Coastal wetlands are the main distribution of blue carbon in coastal zones and well known for their high carbon sequestration capacity. Investigating the variation of carbon budget is crucial for understanding the functionality of coastal wetlands and effectively addressing climate change. In this study, a bibliometric analysis of 4,509 articles was conducted to reveal research progress, hot issues, and emerging trends in the coastal wetland carbon budget field. The number of publications and citations in this field increased exponentially from 1991 to 2022. The leading subject category was Environmental Sciences with 1,844 articles (40.9%). At present, studies have been focused on blue carbon, the effects of climate change and man-made disturbances on carbon cycle, and the restoration of coastal wetlands. Based on the hotspots and trends in this field, the future researches should include (1) exploring the functional mechanisms of various factors affecting carbon cycle and establishing a methodological system for the estimation of blue carbon in coastal wetlands; (2) researching restoration techniques of coastal wetland and constructing wetland restoration evaluation index system; and (3) formulating enforceable carbon trading policy and strengthening international cooperation.

  • Bibliometric analysis of the carbon budget of coastal wetlands was conducted.

  • The number of publications and citations increased exponentially from 1991 to 2022.

  • Research hotspots of coastal wetlands shifted from greenhouse gases to blue carbon.

  • More attention should be paid to restoration techniques of coastal wetlands.

Coastal wetlands serve as transitional zones connecting terrestrial and marine ecosystems, which mainly refer to water bodies with depths of less than 6 m at low tide and wetted areas (Jing et al. 2023), including permanent waters, intertidal zones, and coastal lowlands. They provide vital ecosystem services with an estimated value of $194,000 ha−1 yr−1 (Schuerch et al. 2018). Periodic tidal inundation of the land-sea ecotone has created a locally anoxic environment (Bonan 2008), resulting in a slow decomposition rate of organic carbon and low release of carbon dioxide (CO2), thereby increasing carbon sequestration. Coastal wetland ecosystems are the most carbon-dense ecosystems in the biosphere, which can sequester carbon at a significantly higher rate than forests (Duarte et al. 2010). Consequently, they are recognized as an effective carbon sink for mitigating global warming (Macreadie et al. 2019).

The carbon sequestered in coastal wetlands, including seagrass beds, mangroves, and salt marshes, is commonly known as blue carbon (Macreadie et al. 2019). Coastal wetlands contribute to 50% of carbon burial in the ocean, with covering only 0.2% of the ocean surface (Duarte et al. 2013). Bertram et al. (2021) estimated the carbon sequestration potentials, with 24.0 ± 3.2 Mt C yr−1 for mangroves, 13.4 ± 1.4 Mt C yr−1 for salt marshes, and 43.9 ± 12.1 Mt C yr−1 for seagrass beds. Carbon sequestration in coastal wetlands is on par with terrestrial ecosystems, with mangroves similar to terrestrial forests, and seagrass beds comparable to agricultural land (McLeod et al. 2011; Pendleton et al. 2012).

Coastal wetlands are recognized as the most vulnerable and sensitive ecosystems, and their degradation and disappearance will significantly affect the carbon budget. In recent years, coastal eutrophication, siltation, and coastal area development have reduced seagrass beds (Wang et al. 2021). Meanwhile, mangroves and salt marshes have been destroyed by logging (Nwobi et al. 2020), sea-level rise (Nedd et al. 2021), reclamation, and invasive species (Liu et al. 2022). Approximately 29% of seagrasses, 50% of salt marshes, and 35% of mangroves are estimated to have experienced degradation or even disappearance (Barbier et al. 2011), thus affecting the ecological services and potentially releasing CO2 into the atmosphere. CO2 emissions caused by the loss of coastal wetlands have been estimated at 0.2–1.0 Pg CO2 yr−1, equivalent to 3–19% of emissions derived from deforestation (Pendleton et al. 2012). In order to restore ecological functions and effectively manage coastal wetlands, it is essential to deeply explore the factors influencing wetland degradation, greenhouse gas emissions, and carbon storage.

The degradation of coastal wetland ecological function due to the dual stresses of climate change and anthropogenic disturbance has attracted considerable public and scientific attention. However, a lack of understanding of the scientific issues related to wetland restoration has led to the current lack of significant success in the conservation and restoration of coastal wetlands worldwide (Zhao et al. 2016). Hence, there is an urgent need to conduct in-depth scientific measurement studies on greenhouse gas generation and carbon storage of coastal wetlands, so as to systematically and comprehensively review the driving factors affecting carbon emission and carbon storage. Bibliometric analysis, as a statistics-based document processing method (Meseguer-Sánchez et al. 2021), can be used for visual analysis to identify hot issues and thematic research trends in the field.

Therefore, this study conducted a bibliometric analysis on the carbon budget of coastal wetlands. The objectives of this study were (1) to comprehensively grasp the research status and reveal the hotspots; (2) to systematically analyze the key influencing factors; and (3) to propose suggestions for future development.

The data were obtained from the database of the Scientific Citation Index Expanded and Emerging Sources Citation Index of the Web of Science from 1991 to 2022. The search function was defined as ‘TS = (coastal wetland or mangrove or salt marsh or seagrass) AND TS = (blue carbon or greenhouse gas or carbon emission or carbon budget or carbon source or carbon sink)’ and a total of 4,544 documents were collected. The impact factor (IF) was obtained from the Journal Citation Reports (JCR 2022). The in-depth understanding was assessed based on the following characteristics, including publications, subject categories, journals, countries, institutions, authors, and author keywords. Frequency calculations, citations, and co-occurrence analysis were performed utilizing HistCite™ (12.03.17), VOSviewer (1.6.18), and CiteSpace (6.2.R4). Statistical graphs depicting the number of published articles and citation frequencies were generated using Origin 2021.

