Blue Carbon Sequestration Potential of Soils in Degraded Mangroves of Sundarban, India: A Geochemical Approach ()
1. Introduction
Mangrove ecosystems, situated in the intertidal zones of tropical and subtropical regions, are globally recognized for their remarkable ability to sequester carbon, commonly referred to as blue carbon (Alongi, 2012; Kauffman et al., 2020). These ecosystems play a critical role in mitigating climate change by capturing atmospheric carbon dioxide (CO2) and storing it within their biomass and soils (Choudhary et al., 2024; Donato et al., 2011). The Sundarbans are the world’s largest contiguous mangrove forest spanning approximately 10,200 km2, of which 38% is in India and remaining in Bangladesh (Spalding, 2010). This area is characterized by high biodiversity, extensive tidal channels, and mudflats that are influenced by dynamic tidal fluctuations, which affect biogeochemical processes important to carbon sequestration (Al Mahmud et al., 2024; Padhy et al., 2023). Sundarban mangroves contribute substantially to regional and global carbon cycles with estimated carbon sequestration rates ranging from 4.71 to 6.54 Mg C ha−1 yr−1 (Ray et al., 2021). Key areas include the South 24 Parganas Reserve Forest, spanning 1678 km2, and the Sundarban Tiger Reserve, covering 2585 km2. Nearly 13% of these mangroves have been degraded during 1930-2024 due to climate change vagaries and land conversion for rice cultivation and aquaculture systems. This degraded mangrove system, interspersed with rice paddies and aquaculture ponds and subjected to intense anthropogenic pressure, represents a unique ecological matrix prevalent in the Indian Sundarban region. Due to the dense human population and associated socio-demographic constraints, restoring these degraded areas to pristine or core mangrove forests is extremely challenging. Moreover, as the Sundarbans have been designated as a reserve forest, the current land-use configuration—comprising degraded mangroves, rice fields, and aquaculture—is expected to persist in the coming decades. Over the last century, 40% of tropical mangroves globally have been lost, with a significant portion becoming ecologically degraded (Padhy et al., 2020). The Indian Sundarban mangroves constitute approximately 2.84% of the world’s total mangrove area. The Sundarbans biosphere acts as a net carbon sink, sequestering 2.79 teragrams of carbon (Tg C) annually, with around 96% of this carbon stored in below-ground biomass, while the remainder is retained in above-ground biomass (Nayak et al., 2024; Ray et al., 2013).
Although degraded mangrove ecosystems have lower carbon sequestration capacity than core or undisturbed mangroves, they still exhibit higher carbon storage potential than adjacent rice-aquaculture systems. Several studies (Padhy et al., 2021; Royna et al., 2024) indicate that with sustained conservation efforts and replantation strategies, even degraded mangrove systems can outperform rice-aquaculture landscapes in terms of carbon storage. Hence, this mosaic of degraded mangroves, though suboptimal compared to pristine forests, holds relatively higher carbon sequestration potential than surrounding non-mangrove land uses.
However, the geochemical processes influencing the stabilization of soil organic carbon (SOC), particularly through trace elements and elemental oxides, are not fully understood. Elemental oxides like sodium oxide (Na2O), magnesium oxide (MgO), aluminium oxide (Al2O3), silicon dioxide (SiO2), phosphorus pentoxide (P2O5), sulphur trioxide (SO3), potassium oxide (K2O), calcium oxide (CaO), titanium dioxide (TiO2), and ferric oxide (Fe2O3) play a vital role in blue carbon characterization (Cai et al., 2024; Murugesan et al., 2021; Romero-Mujalli & Melendez, 2023). These oxides influence soil mineralogy, pH, and redox potential that are critical for carbon stabilization. Fe2O3 and Al2O3 stabilize organic matter through clay-humus complex formation, while SiO2 and TiO2 enhance organic carbon retention via adsorption surfaces (Frates et al., 2023; Li et al., 2023).
Carbon sequestration in mangrove soils is driven by complex interactions among soil organic matter (SOM), mineralogy, and trace element geochemistry, which together influence carbon stabilization mechanisms. Trace elements such as chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) influence carbon stabilization. These elements can form complexes with organic matter or precipitate as insoluble compounds under anoxic conditions that prevail in mangrove soils (Wang et al., 2024a; Wu et al., 2024). Cr and Pb, for instance, stabilize SOC through adsorption, while Mn, Ni, and Cu catalyse microbial and enzymatic processes that affect carbon turnover (Grey et al., 2023; Ray et al., 2023).
