Large increase in CH4 emission following conversion of coastal marsh to aquaculture ponds caused by changing gas transport pathways

Conversion of coastal wetlands to paddy fields substantially decreases methane oxidation potential and methanotrophic abundance on the eastern coast of China

Coastal wetland ecosystems play a key role in the global carbon cycle and climate mitigation. The land conversion of coastal wetlands to paddy fields, an increasingly common practice to feed the growing population, has been shown to dramatically stimulate the methane emissions of (CH4). However, the knowledge about how such wetland conversion affects the methane oxidation, a key process regulating methane emissions from coastal wetlands, is nearly unknown. In this study, a space-for-time substitution method was employed to investigate the impact of the conversion of coastal wetlands (dominated by Phragmites or mangrove (Kandelia and Bruguiera)) to paddy fields on the methane oxidation process on the eastern coast of China. Our results showed that the average CH4 oxidation potential in the converted paddy soils significantly reduced by 28.4 % and 29.3 %, respectively, and the average abundance of methanotrophic pmoA gene decreased by 77.1 % and 81.9 %, respectively, compared to the original Phragmites and mangrove soils. Significant changes in the methanotrophic community composition were also found after converting Phragmites and mangrove wetlands to paddy fields. Structural equation modeling analysis suggested that the land conversion significantly affected the CH4 oxidation potential by changing the soil physicochemical properties (pH, ammonium content, and nitrate content) and methanotrophic abundance. Overall this study showed significant alterations in CH4 oxidation potential and community composition and abundance of methanotrophs caused by conversion of coastal wetlands to paddy fields, improving the knowledge of the underlying microbial mechanisms of land conversion on methane emissions.

Use of biochar derived from Spartina alterniflora to reduce sediment methane (CH4) production potential during non-farming period in earthen aquaculture ponds

Biochar has been proposed as an effective material for mitigating greenhouse gas emissions from farmlands, but comparable information for earthen aquaculture ponds is limited. A field study was conducted to investigate the effects of adding biochar (200-1600 kg ha-1) derived from the invasive plant Spartina alterniflora on sediment physico-chemical properties, CH4 production potential (PCH4), and the relevant functional gene abundances in earthen aquaculture ponds during the non-farming period. The results indicated that biochar treatments increased sediment porosity and salinity, while decreasing dissolved organic carbon and microbial biomass carbon. Biochar-treated sediments also exhibited a significantly lower abundance of mcrA gene especially in the early drainage stage, and a higher abundance of pmoA gene especially in the intermediate and final drainage stages. Consequently, the mean PCH4 in biochar-treated sediments (1.28-21.12 ng g-1 d-1) was 57-73% lower than in the control group (5.41-39.45 ng g-1 d-1). The reduction in PCH4 did not differ between biochar produced at 300 °C vs. 500 °C and was not dependent on the amount of biochar added. The findings suggest that using biochar derived from S. alterniflora can be a cost-effective method to control the spread of this invasive plant and reduce CH4 production in aquaculture pond sediment during the non-farming period.

Divergent impacts of animal bioturbation on methane and nitrous oxide emissions from mariculture ponds

