Among these wetlands, the place locates near the coastline is called salt marsh. Salt marshes cover less than 1% of the Earth’s coastal regions, from polar to tropical areas, and exhibit high primary productivity (Chmura et al., 2003; Ewers Lewis et al., 2018; Hopkinson et al., 2012). The marshes are known to span an area of 22,000 km² to a maximum of 400,000 km² globally (Chmura et al., 2003; Hopkinson et al., 2012). However, these wetland ecosystems have gradually transitioned to terrestrial habitats by human activities such as agriculture and urbanization. The wetland area has decreased by more than 50% compared to 100 years ago (Belluco et al., 2006; Finlayson and Davidson, 1999; Gallant et al., 2007; Mahdavi et al., 2018). The salt marshes of South Korea have been under constant water pressure from the tidal, and the salt marshes area in Korea has continuously decreased (Cho et al., 2015). In Korea, the conversion of salt marshes to agricultural lands began in the 1930s, and wetlands loss has accelerated further due to additional conversion for industrial development since 1980s (Im et al., 2020).

Recently, mangrove, salt marsh and seagrass ecosystems are globally recognized as major blue carbon sources (Chmura et al., 2003; Duarte et al., 2005; Ewers Lewis et al., 2018; Kristensen et al., 2008). Blue carbon refers to carbon absorbed and stored by vegetation and sediments in marine ecosystems with higher carbon absorption and storage efficiency compared to the terrestrial ecosystems (Lovelock and Reef, 2020; Pendleton et al., 2012). These blue carbon sources absorb carbon dioxide from the atmosphere through photosynthesis and fix carbon into their tissues and sediment (Mcleod et al., 2011). The highest local carbon storage rate value, which is the storage rate of carbon per unit area, was shown in salt marshes (Mcleod et al., 2011). When the comparing net primary production (NPP), the salt marsh had the highest NPP value per unit area among the blue carbon sources (Melillo et al., 1993, Gu et al., 2022). Therefore, the salt marshes have the highest local carbon storage rate and primary productivity among the blue carbon ecosystems and potential for high carbon sequestration (Chmura et al., 2003).

There has been a growing recognition of the importance of salt marsh in providing ecosystem services and sequestering carbon and nitrogen, leading to increased efforts in salt marsh restoration. In salt marshes, the plants can germinate, grow and reproduce in salty soils via physiological mechanisms to remove salt from their tissues (Min, 1998; Waisel, 2012). In Korea, approximately 13 genera of salt marsh plants inhabited (Lee et al., 2019). Among them, Suaeda japonica, Phacelurus latifolius, Carex scabrifolia, Suaeda glauca and Phragmites australis are commonly found in Korea, with Su. japonica being particularly dominant along the west coast (Bang et al., 2018; Lee and Ihm, 2004). Su japonica is distributed at the inner side of salt marshes. Su. japonica primarily grows in coastal habitats with higher water content and salinity compared to other species (Lee et al., 2009). In addition, the Suaeda genus utilizes both C3 and C4 processes that can fix carbon by photosynthesis (Shomer-Ilan et al., 1975). In particular, carbon dioxide fixation by C4 shows high efficiency even in environments with heat, dryness, and salt stress (Haque et al., 2021).

Seedling of salt marsh plant is necessary for salt marsh restoration projects, but research on the growth characteristics of the plants currently found in Korea is very lacking. This study aimed to analyze the potential carbon and nitrogen sequestration capacity of Su. japonica by evaluating changes in vegetation and analyzing photosynthetic efficiency, as well as tissue carbon and nitrogen contents of this plant in different seasons and sites. In this study, biomass varied depending on the season and site. The plant density of Su. japonica affected survival, growth and photosynthetic efficiency, with the carbon content being higher in stems and roots, while the nitrogen content was higher in leaves.