The spatial distribution of trace elements and elemental oxides in mangrove soils is strongly stratified, with higher concentrations typically observed in surface layers (0 - 1 m) due to atmospheric deposition, tidal inputs, and anthropogenic activities. Elements such as Cu, Zn, and Cd are frequently enriched in surface soils, while concentrations decline with depth due to vertical leaching and redox-induced immobilization (Rahman et al., 2024).
In the Indian Sundarbans, anthropogenic activities such as industrial effluents, agricultural runoff, urbanization, and shipping have significantly altered degraded mangrove system’s soil geochemistry. Recent studies highlight trace metal enrichment, including Cr, Mn, Ni, Cu, Zn, Cd, and Pb, emphasizing the impact of industrial and agricultural activities (Islam et al., 2022; Sharma et al., 2021). Higher concentrations of specific trace element concentrations and their interactions with oxides like Fe2O3 and TiO2 reveal complex carbon stabilization mechanisms (Li et al., 2023).
Tidal-driven redox cycling affects trace element movement and reactivity, as the periodic oxidation and reduction of Fe and Mn compounds release and re-adsorb organic matter and trace elements, helping stabilize SOC in deeper layers (Queiroz et al., 2022; Tognella et al., 2016). The effect of tidal influence varied among the estuaries and distance from the sea (Bhattacharyya et al., 2023). However, majority of studies (Cooray et al., 2024; Grey et al., 2023; Rovai et al., 2018) focus on surface soils (0 - 1 m), overlooking significant carbon stocks in deeper layers. Mangrove soils extending to deeper soils (5 metres or so) offer unique insights into the potential of long-term carbon sequestration. Depth-resolved studies are critical for quantifying total carbon (TC) storage and understanding stabilization mechanisms, especially given the vulnerability of deep soils to environmental disturbances like land-use and hydrological changes (Sierra et al., 2024).
This study comprehensively investigates the role of trace elements and elemental oxides in the stabilization of soil organic carbon (SOC) within Sundarban mangrove soils, extending to a depth of 5 meters. To achieve the quantified data and correlation between SOC, elemental oxides and trace elements, depth-wise soil samples were considered from the three major estuaries of Sundarbans, representing distinct geological background and anthropogenic activities. The findings unveil critical geochemical interactions driving SOC stabilization, offering a transformative framework for advancing blue carbon accounting and global carbon management.
2. Methodology
2.1. Study Area and Site Selection
The study was conducted in the degraded mangroves of Indian Sundarban located at three different estuarine systems. The research focused on three river estuarine systems selected based on their drainage patterns, sedimentation rates, and depositional environments: Dayapur (22˚08'02.3" N 88˚50'37.7" E) along Bidyadhari estuarine, Sagar Island (21˚46'15.2" N 88˚10'09.3" E) along Hooghly estuarine and Maipith (21˚51'58.3" N 88˚30'39.7" E) along Matla estuarine (Figure 1). At each location, four sampling sites were chosen to acquire variability arising from both natural environmental gradients and anthropogenic influences. These locations were chosen because they reflect a broad range of environmental conditions, including differences in geology, salinity, mangrove species, tidal patterns, and the extent of human impact. This variety helps us better understand how these factors influence geochemical processes related to carbon storage. While our study offers a snapshot in time, it serves as a valuable starting point for assessing the carbon sequestration potential of degraded mangrove systems in the region.
2.2. Soil Sampling
Soil samples were collected from the surface to a depth of 5 meters using a combination of soil augers and manual excavation. Sampling was performed at 1-meter intervals to capture variations in carbon and geochemical properties across the vertical profile. A total of 12 locations (four from each estuarine site) were sampled to ensure comprehensive spatial representation. The collected samples were air-dried, sieved through 2 mm mesh (0.5 mm sieve was used for total carbon analysis), and homogenized before undergoing laboratory analyses.
Figure 1. Map showing the three study areas (Dayapur along Bidyadhari estuarine, Sagar Island along Hooghly estuarine and Maipith along Matla estuarine) and the degradation of mangroves from 1930 to 2024 in Sundarban, India.
