated soils compared to the denitrifiers or nitrifiers. It appears that anammox reactions mediated by anammox bacteria played a key role in affecting the spatial variation in N₂O fluxes from the mangrove soils in the study area. And an increased level of ammonium in soils tended to promote the activity of anammox bacteria and consequently enhanced N₂O emission from the mangrove soils.

Linking eutrophication to carbon dioxide and methane emissions from exposed mangrove soils along an urban gradient

Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH₄) and carbon dioxide (CO₂), are still poorly understood. Here, we aim to evaluate the relationships between CO₂ and CH₄ efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO₂ and CH₄ fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δ¹³C and δ¹⁵N), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δ¹⁵N (proxy of anthropogenic N enrichment), less negative δ¹³C, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO₂ efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m^-2 h^-1 from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO₂ fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH₄ emissions were relatively low (three orders of magnitude lower than CO₂ fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.

Biodegradation of polyester polyurethane by the marine fungus Cladosporium halotolerans 6UPA1

Lack of degradability and the accumulation of polymeric wastes increase the risk for the health of the environment. Recently, recycling of polymeric waste materials becomes increasingly important as raw materials for polymer synthesis are in short supply due to the rise in price and supply chain disruptions. As an important polymer, polyurethane (PU) is widely used in modern life, therefore, PU biodegradation is desirable to avoid its accumulation in the environment. In this study, we isolated a fungal strain Cladosporium halotolerans from the deep sea which can grow in mineral medium with a polyester PU (Impranil DLN) as a sole carbon source. Further, we demonstrate that it can degrade up to 80% of Impranil PU after 3 days of incubation at 28 ℃ by breaking the carbonyl groups (1732 cm^-1) and C-N-H bonds (1532 cm^-1 and 1247 cm^-1) as confirmed by Fourier-transform infrared (FTIR) spectroscopy analysis. Gas chromatography-mass spectrometry (GC-MS) analysis revealed polyols and alkanes as PU degradation intermediates, indicating the hydrolysis of ester and urethane bonds. Esterase and urease activities were detected in 7 days-old cultures with PU as a carbon source. Transcriptome analysis showed a number of extracellular protein genes coding for enzymes such as cutinase, lipase, peroxidase and hydrophobic surface binding proteins A (HsbA) were expressed when cultivated on Impranil PU. The yeast two-hybrid assay revealed that the hydrophobic surface binding protein ChHsbA1 directly interacts with inducible esterases, ChLip1 (lipase) and ChCut1 (cutinase). Further, the KEGG pathway for "fatty acid degradation" was significantly enriched in Impranil PU inducible genes, indicating that the fungus may use the degradation intermediates to generate energy via this pathway. Taken together, our data indicates secretion of both esterase and hydrophobic surface binding proteins by C. halotolerans plays an important role in Impranil PU absorption and subsequent degradation. Our study provides a mechanistic insight into Impranil PU biodegradation by deep sea fungi and provides the basis for future development of biotechnological PU recycling.

Genomic and transcriptomic dissection of Theionarchaea in marine ecosystem

Theionarchaea is a recently described archaeal class within the Euryarchaeota. While it is widely distributed in sediment ecosystems, little is known about its metabolic potential and ecological features. Here, we used metagenomics and metatranscriptomics to characterize 12 theionarchaeal metagenome-assembled genomes, which were further divided into two subgroups, from coastal mangrove sediments of China and seawater columns of the Yap Trench. Genomic analysis revealed that apart from the canonical sulfhydrogenase, Theionarchaea harbor genes encoding heliorhodopsin, group 4 [NiFe]-hydrogenase, and flagellin, in which genes for heliorhodopsin and group 4 [NiFe]-hydrogenase were transcribed in mangrove sediment. Further, the theionarchaeal substrate spectrum may be broader than previously reported as revealed by metagenomics and metatranscriptomics, and the potential carbon substrates include detrital proteins, hemicellulose, ethanol, and CO2. The genes for organic substrate metabolism (mainly detrital protein and amino acid metabolism genes) have relatively higher transcripts in the top sediment layers in mangrove wetlands. In addition, co-occurrence analysis suggested that the degradation of these organic compounds by Theionarchaea might be processed in syntrophy with fermenters (e.g., Chloroflexi) and methanogens. Collectively, these observations expand the current knowledge of the metabolic potential of Theionarchaea, and shed light on the metabolic strategies and roles of these archaea in the marine ecosystems.

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.

