Stocks and Sources of Soil Carbon and Nitrogen in Non-Native Kandelia obovata Afforestation and Spartina alterniflora Invasion: A Case Study on Northern Margin Mangroves in the Subtropical Coastal Wetlands of China

Abstract

For decades in China, carbon neutrality policies have spurred the establishment of northern margin mangroves as artificial blue carbon ecosystems. However, there has been limited research on the impact of plantation and invasion on the stocks and sources of soil carbon and nitrogen in rehabilitated coastal wetlands. Non-native Kandelia obovata afforestation began on Ximen Island, Zhejiang, China, where Spartina alterniflora invasion had also occurred decades ago. Soil cores were collected from both mangrove and salt marsh habitats with depths from 0 to 50 cm and were analyzed for total carbon (TC), soil organic carbon (SOC), total nitrogen (TN), and the isotope of carbon and nitrogen in sediments. The results indicated that there were no significant differences in the TC, SOC, and C/N ratio between the K. obovata and the S. alterniflora, but there were significant differences in TN, isotope δ¹³C, and δ¹⁵N. The SOC content of both ecosystems in the 0–20 cm layer was significantly higher than that in the 30–50 cm layer. Our study has shown that the main sources of carbon and nitrogen for mangroves and salt marshes are different, especially under the impact of external factors, such as tidal waves and aquaculture. These findings provide insight into the ecological functioning of subtropical coastal wetlands and an understanding of the biogeochemical cycles of northern margin mangrove ecosystems.

1. Introduction

Coastal wetlands are being preserved, restored, and created globally for climate mitigation, especially following the signing of the Paris Agreement [1,2]. They are recognized for their vital ecological functions, serving as critical habitats for various species, nature’s nurseries, defenses against storm surges, frontiers for protecting against coastal erosion, nutrient reservoirs, and carbon banks [3,4,5]. In addition, mangroves, which contain 10–15% of the organic carbon in the global ocean, are identified as blue carbon ecosystems, along with coastal vegetation ecosystems such as salt marshes and seagrass beds, despite accounting for only 0.5% of the world’s ocean area [6,7]. The distribution of mangroves closely aligns with temperature requirements, and historically, the cold winters in Zhejiang Province have prevented the natural establishment of mangroves [8]. Kandelia obovata, identified as the most cold-tolerant among mangrove species and representing the northernmost limit of intentionally planted mangroves, was introduced to Ximen Island, Yueqing Bay, Zhejiang Province, in 1957 [9]. After decades of trials, K. obovata has become one of the dominant surviving species. S. alterniflora was introduced for shore protection in the 1990s. Consequently, the northern margin mangrove (NMM) has been experiencing both intentional afforestation and inadvertent invasion by salt marsh plants like S. alterniflora [10]. As a mono-system, the NMM faces challenges from nature and human activities, such as climate change, typhoons, diseases, and pollution from aquaculture [8,10,11]. The invasion of S. alterniflora has emerged as one of the key issues. These activities continue to be crucial for mangrove rehabilitation, yet their effects on soil carbon and nitrogen stocks were not well understood in the past [12].

Mangroves are naturally highly efficient carbon sinks and long-term pools. Soil carbon accumulation in mangrove ecosystems is a crucial component of overall coastal carbon sequestration [13,14]. The soil surface layer plays an important role in the soil carbon pool. However, its biogeochemical cycles are impacted by environmental conditions, such as climate, invasion, and nutrient limitation. The carbon content within mangrove ecosystems primarily originates from upstream river inflow, precipitation, and marine tidal influences [7,15]. The intricate root systems of mangroves play a pivotal role in sequestering carbon, effectively serving as carbon reservoirs [16]. Simultaneously, mangroves act as food sources, supplying substantial organic matter and nutrients to the surrounding biota, thereby facilitating nutrient cycling [17]. Understanding the carbon stock and sources of mangrove afforestation is key for long-term nature-based solutions, especially those targeted at climate change. Previous studies have shown that soil carbon stock decreases as latitude moves north for natural mangroves [13,16].

Like carbon, nitrogen is an indispensable key growth element within mangrove ecosystems [18]. An excess or deficiency of nitrogen, such as in eutrophic or nutrient-deficient conditions, can impede mangrove development [19]. Mangroves function as both nitrogen reservoirs and nitrogen sources, contributing to the maintenance of overall nitrogen cycling through processes like nitrogen fixation, denitrification, nitrification, anaerobic ammonium oxidation, and nitrogen mineralization [20]. Through nitrogen fixation, they can ultimately bury anthropogenic nitrogen (from sources such as aquaculture, domestic sewage, and industrial waste) in sediments for extended periods [21]. These processes are vital for sustaining mangrove productivity, ecological protection, and restoration efforts [22].