Publications and subject categories analysis

Figure 1(a) shows the number of publications and citations on the carbon budget of coastal wetlands from 1991 to 2022. The development of this field can be divided into three stages, including sprouting stage (1991–2001), slow development stage (2002–2010), and rapid development stage (2011–2022). Since 2011, both publications and citations have risen sharply, reflecting a dramatic increase in public attention and interest in this field. The publication count surged from 99 in 2011 to 543 in 2022, accompanied by a rise in citations from 4,342 to 23,828, which implies that the carbon budget of coastal wetlands has gained considerable attention, foreshadowing sustained growth in the research field in the future.
Figure 1

(a) Temporal variation of publications and citations from 1991 to 2022 and (b) trend of the top 5 published categories in Web of Science.

Figure 1

(a) Temporal variation of publications and citations from 1991 to 2022 and (b) trend of the top 5 published categories in Web of Science.

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Eleven document types were identified among 4,544 publications, where articles (4,287; 94.3%) were the most common document type, followed by reviews (222; 4.9%), and conference proceedings (94; 2.1%). In this study, 4,509 original articles (4,287 articles and 222 reviews) were used for further analysis. The publications were classified into 99 categories, and the top five categories from 2012 to 2022 are manifested in Figure 1(b). Environmental Sciences was the leading category with 1,844 articles (40.9%), followed by Marine Freshwater Biology (1,291; 28.6%), Ecology (945; 21.0%), Oceanography (829; 18.4%), and Geosciences Multidisciplinary (546; 12.1%) (Supplementary Table S1). In addition, the research fields are consistent with the results shown by the Web of Science categories (Supplementary Figure S1). Environmental Sciences Ecology was the leading research field with 2,511 articles (49.9%), followed by Marine Freshwater Biology with 1,420 articles (31.5%), Oceanography with 830 articles (18.4%), Geology with 550 articles (12.2%), and Science Technology Other Topics with 5.5% of 4,509 articles, respectively. The leading category of Environmental Sciences mainly included the research on the carbon cycle processes and factors affecting coastal wetlands. The categories of Marine Freshwater Biology and Ecology implied that many studies were related to blue carbon and ecological restoration of coastal wetlands. The categories of Environmental Sciences, Marine Freshwater Biology, and Ecology have become increasingly popular, while the categories of Oceanography and Geosciences Multidisciplinary have reached a platform since 2019.

Analysis of journals

The subject category, number of citations, and impact factor (IF) of the 20 most active journals are displayed in Table 1. Estuarine Coastal and Shelf Science (243; 5.4%), Science of the Total Environment (199; 4.4%), and Marine Ecology Progress Series (178; 4.0%) were the three journals that published the most articles. The first report of the top three journals occurred relatively early (before 1994), while the initial years of Frontiers in Marine Science, PLoS ONE, and Scientific Report were 2014, 2011, and 2012, respectively. The first to focus on the carbon budget field were the Marine Ecology Progress Series, Limnology and Oceanography, Biogeochemistry, and Journal of Experimental Marine Biology and Ecology journals, which all started in 1991. In addition, most of the top 20 journals were associated with environmental category, indicating that the topic has gained significant interest from environmental, biological, and ecological perspectives.