2.3. Carbon and Geochemical Analyses
2.3.1. Soil Organic Carbon (SOC) and Total Carbon (TC)
The SOC was determined using the Walkley-Black method, a dichromate oxidation technique widely employed for soil analysis. Total carbon (TC) was measured by dry combustion method using an AnalytikJena Multi N/C 2100S analyzer.
2.3.2. Major Oxides and Trace Element Estimation
The geochemical composition of the soils was analyzed using two techniques, namely X-ray fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). XRF analysis was done to quantify the major oxide percentages, including sodium oxide (Na2O), magnesium oxide (MgO), aluminium oxide (Al2O3), silicon dioxide (SiO2), phosphorus pentoxide (P2O5), sulphur trioxide (SO3), potassium oxide (K2O), calcium oxide (CaO), titanium dioxide (TiO2), and ferric oxide (Fe2O3). The ICP-MS analysis was performed to determine the concentrations of trace elements like chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb).
2.3.3. Clay Mineralogy Identification
X-ray Diffraction (XRD) analysis was done to identify dominant clay minerals. Samples from three depths i.e., 1 m, 2.5 m, and 5 m, were analysed for each site. The analysis used a Cu K-α radiation source, operating at 40 kV and 40 mA, with a scanning rate of 4˚/minute. Diffractogram-derived “d” values were matched against the Joint Committee on Powder Diffraction Standards (JCPDS) database using X’pert HighScore Plus (v3.1) software for precise mineral identification.
2.4. Data Analysis
To evaluate spatial and vertical variability in the studied parameters, a two-factor analysis of variance (ANOVA) was performed using the OPSTAT online software. Depth and location were treated as independent variables, with a significance threshold of p ≤ 0.05. The depth-wise variation data analysis was done in RStudio (v2021.09.2). The correlation analyses were done in RStudio (v2021.09.2) using corrplot package.
3. Results
3.1. Soil Carbon Distribution across Depths
The distribution of total carbon (TC) and soil organic carbon (SOC) in 1m to 5m soil profile varied across the studied sites. TC concentrations were highest in surface soils (0 - 1 m), with Maipith (Matla estuarine) recording the maximum value of 0.94%, followed by Sagar Island (Hooghly estuarine, 0.75%) and Dayapur (Bidyadhari estuarine, 0.67%) (Figure 2). TC was found to be decreasing with increasing depth across all the sites. At 4 - 5 m depth, TC values were 0.50% at Maipith, 0.72% at Sagar Island and 0.65% at Dayapur. Similarly, SOC levels were highest in the uppermost layers (0 - 1 m), with Maipith reaching 0.73% in surface soils (Figure 3). SOC values decreased progressively with depth, showing the lowest values at 4 - 5 m (0.41% at Maipith, 0.42% at Sagar Island, and 0.59% at Dayapur). Despite site-specific differences, all locations exhibited a consistent declining trend in carbon concentrations with depth.
Figure 2. Depth-wise and site-wise variation of Total Carbon.
3.2. Variation of Oxide Composition and Concentration
Oxide composition and concentration varied with soil depths among the three study sites located in three distinct estuaries (Figure 4). Silicon dioxide (SiO2) was the predominant oxide, with its concentration increasing with depth at all sites. Surface soils (0 - 1 m) displayed the highest SiO2 concentrations at Maipith (Matla estuarine, 51%), followed by Sagar Island (Hooghly estuarine, 49.2%) and Dayapur (Bidyadhari estuarine, 45.7%). SiO2 exhibited relatively higher concentrations at deeper depths (4 - 5 m) in Maipith (60.2%) and Sagar Island (58.9%) compared to Dayapur (51.3%).
Ferric oxide (Fe2O3) was most concentrated at surface soils (0 - 1 m) but gradually decreased with depth. At the surface layer (0 - 1 m), Fe2O3 concentration
Figure 3. Depth-wise and site-wise variation of Soil Organic Carbon.
Figure 4. Variation of oxide concentrations along different depth at the three locations.
was highest at Sagar Island (8.9%) followed by Maipith (7.3%) and Dayapur (6.4%). At 4 - 5 m depth, Fe2O3 concentrations were highest at Sagar Island (7.3%), followed by Dayapur (6.9%) and Maipith (6.9%). Fe2O3 concentrations varied across depths, with peak values observed at Maipith (8% at 2 - 3 m soil depth) and Sagar Island (8.2% at 3 - 4 m soil depth).