Aquaculture systems are of increasing concern as an important source of atmospheric methane (CH4) and nitrous oxide (N2O). However, the role of animals in regulating CH4 and N2O emissions from aquaculture systems remains unclear. Here, we established mesocosm trials to investigate impacts of bioturbation of different aquaculture species (i.e., clam, shrimp, and crab) on CH4 and N2O fluxes in a mariculture pond. Across the initial, middle, and final culturing stages, mean CH4 flux in mesocosm without animals was 4.81 ± 0.09 µg CH4 m‒2 h‒1, while the existence of clam, shrimp, and crab significantly increased CH4 flux by 35.3 %, 80.6 %, and 138 %, respectively. Bioturbation significantly decreased dissolved oxygen (DO) concentration by 5.19‒44.8 % but increased porewater CH4 concentration by 14.1‒59.9 %, indicating that lowered DO caused by animal respiration promoted CH4 production in sediment. Moreover, bioturbation of animals significantly increased ebullitive CH4 fluxes by 41.0‒216 %, contributing 57.4‒77.2 % of the increased CH4 emission in mesocosms with animals. However, shrimp and crab significantly reduced N2O flux by 30.3 % and 42.5 %, respectively, primarily due to lowered DO conditions suppressing nitrification and limiting NO3‒ supply for denitrification. By contrast, clam significantly increased N2O emission by 181 % because its filter-feeding behavior excreted more NH4+ and NO3‒ into overlying water and thereby facilitating N2O production. The N2O concentration in overlying water was 1.72‒2.83-fold of that in porewater, and the calculated diffusive N2O flux was 1.80‒37.5 % greater than chamber-measured N2O efflux. This implied that N2O might be primarily produced in overlying water rather than sediments, and the produced N2O can either evade as water-air fluxes or diffuse downwards into sediments to be consumed. Overall, our study advocates that aquaculture-related climate mitigation strategies should place more attention on the divergent impacts of animal bioturbation on CH4 and N2O emissions.

Research Status and Development Trend of Greenhouse Gas in Wetlands: A Bibliometric Visualization Analysis

With the intensification of global warming, wetland greenhouse gas (GHG) emissions have attracted worldwide attention. However, the scientific understanding of wetland GHGs is still limited. To gain a comprehensive and systematic understanding of the current research status and development trends in wetland GHGs. We selected 1627 papers related to wetland GHG research from the Web of Science Core Collection database and used the bibliometric visualization analysis method to reveal the annual publication, main core research forces, research hotspots, and trends in this field. The results showed that the research in this field shows a steady upward trend. United States research institutions and scholars play a key role in this field. The research on "climate change" based on three major wetland GHGs (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) has been continuously gaining popularity. In recent years, "water" has become an emerging core topic. More and more studies have focused on enhancing wetland pollutant treatment capacity, improving wetland ecosystem productivity, maintaining water level stability, strengthening blue carbon sink function, exploring remote sensing applications in wetlands, and promoting wetland restoration to reduce GHG emissions. Furthermore, we discussed the influencing factors of the emission of CO2, CH4, and N2O in wetlands and summarized the potential methods to reduce GHG emissions. The findings provide scientific guidance and reference on wetland sustainable development and GHG emission reduction.

Utilization patterns strongly dominated the dynamics of CO2 and CH4 emissions from small artificial lakes

Small lakes are significant sources of CO2 and CH4 emissions to atmosphere. The dynamics and controls of CO2 and CH4 emissions from human-dominated small lakes with diverse functions remain poorly understood. We investigated the spatiotemporal dynamics of CO2 and CH4 concentrations and fluxes in 33 small lakes around the urban area with different landscape properties and utilization patterns, to clarify the impact of human-dominated functional shift on their greenhouse gas emissions. Meanwhile, we used microcosm cultivation methods to assess the CO2 and CH4 production rates of sediments in these lakes. The results indicated that the utilization ways significantly influence the CO2 and CH4 emissions in these lakes, with urban landscape lakes and aquaculture lakes showing significantly higher emissions compared to irrigation water-supplying lakes and drinking-water lakes. Extensive urbanization and aquaculture practices could increase the risk of that small lakes turn into hotspots of CO2 and CH4 emissions, and further complicate their spatial heterogeneity. Meanwhile, the production potential of CO2 and CH4 in sediments, as well as gas fluxes in small lakes, exhibited consistent functional differentiation across different utilization patterns. They were mainly driven by changes in sediment organic carbon and microbial carbon. Additionally, the difference of organic carbon and nitrogen loads were another drives for the variability in CO2 and CH4 emissions. We highlighted that the continuous accumulation of nutrient loads in water and sediments in human-dominated small lakes has greatly enhanced the potential for carbon gas emissions. We also found that utilization ways can significantly affect the key controls of CO2 and CH4 emission from small lakes, and also influence the reliability of carbon emission prediction models based on water chemistry parameters. To accurately estimate the contribution of small lakes to the global greenhouse gas inventory, it is essential to establish adaptive predictive models that consider the uncertainties in lake carbon emissions resulting from human utilization patterns.

