Indirect Emission Factor (EF5) is a practical tool for estimating in situ N2O concentration from N-nutrients in aquatic systems, but published EF5 values varied by orders of magnitude. Coastal creeks are pervasive waterbodies that receive direct abundant nutrients from their catchment, and are identified as potential hotspots for nitrous oxide (N2O) emissions. However, the dynamics of N2O and EF5 in creeks of different trophic states are not well understood and quantified. This study compared N2O dynamics across three coastal creeks in the Shanyutan Wetland, southeastern China, that ranged from mesotrophic, lightly eutrophic to moderately eutrophic. N2O concentrations were positively influenced by NO3 --N, nirK and nirS gene abundances, suggesting that nitrite reduction may play an important role in regulating N2O production in these creeks. N2O concentration in all three creeks followed the same regression relationship with trophic level index (TLI), with the regression explaining 83% of the observed variations. Average EF5 value for the creeks increased linearly from 0.015% to 0.054% along a TLI gradient of 20-150, and was much lower than IPCC default value (0.26%). When combined with the literature data, EF5 generally increased with increasing degree of eutrophication. Overall, the results suggest that information on trophic state could be used to improve EF5 based on specific in situ condition, and that continuous eutrophication of rivers, streams and creeks may accelerate N2O emissions from these habitats.

Many coastal marshes dominated by the invasive species, for example, Spartina alterniflora, had been cleared to create aquaculture ponds for shrimp farming. Efforts were made in recent years to revert aquaculture ponds to wetlands using the native species Phragmites australis and Cyperus malaccensis. This study quantified the impact of this restoration effort on sediment methane production potential (PCH4) and methane emission (FT-CH4) in southeastern China. The results showed that restoration decreased PCH4 from 30.3 to 23.9 ng CH4 g-1 d-1 and decreased FT-CH4 from 13.9 to 2.8 mg m-2 h-1. The abundance of mcrA gene decreased by 58.6 %, whereas pmoA gene abundance increased by 103.1 % in restored wetlands. Structural equation modeling (SEM) showed that decrease in PCH4 was primarily caused by changes in sediment labile organic carbon and SO42-contents, which led to a decrease in mcrA gene and an increase inpmoA gene. Reconnecting the aquaculture ponds to adjacent coastal area improved seawater exchange and oxygenation. Compared to the original Spartina-dominated marshes, the restored wetlands had lower methane contribution, above-ground vegetation biomass and organic deposition. Overall, this study showed that active restoration using native vegetation is preferred over passive restoration for coastal wetlands.

Ecological restoration of coastal wetlands has contributed to reversing biodiversity loss, but its effects on greenhouse gas emissions remain poorly understood. Here, we present three years of field measurements from restored wetlands in Southeast China, comparing areas with and without dense migratory waterbird congregations. Results indicate that carbon dioxide and methane emissions were significantly higher in waterbird-influenced plots, but only during autumn and winter when bird populations peaked (P < 0.001). Bird droppings increased nutrient availability in the soil and water, which in turn stimulated microbial activity and enzymatic reactions. Meanwhile, bird-induced oxygen consumption promoted anaerobic conditions, which increased the abundance of methanogens while suppressing methanotrophs. These combined effects drove seasonal surges in greenhouse gas emissions. Collectively, our findings identify migratory bird aggregation as a key driver of seasonal emission hotspots in restored wetlands, and highlight the need for long-term, high-frequency monitoring frameworks that capture these seasonal biological pulses when assessing the climate benefits of wetland restoration projects. Increased guano deposition and oxygen consumption from migratory birds result in seasonally higher greenhouse gas emissions, which may affect carbon budgets in restored wetlands, according to three years of field measurements in the Shanyutan Wetlands of Southeast China.