Table 1

Top 20 most productive journals during 1991–2022

Total publication
Subject categoryNumber of citationsAverage citation per paperInitial yearImpact factor of 2022
Journal nameNo.%
Estuarine Coastal and Shelf Science 243 5.4 Marine Freshwater Biology 9,155 37.7 1993 2.8 
Science of the Total Environment 199 4.4 Environmental Sciences 4,069 20.5 1994 9.8 
Marine Ecology Progress Series 178 4.0 Ecology 9,693 54.5 1991 2.6 
Frontiers in Marine Science 115 2.6 Marine Freshwater Biology 1,703 14.8 2014 3.7 
Estuaries and Coasts 112 2.5 Environmental Sciences 2,948 26.3 2006 2.7 
Limnology and Oceanography 111 2.5 Limnology 6,074 54.7 1991 4.5 
Marine Pollution Bulletin 91 2.0 Environmental Sciences 3,304 36.3 2001 5.8 
Journal of Geophysical Research Biogeosciences 84 1.9 Environmental Sciences 2,701 32.2 2007 3.7 
Wetlands 79 1.8 Ecology 1,926 24.4 1997 2.0 
Biogeosciences 67 1.5 Ecology 3,367 50.3 2004 4.9 
Global Change Biology 61 1.4 Biodiversity Conservation 3,305 54.2 2000 11.6 
Biogeochemistry 57 1.3 Environmental Sciences 5,098 89.4 1991 4.0 
PLoS ONE 57 1.3 Multidisciplinary Sciences 3,393 59.5 2011 3.7 
Scientific Reports 56 1.2 Multidisciplinary Sciences 1,783 31.8 2012 4.6 
Hydrobiology 53 1.2 Marine Freshwater Biology 2,033 38.4 1993 2.6 
Ecological Engineering 52 1.2 Ecology 1,549 29.8 1995 3.8 
Journal of Experimental Marine Biology and Ecology 44 1.0 Ecology 2,928 66.6 1991 2.0 
Marine Chemistry 44 1.0 Oceanography 2,954 67.1 2001 3.0 
Organic Geochemistry 44 1.0 Geochemistry & Geophysics 1,501 34.1 1992 3.0 
Aquatic Botany 39 0.9 Marine Freshwater Biology 3,102 79.5 1992 1.8 
Total publication
Subject categoryNumber of citationsAverage citation per paperInitial yearImpact factor of 2022
Journal nameNo.%
Estuarine Coastal and Shelf Science 243 5.4 Marine Freshwater Biology 9,155 37.7 1993 2.8 
Science of the Total Environment 199 4.4 Environmental Sciences 4,069 20.5 1994 9.8 
Marine Ecology Progress Series 178 4.0 Ecology 9,693 54.5 1991 2.6 
Frontiers in Marine Science 115 2.6 Marine Freshwater Biology 1,703 14.8 2014 3.7 
Estuaries and Coasts 112 2.5 Environmental Sciences 2,948 26.3 2006 2.7 
Limnology and Oceanography 111 2.5 Limnology 6,074 54.7 1991 4.5 
Marine Pollution Bulletin 91 2.0 Environmental Sciences 3,304 36.3 2001 5.8 
Journal of Geophysical Research Biogeosciences 84 1.9 Environmental Sciences 2,701 32.2 2007 3.7 
Wetlands 79 1.8 Ecology 1,926 24.4 1997 2.0 
Biogeosciences 67 1.5 Ecology 3,367 50.3 2004 4.9 
Global Change Biology 61 1.4 Biodiversity Conservation 3,305 54.2 2000 11.6 
Biogeochemistry 57 1.3 Environmental Sciences 5,098 89.4 1991 4.0 
PLoS ONE 57 1.3 Multidisciplinary Sciences 3,393 59.5 2011 3.7 
Scientific Reports 56 1.2 Multidisciplinary Sciences 1,783 31.8 2012 4.6 
Hydrobiology 53 1.2 Marine Freshwater Biology 2,033 38.4 1993 2.6 
Ecological Engineering 52 1.2 Ecology 1,549 29.8 1995 3.8 
Journal of Experimental Marine Biology and Ecology 44 1.0 Ecology 2,928 66.6 1991 2.0 
Marine Chemistry 44 1.0 Oceanography 2,954 67.1 2001 3.0 
Organic Geochemistry 44 1.0 Geochemistry & Geophysics 1,501 34.1 1992 3.0 
Aquatic Botany 39 0.9 Marine Freshwater Biology 3,102 79.5 1992 1.8 

Publication performance and cooperation network

Papers published from 1991 to 2022 on the carbon budget of coastal wetlands covered 134 countries, where the USA and China were the two most productive countries, contributing 34.7 and 18.4% of the total publications, respectively (Table 2). The USA has been the leading country in terms of annual publications since 1991. However, China was the leading country with 136 articles in 2021, which was the first time that the annual publications of China had exceeded those of the USA since 1991 (Supplementary Figure S2). In terms of the average citations per paper, Spain (57.7) ranked first, followed by the USA (54.6) and Australia (49.5). Notably, China ranked second in the number of publications, but the average citation was relatively low, indicating that the quality of Chinese publications still needed to be improved. The ratio of international collaboration to total publications (ICP ratio) reflects the status of international collaboration (Table 2). Both single country publications and international collaboration publications of the top three countries (the USA, China, and Australia) were higher than other countries (Figure 2(a)), while the ICP ratios were relatively low. On the contrary, countries with relatively low productivity, like France and England, had high ICP ratios, implying the lower proportion of single country publications. International cooperation among countries was explored utilizing the co-occurrence analysis. As shown in Figure 2(b), it can be seen that the international cooperation in the area of coastal wetland carbon budget was mostly concentrated in the most productive countries. The USA had not only the most publications but also the most contacts with other countries. The USA engaged in collaboration with 100 countries, with an international cooperation frequency reaching 670, followed by Australia, which collaborated with 67 countries and had an international cooperation frequency of 434. China cooperated with 62 countries and with an international cooperation frequency of 333, indicating that China's international cooperation in the field needed to be strengthened.
Table 2

Top 10 most productive countries from 1991 to 2022

CountryTotal citations
Total citationsAverage citations per paperSingle country publicationsInternational collaboration publicationsICP ratio
No.%
USA 1,563 34.7 85,372 54.6 893 670 0.4 
China 828 18.4 17,448 21.1 495 333 0.4 
Australia 697 15.5 34,508 49.5 263 434 0.6 
Spain 330 7.3 19,029 57.7 69 261 0.8 
India 284 6.3 6,735 23.7 194 90 0.3 
Germany 247 5.5 9,208 37.3 53 194 0.8 
Brazil 229 5.1 5,350 23.4 84 145 0.6 
France 218 4.8 7,820 35.9 44 174 0.8 
Japan 217 4.8 6,108 28.2 82 135 0.6 
England 197 4.4 8,620 43.8 34 163 0.8 
CountryTotal citations
Total citationsAverage citations per paperSingle country publicationsInternational collaboration publicationsICP ratio
No.%
USA 1,563 34.7 85,372 54.6 893 670 0.4 
China 828 18.4 17,448 21.1 495 333 0.4 
Australia 697 15.5 34,508 49.5 263 434 0.6 
Spain 330 7.3 19,029 57.7 69 261 0.8 
India 284 6.3 6,735 23.7 194 90 0.3 
Germany 247 5.5 9,208 37.3 53 194 0.8 
Brazil 229 5.1 5,350 23.4 84 145 0.6 
France 218 4.8 7,820 35.9 44 174 0.8 
Japan 217 4.8 6,108 28.2 82 135 0.6 
England 197 4.4 8,620 43.8 34 163 0.8 
Figure 2

(a) Top 10 countries in number of publications during 1991–2022 and (b) the cooperation network of the countries. (The circle size represents the total publications from a country; the color depicts publication time variation; the thickness of the lines represents the frequency of cooperation).