Methanogens dominate methanotrophs and act as a methane source in aquaculture pond sediments

Aquaculture pond sediments act as hotspots for methane (CH4) emissions; however, knowledge gaps on the regulation of microorganisms hinder our further understanding of methane dynamics in aquaculture pond sediment. Using field sampling and molecular analysis, we examined CH4 fluxes, the methanogenic community composition, and their interaction with methanotrophs to comprehensively understand the methane cycling in sediments of aquaculture ponds in northern China. Compared with a fishing pond without feed inputs, the abundances of methanogens mcrA and methanotrophs pmoA genes increased significantly in aquaculture ponds sediments. The dominant methanogens were Methanothrix, Methanoregula, and Methanolinea, and the α-diversity indices of methanogens demonstrated higher levels in 0-5 cm surface sediment. The methanotrophs were dominated by Methylocystis, Methylocaldum, and Methylobacter, and the α-diversity indices of methanotrophs showed no significant difference. The total organic carbon (TOC) contents and oxidation reduction potential (ORP) were the key factors driving methanogenic and methanotrophic communities on methane cycle in aquaculture sediment. The inter-domain ecological network (IDEN) analysis revealed that total number of network nodes, links, connectances, and links per species in the aquaculture sediments presented relatively higher levels, whereas the IDEN modules were fewer. The methanogens dominated in the networks and the interaction of methanogens and methanotrophs was more competitive and complex in aquaculture sediments. These findings highlight the marked methane production in aquaculture sediment, primarily due to the abundance, diversity, and competitive advantage of methanogens over methanotrophic communities.

Anaerobic oxidation of methane driven by different electron acceptors: A review

Methane, the most significant reduced form of carbon on Earth, acts as a crucial fuel and greenhouse gas. Globally, microbial methane sinks encompass both aerobic oxidation of methane (AeOM), conducted by oxygen-utilizing methanotrophs, and anaerobic oxidation of methane (AOM), performed by anaerobic methanotrophs employing various alternative electron acceptors. These electron acceptors involved in AOM include sulfate, nitrate/nitrite, humic substances, and diverse metal oxides. The known anaerobic methanotrophic pathways comprise the internal aerobic oxidation pathway found in NC10 bacteria and the reverse methanogenesis pathway utilized by anaerobic methanotrophic archaea (ANME). Diverse anaerobic methanotrophs can perform AOM independently or in cooperation with symbiotic partners through several extracellular electron transfer (EET) pathways. AOM has been documented in various environments, including seafloor methane seepages, coastal wetlands, freshwater lakes, soils, and even extreme environments like hydrothermal vents. The environmental activities of AOM processes, driven by different electron acceptors, primarily depend on the energy yields, availability of electron acceptors, and environmental adaptability of methanotrophs. It has been suggested that different electron acceptors driving AOM may occur across a wider range of habitats than previously recognized. Additionally, it is proposed that methanotrophs have evolved flexible metabolic strategies to adapt to complex environmental conditions. This review primarily focuses on AOM, driven by different electron acceptors, discussing the associated reaction mechanisms and the habitats where these processes are active. Furthermore, it emphasizes the pivotal role of AOM in mitigating methane emissions.