N2O emission dynamics along an intertidal elevation gradient in a subtropical estuary: Importance of N2O consumption

Studying nitrous oxide (N₂O) production and consumption processes along an intertidal elevation gradient can improve the understanding of N₂O dynamics among coastal wetlands. A natural-abundance isotope technique was applied to characterize the processes responsible for N₂O emission in high, middle and low intertidal zones in the Yangtze Estuary. The results showed that N₂O emission rates in high tidal zones (0.84 ± 0.35 nmol g^-1 h^-1) were significantly higher than those in middle (0.21 ± 0.04 nmol g^-1 h^-1) and low tidal zones (0.26 ± 0.05 nmol g^-1 h^-1). Gross N₂O production and consumption rates were greater in high and low tidal zones than in middle tidal zones, whereas N₂O consumption proportions generally increased from high to low tidal zones. N₂O consumption was quite pronounced, implying that N₂O emission in estuarine wetlands accounts for only a small fraction of the total production. Higher degrees of N₂O consumption were the pivotal driver of less N₂O emission in low tidal zones. Bacterial denitrification (>84%) was the dominant pathway, although hydroxylamine (NH₂OH) oxidation/fungal denitrification contributed substantially to N₂O production in high tidal flats. The contribution to N₂O production exhibited a decrease in NH₂OH oxidation/fungal denitrification and an increase in bacterial denitrification with decreasing elevation. Changes in N₂O dynamics along the elevation gradient were affected by carbon and nitrogen substrate availabilities as well as the redox environments. Overall, our findings highlight the importance of N₂O consumption in controlling N₂O emission in intertidal wetlands, especially with higher inundation frequencies and durations.

Net CO2 and CH4 emissions from restored mangrove wetland: New insights based on a case study in estuary of the Pearl River, China

Mangroves have the potential to affect climate via C sequestration and methane (CH₄) emissions. With half of the world's mangroves lost during the 20th century, mangrove restoration in mitigating greenhouse gases has been increasingly recognized. However, the carbon exchanges during restored processes still remain large uncertain. In this study, we analyzed the temporal variations of CO₂ and CH₄ fluxes and their environmental controls during 2019 and 2020 based on a closed-path eddy covariance (EC) system in a 12-year restored subtropical mangrove wetland, in estuary of the Pearl River, southeastern China. We also estimated the CO₂ and CH₄ fluxes and their climate effect from the beginning of restoration by Random Forest algorithm (RF). The EC observations showed that annually the 12-year restored mangrove acted as CO₂ and CH₄ sources, with net CO₂ ecosystem exchange (NEE) of 82-175 gC·m^{- 2}·a^-1 and CH₄ fluxes of 24.7-26.3 gC·m^-2·a^-1. Low vegetation gross primary productivity (GPP) and high ecosystem respiration (Re) caused net CO₂ emissions in the mangroves. The estimation by RF indicated that the mangroves were always a CO₂ source after the beginning of restoration, but the annual NEE was linearly decreased from 233 to 131 gC·m^-2·a^-1 from 2008 to 2020. The annual CH₄ emissions continually increased from 19.0 to 25.8 gC·m^-2·a^-1 after restoration. As a result, the restored mangrove had a positive effect on climate warming, with increased GWP from 1276 to 1386 g CO₂-eq ·m^-2·a^-1 from 2008 to 2020. This is mainly due to lower GPP and higher Re by young restored mangroves, large water area as well as low salinity induced strong CH₄ emissions. Our results indicate new sights that young restored mangrove with large area of water surface may act as carbon sources. However, the long-term climate and ecosystem benefits due to mangrove restoration should not be ignored in future.

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.

Methane Contributions of Different Components of Kandelia candel–Soil System under Nitrogen Supplementation

Kandelia candel is the most widely distributed tree species on the southeast coast of China and is also the main afforestation tree species along the coastal wetland. In recent years, inorganic nitrogen pollution has become increasingly severe, and investigating the effects of nitrogen input on methane emissions in Kandelia candel–soil systems has become significant from a global change perspective. However, the effect of nitrogen input on methane emissions in coastal wetland systems is still uncertain. The field tidal environment is complex and varied, and thus it is difficult to accurately control the amount of nitrogen in the system. Therefore, in order to accurately assess the effects of different concentrations of foreign nitrogen input on methane emission fluxes in a Kandelia candel–soil system, we use indoor tidal simulation experimental devices and design two simulation systems with and without plant planting to explore the difference of methane emission flux in this system under five nitrogen input concentrations: N0 (0 g N·m−2·a−1), N1 (5 g N·m−2·a−1), N2 (10 g N·m−2·a−1), N3 (20 g N·m−2·a−1), and N4 (30 g N·m−2·a−1). The results showed that: (1) The introduction of Kandelia candel promoted methane emissions in coastal wetland ecosystem. Under each nitrogen application concentration, the mean CH4 emission flux in the planting group was 42.98%, 65.59%, 40.87%, 58.93% and 39.23% higher than that in the non-planting group, respectively. (2) Nitrogen input significantly promoted methane emissions in both planted and non-planted environments, and the promoting effect showed as follows: N4 > N3 > N2 > N1 > N0. (3) After the introduction of Kandelia candel, the contribution of Kandelia candel and soil microorganisms to methane emissions was different under different concentrations of nitrogen addition. The contribution rate of Kandelia candel to CH4 emission flux of Kandelia candel–soil system ranged from 10.74% to 60.25%, with an average contribution rate of 37.30%. The changed soil microbes contributed 39.75% to 89.26% to the CH4 emission flux in the Kandelia candel–soil system, with an average contribution rate of 62.60%. Under N3 nitrogen application concentration, the emission flux of plant was the largest, which was significantly higher than that of the soil microbial pathway; at other concentrations, the methane emissions from the soil microbial pathway were greater than that of the plant pathway, and the contribution rate to the plant–soil system reached 60.25%. The results of this study provide an important basis for improving the estimation accuracy of carbon emissions in coastal waters and formulating policies for the restoration and protection of coastal wetlands.