Figure 2

(a) Top 10 countries in number of publications during 1991–2022 and (b) the cooperation network of the countries. (The circle size represents the total publications from a country; the color depicts publication time variation; the thickness of the lines represents the frequency of cooperation).

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Table 3 shows the top 10 most productive institutions. The Chinese Academy of Sciences and the State University System of Florida were the leading institutions with 226 articles, followed by the Centre National De La Recherche Scientifique (CNRS) and the United States Department of the Interior. The highest average citations per article were achieved by Consejo Superior De Investigaciones Cientificas (CSIC) in Spain (83.08), followed by the State University System of Florida (74.3) in the USA, both of which published their first studies earlier in 1995 and 1991, respectively. The first study from the institute of Chinese Academy of Sciences was reported in 2003, relatively late compared to other countries, which explained its low average citations per paper (21.2). The cooperative network of institutes is displayed in Figure 3(a), which indicates that most of the top institutes had close cooperations with other institutes. The centralities of several institutes (including Chinese Academy of Sciences, the State University System of Florida, the United States Department of the Interior, and the University of California System) were larger than 0.1 (Table 3), which indicated the importance of these institutions in this field (Wen et al. 2020).
Table 3

Top 10 most productive institutes from 1991 to 2022

InstitutionCountryTotal publications
CitationsAverage citations per paperCentralityInitial year
No.%
Chinese Academy of Sciences China 226 5.0 4,796 21.2 0.14 2003 
State University System of Florida USA 226 5.0 16,781 74.3 0.32 1991 
Centre National De La Recherche Scientifique (CNRS) France 178 4.0 6,583 37.0 0.03 1993 
United States Department of the Interior USA 178 4.0 10,594 59.5 0.14 1993 
Consejo Superior De Investigaciones Cientificas (CSIC) Spain 175 3.9 14,539 83.1 0.10 1995 
United States Geological Survey USA 161 3.6 10,170 63.2 0.04 1993 
University of California System USA 139 3.1 7,762 55.8 0.17 1992 
UDICE French Research Universities France 133 3.0 4,400 33.1 0.04 1996 
Southern Cross University Australia 129 2.9 5,130 39.8 0.02 2010 
Florida International University USA 120 2.7 8,656 72.1 0.08 1996 
InstitutionCountryTotal publications
CitationsAverage citations per paperCentralityInitial year
No.%
Chinese Academy of Sciences China 226 5.0 4,796 21.2 0.14 2003 
State University System of Florida USA 226 5.0 16,781 74.3 0.32 1991 
Centre National De La Recherche Scientifique (CNRS) France 178 4.0 6,583 37.0 0.03 1993 
United States Department of the Interior USA 178 4.0 10,594 59.5 0.14 1993 
Consejo Superior De Investigaciones Cientificas (CSIC) Spain 175 3.9 14,539 83.1 0.10 1995 
United States Geological Survey USA 161 3.6 10,170 63.2 0.04 1993 
University of California System USA 139 3.1 7,762 55.8 0.17 1992 
UDICE French Research Universities France 133 3.0 4,400 33.1 0.04 1996 
Southern Cross University Australia 129 2.9 5,130 39.8 0.02 2010 
Florida International University USA 120 2.7 8,656 72.1 0.08 1996 
Figure 3

(a) The cooperative relationship map of the institutes and (b) the co-authorship analysis map.

Figure 3

(a) The cooperative relationship map of the institutes and (b) the co-authorship analysis map.

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There were 13,360 authors who contributed to this research field. Table 4 shows the top 10 productive authors. Duarte from the Institute of King Abdullah University Science and Technology was the most productive author, with 128.1 average citations per article, ranking second only to Marba from Edith Cowan University (134.1). It is interesting to find out that the top four authors also have close cooperation with each other, as indicated by the large nodes in Figure 3(b).

