Keywords are a high generalization of the topic of literature. Keywords with high frequency usually indicate that more research has been carried out through this keyword, which is more important and is used to determine the hot spot in a research field. To understand the hot spots of Blue Carbon international research from 2011 to 2021, the sorted 865 literature records were imported into CiteSpace software. The network node was selected as “Keyword,” and appropriate thresholds were set. There were 52 keywords with frequency greater than 20 (as shown in Table 2).

Firstly, in terms of the top 10 keywords, “Blue Carbon” topped the list 273 times. The second most frequent occurrence is “forest,” which can be seen as a specific component of Blue Carbon. In particular, mangrove forests provide a wide range of ecosystem services. Mangrove forests are known for their extraordinary carbon storage capacity, playing an important role in climate change mitigation. Moreover, organic carbon, sediment and other key elements should also belong to this concept. Secondly, storage and sequestration of Blue Carbon are also the focus of scholars. The Blue Carbon ecosystem and its dynamics are also of interest to scholars. In addition, Climate change is also a high-frequency keyword of Blue Carbon in international journals.

To better explain and analyze the academic significance of keywords transmitted in the above international journals, the key points were extracted by LLR algorithm of CiteSpace. Based on this, a cluster map of high-frequency keywords was generated, forming 16 knowledge clusters (Figure 5). They are Spartina Alterniflora, Soil organic carbon, Matter, Livestock, Trophic cascade, Biodiversity, Seagrass ecosystem, Arabian Gulf, Posidonia Oceanica, Sentinel, GIS, Salt marshes, Rehabilitation, Stable isotopes, Brown algae, etc., Modularity = 0.7746 > 0.3, Mean Silhouette = 0.9044 > 0.5, indicating good clustering and a clear silhouette. Seagrass ecosystem, salt marshes and brown algae are the specific components of Blue Carbon. Arabian qulf and Posidonia oceanica are the most frequently studied objects of Blue Carbon. Spartina Alterniflora and Rehabilitation are currently hot spots in Blue Carbon research.

After more than 10 years of practice and development, the research content of Blue Carbon in international journals has been enriched and expanded. The time-zone diagram for keyword co-occurrence analysis is shown in Figure 6. To further explore the evolution of topics in international journals, the nodes in the same period were collected in the same time zone and arranged in order from far to near. From the perspective of the main line, high-frequency keywords mainly appeared from 2011 to 2016, during which the newly emerged keywords occupied the majority of all keywords. From the perspective of newly emerged keywords in recent years, globally significant (2017) and global pattern (2018) confirm that international scholars gradually call for strengthening global cooperation and enhancing the attention of Blue Carbon in the global carbon sink system.

Citation Burst analysis is one of the most important tools in literature content mining, reflecting active or cutting-edge research nodes. This paper uses “Burst” to predict Blue Carbon research frontier and direction. The top 9 keywords with the strongest citation bursts in Blue Carbon research in international journals are shown in Figure 7.

In terms of burst intensity, “Productivity” had the highest burst intensity at 4.29. This term reflects that the important development of Blue Carbon research is to explore the productivity of Blue Carbon, which has become a relatively influential research frontier. At the same time, the burst intensity of Conservation, Respiration and CO₂ was 3.55, 3.15, and 2.82, respectively, which were important frontiers of Blue Carbon.

In terms of burst time, “CO₂” is the theme word with the earliest appearance (2011) and the longest duration (5 years), which has become a frontier issue of continuous research. “Conservation” first appeared in 2012 and lasted for 3 years; “Productivity” first appeared in 2014 and lasts 4 years; “Bed” and “Flow” first appeared in 2015 and lasted for 3 years. They are also frontier studies in the field of Blue Carbon. At the same time, from the keywords with the strongest citation bursts in recent 3 years (2019–2021), “Respiration” is the research frontier in the field of Blue Carbon.

To understand the domestic research hotspots of Blue Carbon from 2009 to 2021, 162 pieces of literature were imported into CiteSpace. The network nodes selected “Keyword,” and appropriate thresholds are set. There were 40 keywords with frequency greater than 3 in total (as shown in Table 3). “Mangrove” is an important ecosystem of marine Blue Carbon sink, and its carbon pool includes vegetation biomass and soil carbon pool. “Blue Carbon” came in second with 45. Blue Carbon sink systems such as seagrass beds, coastal wetlands and coastal zones were also in the top 10.