Methane and nitrous oxide emissions in the rice-shrimp rotation system of the Vietnamese Mekong Delta

Rice-shrimp rotation systems are one of the widespread farming practices in the Vietnamese Mekong Delta coastal areas. However, greenhouse gas (GHG) emissions in the system have remained unclear. This study aimed to examine methane (CH4) and nitrous oxide (N2O) emissions from the system, including (i) land-based versus high-density polyethylene-lined (HDPE) nursery ponds and (ii) conventional versus improved grow-out ponds inoculated with effective microorganisms (EM) bioproducts. The results showed that CH4 flux in land-based and HDPE-lined nursery ponds were 1.04 and 0.25 mgCH4 m-2 h-1, respectively, while the N2O flux was 8.37 and 6.62 μgN2O m-2 h-1, respectively. Global warming potential (GWP) from land-based nursery ponds (18.3 g CO2eq m-2) was approximately 3 folds higher than that of the HDPE-lined nursery pond (6.1 g CO2eq m-2). Similarly, the mean CH4 and N2O fluxes were 15.84 mg CH4 m-2 h-1 and 7.17 μg N2O m-2 h-1 for the conventional ponds, and 10.51 mg CH4 m-2 h-1 and 7.72 μg N2O m-2 h-1 for the improved grow-out ponds. Conventional practices (2388 g CO2eq m-2) had a higher 1.5-fold GWP compared to the improved grow-out pond (1635 g CO2eq m-2). The continuation of the land-based nursery pond and conventional aquacultural farming practices increase CH4 emission and GWP, while applying HDPE-lined nursery ponds combined with improved grow-out ponds could be a promising approach for reducing GHG emissions in rice-shrimp rotation systems. This study recommends further works in the rice-shrimp rotation systems, including (i) an examination of the effects of remaining rice stubbles in the platform on the availability of TOC levels and GHG emissions and (ii) ameliorating dissolved oxygen (DO) concentration on the effectiveness of GHG emission reduction.

The methane-oxidizing microbial communities of three maar lakes in tropical monsoon Asia

Methane-oxidizing bacteria (MOB) is a group of planktonic microorganisms that use methane as their primary source of cellular energy. For tropical lakes in monsoon Asia, there is currently a knowledge gap on MOB community diversity and the factors influencing their abundance. Herewith, we present a preliminary assessment of the MOB communities in three maar lakes in tropical monsoon Asia using Catalyzed Reporter Deposition, Fluorescence In-Situ Hybridization (CARD-FISH), 16S rRNA amplicon sequencing, and pmoA gene sequencing. Correlation analysis between MOB abundances and lakes' physicochemical parameters following seasonal monsoon events were performed to explain observed spatial and temporal patterns in MOB diversity. The CARD-FISH analyses detected the three MOB types (I, II, and NC10) which aligned with the results from 16S rRNA amplicons and pmoA gene sequencing. Among community members based on 16S rRNA genes, Proteobacterial Type I MOB (e.g., Methylococcaceae and Methylomonadaceae), Proteobacterial Type II (Methylocystaceae), Verrucomicrobial (Methylacidiphilaceae), Methylomirabilota/NC10 (Methylomirabilaceae), and archaeal ANME-1a were found to be the dominant methane-oxidizers in three maar lakes. Analysis of microbial diversity and distribution revealed that the community compositions in Lake Yambo vary with the seasons and are more distinct during the stratified period. Temperature, DO, and pH were significantly and inversely linked with type I MOB and Methylomirabilota during stratification. Only MOB type I was influenced by monsoon changes. This research sought to establish a baseline for the diversity and ecology of planktonic MOB in tropical monsoon Asia to better comprehend their contribution to the CH4 cycle in tropical freshwater ecosystems.

Plant-mediated CH4 exchange in wetlands: A review of mechanisms and measurement methods with implications for modelling