The carbon and nitrogen cycles in mangrove and salt marsh ecosystems are inextricably linked, driving the functioning and resilience of these wetlands [20]. This interplay between the two cycles is a key feature of these ecosystems and is essential to their ecological functions. Understanding these cycles is fundamental to comprehending the functioning and resilience of these critical coastal ecosystems, particularly in the context of non-native plant species and ecological changes, such as the invasion of S. alterniflora and afforestation with K. obovata in the subtropical coastal wetlands of China [13].

The objective of this study is to evaluate and compare soil carbon and nitrogen stocks and their sources in the topsoil of areas where non-native K. obovata afforestation and S. alterniflora invasion have occurred in the subtropical coastal wetlands of China. Specifically, this research aims to (1) assess disparities in soil carbon and nitrogen stocks between K. obovata afforestation areas and S. alterniflora-invaded regions; (2) identify the primary sources contributing to these stocks in each type of area. By addressing these research aims, this study endeavors to advance our understanding of how different vegetation types influence biogeochemical cycles. The outcomes hold the potential to inform mangrove conservation and rehabilitation strategies, not only in China but also in regions undergoing analogous ecological transitions [23].

2. Materials and Methods

2.1. Study Area

Yueqing Bay, located on the northern fringes of the Oujiang River estuary in southern Zhejiang, China (27°5′–28°23′ N, 120°57′–121°16′ E), is characterized by its exceptional natural surroundings, which have facilitated the development of marine aquaculture as one of the major economic sectors in the area [24] (Figure 1). As a semi-enclosed tidal bay, Yueqing Bay is significantly influenced by tidal dynamics, exhibiting a maximum tidal range of 8.34 m and an average tidal range of 4.2 m. This bay is subject to a subtropical maritime monsoon climate, featuring an average annual temperature of 17.9 °C, average annual precipitation of 1556.3 mm, and an average annual frost-free period of 258 days (http://www.yueqing.gov.cn/, accessed on 15 August 2023).

Ximen Island (28°20’ N, 121°10’ E), located in the northern region of Yueqing Bay within Yueqing City, Zhejiang Province, was designated as the first national marine special reserve in Zhejiang Province in 2005. Since 2006, it has received significant national funding aimed at the plantation and rehabilitation of mangroves. According to data from the Zhejiang Provincial Forestry Bureau, the Zhejiang Provincial Government has invested RMB 7.6668 million in the “Ximen Island Mangrove Transplantation Project”, which was completed in 2013 and 2015. The mangrove species K. obovata, introduced from Fujian Province on five occasions, now covers approximately 5% of the coastal wetlands on the island [25]. However, since 1999, S. alterniflora has expanded rapidly in Yueqing Bay, dominating most of the mudflats on Ximen Island [26]. Consequently, clearing S. alterniflora and planting and protecting mangroves have emerged as significant conservation tasks in this area. Currently, both K. obovata and S. alterniflora coexist in the intertidal zone of Ximen Island, necessitating a comprehensive examination of their soil carbon and nitrogen dynamics.

2.2. Soil Sampling and Analysis

In May 2021, soil sampling was conducted in both the K. obovata mangrove area and the S. alterniflora salt marsh area on Ximen Island. At each sampling site, three replicate 1 m × 1 m sampling plots were established randomly with a minimum separation of 100 m between plots. Surface litter was removed from each site before collecting six soil cores, each with a depth of 50 cm and diameter of 5 cm, using Russian Sediment/Peat Borer. The cores were then sectioned into seven layers at depths of 0–5 cm, 5–10 cm, 10–15 cm, 15–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm in the field. A total of 42 soil samples were collected, placed into plastic bags, refrigerated, and transported to the laboratory.