Sea-level rise is salinizing estuarine wetlands, yet its impact on Fe-bound organic carbon (Fe-OC) persistence remains unclear. Here, we analyzed topsoil (0–10 cm) and subsoil (40–50 cm) samples from paired Phragmites australis-dominated saltwater and freshwater marshes at six Chinese estuaries spanning 18° of latitude, to determine how saltwater intrusion modulates Fe-OC sequestration and soil organic carbon (OC) degradation. Freshwater marshes stored 18.7

Microbial carbon use efficiency (CUE) regulates soil carbon turnover; its response to the combined pressure of warming and saltwater intrusion would determine the carbon storage capacity of wetland. We collected surface soils from Phragmites australis marshes in five major estuaries in China, spanning 18 degrees of latitude. Sampling sites included mid-salinity (4.7-8.6 ppt) and high-salinity (11.0-14.8 ppt) sites within the marshes. Microbial CUE estimated from extracellular enzyme stoichiometric model ranged from 0.11 to 0.33, and was influenced primarily by soil carbon supply and phosphorus limitation. Both temperature and salinity affected extracellular enzyme activities and microbial CUE significantly. Across the latitudinal gradient, microbial CUE showed a significant negative association with mean annual temperature while stimulating enzyme activities, whereas elevated salinity inhibited enzyme activities. Microbial CUE remained stable across the mid-salinity range but increased with salinity in the high-salinity range. Microbial CUE at the mid-salinity sites was significantly higher than that at high-salinity sites. Structural equation modeling revealed that the indirect effects of temperature and salinity via soil properties and microbial nutrient limitation were stronger than their direct effects on microbial CUE. Together, these findings improve our ability to predict global change effects on soil CUE in salt marshes, and the consequences on carbon sink.

Biochar has been used to reduce carbon mineralization and enhance carbon sequestration in agricultural soils, but its use in aquaculture ponds remains rare. This study was conducted in coastal earthen aquaculture ponds in Southeast China to examine the influences of biochar derived from Spartina alterniflora on sediment carbon content, sediment organic carbon (SOC) mineralization, CO2 emission and carbon storage during the non-farming season. Biochar was produced at 300 degrees C and 500 degrees C terminal temperatures and applied at 200-1600 kg ha(-1). Relative to the control, biochar applications during the non-farming season increased total carbon by 3.4 - 82.6%, SOC by 7.2 - 55.7%, but decreased easily oxidized carbon by 33.8 - 53.4% and CO2 emission by 0.22 - 0.78 Mg ha(-1) in a dose-dependent manner. In biochar-treated sediments, activities of beta-1,4-glucosidase, cellulobiohydrolase and peroxidase all decreased significantly; SOC mineralization rate decreased by 31.6 - 62.9%, and bioavailable carbon estimated from first-order kinetic model sediments was 29.0-55.2% lower relative to the control. The results suggest that biochar affected sediment carbon dynamics primarily by limiting bioavailability of labile carbon to microbes, and secondarily by raising sediment pH that hindered enzyme activities. The strongest effect was attained by applying biochar produced at 300 degrees C at 1600 kg ha(-1), which increased the total sediment carbon storage by up to similar to 20.6 Mg C ha(-1), from an initial carbon stock of approximately 1.6 Mg C ha(-1), equivalent to a net gain in carbon storage by 12.8-fold. Producing and utilizing biochar in the non-farming season can avoid disruption to farming operation, and turns the invasive plant S. alterniflora into a useful product to mitigate the climate impacts of the aquaculture sector.

Aquaculture ponds are an important anthropogenic source of the potent greenhouse gas nitrous oxide (N2O). Biochar application is an effective strategy for mitigating N2O emission in agricultural systems, yet its effects on aquaculture systems are unclear. This study conducted in situ investigation to examine the impact of applying biochar (200-1600 kg ha-1) produced from the invasive species Spartina alterniflora on sediment N2O dynamics in coastal aquaculture ponds, and the underlying mechanisms. The results showed that biochar application significantly lowered sediment N2O production potential (PN2O) and emission (FN2O), but did not substantially alter the temporal patterns of N2O. Compared to the control (without biochar addition), PN2O and FN2O of biochar-treated sediments were decreased respectively by 22.8-69.4% and 19.6-44.9%, proportional to the amount of biochar added, but the effect was not influenced by the pyrolysis temperature. Structural equation modeling results indicated that reductions in PN2O and FN2O following biochar addition were principally mediated by shifts in soil nitrogenous substrates availability and porosity, which drove the decrease in nitrite reductase genes (nirS and nirK) and increase in N2O reductase gene nosZ. Overall, the results suggest that transforming S. alterniflora into biochar would be a win-win strategy for controlling the invasion of this species, repurposing 'waste' biomass and mitigating N2O emission from the aquaculture sector, thereby contributing to sustainability.