Living in salt marshes requires the ability to adapt to seasonal fluctuations in salinity as well as other environmental challenges such as precipitation, evaporation, and seawater inflow (Waisel, 2012). First of all, while temperature and humidity varied with the seasons, insolation, wind and sediment water content remained unchanged ( Table 1 ). In addition, the dissolved inorganic nitrogen (DIN) dissolved inorganic phosphorus (DIP), and salinity were measured by Incheon Metropolitan City Health and Environment Research Institute (https://www.incheon.go.kr/ecopia/) at the nearest locations, approximately 2 and 9 km from YJ and SR, respectively. The DIN, DIP and salinity of seawater flowing into YJ and SR showed no differences in summer and autumn. However, in spring, the DIN concentration was nearly twice as high in YJ (760.7 μg/L) compared to SR (419.2 μg/L). On the other hand, the present study revealed that the plant density at SR was 851 individuals m^-2 in spring and significantly decreased to 213 individuals m^-2 in autumn. In contrast, YJ, which had a lower initial plant density did not show any significant changes between spring and autumn. The observed difference in plant density between YJ and SR is likely attributed to variations in seawater inflow rather than other environment conditions. The YJ site is directly affected by tidal action, with the seawater flowing in and out twice a day. In contrast, the SR site, an artificial wetland ecological park experiences flooding only when the high tide level exceeds 9 m, occurring approximately 2-3 times per month (Park et al., 2023).

Plant density also affects the plant survival and growth (Johnson et al., 2017; Comita et al., 2014; Terborgh, 2012; Lu et al., 2015). In general, as plant density increases, the number of branches decreases (Leach et al., 1999). In this study, S. japonica of SR grew thinner and elongated, whereas the plants at YJ grew thicker and wider with more branches ( Figure 4 ). A previous study was conducted to determine changes in the biomass of Suaeda salsa in relation to plant density (Xu et al., 2023). Initially, when the plant density was 30, 40, 50 and 60 plants m^-2, the height of S. salsa increased with plant density. However, as the plants grew, their height, stem diameter, canopy width and biomass decreased with increasing plant density (Xu et al., 2023).

Photosynthesis was also affected by the plant density. In summer, the values of ETRmax and Ik of S. japonica significantly decreased at SR compared to spring. SR showed lower light saturation than YJ in both summer and autumn. The lower photosynthetic rate is due to light interruption by higher plant density, which reduces the electron transfer rate and light saturation. For example, Atriplex prostrata, an annual plant in the Amaranthaceae family, showed increased intraspecific competition at a higher plant density (Wang et al., 2005). The net photosynthetic rate of this plant decreased from 7.6 ± 0.9 μmol CO₂ m^-2 s^-1 at a lower plant density to 3.5 ± 0.4 μmol CO₂ m^-2s^-1 at a higher density (Wang et al., 2005). Short-term light inhibition reduces electron transfer rate or light saturation, while long-term light restrictions can inhibit plant growth, leaf area expansion, leaf gas exchange and biomass increase (Ballaré et al., 1994; Ren et al., 2017; Wells, 1991).

In Yeongjong (YJ) and Sorae (SR) salt marshes, seasonal changes were observed from May to October ( Table 1 ). In addition, Suaeda japonica showed some morphological changes across seasons. In spring, when new plants begin to grow, the stems and leaves are green and red. As summer and autumn progresses, the plant develops many branches and turns predominantly red. This color change is due to the accumulation of betacyanin, a red pigment, in the plants during summer to autumn (Hayakawa and Agarie, 2010; Akbar Hussain et al., 2018; Azeredo, 2009). Hayakawa and Agarie (2010) reported that in S. japonica, chlorophyll content sharply decreased in autumn compared to summer, while betacyanin contents increased significantly in autumn. As the chlorophyll content of the leaves decreases, photosynthetic efficiency can also be significantly reduced.