Plant-mediated CH4 transport (PMT) is the dominant pathway through which soil-produced CH4 can escape into the atmosphere and thus plays an important role in controlling ecosystem CH4 emission. PMT is affected by abiotic and biotic factors simultaneously, and the effects of biotic factors, such as the dominant plant species and their traits, can override the effects of abiotic factors. Increasing evidence shows that plant-mediated CH4 fluxes include not only PMT, but also within-plant CH4 production and oxidation due to the detection of methanogens and methanotrophs attached to the shoots. Despite the inter-species and seasonal differences, and the probable contribution of within-plant microbes to total plant-mediated CH4 exchange (PME), current process-based ecosystem models only estimate PMT based on the bulk biomass or leaf area index of aerenchymatous plants. We highlight five knowledge gaps to which more research efforts should be devoted. First, large between-species variation, even within the same family, complicates general estimation of PMT, and calls for further work on the key dominant species in different types of wetlands. Second, the interface (rhizosphere-root, root-shoot, or leaf-atmosphere) and plant traits controlling PMT remain poorly documented, but would be required for generalizations from species to relevant functional groups. Third, the main environmental controls of PMT across species remain uncertain. Fourth, the role of within-plant CH4 production and oxidation is poorly quantified. Fifth, the simplistic description of PMT in current process models results in uncertainty and potentially high errors in predictions of the ecosystem CH4 flux. Our review suggest that flux measurements should be conducted over multiple growing seasons and be paired with trait assessment and microbial analysis, and that trait-based models should be developed. Only then we are capable to accurately estimate plant-mediated CH4 emissions, and eventually ecosystem total CH4 emissions at both regional and global scales.

Significant inter-annual fluctuation in CO2 and CH4 diffusive fluxes from subtropical aquaculture ponds: Implications for climate change and carbon emission evaluations

Aquaculture ponds are potential hotspots for carbon cycling and emission of greenhouse gases (GHGs) like CO2 and CH4, but they are often poorly assessed in the global GHG budget. This study determined the temporal variations of CO2 and CH4 concentrations and diffusive fluxes and their environmental drivers in coastal aquaculture ponds in southeastern China over a five-year period (2017-2021). The findings indicated that CH4 flux from aquaculture ponds fluctuated markedly year-to-year, and CO2 flux varied between positive and negative between years. The coefficient of inter-annual variation of CO2 and CH4 diffusive fluxes was 168% and 127%, respectively, highlighting the importance of long-term observations to improve GHG assessment from aquaculture ponds. In addition to chlorophyll-a and dissolved oxygen as the common environmental drivers, CO2 was further regulated by total dissolved phosphorus and CH4 by dissolved organic carbon. Feed conversion ratio correlated positively with both CO2 and CH4 concentrations and fluxes, showing that unconsumed feeds fueled microbial GHG production. A linear regression based on binned (averaged) monthly CO2 diffusive flux data, calculated from CO2 concentrations, can be used to estimate CH4 diffusive flux with a fair degree of confidence (r2 = 0.66; p < 0.001). This algorithm provides a simple and practical way to assess the total carbon diffusive flux from aquaculture ponds. Overall, this study provides new insights into mitigating the carbon footprint of aquaculture production and assessing the impact of aquaculture ponds on the regional and global scales.

Community assembly of comammox Nitrospira in coastal wetlands across southeastern China

Complete ammonia oxidizers (comammox Nitrospira) are ubiquitous in coastal wetland sediments and play an important role in nitrification. Our study examined the impact of habitat modifications on comammox Nitrospira communities in coastal wetland sediments across tropical and subtropical regions of southeastern China. Samples were collected from 21 coastal wetlands in five provinces where native mudflats were invaded by Spartina alterniflora and subsequently converted to aquaculture ponds. The results showed that comammox Nitrospira abundances were mainly influenced by sediment grain size rather than by habitat modifications. Compared to S. alterniflora marshes and native mudflats, aquaculture pond sediments had lower comammox Nitrospira diversity, lower clade A.1 abundance, and higher clade A.2 abundance. Sulfate concentration was the most important factor controlling the diversity of comammox Nitrospira. The response of comammox Nitrospira community to habitat change varied significantly by location, and environmental variables accounted for only 11.2% of the variations in community structure across all sites. In all three habitat types, dispersal limitation largely controlled the comammox Nitrospira community assembly process, indicating the stochastic nature of these sediment communities in coastal wetlands. IMPORTANCE Comammox Nitrospira have recently gained attention for their potential role in nitrification and nitrous oxide (N2O) emissions in soil and sediment. However, their distribution and assembly in impacted coastal wetland are poorly understood, particularly on a large spatial scale. Our study provides novel evidence that the effects of habitat modification on comammox Nitrospira communities are dependent on the location of the wetland. We also found that the assembly of comammox Nitrospira communities in coastal wetlands was mainly governed by stochastic processes. Nevertheless, sediment grain size and sulfate concentration were identified as key variables affecting comammox Nitrospira abundance and diversity in coastal sediments. These findings are significant as they advance our understanding of the environmental adaptation of comammox Nitrospira and how future landscape modifications may impact their abundance and diversity in coastal wetlands.