Table 4

List of the top 10 authors

AuthorInstitutionTotal publications
CitationsAverage citations per paperCentralityInitial year
No.%
Duarte, C.M. King Abdullah Univ Sci & Technol 97 2.2 12,429 128.1 0.04 1995 
Lovelock, C.E. Univ Queensland 76 1.7 5,776 76.0 0.02 2006 
Maher, D.T. Southern Cross Univ 67 1.5 3,114 46.5 0.03 2010 
Santos, I.R. Southern Cross Univ 65 1.4 3,207 49.3 0.01 2012 
Macreadie, P.I. Univ Technol Sydney, Deakin Univ 62 1.4 2,757 44.5 0.02 2015 
Marba, N. Edith Cowan Univ 46 1.0 6,170 134.1 0.02 2004 
Lavery, P.S. Edith Cowan Univ 45 1.0 2,820 62.7 0.00 2005 
Connolly, R.M. Griffith Univ 38 0.8 2,806 73.8 0.00 1999 
Sanders, C.J. Southern Cross Univ 35 0.8 650 18.6 0.01 2019 
Krauss, K.W. US Geol Survey 34 0.8 1,170 34.4 0.01 2006 
AuthorInstitutionTotal publications
CitationsAverage citations per paperCentralityInitial year
No.%
Duarte, C.M. King Abdullah Univ Sci & Technol 97 2.2 12,429 128.1 0.04 1995 
Lovelock, C.E. Univ Queensland 76 1.7 5,776 76.0 0.02 2006 
Maher, D.T. Southern Cross Univ 67 1.5 3,114 46.5 0.03 2010 
Santos, I.R. Southern Cross Univ 65 1.4 3,207 49.3 0.01 2012 
Macreadie, P.I. Univ Technol Sydney, Deakin Univ 62 1.4 2,757 44.5 0.02 2015 
Marba, N. Edith Cowan Univ 46 1.0 6,170 134.1 0.02 2004 
Lavery, P.S. Edith Cowan Univ 45 1.0 2,820 62.7 0.00 2005 
Connolly, R.M. Griffith Univ 38 0.8 2,806 73.8 0.00 1999 
Sanders, C.J. Southern Cross Univ 35 0.8 650 18.6 0.01 2019 
Krauss, K.W. US Geol Survey 34 0.8 1,170 34.4 0.01 2006 

Author keywords analysis

In this study, only 213 keywords appeared more than 10 times and were used for further analysis. Figure 4(a) shows the four main keyword clusters, including ‘stable isotopes’, ‘blue carbon’, ‘greenhouse gas’, and ‘degradation’. The timeline of the keywords is shown in Figure 4(b). For the ‘stable isotopes’ cluster, the keywords mainly included nitrogen, carbon isotopes, food web, δ15N, δ13C, eutrophication, dissolved organic matter, etc. The ‘blue carbon’ cluster included keywords of climate change, carbon sequestration, carbon stock, restoration, ecosystem services, remote sensing, etc. Research on blue carbon in coastal wetlands had changed from only measuring carbon storage impacted by climate change to the management of coastal wetlands and the estimation of carbon storage by advanced means such as remote sensing. For the ‘greenhouse gas’ cluster, the keywords were mainly Spartina alterniflora, nitrous oxide (N2O), methane (CH4), carbon dioxide (CO2), denitrification, salinity, microbial community, etc. Researchers have gradually paid attention to the carbon and nitrogen cycle of coastal wetlands and the coupling mechanisms (Asanopoulos et al. 2021), as well as the influencing factors of the carbon cycle.
Figure 4

(a) Top 4 cluster of keywords co-occurrence map and (b) the timeline view of keywords of top 4 cluster.

Figure 4

(a) Top 4 cluster of keywords co-occurrence map and (b) the timeline view of keywords of top 4 cluster.

Close modal
The keywords co-occurrence network is manifested in Figure 5. Carbon budget of coastal wetlands could be clustered into four sections and distinguished by four different colors. The keywords shown as red mainly included stable isotopes, sediments, food web, etc. These words implied that stable isotopes could be used for carbon cycle research (Tian et al. 2020). The keywords shown as green included blue carbon, ecosystem services, carbon storage, carbon sequestration, climate change mitigation, etc. This theme dealt with changes in vegetation and soil carbon pools in the context of climate change and focused on changes in coastal wetland carbon stocks in response to sea-level rise and climate warming. The keywords in blue included carbon dioxide, methane, nitrous oxide, etc. This section involved greenhouse gas emission fluxes, temporal variation characteristics, and influencing factors. The keywords highlighted in yellow color were a relatively minor portion, including carbon budget, decomposition, which focused on carbon decomposition. These topics are closely related to each other. The carbon budget of coastal wetland ecosystems includes carbon input, output, and fixation, where greenhouse gas fluxes are related to carbon decomposition and carbon pools to carbon fixation.
Figure 5

(a) Visualization of keywords co-occurrence network and (b) the overlay visualization of keywords co-occurrence network. (Each node denotes one keyword, the size depicts the number of publications, and the line represents the connection between keywords.)

Figure 5

(a) Visualization of keywords co-occurrence network and (b) the overlay visualization of keywords co-occurrence network. (Each node denotes one keyword, the size depicts the number of publications, and the line represents the connection between keywords.)

Close modal

The changes in keywords represent the evolution of hotspots in the research field of coastal wetland carbon budget (Figure 5(b)). According to the different colors, the research hotspots could be roughly divided into three stages. Before 2011, the carbon source, and the nitrogen and phosphorus cycles related to the eutrophication of coastal wetlands were preliminarily studied. In the second stage, from 2011 to 2017, keywords such as carbon dioxide, methane, nitrous oxide, and organic carbon gradually appeared. Compared with the previous stage, the research on greenhouse gas fluxes was strengthened, and the attention was paid to the temporal rule, and the influence mechanism of greenhouse gas emissions. After 2017, keywords such as blue carbon, carbon sequestration, soil organic carbon (SOC), sea-level rise, climate change mitigation, carbon sink, carbon storage, ecosystem services remote sensing, and restoration appeared frequently. Based on the above analysis, studies in the current stage focus on blue carbon sequestration and storage, the influence of various factors on carbon cycle, as well as the restoration of coastal wetlands (Macreadie et al. 2021).