Aquaculture is one of the fastest-growing food production sectors and a significant yet underappreciated source of greenhouse gases (GHGs). Using the most comprehensive aquaculture greenhouse gas dataset to-date and robust modeling, we estimated that aquaculture systems globally emitted 63.90 (19.72 - 103.16) Tg CO2-equivalent per year, with methane being the main contributor. Although aquaculture systems account for <2% of the global lentic habitat area, they contribute 8.8% of the total CH4 emissions from these ecosystems. Per unit area, GHG emissions were lower in coastal systems than inland systems, and shellfish aquaculture had lower emissions than fish and crustacean farming. Geographically, the majority of emissions came from traditionally aquaculture-intensive regions in the mid-low latitudes, particularly East Asia, Southeast Asia, and South Asia. Relative to other major animal products, aquaculture had a lower CO2-eq per unit of protein produced, but its total emissions could still offset 4.0% of the terrestrial carbon sink. Variations in biogeochemical conditions, induced by aeration and nutrient inputs, play a crucial role in regulating GHG emissions from aquaculture systems. Management strategies such as improving aeration, installing bottom lining and optimizing feeding practices would be key to mitigating GHG emissions and lowering the sector's climate impact.

Restoration of coastal earthen aquaculture ponds to wetlands revived the local biodiversity including avian species, but dense congregation of waterfowl especially during the migrating seasons may lead to unintended ecological consequences. A three-year study was conducted in restored coastal wetlands in southeastern China, where a seawall separated two neighboring open-water areas with and without waterfowl, allowing a natural experiment to investigate the effects of waterfowl on nitrogen dynamics including N2O production and emissions. Migratory species were estimated to account for 79-85 % of the observed waterfowl. Relative to the control plots without waterfowl, plots frequented by waterfowl had significantly lower dissolved oxygen and sediment pH, higher levels of NO3--N, NH4+-N, total dissolved nitrogen, microbial biomass nitrogen, abundances of denitrification-related genes (nirS, nirK) and sediment N2O production potential. N2O emissions were elevated by about 121 % in waterfowl-influenced plots, with most of the excess emissions occurring in months when migratory waterfowl abundance peaked. The between-habitat differences were consistent over the three years. Our findings reveal a hidden ecological cost of waterfowl recovery, i.e., avian-derived nutrient deposition and associated greenhouse gas emissions may partially offset the climate benefits of restored wetlands. These results have direct implications for coastal wetland management and blue carbon accounting, suggesting that biodiversity conservation and greenhouse gas mitigation do not always align.

Coastal wetlands, as key “blue carbon” ecosystems, have suffered extensive global loss and degradation driven primarily by land-use change for agriculture and aquaculture, and initiatives to restore earthen aquaculture ponds to wetlands has helped recover lost wetlands and restore their ecosystem functions. This study examined the effects of this restoration effort on soil organic carbon (SOC) chemical composition, mineralization, stability and storage in a coastal wetland in southeast China. Our results showed that, relative to the earthen aquaculture ponds, restored wetlands had more total carbon by 35.8%, SOC by 46.4% and easily oxidized carbon by 106.2%, but less microbial biomass carbon and dissolved organic carbon by 26.1% and 32.7%, respectively. Following restoration, total carbon and organic carbon stocks in the top 30 cm of soil increased by 6.9 Mg C ha−1 (+14.5%) and 9.3 Mg C ha−1 (+24.7%), respectively. In restored wetlands, enzyme activities of β-1,4-glucosidase and cellulobiohydrolase decreased significantly; SOC mineralization rate decreased by 22.1%–40.0% across all seasons, and bioavailable carbon estimated from first-order kinetic model sediments was 44.3% less than earthen aquaculture ponds. Spectral analyses showed that restoration led to an increased proportion of stable carbon groups, for example, O-alkyl C and aromatic C, along with a significantly higher O-alkyl C/aromatic-C ratio. Structural equation modeling analysis indicated that restoration affected SOC dynamics primarily by elevating ionic stress and reducing microbial substrate availability. Overall, this study demonstrated that converting earthen aquaculture ponds into coastal wetlands with native vegetation can effectively increase blue carbon sequestration, enhance the long-term stability of stored carbon, and strengthen the associated climate mitigation benefits.