In the laboratory, the soil samples underwent freeze-drying and were cleared of impurities such as gravel and roots. Soil samples from the same layer of three cores within the community area were mixed to create a composite sample. These composite soil samples were then ground, passed through a 100-mesh sieve, and air-dried. Total carbon (TC) and total nitrogen (TN) contents were determined using the dry combustion method (Vario PYRO cube, Elementar, Langenselbold, Germany). To analyze the soil organic carbon (SOC) content and stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), inorganic carbon was removed by acidification with 1 mol L⁻¹ hydrochloric acid, followed by drying at 60 °C. Finally, the SOC content and stable isotopes were measured using elemental analysis coupled with stable isotope mass spectrometry (Elementar isoprime100, Langenselbold, Germany). The isotopic compositions values of δ¹³C and δ¹⁵N were expressed as follows:

$$\delta (\mathrm{‰})=\left[\left(\frac{R_{sample}}{R_{\mathit{standard}}}\right)-1\right]\times 1000$$

(1)

where R_sample is the ¹³C/¹²C or ¹⁵N/¹⁴N isotope ratio of the sample, and R_standard is the δ¹³C characteristic value relative to the international standard material Vienna-Pee Dee Belemnite (V-PDB) for carbon and the δ¹⁵N characteristic value relative to atmospheric nitrogen for nitrogen. The C/N ratio is expressed as the ratio of soil organic carbon to total nitrogen content. The calculation of organic carbon stock and nitrogen stock within each soil layer, spanning depths of 0–50 cm, was performed using the following equations:

$$SO{C}_{stock}=\frac{BD\times SO{C}_c\times D}{10}$$

(2)

$$T{N}_{stock}=\frac{BD\times T{N}_c\times D}{10}$$

(3)

where SOC_stock is the soil organic carbon stock (Mg C_org ha⁻¹), and BD is assumed based on a reference value of 0.85 g cm⁻³ from previous relevant studies [25,27]. SOC_c is the soil organic carbon content (g kg⁻¹), and D is the thickness of the soil layer (cm). TN_stock is the soil nitrogen stock (Mg N ha⁻¹) and TN_c is the soil total nitrogen content.

2.3. Data Analysis

Statistical analysis began with the evaluation of normality for soil parameter data from mangrove and S. alterniflora habitats, utilizing the Shapiro–Wilk test. For data adhering to a normal distribution, Student’s t-test for independent samples was applied to identify differences between groups. Conversely, for data not following a normal distribution, the Mann–Whitney U test was utilized. Pearson correlation coefficients were calculated to explore the relationships among soil SOC, TIC, TN, and C/N ratio. A significance level of p < 0.05 was adopted to determine statistically significant differences. All statistical analyses were conducted using IBM SPSS Statistics (version 27.0.1.0).

3. Results

3.1. Vertical Distribution Characteristics of Soil TC, SOC, and TN

The mean TC and SOC contents in the soil samples were 13.26 ± 0.39 g Kg⁻¹ and 6.60 ± 0.15 g Kg⁻¹, respectively. The TC contents ranged from 8.7 g Kg⁻¹ to 21.7 g Kg⁻¹ in mangrove soils and ranged from 9.6 g Kg⁻¹ to 16.2 g Kg⁻¹ in S. alterniflora soils. Vertically, the TC content of mangrove soil exhibited an upward trend from the profile base to the surface and was higher than those in S. alterniflora soils at the depths of 0–10 cm and 15–20 cm (Figure 2A). However, no significant difference was observed in TC content between mangrove soil (13.36 ± 0.73 g Kg⁻¹) and S. alterniflora soil (13.15 ± 0.30 g Kg⁻¹). The contents of SOC in mangrove and S. alterniflora soils were 5.3 g Kg⁻¹–9.3 g Kg⁻¹ (6.40 ± 0.21 g Kg⁻¹) and 5.7 g Kg⁻¹–8.7 g Kg⁻¹ (6.80 ± 0.20 g Kg⁻¹), respectively. The SOC content of S. alterniflora soil decreased with depth and was higher in the surface layer than in mangrove soils (Figure 2C).

The average TN content in the soil samples was 1.00 ± 0.03 g Kg⁻¹. The TN contents in mangrove and S. alterniflora soils were 0.6 g Kg⁻¹–1.4 g Kg⁻¹ (0.95 ± 0.05 g Kg⁻¹) and 0.8 g Kg⁻¹–1.3 g Kg⁻¹ (1.06 ± 0.03 g Kg⁻¹), respectively. TN content showed a decreasing trend with soil depth, with mangrove soil exhibiting higher TN content than S. alterniflora soil only at the 15–20 cm layer (Figure 2B).