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.

Desulfobacterales stimulates nitrate reduction in the mangrove ecosystem of a subtropical gulf

The amount of nitrogen compounds discharged into the natural environment has increased drastically due to frequent human activities and led to worsening pollution. The mangrove ecosystem can remove nitrogen pollution, in this regard, few studies had focused on the relationship among nitrogen cycling genes, environmental factors, and taxonomic composition. In this study, shotgun metagenomic sequencing and quantitative polymerase chain reaction were used to understand the nitrogen cycle in the subtropical mangrove ecosystem in the Beibu Gulf of China. Eight nitrogen cycling pathways were annotated. Nitrogen metabolism activities were significantly higher in the wet season than those in the dry season. The most abundant genes were those related to the synthesis and degradation of organic nitrogen, followed by the genes involved in nitrate reduction (denitrification, dissimilation/assimilation nitrate reduction). Furthermore, dissimilation nitrate reduction was the main nitrate reduction pathway. Desulfobacterales plays an important role in nitrogen cycling and contributes 12% of the genes of nitrogen pathways on average; as such, a strong coupling relationship exists among nitrogen cycling, sulfur cycling, and carbon cycling in the mangrove ecosystem. Nitrogen pollution in the mangrove wetland can be efficiently alleviated by nitrate reduction of Desulfobacterales. Nevertheless, only 50% of genes can be matched among the known species, suggesting that many unknown microorganisms in the mangrove ecosystem can perform nitrogen cycling. Total phosphorus, available iron, and total organic carbon are the key environmental factors that influence the distribution of nitrogen cycling genes, related pathways, and the taxonomic composition. Our study clearly illustrates how the mangrove ecosystem mitigates nitrogen pollution through Desulfobacterales. This finding could provide a research reference for the whole nitrogen cycling in the mangrove ecosystem.

Heavy metal pollution increases CH4 and decreases CO2 emissions due to soil microbial changes in a mangrove wetland: Microcosm experiment and field examination

Mangrove plays an important role in modulating global warming through substantial blue carbon storage relative to their greenhouse gas emission potential. The presence of heavy metals in mangrove wetlands can influence soil microbial communities with implications for decomposition of soil organic matter and emission of greenhouse gases. In this study, field monitoring and a microcosm experiment were conducted to examine the impacts of heavy metal pollution on soil microbial communities and greenhouse gas fluxes. The results show that heavy metal pollution decreased the richness and diversity of the overall soil microbial functional groups (heterotrophs and lithotrophs); however, it did not inhibit the activities of the methanogenic communities, possibly due to their stronger tolerance to heavy metal toxicity compared to the broader soil microbial communities. Consequently, the presence of heavy metals in the mangrove soils significantly increased the emission of CH₄ while the emission of CO₂ as a proxy of soil microbial respiration was decreased. The soil organic carbon content could also buffer the effect of heavy metal pollution and influence CO₂ emissions due to reduced toxicity to microbes. The findings have implications for understanding the complication of greenhouse gas emissions by heavy metal pollution in mangrove wetlands.