Dynamics of the carbon sequestration in coastal wetlands

Coastal wetlands are vulnerable ecosystems due to their special geographical location, which is sensitive to climate change. With the continuous rise of sea-level, a large number of coastal wetlands were gradually inundated or even disappeared, and the carbon sink capacity was also reduced (Singh et al. 2022). Moreover, sea-level rise altered wetland wetness, dryness, and submergence depth, which affected the wetland vegetation types and increased the vegetation vulnerability, leading to the decreased carbon storage (Rodriguez et al. 2017). However, previous studies showed that due to vertical deposition in salt marshes, carbon accumulation continues to increase with the sea-level rise until seawater completely submerges salt marsh vegetation (Mudd et al. 2009). Rogers et al. (2019) also suggested that sea-level rise increased the rate of organic carbon burial rate. Seawater intrusion increased the salinity of wetland water and the concentration of electron acceptor sulfate () in sediment pore water, which affected the process of carbon mineralization and led to the accumulation of SOC (Zhao et al. 2023). Furthermore, the rising groundwater levels caused by sea-level rise could also change the magnitude and direction of carbon flux, and even contribute to organic carbon sequestration (Zhao et al. 2020).

The deforestation will lead to a large area of mangrove loss, thereby reducing the carbon sequestration, since mangroves are one of the most productive and carbon-intensive ecosystems (Hamilton & Casey 2016). Global mangrove carbon stocks were estimated to decline by 158.4 Mt, representing a 1.8% reduction from the existing stocks in 1996 (Richards et al. 2020). Moreover, mangrove deforestation resulted in the carbon emission rate of 23.5–38.7 Tg yr−1 (Ouyang & Lee 2020). Fortunately, some countries in Southeast and South Asia had invested heavily in mangrove restoration, and the loss of mangrove forests almost halved between 2000–2010 and 2010–2020 (Contessa et al. 2023). Global warming also led to the natural expansion of mangroves in areas such as Mexico and northeastern Brazil, resulting in net increase in mangrove carbon stocks (dos Reis-Neto et al. 2019; Richards et al. 2020).

To date, there is still a significant knowledge gap on the carbon sequestration capacity of coastal ecosystems, and many uncertainties existed in its assessment. In recent years, remote sensing, machine learning techniques, and geographic information system have drawn wide attention in monitoring coastal wetlands due to their advantages such as lower cost, higher accuracy, and wider coverage (Pham et al. 2019). Previous studies utilized these techniques to monitor mangroves and determine the attributes such as species and biomass (Farzanmanesh et al. 2021), which could estimate mangrove biomass more accurately than conventional field surveys (Pham et al. 2019). However, in order to more accurately assess the carbon sequestration of global coastal wetlands, some basic scientific issues should be prioritized, including: (1) the exact distribution area of global coastal wetlands; (2) the differences in carbon sink capacity of coastal wetlands in different regions; and (3) the effects of anthropogenic disturbance and global climate change on the carbon sequestration capacity of coastal wetland ecosystems.

Key factors influencing carbon cycle of coastal wetlands

Temperature

According to the IPCC, the earth's surface temperature is anticipated to increase by 1.8–4.0 °C by the end of this century (Guo et al. 2019). Nevertheless, there have been only few studies specifically addressing the direct effects of temperature rise on the carbon cycle of coastal wetlands. Rising temperatures not only increased the accumulation of organic matter, but also promoted the decomposition rates of organic matter (Tao et al. 2019). Rising temperatures altered the structure and geographical distribution of vegetation communities, thus affecting the plant growth and carbon sequestration (Bhattarai et al. 2021). In addition, the changes in environmental impact factors caused by climate warming could affect CO2, CH4, N2O emissions, and the source-sink effect of soil carbon and nitrogen through changes in microbial activity, redox potential, aeration rate, and others (Liu et al. 2019). Hopple et al. (2020) reported a linear increase in CH4 emissions with rising temperatures. Overall, temperature increases have complex effects on the carbon cycle of coastal wetland ecosystems, necessitating further research to enhance our understanding of the underlying mechanisms.

Plant species

The effects of vegetation characteristics on the carbon cycle of coastal wetlands have been extensively studied. The soil carbon in coastal wetlands (especially salt marsh) is primarily derived from the plants growing in the system. Changes in plant community could alter the structure and material cycling processes of the ecosystems, thereby leading to the changes in carbon mineralization rates (Lin et al. 2023). Osland et al. (2018) found higher levels of soil organic matter in mangroves and salt marshes where gramineous plants were dominated, but lower levels in salt marshes lacking vascular plants and dominated by succulents. Vegetation type and SOC were also the main factors affecting CH4 and CO2 emissions from coastal wetlands. For example, wetland plants with abundant aeration tissues were more favorable for greenhouse gas transport (Hameed et al. 2010). Wetland plants with vigorous root growth consumed large amounts of CH4 due to radial oxygen loss, thus reducing CH4 emissions (Luo et al. 2017). SOC, the substrate for microbial mineralization, was positively correlated with greenhouse gas emissions (Xu et al. 2021a).