3.2. Vertical Distribution Characteristics of Soil δ¹³C, δ¹⁵N, and C/N Ratio

The δ¹³C values of the collected soil samples ranged from −24.38‰ to −20.53‰, with a mean of −21.93 ± 0.14‰ (

Table 1. Normality test of soil composition variables: carbon, nitrogen, and isotopic ratios in coastal wetlands on Ximen Island, Zhejiang, China.

Variable	Mean	Standard Error	n	Skewness	Kurtosis	Shapiro–Wilk Test of Normality
						Statistic W Value	p
Total Carbon (%)	1.33	0.04	42	1.04	2.20	0.931	0.014 *
Soil Organic Carbon (%)	0.66	0.01	42	1.21	0.98	0.884	0.000 **
Total Nitrogen (%)	0.10	0.00	42	0.00	−0.47	0.972	0.377
C/N	6.75	0.21	42	0.97	1.59	0.934	0.018 *
δ13C (‰)	−21.93	0.14	42	−1.05	1.36	0.917	0.005 **
δ15N (‰)	3.47	0.07	42	0.73	1.53	0.954	0.092

Note: * p < 0.05 ** p < 0.01.

). And the δ¹³C values of mangroves and S. alterniflora soils ranged from −24.38‰ to −21.02‰ and −24.09‰ to −20.53‰, respectively. In vertical distribution, the δ¹³C values all showed a decrease followed by an increase with the increase in soil depth. Except for the 15–20 cm and 40–50 cm depths, the δ¹³C values of S. alterniflora were higher than those of mangroves (Figure 2E). The δ¹⁵N values of mangrove soil and S. alterniflora soil ranged from 3.11‰ to 3.93‰ and 2.52‰ to 4.85‰, respectively, while showing an initial increase from the profile base to the 10–15 cm depth, followed by a decrease towards the surface layer (Figure 2F).

The average C/N ratio of the collected soil samples was 6.75 ± 0.21 (Table 1). The C/N ratios of mangrove and S. alterniflora soils were 4.29–10.83 (7.07 ± 0.39) and 5.27–7.91 (6.42 ± 0.16), respectively. The C/N ratio of mangrove soil showed significant fluctuations from 30 cm to 10 cm, peaking at 15 cm depth. However, the C/N ratio in S. alterniflora soils slightly decreased with increasing soil depth (Figure 2D). No significant difference was found in the C/N ratio between the soils of the two vegetation soils.

3.3. Differences between Ecotypes and Soil Layers

Among the soil composition variables analyzed, only TN content and δ¹⁵N were normal distribution, whereas TC, SOC content, C/N ratio, and δ¹³C values exhibited non-normal distribution (Table 1). In terms of vegetation types, the SOC content in salt marsh soil (0.68 ± 0.02%) was higher than in mangrove soil (0.64 ± 0.02%) (p < 0.05), and the TN content in salt marsh soil (0.11 ± 0.003%) was also significantly greater than in mangrove soil (0.09 ± 0.005%) (p < 0.05) (Figure 3A,B). The difference in the δ¹³C value between mangrove soil (−22.33 ± 0.18‰) and salt marsh soil (−21.54 ± 0.17‰) was very significant (p < 0.01) (Figure 3C). However, the δ¹⁵N value did not significantly differ between mangrove soil (3.51 ± 0.05‰) and salt marsh soil (3.43 ± 0.14‰) (Figure 3D).

Analyzing the soil profile stratification, the difference in the SOC content between the soil surface layer (0–20 cm) (0.69 ± 0.02%) and soil subsurface layer (30–50 cm) (0.61 ± 0.01%) was very significant (p < 0.01) (Figure 3E), with higher values in the surface layer. Similarly, TN content was also significantly higher in the soil surface layer (0.11 ± 0.004%) compared to the soil subsurface layer (0.09 ± 0.004%) (p < 0.05) (Figure 3F). Nevertheless, there were no significant differences between the δ¹³C values (−22.13 ± 0.21‰) and δ¹⁵N values (3.57 ± 0.11‰) of the soil surface layer and the δ¹³C values (−21.66 ± 0.14‰) and δ¹⁵N values (3.33 ± 0.08‰) of the soil subsurface layer (Figure 3G,H).

In the 0–50 cm soil layer, the SOC stock of S. alterniflora (28.14 ± 0.35 Mg C_org ha⁻¹) was higher than that of mangrove soils (26.76 ± 0.82 Mg C_org ha⁻¹) (Figure 4), although this difference was not statistically significant. Similarly, no significant difference was found in TN stock between two vegetation types, with mangrove soils having a TN_stock of 3.95 ± 0.02 Mg N ha⁻¹ compared to 4.42 Mg N ha⁻¹ in S. alterniflora soils.