Effect of inundation on greenhouse gas emissions from temperate coastal wetland soils with different vegetation types in southern Australia

Predicted sea level fluctuations and sea level rise with climate change will lead to inundation of coastal and estuarine soils. Coastal wetlands usually contain large amounts of organic matter, which can be potential sources of greenhouse gas emissions (GHGs; CO₂, CH₄, N₂O) during decomposition, but there are limited studies on the effects of sea level variation on GHGs in coastal wetlands. We measured the effect of brackish water inundation and wetting and drying cycles on GHG emissions from coastal wetland soil cores that supported four different vegetation types: Apium gravedens (AG), Leptospermum lanigerum (LL), Phragmites australis (PA) and Paspalum distichum (PD) from the estuarine floodplain of the Aire River in south-western Victoria, Australia. Intact soil cores were incubated under either dry, flooded, or a 14 day wet-dry cycle treatments for a total of 56 days at a constant temperature of 23 °C. CO₂, CH₄, and N₂O fluxes were investigated in closed chambers and measured with gas chromatography. In the dry treatment, a positive correlation was found between soil organic carbon (SOC) and CO₂ flux, and between SOC and CH₄ flux. Higher SOC is indicative of higher amounts of soil organic matter (SOM) which acts as a source of substrate for microbes to produce CO₂ or CH₄ emissions under aerobic or anaerobic conditions. The NO₂^- and NO₃^- concentrations were positively correlated with N₂O emissions in the wet-dry cycle treatment. NO₂^- and NO₃^- provide a supply of substrate for denitrification. The flooded treatment decreased cumulative CO₂ emissions by 34%, 25% and 14% at the LL, PA, PD sites, respectively, and decreased cumulative N₂O emissions by 42%, 39% and 43% at the AG, LL and PA sites, compared to the dry treatment. The wet-dry cycle treatment and dry treatment decreased cumulative CH₄ emissions for all vegetation types compared to the flooded treatment. The redox potential (Eh) was negatively correlated with CH₄ flux and positively correlated N₂O flux at all sites. This study highlights the significance of sea level fluctuations when estimating GHG flux from coastal and estuarine floodplains which are highly vulnerable to inundation, and the role of SOC and mineral N as important drivers affecting GHG flux.

Effects of irrigation methods and salinity on CO2 emissions from farmland soil during growth and fallow periods

Drip irrigation and brackish water irrigation are considered to be the two main ways to alleviate the current shortage of agricultural freshwater resources and have been widely used in countries around the world. Our purpose is to evaluate the effects of different irrigation methods (flood irrigation and drip irrigation) and irrigation water salinities (1.1 g·L^-1, 2.0 g·L^-1, 3.5 g·L^-1, and 5.0 g·L^-1) on the soil CO₂ emissions during the growth and fallow periods of spring maize. Therefore, a two-year field experiment was conducted in Hetao Irrigation District in China from 2017 to 2019. The results showed that compared with flood irrigation, drip irrigation significantly decreased the soil CO₂ emissions in the growth period. After irrigation with a salinity of 5.0 g·L^-1, the soil moisture was the highest because the plant water absorption was inhabited by soil salt, and the soil CO₂ emissions were also promoted during the growth period. Irrigation method and irrigation water salinity had no effect on the soil CO₂ emissions in the fallow period. Soil temperature, moisture and inorganic nitrogen content were the main factors affecting the daily CO₂ emission flux. The soil CO₂ emissions during the growth period accounted for more than 83.93% of the annual soil emissions. Based on the goal of saving freshwater resources, ensuring soil safety, reducing soil CO₂ emissions and increasing the size of the carbon pool, adoption of drip irrigation with 2.0 g·L^-1 brackish water could be adopted to ensure the sustainable development of local agriculture.

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.

Methane emissions reduce the radiative cooling effect of a subtropical estuarine mangrove wetland by half

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide (CO₂ ) and mitigating climate change has received increasing attention in recent years. While recent studies have shown that methane (CH₄ ) emissions can potentially offset the carbon burial rates in low-salinity coastal wetlands, there is hitherto a paucity of direct and year-round measurements of ecosystem-scale CH₄ flux (F_CH4 ) from mangrove ecosystems. In this study, we examined the temporal variations and biophysical drivers of ecosystem-scale F_CH4 in a subtropical estuarine mangrove wetland based on 3 years of eddy covariance measurements. Our results showed that daily mangrove F_CH4 reached a peak of over 0.1 g CH₄ -C m^-2  day^-1 during the summertime owing to a combination of high temperature and low salinity, while the wintertime F_CH4 was negligible. In this mangrove, the mean annual CH₄ emission was 11.7 ± 0.4 g CH₄ -C m^-2  year^-1 while the annual net ecosystem CO₂ exchange ranged between -891 and -690 g CO₂ -C m^-2  year^-1 , indicating a net cooling effect on climate over decadal to centurial timescales. Meanwhile, we showed that mangrove F_CH4 could offset the negative radiative forcing caused by CO₂ uptake by 52% and 24% over a time horizon of 20 and 100 years, respectively, based on the corresponding sustained-flux global warming potentials. Moreover, we found that 87% and 69% of the total variance of daily F_CH4 could be explained by the random forest machine learning algorithm and traditional linear regression model, respectively, with soil temperature and salinity being the most dominant controls. This study was the first of its kind to characterize ecosystem-scale F_CH4 in a mangrove wetland with long-term eddy covariance measurements. Our findings implied that future environmental changes such as climate warming and increasing river discharge might increase CH₄ emissions and hence reduce the net radiative cooling effect of estuarine mangrove forests.