Salt marshes called “blue carbon”, absorbed carbon through primary production and plays an important role in sequestering carbon dioxide in sediment (Duarte et al., 2013). In this study, the dry weight of the underground parts was 7-19% of whole plant ( Figure 5 ). Photosynthetic rates were higher in spring and summer than in autumn, but the carbon content in stems and roots (41.3-42.8%) were higher in autumn than in spring (34.7-38.1%). These results suggest that carbon absorbed through photosynthesis in leaves is transferred to and stored in stems and roots. In Table 1 , when the TOC content of surface sediments was analyzed by seasons, a higher content (0.35-38%) was observed in the autumn as biomass of plants was high. Additionally, sedimentation rates of 1.53 and 2.53 cm were observed annually in YJ and SR, respectively. As a result, the carbon stored in the underground parts and dead tissue of the plants can be sequestered in sediments.

Tissue carbon content was higher in the roots (34.7-42.8%) compared to other parts, while nitrogen content was significantly higher in leaves (0.92-1.70%) than in the roots and stems (0.32-0.92%). Both YJ and SR showed higher nitrogen content in all part in spring than other seasons (p < 0.05). In spring, the nitrogen content of leaves in YJ was higher than at SR (p < 0.001), but there was no significant difference in autumn (p > 0.05). Cartaxana and Catarino (1997) compared nitrogen content in different parts of Spartina maritima, Halimione portulacoides and Arthrocnemum perenne and found higher nitrogen content in the leaves probably because leaves are the major plant part for photosynthesis. These results suggest that chlorophylls in leaves are a major storage for nitrogen. When S. japonica dies in late autumn, the stored nitrogen in its tissues can be buried underground and accumulated in sediments.

Carbon sequestration by S. japonica did not differ between sites in spring and summer. In autumn, however, the amount of carbon sequestration by S. japonica was much higher at YJ (114 g C m^-2) than SR (65 g C m^-2) ( Table 2 ). Nitrogen sequestration also did not differ in spring and summer, but the nitrogen sequestration of YJ (3.0 g N m^-2) was higher compared to SR (1.2 g N m^-2). The area of the salt marsh ecosystem at YJ and SR is approximately 3,905,000 m² and 3,500,000m², respectively. Considering these areas, along with the tissue carbon and nitrogen contents of the plants, the total carbon and nitrogen sequestration is estimated to be approximately 440 (1,630 CO₂) tons and 11 tons, respectively at YJ and 230 (830 CO₂) tons and 4 tons, respectively at SR. These values are very significant, approximately 136.0% of total organic carbon and 1.6% of total nitrogen emitted by the closest wastewater treatment plant (WTP) from SR, Seunggee WTP, Incheon in 2022 (Environmental Corporation of Incheon, 2024).

Yeongjong (YJ) experiences tidal inflow and outflow twice a day, whereas Sorae (SR), an artificial ecological park, is affected by tides only 2-3 times a month. The differences in seawater inflow may have influenced the plant density and biomass of Suaeda japonica. Consequently, in spring, the plant density at SR was approximately four times higher than at YJ. However, the higher plant density led to morphological variations and inhibited the survival, growth and photosynthetic efficiency of S. japonica. At SR, both the plant density and photosynthetic efficiency significantly decreased in summer compared to spring. S. japonica at YJ (lower density) grew wider with many branches, while at SR (higher density), the plants grew longer with fewer branches. These morphological differences were reflected in their carbon sequestration capacity. YJ exhibited a higher carbon sequestration per unit area, approximately 113.70 g/m², which was twice that of SR. Tissue nitrogen content was higher in leaves than in roots or stems, while tissue carbon content was higher in roots and stems than in leaves. The high carbon content in roots and stems suggest that carbon absorbed via photosynthesis in leaves can be transferred and stored underground. This study emphasizes the critical role of salt marshes as blue carbon resources and advocates for continued and enhanced restoration efforts. By quantifying the carbon sequestration capabilities of S. japonica, we provide valuable insights that support the need for targeted conservation and restoration projects to mitigate climate change impacts.