Surface soils (0 - 1 m) recorded the highest Na2O concentrations at Maipith (Matla estuarine, 7.1%), followed by Sagar Island (Hooghly estuarine, 6.7%) and Dayapur (Bidyadhari estuarine, 6.5%). Na2O concentrations declined sharply with depth at all sites, reaching their lowest values at 4 - 5 m, with 3.8% at Sagar Island, followed by 2.5% at Maipith and 1.4% at Dayapur.
Surface soils (0 - 1 m) showed the highest Al2O3 concentration at Dayapur (Bidyadhari estuarine, 23.6%), followed by Maipith (Matla estuarine, 19.5%) and Sagar Island (Hooghly estuarine, 18.1%). At 4 - 5 m depth, Al2O3 was again highest at Dayapur (25.3%), followed by Maipith (19.5%) and Sagar Island (15.7%). Al2O3 consistently increased with depth at Dayapur, with lower but relatively stable values at Maipith and Sagar Island.
At surface layer (0 - 1 m), K2O concentrations were highest at Maipith (Matla estuarine, 4.3%), followed by Dayapur (Bidyadhari estuarine, 3.8%) and Sagar Island (Hooghly estuarine, 3.7%). At 4 - 5 m depth, K2O concentrations were highest at Maipith (4.4%), followed by Dayapur (4.1%) and Sagar Island (3.9%). K2O showed a gradual increase with depth at all sites.
Phosphorus pentoxide (P2O₅) exhibited minor concentrations, ranging from 0.11 - 0.14%, while other oxides, including magnesium oxide (MgO) and calcium oxide (CaO), showed minimal fluctuations across the depths in all studied sites. These trends indicate the potential role of site-specific factors in shaping the vertical distribution of major oxides.
The correlation analysis between Soil Organic Carbon (SOC) and major soil oxides reveals distinct trends across the dataset (Figure 5). A strong positive correlation was observed between SOC and SO3 (r ≈ 0.56), followed by a moderate correlation with Fe2O3 (r ≈ 0.51) and TiO2 (r ≈ 0.36). SOC content also showed a weak positive trend with MnO (r ≈ 0.17) and Al2O3 (r ≈ 0.04). In contrast, SOC exhibited a clear negative correlation with SiO2 (r ≈ −0.26), and no significant correlation was observed with K2O. These patterns suggest that the distribution of SOC is closely associated with specific oxide concentrations, particularly those of iron, sulphur and aluminium.
3.3. Trace Elements Composition and Concentrations
Trace element concentrations varied notably with soil depth among the three study sites, reflecting the influence of site-specific geochemical processes (Figure 6). Surface soils (0 - 1 m) exhibited the highest Cr concentrations at Dayapur (Bidyadhari estuarine, 89.2 ppm), followed closely by Sagar Island (Hooghly estuarine, 89.1 ppm) and Maipith (Matla estuarine, 88.3 ppm). At 4 - 5 m depth, Cr concentrations were highest at Sagar Island (80.8 ppm), followed by Maipith (54.4 ppm) and Dayapur (52.5 ppm). With increasing depth, Cr concentrations generally declined at all sites, reaching the lowest values at Maipith (49.5 ppm at 3 - 4 m) and Dayapur (52.4 ppm at 4 - 5 m). Sagar Island displayed higher (95.2 ppm) Cr values at 2 - 3 m depth.
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Figure 5. Pearson correlation matrix between Soil Organic Carbon (SOC) content and major oxides (Fe2O3, Al2O3, SiO2, TiO2, CaO, MgO, MnO) in mangrove soils at all three study areas. Shades of blue represent positive correlations, while red shades indicate negative correlations. The colour intensity and the circular sizes reflect the strength of correlation, with values ranging from –1 to +1.
Manganese (Mn) concentrations were highest in surface soils (0 - 1 m) at Sagar Island (Hooghly estuarine, 696.1 ppm), followed by Dayapur (Bidyadhari estuarine, 553 ppm) and Maipith (Matla estuarine, 279.6 ppm). At 4 - 5 m depth, Mn concentrations were highest at Sagar Island (615.4 ppm), followed by Dayapur (241.6 ppm) and Maipith (238.1 ppm). With depth, Mn concentration declined sharply, reaching minimum values at Maipith (171 ppm at 3 - 4 m) and Dayapur (241.6 ppm at 4 - 5 m). However, Sagar Island consistently showed higher Mn concentration across all depths with maximum (787.7 ppm) at 1 - 2 m depth interval.