Finding the optimal fertilizer type and rate to balance yield and soil GHG emissions under reclaimed water irrigation

Water and inorganic nitrogen fertilizer have a notable impact on crop yield and greenhouse gas (GHG) emissions from soil. Reclaimed water (RW) is widely used for irrigation when there are shortages of water resources. It is very important to control yield and greenhouse gas emissions by fertilization under reclaimed water irrigation (RWI). The study consisted of a continuous test that evaluated three types of fertilizer treatments (urea, amine, and slow-release fertilizer) and a no-fertilizer treatment under three-year RWI and four fertilizer levels (150, 200, 250 and 300 kg.N.ha^-1) under one-year RWI to determine the best fertilizer to support maize production and reduce GHG (CO₂ and N₂O) emissions from soil; further, the applicability of RWI in the DNDC model was verified. For many years, GHG emissions under RWI showed an increasing trend, but the effect was not significant. A strong correlation was found between the GHG emissions flux and fertilizer amount, and a threshold fertilization amount existed between 220 and 260 kg.N.ha^-1 that minimized yield-scaled N₂O emissions and the ratio of GHG cumulative emission to yield (GHG/Y). The results indicated that the optimal amounts of SF and UF under RWI were 240 and 225 kg.N.ha^-1 by second-order equation and the DNDC model, respectively, and the rate better balanced the yield and GHG emissions.

Carbon sequestration, kinetics of ammonia volatilization and nutrient availability in alkaline sandy soil as a function on applying calotropis biochar produced at different pyrolysis temperatures

This incubation study assessed the effects of unpyrolyzed Calotropis procera and its biochar produced at different pyrolysis temperatures as well as incubation periods on carbon (C) emission, ammonia (NH₃) volatilization, soil quality indicators and nutrient availability of alkaline sandy soil. Five treatments were studied in this experiment: unamended soil (CK), unpyrolyzed calotropis (UPC), calotropis biochar at 250 °C (CB250), calotropis biochar at 400 °C (CB400), and calotropis biochar at 650 °C (CB650). These amendments were applied to the soil at level of 4% (w/w). The results of this study showed that applying unpyrolyzed calotropis residues increased cumulative CO₂ emission from the soil by 117.3, 239.4 and 232.0% over CB250, CB400, and CB650, respectively, by the end of incubation. Compared to the unamended soil, applying CB250 reduced cumulative NH₃ volatilization in soil by 71.5%, which attributed to ammonia adsorption because of increased cation exchange capacity and decreased soil pH, but CB650 increased cumulative NH₃ volatilization by 73.3% after the 3-day incubation as a result of high soil pH. The available phosphorus in soil improved significantly (p ≤ 0.01) with adding unpyrolyzed calotropis residues and its biochar produced at different pyrolysis temperatures compared to the unamended soil. The values of available phosphorus in the soil under study influenced significantly by pyrolysis temperatures of produced biochar; this is due to the pyrolysis of feedstocks increases labile phosphorus. Thenceforth, using biochar is an important strategy for enhancing carbon sequestration, decreasing ammonia volatilization and improving soil quality parameters in arid regions.

A stable simultaneous anammox, denitrifying anaerobic methane oxidation and denitrification process in integrated vertical constructed wetlands for slightly polluted wastewater

Anaerobic ammonium oxidation (anammox), denitrifying anaerobic methane oxidation bacteria (DAMO) have received great attention for their excellent performance in nitrogen removal. However, not much study focused on the co-existence of anammox, DAMO, and denitrification in constructed wetlands, not to mention the advantage of their application in mitigating the necessary byproduct nitrous oxide (N₂O), methane (CH₄) from the biodegradation process. In this study, the result indicated the construction of integrated vertical constructed wetlands (IVCWs) contributed to the high-efficient stable simultaneous anammox, DAMO and denitrification (SADD) process for the nutrients removal, with denitrification being the least contributor to nitrogen reduction. Besides the succession of SADD process was largely the driver for the variation of N₂O, CH₄ emission. The structural equation method (SEM) further suggested that the three biological pathways of qnorB/bacteria, archaea/qnorB, and anammox/nirK accounted for the N₂O production, as were top-controlled by mcrA/DAMO in IVCWs. Besides the anammox-associated nitrifier denitrification was the main source for N₂O production. And that the trade-off effect between the CH₄ and N₂O production was exerted by the DAMO, while the influence was far from satisfactory under the methane constraints.