Effects of conversion of coastal marshes to aquaculture ponds on sediment anaerobic CO2 production and emission in a subtropical estuary of China

The extensive conversion of carbon-rich coastal wetland to aquaculture ponds in the Asian Pacific region has caused significant changes to the sediment properties and carbon cycling. Using field sampling and incubation experiments, the sediment anaerobic CO₂ production and CO₂ emission flux were compared between a brackish marsh and the nearby constructed aquaculture ponds in the Min River Estuary in southeastern China over a three-year period. Marsh sediment had a higher total carbon and lower C:N ratio than aquaculture pond sediment, suggesting the importance of marsh vegetation in supplying labile organic carbon to the sediment. Conversion to aquaculture ponds significantly decreased sediment anaerobic CO₂ production rates by 69.2% compared to the brackish marsh, but increased CO₂ emission, turning the CO₂ sink (-490.8 ± 42.0 mg m^-2 h^-1 in brackish marsh) into a source (6.2 ± 3.9 mg m^-2 h^-1 in aquaculture pond). Clipping the marsh vegetation resulted in the highest CO₂ emission flux (382.6 ± 46.7 mg m^-2 h^-1), highlighting the critical role of marsh vegetation in capturing and sequestering carbon. Sediment anaerobic CO₂ production and CO₂ uptake (in brackish marsh) and emission (in aquaculture ponds) were highest in the summer, followed by autumn, spring and winter. Redundancy analysis and structural equation modeling showed that the changes of sediment temperature, salinity and total carbon content accounted for more than 50% of the variance in CO₂ production and emission. Overall, the results indicate that vegetation clearing was the main cause of change in CO₂ production and emission in the land conversion, and marsh replantation should be a primary strategy to mitigate the climate impact of the aquaculture sector.

Comparing the variations and influencing factors of CH4 emissions from paddies and wetlands under CO2 enrichment: A data synthesis in the last three decades

Understanding and quantifying the impact of elevated tropospheric carbon dioxide concentration (e [CO₂]) on methane (CH₄) globally is important for effectively assessing and mitigating climate warming. Paddies and wetlands are the two important sources of CH₄ emissions. Yet, a quantitative synthetic investigation of the effects of e [CO₂] on CH₄ emissions from paddies and wetlands on a global scale has not been conducted. Here, we conducted a meta-analysis of 488 observation cases from 40 studies to assess the long-term effects of e [CO₂] (ambient [CO₂]+ 53-400 μmol mol^-1) on CH₄ emissions and to identify the relevant key drivers. On aggregate, e [CO₂] increased CH₄ emissions by 25.7% (p < 0.05) from paddies but did not affect CH₄ emissions from wetlands (-3.29%; p > 0.05). The e [CO₂] effects on paddy CH₄ emissions were positively related to that on belowground biomass and soil-dissolved CH₄ content. However, these factors under e [CO₂] resulted in no significant change in CH₄ emissions in wetlands. Particularly, the e [CO₂]-induced abundance of methanogens increased in paddies but decreased in wetlands. In addition, tillering number of rice and water table levels affected e [CO₂]-induced CH₄ emissions in paddies and wetlands, respectively. On a global scale, CH₄ emissions changed from an increase (+0.13 and + 0.86 Pg CO₂-eq yr^-1) under short-term e [CO₂] into a decrease and no changes (-0.22 and + 0.03 Pg CO₂-eq yr^-1) under long-term e [CO₂] in paddies and wetlands, respectively. This suggested that e [CO₂]-induced CH₄ emissions from paddies and wetlands changed over time. Our results not only shed light on the different stimulative responses of CH₄ emissions to e [CO₂] from paddy and wetland ecosystems but also suggest that estimates of e [CO₂]-induced CH₄ emissions from global paddies and wetlands need to account for long-term changes in various regions.