The centrality represents the status of the keyword in the field, and the higher the value is, the more important it is. The centrality of coastal wetland was 0.40, ranking first among all the keywords. Salt marsh appeared only 11 times, but its centrality ranked second, with 0.38. In addition, seagrass appeared 3 times, with 0.37. Then mangrove forests (centrality = 0.36), organic carbon (centrality = 0.26), seagrass beds (centrality = 0.25), etc.

To better explain and analyze the academic significance of Blue Carbon research keywords transmitted from Chinese journals, the LLR, algorithm of CiteSpace was used to extract keywords and generate high-frequency keyword clustering map. Nine knowledge clusters were formed (Figure 8), including organic carbon, seagrass conservation, methodology, sustainable development, climate change, carbon neutrality, coastal zone, carbon storage, etc. Modularity = 0.8461 > 0.3, Mean Silhouette = 0.9578 > 0.5, indicating that the keyword clustering is in good condition and has a clear outline.

Co-occurrence analysis of keywords in Blue Carbon research in domestic journals was conducted through CiteSpace to generate time-zone map. The map sets the keywords that appear for the first time at the same time in the same time-zone in order from far to near (Chen, 2017). It can be seen that keywords with high frequency (large node size) are mainly concentrated in the time zone of 2009. As the central theme, the nodes of “mangrove,” “Blue Carbon” and “carbon neutrality” are the largest, and they have intensive links with other keywords, indicating that the research on Blue Carbon in China CNKI database has been inherited since 2009. From the perspective of the evolution of theme in China, although there were many research topics in 2009, the most core themes were Blue Carbon subsystems such as “seagrass bed,” “mangrove,” “coastal wetland,” “salt marsh,” and other realistic backgrounds such as “carbon neutrality.” These mainly focused on the conceptual and policy. In 2012, the focus was on “organic carbon;” In 2013, “Eelgrass,” “Chiu-lung River Estuary” and other specific objects began to appear. In 2014, the emergence of “World First,” “China Blue,” “Land-sea integration” and others showed that the focus of Blue Carbon in China has shifted from simply focusing on marine regions to land-sea integration, and also began to pay attention to China’s position in the world’s Blue Carbon resources and research. In 2015, it began to focus on nitrogen research. From 2017 to 2018, it began to pay attention to the impact of environmental pollution on Blue Carbon. In 2019, the main focus was on the construction of marine pastures.

Blue Carbon has important ecological service functions, including fish and wildlife habitat, coastal erosion and flood prevention, food resources, water purification, carbon sequestration and storage. Blue Carbon has high ecological and economic development value. Among them, mangroves are special wetland forests growing in the intertidal zone of tropical and subtropical regions, which play an important role in windbreak and field retention, promoting silt deposition, resisting natural disasters such as tsunami and typhoon, and protecting coastlines (Zhang et al., 2013).

However, the main function of Blue Carbon is carbon sequestration and carbon sink. Carbon sequestration refers to measures to increase the carbon content of carbon pools other than the atmosphere. Coastal Blue Carbon ecosystem has efficient carbon sequestration capacity, potentially mitigating the increase of atmospheric CO₂ concentration (Zhou et al., 2016). Although the area of mangroves is relatively small, their carbon sequestration capacity per unit area is very strong. Mangroves are tropical ecosystems with high carbon sequestration capacity and high light efficiency, and are important “Blue Carbon” carbon pools. Mangrove organic carbon storage and dynamics have an important impact on global carbon balance (Ray et al., 2011; Zhu et al., 2012; Zhao et al., 2013). In addition, macroalgae may also be an important source of carbon sequestration in marine sediments and deep oceans (Krause-Jensen and Duarte, 2016; Ortega et al., 2019). Carbon sink refers to the unstable state of carbon balance in an ecosystem, in which the carbon input (the sum of plant productivity and external input carbon) is greater than the carbon output (carbon consumed by respiration and external output carbon), namely the state of carbon accumulation. Mangrove is an important carbon sink ecosystem because of its high plant productivity and soil carbon sequestration rate. Therefore, the protection and restoration of mangroves has also become an important means of addressing climate change in the marine field (Zhang et al., 2013; Chen et al., 2021).