The invasion of plants represents a substantial menace to the sustainability of native ecosystems, exerting a noteworthy influence on the dynamics of carbon cycle in coastal wetlands. The invasive C4 plant Spartina alterniflora had a longer growing season and higher productivity, and released more secretions than native species, thus increasing the carbon inputs to the ecosystem (Chen et al. 2015; Zhang et al. 2021). The invasion of Spartina alterniflora significantly increased the microbial biomass and phospholipid fatty acid (PLFA) species, leading to the formation of organic carbon pools (Zhang et al. 2021). However, some studies considered the trend of the reduced soil bulk density after Spartina alterniflora invasion, so they believed that the organic carbon density of Spartina alterniflora invaded ecosystems was lower than or similar to most native ecosystems (Xu et al. 2022). Meanwhile, Spartina alterniflora invasion also increased greenhouse gas emissions during cold seasons. The CH4 production potential of marshes invaded by Spartina alterniflora was 10 times higher than that of other areas, displaying a notable correlation with SOC and dissolved organic carbon content (Yuan et al. 2016).

Land reclamation

Reclamation, a common way of land use change in estuarine and coastal areas, is one of the foremost human activities affecting coastal wetlands. Globally, about 70% of coastal wetlands were lost due to anthropogenic disturbance, and around 25–50% were reclaimed for other land uses (Kirwan & Megonigal 2013; Davidson 2014). In recent years, there has been a persistent decline in coastal wetlands, experiencing an annual loss rate of 1–7% (Hopkinson et al. 2012), leading to the decreased carbon sequestration. Compared to natural coastal wetlands, the soil properties, and microbial community structure of reclaimed coastal wetlands were significantly altered, thereby affecting its carbon cycling process. The degradation of coastal habitats had the potential to destroy soil carbon up to 1 m deep, resulting in its conversion into CO2 through mineralization (Fourqurean et al. 2012; Atwood et al. 2017). Previous studies also showed that the reclaimed creeks had a higher global warming potential (0.6 ± 0.2 g CO2-eq m−2·d−1) than the natural tidal creeks (Tan et al. 2021). Reclaimed coastal wetlands could block periodic tidal flooding, and thus produce more CH4 (Tan et al. 2020). Moreover, seawalls built along the shoreline also affected greenhouse gas emissions, and significantly reduced soil total organic carbon content by about 57.0% (Zhou & Bi 2020), as well as plant biomass and soil moisture (Li et al. 2021).

Reclamation of coastal wetlands often led to salinization, which affected the physiological and ecological status of plants, as well as the plant diversity and community structure of wetlands, thus further altering the primary productivity (carbon sequestration capacity) of plant communities (Li et al. 2018). Moreover, the construction of reclamation facilities basically stopped the exchange of substances and energy between coastal wetlands and aquatic ecosystems, which greatly interfered with the carbon sequestration process and lateral carbon fluxes (Spivak et al. 2019). It can be seen that with the development of urbanization and land use change in coastal areas, the area of coastal wetlands will continue to decrease, seriously affecting the ecological function and carbon cycle process of wetland ecosystems.

Pollutant input

Human activities such as mining, aquaculture and discharge of domestic, industrial, and agricultural sewage cause pollution and eutrophication of coastal wetlands. Eutrophication accelerated plant growth (Moseman-Valtierra et al. 2022), and Geoghegan et al. (2018) reported that nitrogen enrichment could increase the gross primary productivity of the ecosystem by 7.7%, leading to increased soil carbon content. However, pollutant inputs also adversely affected the coastal wetlands, increasing soil greenhouse gas emissions and transforming coastal wetlands from a terrestrial carbon sink to a negative feedback loop that enhanced the atmospheric carbon sink. For example, nitrogen inputs tended to accelerate SOC decomposition and stimulated CO2 emissions in coastal wetlands (Tao et al. 2018). This result was influenced by multiple factors, such as pollutant concentration, water level height, and temperature (Chen et al. 2020). At low nitrogen input level, SOC decomposition was stimulated, which might be related to changes in soil nutrients, and carbon storage (Qu et al. 2020). Chen et al. (2020) found that addition lower than 100 kg ha−1yr−1 significantly stimulated greenhouse gas emissions. The form of nitrogen input was also important in stimulating organic carbon decomposition, and nitrate () had a stronger stimulating effect than ammonium () (Qu et al. 2020). However, at high nitrogen input level, the SOC decomposition would be inhibited (Xu et al. 2017), indicating the reduced availability of soil carbon. In addition, human activities led to a large number of microplastics entering coastal wetlands. Microplastics could change the soil properties, thus increasing cumulative CO2 emissions by 13.9–49.5% (Tang et al. 2022).

In-depth understanding on carbon cycle of coastal wetland

The blue carbon sink of coastal wetlands is an important ecological resource, and the existing studies predominantly focused on the carbon sequestration capacity of coastal wetlands (Bertram et al. 2021), as well as the ecological functions (Tan et al. 2021; Singh et al. 2022). However, systematic exploration on the interaction of multiple factors affecting carbon exchange and carbon cycling, especially its functional mechanisms are still lacking. Therefore, it is essential to conduct in-depth research on the carbon cycling mechanisms of coastal wetlands, including comprehensive analysis related to impacts of natural environmental factors and human activities on the carbon budget of coastal wetlands. Isotope tracer technology might be a promising approach, since it could identify the sources of carbon and reveal the deposition and recycling process of carbon in wetlands.