Genomic and transcriptomic insights into methanogenesis potential of novel methanogens from mangrove sediments

BACKGROUND

Methanogens are crucial to global methane budget and carbon cycling. Methanogens from the phylum Euryarchaeota are currently classified into one class and seven orders, including two novel methanogen taxa, Methanofastidiosa and Methanomassiliicoccales. The relative importance of the novel methanogens to methane production in the natural environment is poorly understood.

RESULTS

Here, we used a combined metagenomic and metatranscriptomic approach to investigate the metabolic activity of methanogens in mangrove sediments in Futian Nature Reserve, Shenzhen. We obtained 13 metagenome-assembled genomes (MAGs) representing one class (Methanofastidiosa) and five orders (Methanomassiliicoccales, Methanomicrobiales, Methanobacteriales, Methanocellales, and Methanosarcinales) of methanogens, including the two novel methanogens. Comprehensive annotation indicated the presence of an H₂-dependent methylotrophic methanogenesis pathway in Methanofastidiosa and Methanomassiliicoccales. Based on the functional gene analysis, hydrogenotrophic and methylotrophic methanogenesis are the dominant pathways in mangrove sediments. MAG mapping revealed that hydrogenotrophic Methanomicrobiales were the most abundant methanogens and that methylotrophic Methanomassiliicoccales were the most active methanogens in the analyzed sediment profile, suggesting their important roles in methane production.

CONCLUSIONS

Partial or near-complete genomes of two novel methanogen taxa, Methanofastidiosa and Methanomassiliicoccales, in natural environments were recovered and analyzed here for the first time. The presented findings highlight the ecological importance of the two novel methanogens and complement knowledge of how methane is produced in mangrove ecosystem. This study implies that two novel methanogens play a vital role in carbon cycle. Video Abstract.

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.

Nitrogen enrichment increases greenhouse gas emissions from emerged intertidal sandflats

Unvegetated, intertidal sandflats play a critical role in estuarine carbon and nutrient dynamics. However, these ecosystems are under increasing threat from anthropogenic stressors, especially nitrogen enrichment. While research in this area typically focuses on sediment-water exchanges of carbon and nutrients during tidal inundation, there remain significant gaps in our understanding of GHG (Greenhouse Gas) fluxes during tidal emergence. Here we use in situ benthic chambers to quantify GHG fluxes during tidal emergence and investigate the impact of nitrogen enrichment on these fluxes. Our results demonstrate significant differences in magnitude and direction of GHG fluxes between emerged and submerged flats, demonstrating the importance of considering tidal state when estimating GHG emissions from intertidal flats. These responses were related to differences in microphytobenthic and macrofaunal activity, illustrating the important role of ecology in mediating fluxes from intertidal flats. Our results further demonstrate that nitrogen enrichment of 600 gN m-2 was associated with, on average, a 1.65x increase in CO2 uptake under light (photosynthetically active) conditions and a 1.35x increase in CO2 emission under dark conditions, a 3.8x increase in CH4 emission and a 15x increase in N2O emission overall. This is particularly significant given the large area intertidal flats cover globally, and their increasing exposure to anthropogenic stressors.

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.

Seasonal fluctuation in three mode of greenhouse gases emission in relation to soil labile carbon pools in degraded mangrove, Sundarban, India

Tropical mangrove represents one of the most threatened ecosystems despite their huge contribution to ecosystem services, carbon (C) sequestration and climate change mitigation. Understanding the system in light of seasonal fluctuations on greenhouse gases (GHGs) emissions due to human interferences and the tidal effect is important for devising site-specific real-time climate change mitigation strategies. In order to capture the seasonal variations, the three modes of transport of GHGs through pneumatophore, ebullition as bubbles and water-soluble diffusion was quantified. The three unique techniques for the gas collection were used to estimate the GHGs [methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂)] emission, at three degraded-mangrove sites in Sundarban, India. We identified three degraded mangrove ecologies based on the remote sensing data of 1930 and 2013 (mangrove-covered area in Sundarban; 2387, 2136 km², respectively). Samples were collected and analyzed for four seasons [winter (November-January), summer (February-April), pre-monsoon (May-June) and monsoon (July-October)], at three representative sites (Sadhupur, Dayapur, and Pakhiralaya). Monsoonal CH₄ and CO₂ fluxes (0.353 ± 0.026 and 64.5 ± 6.1 mmol m^-2 d^-1, respectively) were higher than winter and summer. However, the soil labile C pools showed the opposite trend i.e. more in summer followed by winter and monsoon. In contrast, the N₂O fluxes were more during summer (54.2 ± 3.2 μmol m^-2 d^-1). The stagnant water had higher dissolved GHGs concentration compared to tidewater due to less salinity and a long time of stagnation. The mode of transport of GHGs through pneumatophore, ebullition, and water-soluble diffusion was also significantly varied with seasons, soil‑carbon status and tidewater intrusion. Therefore, seasonal fluctuations of GHGs emission and tidal effect must be considered along with soil labile C pools for GHG-C budgeting and climate change mitigation in the mangrove ecosystem.