Studies have estimated the carbon storage. For example, Li et al. (2019) conducted methodological analysis on measuring carbon sinks in mangrove, seagrasses bed and salt marsh based on the recommended methods of carbon sink measurement given by IPCC (Intergovernmental Panel on Climate Change) and clean energy mechanism measurement standards. Phang et al. (2015) evaluated the ecosystem carbon storage of mangroves, seagrass meadows, tidal flats and sandbanks in tropical intertidal zone based on field carbon storage.

Furthermore, most existing scholars are committed to measuring the total amount of carbon stored by mangroves. Currently, the research methods of carbon storage and carbon sink capacity of mangrove wetlands are based on measured methods, including conversion based on biomass (Donato et al., 2011; Peng et al., 2016), soil sampling (Xin et al., 2014), allometric equations (Bulmer et al., 2016), remote sensing inversion (Heumann, 2011), model simulation, field analysis and laboratory determination combined (Li et al., 2014; He et al., 2017), UAV imagery (Jones et al., 2020), LiDAR (Salum et al., 2020), etc.

Seagrass provides many valuable ecosystem services and requires accurate mapping of existing meadows and biomass. For example, Ha et al. (2021) detected and quantified aboveground biomass of seagrass using radar and optical satellite imagery, combined with advanced machine learning and meta-heuristic optimization techniques. Monnier et al. (2021) applied high-resolution seismic reflection technology to estimate the carbon sink of seagrass meadow.

Salt marshes are of unique importance in coastal ecosystems. Van de Broek and Govers (2019) used mid-infrared spectroscopy combined with soil remote sensing to predict the concentration and storage of organic carbon in sediments of tidal marshes. Owers et al. (2020) quantified the change of near-surface carbon storage in salt marshes using stratified sampling method to identify vegetation structure.

Macroalgal communities as donors may contribute ecologically to global Blue Carbon sequestration. However, as the fate of isolated macroalgal biomass remains unclear, some researchers have attempted to quantify the total amount of carbon stored by macroalgae. For example, Hill et al. (2015) used carbon sequestration, macroalgae distribution, abundance and productivity to estimate the total amount of carbon stored by macroalgae.

Scholars are devoted to the protection and restoration of Blue Carbon. Mangrove conservation should be a high priority component of climate change mitigation strategies (Murdiyarso et al., 2015). In order to reduce losses, rehabilitation is the top priority of protection work (Duncan et al., 2016). Restoration Projects can sequester large amounts of additional carbon and reduce emissions. At present, the conservation and restoration measures discussed mainly include policy and legislation, restoration of abandoned ponds, positive interaction between species and payment mechanism of ecosystem services.

In recent years, Blue Carbon has been involved in carbon trading, finance and other fields, driving the protection and development of Blue Carbon ecosystem through economic leverage. The topic of Blue Carbon sink has been expanded from scientific research to the global economy.

Although Blue Carbon is beginning to be incorporated into the carbon market, it is still a relatively new concept. Blue Carbon research is not enough to understand the research methodology and development process, resulting in the development of Blue Carbon sink project constrained (Chen S. Y. et al., 2021). In order to make Blue Carbon mechanism play a reasonable role, the international mainstream approach is to moderately include Blue Carbon into the international carbon trading market. In 2011, the Blueprint for Sustainable Development of Oceans and Coastal Zones set out the goal of protecting marine ecosystems and establishing a global Blue Carbon market. The report aims to increase carbon sinks by protecting and restoring coastal wetlands for use in trade. At present, there are two types of carbon trading markets: compliance markets and voluntary markets. Compliance markets deal with mandatory emission reductions under regulations. It is driven by the need of regulated greenhouse gas emitters for quotas and offsets (Sapkota and White, 2020). In voluntary markets, compensation prices are mainly determined by supply and demand. In some cases, prices also depend on whether buyers are willing to pay more to promote environmentally sustainable carbon markets (Xia et al., 2020). Mangrove forests, for example, provide many benefits for human welfare, but they are rapidly disappearing. One reason for this disruption is that policymakers and markets have not understandably evaluated the benefits of mangroves (Tanner et al., 2019). Using the value of carbon stored in mangroves on the voluntary carbon market can generate revenue. This helps support and motivate sustainable mangrove management, improve livelihoods and reduce anthropogenic stress (Benson et al., 2017).