Nickel (Ni) concentrations in surface soils (0 - 1 m) were highest at Sagar Island (Hooghly estuarine, 49 ppm), followed by Dayapur (Bidyadhari estuarine, 46.7 ppm) and Maipith (Matla estuarine, 43.4 ppm). At 4 - 5 m depth, Ni concentrations were highest at Sagar Island (44.5 ppm), followed by Maipith (27.5 ppm) and Dayapur (21.8 ppm). Ni concentrations showed a decreasing trend with depth, with the lowest values observed at Dayapur (21.8 ppm at 4 - 5 m) and Maipith (23.6 ppm at 3 - 4 m).
Figure 6. Variation of trace element concentration along different depths at the three locations.
Copper (Cu) concentrations in surface soils (0 - 1 m) were highest at Sagar Island (Hooghly estuarine, 50.5 ppm), followed by Maipith (Matla estuarine, 47.9 ppm) and Dayapur (Bidyadhari estuarine, 40.1 ppm). At 4 - 5 m depth, Cu concentrations were highest at Sagar Island (35.6 ppm), followed by Maipith (27.1 ppm) and Dayapur (26.6 ppm). With increasing depth, Cu concentration declined, reaching minimum values at Dayapur (25.3 ppm at 3 - 4 m) and Maipith (27.1 ppm at 4 - 5 m). Sagar Island maintained higher (60.2 ppm) Cu concentrations at 2 - 3 m depth.
Zinc (Zn) concentrations in surface soil (0 - 1 m) were highest at Dayapur (Bidyadhari estuarine, 595.1 ppm), followed by Sagar Island (Hooghly estuarine, 327.8 ppm) and Maipith (Matla estuarine, 113.9 ppm). At 4 - 5 m depth, Zn concentrations were highest at Sagar Island (131.8 ppm), followed by Maipith (97.8 ppm) and Dayapur (94.5 ppm). Zn concentration declined with depth, reaching minimum values at Dayapur (94.5 ppm at 4 - 5 m) and Maipith (97.8 ppm at 4 - 5 m). Sagar Island maintained moderate Zn levels, showing peak value (394.7 ppm) at 2 - 3 m.
Cadmium (Cd) concentrations in surface soil (0 - 1 m) were highest at Dayapur (Bidyadhari estuarine, 9.7 ppm), followed closely by Maipith (Matla estuarine, 9.1 ppm) and Sagar Island (Hooghly estuarine, 7.8 ppm). At 4 - 5 m, Cd concentrations were highest at Maipith (7.2 ppm), followed by Sagar Island (5.1 ppm) and Dayapur (3.8 ppm). Cd concentration peaked at mid-depths (2 - 3 m) in Sagar Island (16.8 ppm) but generally declined with depth across all sites.
Lead (Pb) concentrations in surface soils (0 - 1 m) were highest at Dayapur (Bidyadhari estuarine, 82.9 ppm), followed by Maipith (Matla estuarine, 42.3 ppm) and Sagar Island (Hooghly estuarine, 33.1 ppm). At 4 - 5 m depth, Pb concentrations were highest at Sagar Island (29.4 ppm), followed by Maipith (24.1 ppm) and Dayapur (11.5 ppm). Pb concentrations declined with depth, reaching the lowest values at Dayapur (11.5 ppm at 4 - 5 m) and Maipith (22.9 ppm at 3 - 4 m). Sagar Island showed relatively stable Pb concentrations, with the highest value of 46.8 ppm at 2 - 3 m.
The correlation matrix (Figure 7) revealed several notable relationships between Soil Organic Carbon (SOC) and trace elements in Sundarban mangrove soils. A strong positive correlation was observed between SOC and Copper (Cu) (r ≈ 0.43), indicating a substantial association. Nickel (Ni) also showed moderate to strong positive correlations with SOC (r ≈ 0.26). In contrast, Manganese (Mn), Zinc (Zn), and Lead (Pb) exhibited weak or negligible correlations with SOC, suggesting minimal co-variation.