Sea-level rise characterized by salinization and inundation can reshape wetland microbial communities and functions, thereby altering CO2 emissions, but time-scale-dependent mechanisms are poorly resolved. We conducted a one-year mesocosm experiment at Shanyutan Phragmites australis marsh in Min River Estuary, SE China, simulating sea-level rise via additions of 15‰ saline water (SW) and ambient tidal water (TW), with a control (CK). We found that whether 1 month or 12 months after the simulation experiment beginning, SW significantly increased microbial biomass carbon (MBC) relative to CK and TW (p < 0.05), but microbial biomass nitrogen (MBN) declined under SW (p < 0.05) and the MBC/MBN ratio under TW decreased compared with that at 1 month, suggesting progressive nitrogen constraint and more conservative microbial carbon-use strategy. TW and SW decreased Bacteroidota while increasing Proteobacteria and Firmicutes after 1 month, whereas Nitrospirota was enriched under TW and SW after 12 months (p < 0.05), and both TW and SW significantly suppressed soil CO2 fluxes relative to CK during the whole experiment (p < 0.05). Correlation analyses suggested that CO2 emission reduction was associated with Bacteroidota and Myxococcota in the early simulation experiment (1 month), and linked to taxa such as Verrucomicrobiota in the later simulation experiment (12 months), indicating a change of carbon-decomposition rate due to nutrient regulation and stress adaptation under chronic disturbance. Partial least squares path modeling indicated that bacterial succession driven by sea-level rise altered microbial biomass and stoichiometric characteristics, which in turn mediated changes in soil CO2 emissions. Overall, prolonged salinization and inundation reduced soil CO2 emissions in this estuarine wetland by driving a temporal change of bacterial communities, accompanied by the readjustment of microbial nutrient allocation strategies.

Creeks are widespread in coastal areas and have the potential to emit substantial amounts of greenhouse gases (GHGs), as they often receive abundant organic matters from surrounding areas. However, high-resolution spatial data on GHGs in coastal creeks are scarce, which hampers assessments of their role in global aquatic system GHG budgets. This study quantified the spatial variabilities in dissolved methane (CH4) and diffusive CH4 fluxes across different seasons in a coastal creek in the Shanyutan Wetland, southeast China. Surface-water hydrographical variables and abundances of pmoA gene (for methanotrophs) and mcrA gene (for methanogens) were also measured. The persistent supersaturation of surface water CH4 relative to the atmosphere confirmed that the entire creek served as a CH4 emitter. CH4 concentrations and efflux decreased along the creek across all seasons. The spatial variations in CH4 correlated positively with levels of DOC, NH4+-N and mcrA gene abundance. In contrast, CH4 was negatively correlated with salinity, DO and pmoA gene abundance. Seasonally, the concentration and efflux of CH4 were highest in autumn and lowest in winter. The temporal variations in both CH4 concentration and efflux were driven by the combined effect of temperature, rainfall and tidewater. Based on the finding, controlling nutrient discharge from land would be an effective strategy to mitigate CH4 emissions from creeks.

The IPCC's estimates of N2O emissions focus on soils in forests, grasslands and agricultural lands, but often neglect the contributions from aquatic systems, especially small coastal water bodies in built-up areas. In this study, we conducted seasonal analyses of N2O concentrations, N2O fluxes and the relevant functional gene abundances in various water bodies (farmland ditches, aquaculture sewage ditches, tidal creeks, aquaculture ponds, town sewage ditches, and restored aquaculture ponds-to-wetlands) in the Min River Estuary (MRE) in Southeast China. The results showed that all the water bodies were consistently oversaturated in N2O relative to the overlying atmosphere. The town sewage ditches were hotspots for N2O production, with a mean dissolved N2O concentration of -42.9 nmol/L (range 24.7-62.5 nmol/L), which is 2.9-13.7 times greater than those in the other water bodies. The estimated N2O emission from town sewage ditches was -1097.6 nmol m-2h- 1, which was -28.2, 10.7, 4.4 and 3.4 times those from farmland ditches, aquaculture sewage ditches, tidal creeks and aquaculture ponds, respectively. Nitrogen substrate availability and abundance of AOB amoA and nirS genes were the key factors driving the variations in N2O concentration and emission among the various water bodies. Our results highlighted that coastal small water bodies were strong N2O emission source per unit area, but they tend to be poorly surveyed and need to be considered in the national greenhouse gas inventory.