4. Discussion

4.1. Stocks and Sources of Soil Carbon in Non-Native K. obovata Afforestation and S. alterniflora Invasion

Previous studies have indicated that the carbon pools in mangrove ecosystems increase with age, peaking in mature mangroves [16]. Compared with other subtropical mangrove species, such as Avicennia marina and Aegiceras corniculatum, K. obovata exhibits relatively weaker carbon fixation capabilities (

Table 2. Stocks and sources of soil carbon and nitrogen in subtropic coastal wetlands of China.

Subtropic Coastal Wetlands		Latitude	Dominate Species	Soil Depth	Soil Organic Carbon	Soil Total Nitrogen	C/N	δ13C	δ15N	References
				cm	Mg C ha−1	Mg N ha−1		‰	‰
Native + Invasive species	Mangrove	23°53′ N	Kandelia obovate, Avicennia marina, and Aegiceras corniculatum	0–30	65.6	5.6	5.62~20.04	−28.42~−21.74	3.95~7.59	[31]
		24°24′ N	K. obovata	0–60	39.7	4.5				[29]
		23°53′ N	S. alterniflora	0–30	52.1	5.4	6.82~14.62	−23.02~−18.15	4.51~7.54	[31]
		24°24′ N	S. alterniflora	0–60	39.5	4.9				[29]
	Mudflat	23°53′ N		0–30	33.3	4.1	7.17~26.25	−25.70~−23.21	3.53~5.72	[31]
	Mudflat	24°24′ N		0–60	41	4.45				[29]
Non-native + Invasive species	Northern Margin Mangrove	28°20′ N	K. obovata	0–100	65.53		6.7	−22.30		[27]
		28°20’ N	K. obovata	0–50	26.76	3.95	4.29~10.83	−24.38~−21.02	3.11~3.93	This study
		28°21′ N	S. alterniflora	0–100	71.72					[27]
		28°20’ N	S. alterniflora	0–50	28.14	4.42	5.27~7.91	−24.09~−20.53	2.52~4.85	This study
	Mudflat	28°20′ N		0–100	69.52			−21.6~−22.5		[27]
Invasive species	Salt marsh	31°30′ N	Phragmites australis	0–20		1.6	19.7			[32]
		32°48′ N	Phragmites australis	0–30	16.84			−24.97	2.37	[33]
		31°30′ N	S. alterniflora	0–20		1.78	18.6			[32]
		32°48′ N	S.alterniflora	0–30	37.83			−18.41	3.93	[33]
	Mudflat	32°48′ N		0–30	4.49			−21.11	0.48	[33]

) [28]. Nonetheless, some studies show that the impacts of S. alterniflora invasion and mangrove restoration on soil carbon stocks are different (Table 2) [29,30]. Gu et al. [30] suggested that both S. alterniflora invasion and mangrove restoration contribute to increased soil carbon stocks, whereas Feng et al. [29] reported no significant changes in soil organic carbon stocks as a result of these interventions.

In this study, the SOC stocks between mangrove and S. alterniflora soils did not significantly differ, aligning with results observed in Zhangjiakou but diverging from those in the Jiulong River estuary, Fujian Province [34,35]. Moreover, the SOC content in both vegetation types was found to be lower than that reported for Qi’ao Island, Guangdong Province [36]. Similarly, the soil organic carbon stocks in mangroves from this study were lower than those in natural mangroves in Zhangjiangkou, Fujian, and SOC stocks in S. alterniflora were also lower compared to those reported in other regional studies (Table 2). This discrepancy could be attributed to variations in latitude, geographical location, and climate conditions [37]. The mangroves in the study area, being recently restored, are sparse and immature, leading to reduced soil carbon accumulation. Additionally, tidal erosion may contribute to the loss of surface litter, further affecting carbon stock levels.