Figure 7. Pearson correlation matrix between Soil Organic Carbon (SOC) and selected trace elements (Fe, Mn, Zn, Cu, Cr, Ni, Pb, Ti, and Cd) in mangrove soils at all three study areas. Shades of blue represent positive correlations, while red shades indicate negative correlations. The colour intensity and the circular sizes reflect the strength of correlation, with values ranging from –1 to +1.
3.4. Variation of Clay Mineral Composition
At 1 m depth, glauconite was consistently detected across all three estuarines (Figure 8(a), Figure 8(d), Figure 8(g)). At 5 m depth, argentopyrite and pyrite were prominent across all sites (Figure 8(c), Figure 8(f), Figure 8(i)). Distinct site-specific minerals were observed at intermediate and deeper depths. At 2.5 m
Figure 8. XRD diffractograms depicting the clay mineralogy of soil samples collected from different depths across three study areas: (a) Dayapur, 1 m depth; (b) Dayapur, 2.5 m depth; (c) Dayapur, 5 m depth; (d) Sagar Island, 1 m depth; (e) Sagar Island, 2.5 m depth; (f) Sagar Island, 5 m depth; (g) Maipith, 1 m depth; (h) Maipith, 2.5 m depth; (i) Maipith, 5 m depth.
depth in Dayapur (Bidyadhari estuarine), zinnwaldite, a lithium-rich mica, was detected (Figure 8(b)). In the 5 m soil sample from Dayapur (Bidyadhari estuarine), sepiolite, a magnesium-rich silicate, was identified (Figure 8(c)). Soil samples at a depth of 5 meters in Sagar Island (Hooghly estuarine) revealed the presence of tamarugite, a sodium aluminium sulphate mineral (Figure 8(f)). In Maipith (Matla estuarine), chlorite-vermicullite was identified at a depth of 1 meter, indicating partial weathering of chlorite under alternating wet and dry conditions (Figure 8(g)). At a depth of 5 meters in Maipith (Matla estuarine), argentopyrite and montmorillonite were detected (Figure 8(i)).
3.5. Mangrove Species Distribution and Density Patterns across Estuarine Systems
The comparative assessment of mangrove species density across three estuarine systems—Dayapur (Bidyadhari estuary), Sagar Island (Hooghly estuary), and Maipith (Matla estuary)—revealed prominent differences in species composition and total vegetation density (independent tree ha−1) (Table 1). Dayapur exhibited the highest species richness (six species), followed by Sagar Island and Maipith, each with five species. Among the studied species, Avicennia alba demonstrated pervasive presence and dominance across all sites, with the highest density observed at Dayapur (1092.59 Ind. ha−1), followed by Sagar Island (722.22 Ind. ha−1) and Maipith (703.70 Ind. ha−1). Avicennia officinalis was the second most prevalent species, present across all sites with a moderately high density. In contrast, Ceriops decandra was exclusively recorded at Dayapur with a low density of 127.78 Ind. ha−1). Sagar Island recorded the highest total vegetation density (2677.77 Ind. ha−1), primarily attributed to the overwhelming dominance of Sonneratia apetala (1044.44 Ind. ha−1). This species was absent in Dayapur but found in both Sagar Island and Maipith, indicating site-specific preference. Notably, Avicennia marina was exclusively found in Maipith (244.44 Ind. ha−1), reflecting localized environmental conditions conducive to its establishment.
Table 1. Spatial distribution and species-wise density (individuals per hectare) of dominant mangrove species across the three estuarine systems—Bidyadhari (Dayapur), Hooghly (Sagar Island), and Matla (Maipith)in Sundarban, India.