Coastal creeks are ubiquitous in wetland ecosystems, and they act as conduits for significant inputs of nutrients and anthropogenic carbon from surrounding areas, making them potential hotspots for greenhouse gas (GHG) emission. To date, the spatiotemporal variations in GHG emission across different types of coastal creeks and their environmental drivers remain poorly understood due to the lack of observational data. A field investigation was carried out to analyze the concentrations and fluxes of CO2 and CH4 across three coastal creeks (designated as CC1, CC2 and CC3) within the Shanyutan Wetland in southeastern China. These creeks received exogenous input from different sources. The results indicated that CO2 and CH4 concentrations in all three creeks remained persistently oversaturated, with concentrations in the range of 14.5-61.5 mu mol L- 1 and 1.1-11.8 mu mol L- 1, respectively. The estimated emission fluxes varied in the range of 0.4-3.6 mmol CO2 m- 2 h- 1 and 40.2-581.1 mu mol CH4 m- 2 h-1. The mean CO2 efflux over the four seasons was highest in CC1 (1.9 mmol m- 2 h-1) and lowest in CC2 (0.8 mmol m- 2 h-1). For CH4 efflux, the highest value was in CC2, followed by CC3 and CC1. PO4 3availability was the primary factor affecting the change of CO2 concentration and emission, while CH4 were primarily regulated by DOC, DO, TDN and abundances of mcrA and pmoA genes. These results highlighted that coastal creeks are significant atmospheric GHG sources and exogenous inputs substantially influenced their variabilities.

Coastal wetlands along southeastern China have undergone extensive habitat transformations, notably from natural mudflats (MFs) to Spartina alterniflora marshes (SAs) and aquaculture ponds (APs), yet the ecological consequences for sediment protistan communities remain largely unknown. Here, we systematically examined protistan diversity, community composition, functional groups, and assembly processes across 21 wetlands spanning five provinces using amplicon sequencing. Although alpha diversity remained stable across habitat types, community composition and functional group distributions were significantly altered. In particular, phototrophic protists declined markedly following S. alterniflora invasion and then increased after aquaculture pond reclamation. Meanwhile, consumer and phototroph assemblages were strongly influenced by sediment grain size. Salinity emerged as the key environmental driver of protistan diversity and community structure. Distance-decay relationships indicated elevated spatial turnover in SAs, suggesting increased environmental filtering after plant invasion. Assembly process analysis revealed a dominance of deterministic mechanisms in shaping community structure across all habitats, with the strongest signal observed in SAs. These findings demonstrate that land-use change reshapes protistan communities through altered environmental constraints, highlighting the ecological sensitivity of microbial eukaryotes to anthropogenic disturbance in coastal wetland ecosystems. IMPORTANCE:Protists play essential roles in nutrient cycling, energy transfer, and microbial food web dynamics, yet their responses to anthropogenic habitat transformation in coastal wetlands remain underexplored. This study offers the first large-scale biogeographic assessment of sediment protistan communities across three contrasting coastal habitat types in southeastern China. We show that while alpha diversity remains resilient, profound shifts in community composition, functional group structure, and spatial turnover occur following Spartina alterniflora invasion and aquaculture conversion. Our findings underscore the primacy of environmental filtering, driven by salinity and sediment texture, in mediating these patterns and shaping community assembly. These insights not only expand our understanding of protistan ecology under coastal land-use change but also highlight their potential as sensitive bioindicators for monitoring ecological integrity and resilience in dynamic coastal systems.