In this investigation, salt marsh soils exhibited significantly higher δ¹³C values compared to mangrove soils, consistent with findings reported by Xia et al. (Figure 5) [20]. Wetland soil organic matter (SOM) primarily originates from two sources: allochthonous inputs, such as river and ocean contributions, and autochthonous production. Allochthonous sources, enriched with nutrients from riverine inputs, are typically more substantial contributors to wetland soil SOC than autochthonous origins [31]. The differentiation between terrestrial and aquatic plant sediment sources has been effectively achieved through the analysis of stable carbon isotopes (δ¹³C) and C/N ratios [38]. C3 plants (like mangroves) have δ¹³C values ranging from −34‰ to −23‰, C4 plants (like S. alterniflora) range from −17‰ to −9‰, and marine phytoplankton range from −17‰ to −23‰ [39,40,41]. Terrestrial plants predominantly consist of high-nitrogen compounds like lignin and cellulose, leading to C/N ratios above 12, in contrast to marine phytoplankton, which typically possess C/N ratios below 10 [42,43].

In contrast to Hu et al.’s findings on plant tissue δ¹³C and C/N ratios on Ximen Island, our results indicate that mangrove soils feature a higher δ¹³C value (−22.33 ± 0.18‰) and a lower C/N ratio (7.07 ± 0.39), whereas S. alterniflora soils present a lower δ¹³C value (−21.54 ± 0.17‰) and C/N ratio (6.42 ± 0.16). This pattern suggests a reduced contribution of plant tissues to the soil carbon pool, implying a significant influence of marine phytoplankton on soil organic matter composition [27,44]. Additionally, the δ¹³C value in the mangrove soil surface layer (0–20 cm) was found to be lower than in the subsurface layer, indicating the substantial role of mangrove litter decomposition in contributing to the SOC of the surface layer [27]. Tidal dynamics on Ximen Island facilitate the influx of external carbon sources, supporting evidence from studies along the Zhejiang coast of the East China Sea that muddy surface sediments primarily derive from marine organic matter [45]. Furthermore, human activities, such as aquaculture, may also contribute to the carbon sources in vegetation soils. Further research is warranted to explore the impact of such activities on the carbon sources of mangrove and S. alterniflora soils on Ximen Island.

4.2. Stocks and Sources of Soil Nitrogen in Non-Native K. obovata Afforestation and S. alterniflora Invasion

In both vegetation types, a positive correlation was observed between soil TN and TIC contents (r = 0.73, p < 0.01), whereas no significant correlation was found with SOC content (Figure 6), suggesting that mangroves and S. alterniflora may share similar soil nutrient sources. The invasion of S. alterniflora has been shown to significantly enhance the TN content in the soil of sparse mangrove wetlands, aligning with findings from previous research [34,46]. This increase can be attributed to S. alterniflora’s capacity to utilize ammonium and nitrate nitrogen forms less accessible to other wetland species and its ability to alter the nitrogen-fixing microbial community, thus exhibiting a higher nitrogen fixation rate compared to native plants [47,48,49]. Nitrogen, a critical limiting nutrient for S. alterniflora, finds an abundant source in the extensive aquaculture practices on Ximen Island, facilitating its growth [50,51].

The δ¹⁵N values in the surface layers of both mangrove and S. alterniflora soils were found to be higher than those in the subsurface layers (Figure 5), a finding that corroborates the study by Wang et al. but contrasts with Xia et al. [20,31]. This discrepancy may result from more intense microbial decomposition activity in the surface layer, whereas subsurface δ¹⁵N enrichment is mitigated by temperature and tidal influences. As an indicator of soil organic matter quality, the δ¹⁵N value increases with the progression of organic matter decomposition and humification [32]. Although the δ¹⁵N value in S. alterniflora soils was slightly lower than in mangrove soils, the significantly higher TN content suggests a greater contribution of external inputs to soil organic matter.

5. Conclusions

This study on the soil carbon and nitrogen stocks and sources within the subtropical coastal wetlands of Yueqing Bay, China, especially in areas affected by the afforestation of non-native K. obovata and the invasion of S. alterniflora, provides essential insights into the biogeochemical processes shaping these ecosystems. Our findings reveal notable similarities in soil carbon and nitrogen stocks between K. obovata and S. alterniflora habitats, indicating that the ecological impacts of these two vegetation types on soil biogeochemical properties are comparable. The observed vertical distribution patterns of TC, SOC, TN, δ¹³C, and δ¹⁵N within the soil profiles highlight complex biogeochemical interactions, modulated by both natural processes and human activities. Merging biogeochemical research with an evaluation of ecosystem services could elucidate the economic and societal advantages of wetland conservation and restoration efforts. This integrative approach offers a persuasive rationale for policy endorsement, aimed at rejuvenating and expanding natural blue carbon ecosystems.