Effects of fertilizer types and water quality on carbon dioxide emissions from soil in wheat-maize rotations

The use of fertilizers as addition inputs in agricultural systems can increase the yield of wheat and maize, while also stimulating the emission of carbon dioxide from soil that the main man-made greenhouse gas. Our objectives focused on the impact of different types of synthetic fertilizers and water quality. The purposes were to determine the feasibility of using wasted water for irrigation and to relate CO₂ fluxes to the yield of maize and wheat, as well as to select the best fertilizer type with low CO₂ emission and high yield. The experiment consisted of a double factors test focusing on four forms of fertilizer (urea, amine and slow release fertilizer) and the quality of water (reclaimed water and underground water). The results showed that the reclaimed water was not significant on the CO₂ discharge rate, the maize-wheat yield or the soil properties in 2014 or 2015; however, the CO₂ emission increased slightly in 2015. Focusing on fertilizer treatments, the reclaimed water & amine fertilizer treatment (CAF) that had higher cumulative CO₂ emissions was 32.75 t·ha^-1 in 2014 and 33.86 t·ha^-1 in 2015. According to the ratio CO₂/Y, the slow released fertilizer that reduces CO₂ emissions and keeps the yield high is the preferred choice.

Comparison of methane emissions among invasive and native mangrove species in Dongzhaigang, Hainan Island

The strength of methane (CH₄) source of mangroves is not well understood, especially when including all CH₄ pathways in consideration. This study measured CH₄ fluxes by five pathways (sediments, pneumatophores, water surface, leaves, and stems) from four typical mangrove forests, including Kandelia candel without pneumatophores and three species with pneumatophores: Sonneratia apetala, Laguncularia racemosa and Bruguiera gymnorhiza-Bruguiera sexangula. The CH₄ fluxes from sediments were 4.82±1.46mgCH₄m^-2h^-1 for K. candel and 1.36±0.17mgCH₄m^-2h^-1 for the other three with pneumatophores. Among the three communities with pneumatophores, S. apetala community had significantly greater emission rate than the other two (P<0.05). Pneumatophores in S. apetala were found to significantly decrease CH₄ emission from sediments (P<0.01), while those in B. gymnorhiza-B. sexangula were significantly increase it (P<0.05). CH₄ fluxes from waters were 3.48±1.11mgCH₄m^-2h^-1, with the highest emission rate in the K. candel community for the duck farming. Leaves of mangroves except for those of K. candel were a weak CH₄ daytime sink, but stems were a weak source. The total 72ha of mangroves in the Changning river basin emitted about 8.10Gg CH₄ per year, with a weighted emission rate of about 1.29mgCH₄m^-2h^-1. Our results suggested that mangroves are only a small methane source to atmosphere with great contribution from sediments and waters, only slight contribution from leaves and stems. Pneumatophores of different mangrove species played different roles in CH₄ fluxes from sediments.

Greenhouse gas emissions from intact riparian wetland soil columns continuously loaded with nitrate solution: a laboratory microcosm study

In this study, we aimed at determining greenhouse gas (GHG) (CO₂, CH₄, and N₂O) fluxes exchange between the soil collected from sites dominated by different vegetation types (Calamagrostis epigeios, Phragmites australis, and Carex schnimdtii) in nitrogenous loaded riparian wetland and the atmosphere. The intact soil columns collected from the wetland were incubated in laboratory and continuously treated with [Formula: see text]-enriched water simulating downward surface water percolating through the soil to become groundwater in a natural system. This study revealed that the soil collected from the site dominated by C. epigeios was net CO₂ and N₂O sources, whereas the soil from P. australis and C. schnimdtii were net sinks of CO₂ and N₂O, respectively. The soil from the site dominated by C. schnimdtii had the highest climate impact, as it had the highest global warming potential (GWP) compared with the other sites. Our study indicates that total organic carbon and [Formula: see text] concentration in the soil water has great influence on GHG fluxes. Carbon dioxide (CO₂) and N₂O fluxes were accelerated by the availability of higher [Formula: see text] concentration in soil water. On the other hand, higher [Formula: see text] concentration in soil water favors CH₄ oxidation, hence the low CH₄ production. Temporally, CO₂ fluxes were relatively higher in the first 15 days and reduced gradually likely due to a decline in organic carbon. The finding of this study implies that higher [Formula: see text] concentration in wetland soil, caused by human activities, could increase N₂O and CO₂ emissions from the soil. This therefore stresses the importance of controls of [Formula: see text] leaching in the mitigation of anthropogenic N₂O and CO₂ emissions.