Site |
Estuarine System |
Coordinates |
Species |
Density (Ind ha−1) |
Dayapur |
Bidyadhari |
22˚08'02.3" N 88˚50'37.7" E |
Avicennia alba |
1092 |
Avicennia officinalis |
796 |
Excoecariaa agallocha |
294 |
Bruguiera gymnorrhiza |
259 |
Ceriops decandra |
127 |
Sagar
Island |
Hooghly |
21˚46'15.2" N 88˚10'09.3" E |
Avicennia alba |
722 |
Avicennia officinalis |
311 |
Excoecariaa agallocha |
244 |
Bruguiera gymnorrhiza |
355 |
Sonneratia apetala |
1044 |
Maipith |
Matla |
21˚51'58.3" N 88˚30'39.7" E |
Avicennia alba |
703 |
Avicennia officinalis |
444 |
Bruguiera gymnorrhiza |
333 |
Avicennia marina |
244 |
Sonneratia apetala |
277 |
4. Discussion
4.1. Soil Carbon Distribution across Depths
The observed depth-wise decline in total carbon (TC) and soil organic carbon (SOC) across all degraded mangrove sites reflects well-established patterns of organic matter dynamics in mangrove ecosystems. Surface soils (0 - 1 m), particularly at Maipith (Matla estuarine), exhibited the highest TC and SOC concentrations. This finding aligns with global studies (Ragavan et al., 2023; Wang et al., 2024b), which emphasize the critical role of mangrove vegetation in enriching surface soils with organic carbon. The higher carbon stocks at Maipith can be attributed to its dense mangrove vegetation, which promotes substantial organic matter inputs through litter deposition and root biomass accumulation (Nayak et al., 2024). This enhances organic carbon preservation in surface soils, aligning with the observed elevated TC and SOC values in Maipith. The progressive decline in TC and SOC with depth reflects reduced organic matter inputs and enhanced mineralization and geochemical alteration in deeper layers. These trends are corroborated by earlier studies (Hicks Pries et al., 2023; Qin et al., 2024) who reported similar depth-wise carbon losses in tropical mangrove systems.
However, site-specific variations in carbon stocks likely stem from differences in vegetation density, sedimentation rates, and tidal inundation patterns, which govern organic matter deposition and preservation (Padhy et al., 2022). For example, higher SOC concentrations across depths at Maipith (Matla estuarine) may be attributed to better sediment trapping and prolonged tidal inundation, which enhance organic carbon burial and preservation. Anthropogenic activities, including industrial effluent discharge and land-use changes, further aggravate carbon losses in degraded mangroves (Bhattacharyya et al., 2023; Sharma et al., 2021).
4.2. Variation of Oxide Composition and Concentration
The predominance of silicon dioxide (SiO2) across all depths at all three sites emphasizes the sedimentary nature of mangrove soils, with its increasing concentration at deeper layers indicating reduced organic inputs and greater mineral dominance. This trend aligns with earlier studies (Chen et al., 2025; Marchand, 2017), which highlight the role of silicate-rich sediment deposition in mangrove ecosystems. Higher SiO2 concentrations in deeper soils at Maipith (Matla estuarine) and Sagar Island (Hooghly estuarine) reflect enhanced sedimentation processes and lower biological activity in these layers.
Ferric oxide (Fe2O3) concentration is highest in surface soils across all three sites, suggesting its pivotal role in stabilizing organic carbon through co-precipitation and adsorption mechanisms. This observation is consistent with findings (Ruiz et al., 2024), which emphasized the contribution of Fe2O3 to organic carbon preservation in mangrove soils. The depth-wise decline in Fe2O3 suggests redox-driven mobilization of iron under anoxic conditions, which is characteristic of water-logged mangrove environments (Cheng et al., 2021). This mobilization may lead to decreased iron availability for organic carbon stabilization in deeper layers.
The relatively stable concentrations of aluminium oxide (Al2O3) and potassium oxide (K2O) across depths highlight their significance in forming clay-humus complexes, which contribute to long-term carbon stabilization. Similar patterns have been observed in mangrove systems globally (Wang et al., 2024b).
The minor fluctuations in magnesium oxide (MgO), sodium oxide (Na2O), and calcium oxide (CaO) indicate relatively stable geochemical conditions in deeper layers, as suggested by (Das et al., 2023). The sharp decline in Na2O with depth across all sites may result from reduced contributions from tidal inundation in subsurface soils. These findings highlight the influence of site-specific geochemical and hydrological factors in shaping the vertical distribution of major oxides (Chen et al., 2025; Qin et al., 2024).
The observed correlations between Soil Organic Carbon (SOC) and major oxides highlight the complex geochemical interactions governing carbon stabilization in Sundarbans mangrove soils. The strong positive correlation between SOC and Fe2O3 indicates the role of iron oxides in promoting long-term carbon sequestration. In mangrove ecosystems, the dynamic redox environment enhances the formation of Fe-organic matter complexes via mechanisms such as co-precipitation and ligand exchange, effectively protecting SOC from microbial decomposition.