Labile organic carbon (LOC) plays a pivotal role in soil biogeochemistry and ecological functions. China’s coastal wetlands have been profoundly impacted due to plant invasion and land use change, but the effects on soil LOC quantity and composition are unclear. This study analyzed the soil LOC components—namely, dissolved organic carbon (DOC), easily oxidizable carbon (EOC), and microbial biomass carbon (MBC)—across twenty-one coastal wetlands in southeastern China. These wetlands underwent a uniform land cover transition from native mudflats (MFs) to Spartina alterniflora marshes (SAs), and eventually to aquaculture ponds (APs). The results indicated that EOC was the dominant component of soil organic carbon (SOC) (57.5

Tidal marshes serve as critical carbon (C) sinks, yet face increasing threats from global environmental changes. While previous research has documented how nitrogen (N) loading and sea-level rise affect total C pools individually, their impacts on soil organic carbon (SOC) stabilization remain critically underexplored, particularly when these factors co-occur in tidal marsh ecosystems. Through a 3-yr field experiment, we analyzed how these factors, alone and combined, impact SOC stabilization by examining SOC fraction dynamics. Results showed that N loading increased particulate organic carbon (POC) by 18% and decreased mineral-associated organic carbon (MAOC) by 13%, reducing SOC stabilization. Conversely, increased inundation raised MAOC by 31% and decreased POC by 19%, promoting SOC stabilization. The decreased MAOC under N loading stemmed from reduced fungal necromass C, while the increased POC related to lower phenol oxidase activity. In contrast, with increased inundation, MAOC rose due to iron-bound organic C (Fe-OC) accumulation, while POC declined from increased phenol oxidase activity. When both factors were applied together, SOC stabilization remained at control levels. This occurred because the combined effect maintained oxidative enzyme activities and thus retained POC levels. The simultaneous reduction in fungal necromass C and enhancement of Fe-OC associations established complementary mechanisms that maintained MAOC at levels equivalent to control. Our findings reveal that N loading and increased inundation drive contrasting patterns of SOC stabilization, while their combination produces uniquely stabilized C dynamics. This insight challenges single-factor predictions and underscores the importance of multi-factor experiments in understanding ecosystem responses under concurrent global change scenarios.

Existing projections of wetland methane emissions usually neglect feedbacks from global biogeochemical cycles. Using data-driven approaches, we estimate wetland methane emissions from 2000 to 2100, considering effects of meteorological changes and biogeochemical feedbacks from atmospheric sulfate deposition and CO 2 fertilization. In low-CO 2 scenarios (1.5° and 2°C warming pathways), the suppressive effect of sulfate deposition on wetland methane emissions largely diminishes by 2100 due to clean air policies, with resulting emission increases (7 ± 2 Tg a −1 ) being 35 and 22% of total wetland emission changes. In mid-CO 2 scenarios (2.4° to 3.6°C warming pathways), sulfate deposition changes modestly, and CO 2 fertilization contributes >30% of wetland emission increases. Across all scenarios, biogeochemical feedbacks can stimulate 30 to 45% of future wetland emission rises. Under 1.5° and 2°C pathways, wetland methane emissions will likely increase by 20 to 34 Tg a −1 by 2100, representing 8 to 15% of the allowable space for anthropogenic methane emissions, a factor not yet considered by current assessments.

The transport and transformation of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3–-N) in estuarine wetland sediments are influenced by tidal fluctuations. However, current research on this topic primarily relies on field experiments, which are time-consuming and often lack continuity, particularly regarding the dynamic changes in nitrogen (N) transformation during tidal cycles. In this study, the Minjiang Estuary wetland in China was selected as the research area to investigate the transport and transformation of NH4+-N and NO3−-N in sediments under tidal influence. A numerical model based on HYDRUS was developed, and its simulation accuracy was within acceptable limits. Transformation parameters for N at different tidal flats and months were successfully obtained. Results suggested that nitrification, mineralization, and denitrification coefficients tended to be higher in summer (August) than in autumn (November), although the seasonal pattern varied across tidal flat positions and soil depths. Spatially, nitrification and mineralization often decreased with depth, whereas denitrification tended to increase. With greater inundation depth, denitrification and mineralization often showed an increasing trend. Model simulations indicated that sediment inundation depth and solute concentration were key factors controlling NO3−-N leaching, which increased with tidal level but with a certain degree of lag, especially during spring tides, while neap tides showed greater variability at intermediate tidal flat. This study provides theoretical insights into N transport parameters in estuarine wetland sediments and offers a modeling approach exemplified by the Minjiang Estuary, contributing to the sustainable management of estuarine wetland ecosystems.