Optimizing Blue Carbon sequestration products and developing trading mechanisms are important ways for China and the world to deal with global climate change and achieve carbon neutrality. So far, China has successfully developed and traded its first Blue Carbon sequestration project, the Zhanjiang Mangrove Forest Project. In the future, it will continue to promote the development of Blue Carbon sink projects and the development of trading market (Chen et al., 2021), to promote ecological protection and restoration through market mechanism (Tang et al., 2018).

Monitoring can be divided into flux monitoring and carbon pool monitoring. First, the monitoring of carbon flux. It is to observe all kinds of fluxes and calculate the total amount. For example, Dahl et al. (2020) measured total organic carbon (TOC) deposited in seagrass meadows at two Z-dock sites to assess seasonal variations in sediment carbon storage of seagrass meadows. Potouroglou et al. (2017) conducted a multi-site study on sediment surface elevation of seagrass in different environments and species through long-term monitoring. Macklin et al. (2019) explored the driving factors of CO₂ along the mangrove seagrass transect through 8 seasonal surveys and 55 h fixed location time series. This method——the monitoring of carbon flux——can understand the mechanism and process of carbon pool changes and provide data basis for modeling. For example, using more than 900 soil carbon measurements data, Jardine and Siikamaeki (2014) established a global prediction model for mangrove soil carbon.

Second, the monitoring of carbon pools. It involves directly observing changes in the carbon pool. This method is relatively simple and can obtain the changes of 1 year or several years, but fails to give seasonal changes and the contribution of each carbon flux. For example, Peng et al. (2016) calculated the vegetation carbon storage of each community by investigating the components of each vegetation carbon pool in four representative mangrove communities.

Experiment is to change one or more environmental factors artificially through the comparison with the control group, explore the response of the ecosystem to this kind of factors, and serve the construction of the model. In the existing studies, the simulated factors include temperature, precipitation, nitrogen and phosphorus content, soil oxygen content, etc. For example, in order to further understand how salt marshes respond to warming and precipitation changes, Charles and Dukes (2009) conducted a manipulation experiment in tidal salt marshes through passive open indoor warming, double precipitation and warming, etc. Deegan et al. (2012) conducted a 9-year experiment on nutrient enrichment across the ecosystem in coastal wetlands, which showed that nutrient enrichment is a global problem in coastal ecosystems, and it may be also a driving factor of salt marsh loss. Using nutrient enrichment (N & P) experiments, Armitage and Fourqurean (2016) assessed the effects of long-term and near-term nutrient inputs on aboveground and underground carbon accumulation in seagrass beds. Wei et al. (2020b) explored the effects of tidal flood on CO₂ and CH₄ fluxes of salt marsh ecosystem through field experiments. Chapman et al. (2019) points out that changes in soil oxygen content may be a vital driving factor for Blue Carbon storage in coastal wetlands in the future by manipulating soil oxygen state in real soil.

The purpose of the long-term observation and simulation study of the ecosystem in Blue Carbon research is to build a process-based mathematical model to understand the system process and predict and analyze the future. Most models use input parameters that are easy to measure, such as meteorological factors and vegetation structure, and other data obtained with remote sensing data. For example, Morris et al. (2016) used marsh equilibrium model to predict the carbon sequestration capacity of salt marsh wetlands and their response to sea-level rise. Owers et al. (2018) collects high-resolution three-dimensional point cloud through terrestrial laser scanning and proposes the point cloud elevation histogram model. So, the aboveground biomass of coastal wetland vegetation with complex structures can be quantified. Sohma et al. (2018) established a coastal ecosystem model (Ecosystem Model for Aquatic Integrated Network for Blue Carbon) to estimate climate change mitigation. In addition, Jardine and Siikamaeki (2014) explored several predictive modeling methods using climate and location data for prediction. Based on these models, an estimated global soil carbon concentration and storage dataset was constructed.