The quantification of blue carbon sink of coastal wetlands is prerequisite for its development and utilization, but currently there is no unified estimation standard and method. Therefore, taking the high uncertainty existing in the assessment of carbon sink capacity, a methodological system for the investigation, monitoring and evaluation of blue carbon sinks in coastal wetlands should be established in the future, with the impact of natural and human activities on blue carbon sink fully considered. By strengthening the comprehensive application of wetland observation and model simulation, combined with geographic information system (GIS), remote sensing technology and data model, coastal wetlands will be dynamically monitored, and the assessment of wetland carbon sink will be more accurate.

Ecological restoration technology of coastal wetland

Choosing suitable ecological restoration techniques is a crucial step, and the applicability and goals of restoration vary among different restoration methods. At present, coastal wetland restoration techniques are focusing on the hydrological connectivity restoration, as well as vegetation restoration. Hydrological connectivity is one of the main abiotic drivers of wetland ecological processes, and enhancing it can contribute to water quality improvement (Cui et al. 2021). Vegetation planting and seeding techniques are common engineering measures in coastal zone vegetation restoration projects (Fivash et al. 2021). After vegetation restoration, soil moisture content and vegetation coverage would increase by 45.9 and 5.0%, respectively, while soil salinity would decrease by 70.5% (Pang et al. 2023). However, the coastal wetland restoration project usually ignored the importance of carbon sequestration. In the future, the eco-seawalls with optimized structure should be built to replace the traditional breakwaters (Xu et al. 2021b), which promote tidal hydrological connectivity and expand the area of coastal wetlands, thus inhibiting CO2 and CH4 emissions and increasing carbon sequestration. Vegetation restoration should not only consider landscape, but also choose plants with higher carbon sequestration capacity and primary productivity. Moreover, strengthening the carbon sequestration function of bacteria and symbiotic plants through artificial regulatory means such as adding functional microbial agents will also become a new direction of restoration techniques. Meanwhile, technical systems should be constructed to strengthen the technical coupling optimization and synergies of carbon sequestration in coastal wetlands to promote wetland restoration and maximize social and ecological co-benefits.

Numerous restoration projects of coastal wetlands have been carried out around the world. It is undoubtedly important to scientifically evaluate whether coastal wetland restoration is successful. The existing evaluation systems mainly considered community structure integrity, hydrogeomorphic conditions, soil properties, and water quality. In addition, economic valuation of the benefits generated by coastal wetlands protection and restoration is also receiving increasing attention. Tan et al. (2018) found that the compensating surplus showed an increasing trend with the change of coastal wetland restoration scenarios from modest to ambitious. Quantifying the economic benefits and ecological value of coastal wetland restoration will be beneficial to social decision-making and the implementation of carbon trading policies. However, to date, there is no comprehensive evaluation index system for coastal wetland restoration, and the quantitative evaluation of carbon sequestration capacity is lacking. In the future, the changes in coastal wetland carbon sequestration can be dynamically monitored by remote sensing and other technologies to characterize restoration status. Subsequentially, the evaluation index system that takes ecological functions and socio-economic impacts into account could be established.

Improvement of the coastal wetland blue carbon trading system

The carbon trading market provides a platform for enterprises, individuals, and organizations that commit to carbon emission reduction or carbon neutralization (Sapkota & White 2020). However, at present, there are relatively few studies focusing on the value of blue carbon sinks in coastal wetlands from the perspective of carbon sink trading, and the research on emission reduction and value accounting and evaluation of blue carbon sink projects still needs to be strengthened. In addition, in order to incorporate wetland carbon sinks into the voluntary emission reduction market or to offset some of the mandatory emission reductions, it is also necessary to formulate enforceable policies for the unified quantitative accounting of wetland carbon sinks and the formation of scientific pricing mechanisms.

Actively responding to global climate change is a common responsibility of all countries in the world. Meanwhile, actively carrying out international cooperation on blue carbon sinks is an important manifestation of promoting the development of a community with a shared future for mankind. The international carbon trading market aims to provide economic incentives to reduce global greenhouse gas emissions and promote the development of clean energy and low-carbon technologies. The effectiveness of an international carbon market mechanism depends on the guarantees of international law, the degree of global cooperation, market transparency, and regulatory mechanism. Hence, it is important to study how different countries and regions can implement carbon trading more effectively through international cooperation mechanisms, and the impact of carbon trading cooperation on global climate governance.

A bibliometric analysis of 4,509 articles was conducted to reveal current research hotspots and emerging trends in the field of carbon budget of coastal wetlands. The development of the topic could be divided into three stages, with the rapid development starting from 2011. The most active research areas of carbon budget in coastal wetlands are the dynamics of carbon sequestration caused by degradation and disappearance of coastal wetlands, and factors affecting the carbon cycle process and their functional mechanisms. The future research trends mainly include: (1) comprehensively analyzing the effects of natural environmental factors and human activities on the carbon budget of coastal wetlands, and conducting the unified estimation methods of blue carbon; (2) researching the restoration techniques of coastal wetlands and establishing the wetland restoration evaluation index system; (3) formulating an enforceable carbon trading policy and scientific pricing mechanism for the unified quantitative accounting of wetland carbon sinks, and strengthening international cooperation.

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by X.Z. and W.L. The first draft of the manuscript was written by X.Z. and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

This work was supported by the National Key Research and Development Program of China (No. 2021YFC3200602), the National Science Foundation of China (No. 52170043 and No. 51925803), the Natural Science Foundation of Shandong Province (No. ZR2020YQ42), and the Future Plan for Young Scholar of Shandong University.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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