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.

Plant biomass and soil organic carbon are main factors influencing dry-season ecosystem carbon rates in the coastal zone of the Yellow River Delta

Coastal wetlands are considered as a significant sink of global carbon due to their tremendous organic carbon storage. Coastal CO2 and CH4 flux rates play an important role in regulating atmospheric CO2 and CH4 concentrations. However, the relative contributions of vegetation, soil properties, and spatial structure on dry-season ecosystem carbon (C) rates (net ecosystem CO2 exchange, NEE; ecosystem respiration, ER; gross ecosystem productivity, GEP; and CH4) remain unclear at a regional scale. Here, we compared dry-season ecosystem C rates, plant, and soil properties across three vegetation types from 13 locations at a regional scale in the Yellow River Delta (YRD). The results showed that the Phragmites australis stand had the greatest NEE (-1365.4 μmol m-2 s-1), ER (660.2 μmol m-2 s-1), GEP (-2025.5 μmol m-2 s-1) and acted as a CH4 source (0.27 μmol m-2 s-1), whereas the Suaeda heteroptera and Tamarix chinensis stands uptook CH4 (-0.02 to -0.12 μmol m-2 s-1). Stepwise multiple regression analysis demonstrated that plant biomass was the main factor explaining all of the investigated carbon rates (GEP, ER, NEE, and CH4); while soil organic carbon was shown to be the most important for explaining the variability in the processes of carbon release to the atmosphere, i.e., ER and CH4. Variation partitioning results showed that vegetation and soil properties played equally important roles in shaping the pattern of C rates in the YRD. These results provide a better understanding of the link between ecosystem C rates and environmental drivers, and provide a framework to predict regional-scale ecosystem C fluxes under future climate change.

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.

Methane emissions partially offset "blue carbon" burial in mangroves

Organic matter burial in mangrove forests results in the removal and long-term storage of atmospheric CO₂, so-called "blue carbon." However, some of this organic matter is metabolized and returned to the atmosphere as CH₄. Because CH₄ has a higher global warming potential than the CO₂ fixed in the organic matter, it can offset the CO₂ removed via carbon burial. We provide the first estimate of the global magnitude of this offset. Our results show that high CH₄ evasion rates have the potential to partially offset blue carbon burial rates in mangrove sediments on average by 20% (sensitivity analysis offset range, 18 to 22%) using the 20-year global warming potential. Hence, mangrove sediment and water CH₄ emissions should be accounted for in future blue carbon assessments.

Seasonal variations of nitrous oxide fluxes and soil denitrification rates in subtropical freshwater and brackish tidal marshes of the Min River estuary

Estuarine tidal marshes provide favorable conditions for nitrous oxide (N₂O) production. Saltwater intrusion caused by sea-level rise would exert complex effects on the production and emission of N₂O in estuarine tidal marshes; however, few studies have been conducted on its effects on N₂O emissions. Salinity gradients are a common occurrence in estuarine tidal marshes. Studies on production and emission of N₂O in tidal marshes with different salinities may elucidate the impact of saltwater intrusion on the emission of greenhouse gases. This study explores the seasonal variations of N₂O fluxes and soil denitrification rates in freshwater (Daoqingzhou wetland) and brackish (Shanyutan wetland) tidal marshes dominated by Cyperus malaccensis var. brevifolius (shichito matgrass) in the Min River estuary, southeastern China. N₂O fluxes in both marshes showed strong temporal variability. The highest N₂O fluxes were observed in the hot and wet summer months, whereas the lowest fluxes were observed in the cold winter and autumn months. N₂O fluxes from the freshwater marsh (48.81±9.01μgm^-2h^-1) were significantly higher (p<0.05) than those from the brackish-water marsh (27.69±4.01μgm^-2h^-1). Soil denitrification rates showed a similar temporal pattern, with the highest rates observed in summer and the lowest in winter. Similarly, soil denitrification rates were significantly higher (p<0.05) in the freshwater marsh (32.72±19.15μmolNm^-2h^-1) than in the brackish-water marsh (4.97±2.64μmolNm^-2h^-1). Temperature and the salinity, sulfate (SO₄^2-), and ammonia nitrogen (NH₄⁺-N) concentrations of the overlying water were key factors affecting soil denitrification rates. N₂O fluxes and soil denitrification rates demonstrated negative correlations with salinity and SO₄^2- concentrations in both marshes. The results indicate that increased seawater intrusion would reduce N₂O emissions from estuarine tidal wetlands and exert a negative feedback on the climate system.