Substantial increase in P release following conversion of coastal wetlands to aquaculture ponds from altered kinetic exchange and resupply capacity

The reclamation of wetlands and its subsequent conversion to aquaculture may alter regional nutrient (im)mobilization and cycling, although direct assessments of phosphorus (P) cycling and its budget balance following wetland conversion are currently scarce. Here, parallel field experiments were conducted to investigate and compare the availability and mobilization mechanisms of P from natural coastal wetlands and the adjacent converted aquaculture ponds based on high-resolution diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques and the DGT-induced fluxes in sediments (DIFS) model. The study found that the conversion of wetland to pond strongly reduced the sediment P pool by changing its forms and distribution. High-resolution data showed that concentrations of labile P and soluble reactive P across the sediment-water profiles were markedly enhanced by the converted aquaculture pond, although they exhibited large spatiotemporal heterogeneity. Moreover, the synchronous distribution of labile P, iron (Fe) and sulfur (S) across profiles in coastal wetlands indicated that the dissolution of Fe (III) oxyhydroxide-phosphate complexes coupled with sulfate reduction were the main mechanisms regulating sediment P mobilization in coastal areas. However, the converted aquaculture pond weakened or even reversed this dependence by decoupling the Fe-S-P reactions by changing the sediment structure and nutrient balance. Substantial increases in labile P, Fe and S fluxes in the pond suggested the conversion of wetland to aquaculture facilitated the internal release of P, Fe and S from sediment into water. The high resupply parameter (R) and desorption rate (k_-1) combined with low response time (T_c) in the pond, as fitted by DIFS model, indicated the strong resupply capacity and fast kinetic exchange of sediment P across the sediment-water interface, which is consistent with the high P diffusion fluxes recorded in the pond. It was concluded that converted aquaculture ponds act as an important source of P release in coastal areas, potentially exacerbating water quality degradation and eutrophication. Specific initiatives and actions are therefore urgently needed to alleviate the internal P-loading during aquaculture.

Conversion of coastal wetland to aquaculture ponds decreased N2O emission: Evidence from a multi-year field study

Land reclamation is a major threat to the world's coastal wetlands, and it may influence the biogeochemical cycling of nitrogen in coastal regions. Conversion of coastal marshes into aquaculture ponds is common in the Asian Pacific region, but its impacts on the production and emission of nitrogen greenhouse gases remain poorly understood. In this study, we compared N₂O emission from a brackish marsh and converted shrimp aquaculture ponds in the Shanyutan wetland, the Min River Estuary in Southeast China over a three-year period. We also measured sediment and porewater properties, relevant functional gene abundance, sediment N₂O production potential and denitrification potential in the two habitats. Results indicated that the pond sediment had lower N-substrate availability, lower ammonia oxidation (AOA and comammox Nitrospira amoA), nitrite reduction (nirK and nirS) and nitrous oxide reduction (nosZ Ⅰ and nosZ Ⅱ) gene abundance and lower N₂O production and denitrification potentials than in marsh sediments. Consequently, N₂O emission fluxes from the aquaculture ponds (range 5.4-251.8 μg m^-2 h^-1) were significantly lower than those from the marsh (12.6-570.7 μg m^-2 h^-1). Overall, our results show that conversion from marsh to shrimp aquaculture ponds in the Shanyutan wetland may have diminished nutrient input from the catchment, impacted the N-cycling microbial community and lowered N₂O production capacity of the sediment, leading to lower N₂O emissions. Better post-harvesting management of pond water and sediment may further mitigate N₂O emissions caused by the aquaculture operation.