The moderate positive correlation with SO3 suggests a potential link between sulphur-bearing compounds and organic matter dynamics. In anaerobic environments like mangrove sediments, sulphur may contribute to SOC stabilization through the formation of organo-sulphur compounds or indirectly via iron-sulphide complexation. Additionally, the presence of sulphates can support microbial sulfate reduction, which may enhance the preservation of organic matter under reducing conditions.
The weak positive correlation between SOC and MnO supports the secondary role of manganese oxides in SOC stabilization. Like iron, manganese participates in redox reactions, albeit to a lesser extent, contributing to transient organic matter protection during redox cycling. Al2O3, showing a weak but positive trend, points to its role in stabilizing SOC through sorption processes and the formation of organo-metallic complexes, particularly in acidic, weathered tropical soils where aluminosilicate clays dominate. In contrast, the negative correlation between SOC and SiO2 suggests that sandy, quartz-rich sediments—typical of deltaic environments—have a diminished capacity to retain organic carbon due to low surface reactivity and limited formation of stable mineral-organic associations.
Overall, these relationships suggest that iron, sulphur, and to a lesser extent, aluminium and manganese oxides, contribute to the geochemical stabilization of SOC in mangrove soils.
4.3. Trace Elements Composition and Concentrations
The depth-wise distribution of trace elements across the three studied sites, namely Maipith (Matla estuarine), Sagar Island (Hooghly estuarine), and Dayapur (Bidyadhari estuarine), revealed significant spatial and geochemical variations shaped predominantly by anthropogenic activities, with tidal and geological influences playing secondary roles (Ghosh et al., 2024).
At Maipith (Matla estuarine), surface soils (0 - 1 m) showed moderate Cr and Mn concentrations, which declined sharply with depth. The relatively low Mn levels suggest limited tidal inundation and reduced sediment trapping compared to the other sites (Madi et al., 2015; Mapanao-Villar et al., 2024). Ni and Cu concentrations followed a similar trend, with Cu stabilizing at mid-depths, likely due to clay-organic complexation (Rahman et al., 2024). Elevated Cd levels at intermediate depths indicate sulphide phase formation under reducing conditions, consistent with localized geochemical stability (Robin et al., 2024).
Sagar Island (Hooghly estuarine) exhibited the highest trace element concentrations across all depths, suggesting significant anthropogenic inputs combined with efficient sediment trapping under prolonged tidal inundation (Fu et al., 2023; Ghosh et al., 2024). Surface Mn (696.10 ppm) and Cr (89.13 ppm) levels were among the highest, with Mn peaking at 1 - 2 m (787.76 ppm). The higher concentrations of Mn and Cr at mid-depths suggests active redox cycling facilitated by anthropogenic sediment contributions (Akhand et al., 2024; Mapanao-Villar et al., 2024). Ni and Cu levels peaked at 2 - 3 m, likely due to stabilization by clay minerals and organic matter, while Cd concentrations peaked at mid-depths (16.83 ppm), consistent with redox-driven sulphide formation (Guo et al., 2023; Islam et al., 2023).
Dayapur (Bidyadhari estuarine) exhibited significant surface-level enrichment of Zn and Pb, indicative of substantial anthropogenic inputs, such as agricultural runoff and industrial effluents (Badawy et al., 2024; Tang et al., 2022). With depth, Zn and Pb concentrations declined sharply, while the lowest Mn concentrations (241.65 ppm at 4 - 5 m) indicates reduced sedimentation rates and limited tidal influence (Chakraborty, 2024).
Overall, the site-specific variations highlight the dynamic relationship between redox conditions, tidal regimes, and anthropogenic inputs, with anthropogenic influences emerging as the dominant factor shaping trace element distributions across all sites (Pavoni et al., 2021). The anthropogenic impact at Dayapur (Bidyadhari estuarine), although lower than at Sagar Island, is more pronounced than at Maipith (Matla estuarine), further emphasizing the dominance of human activities over natural tidal processes in influencing trace element distributions (Selvaraj et al., 2024). Elevated surface levels of Zn and Pb highlight the urgent need for regulating human activities, particularly at Sagar Island (Hooghly estuarine) and Dayapur (Bidyadhari estuarine).
The correlation analysis between Soil Organic Carbon (SOC) and trace elements in Sundarban mangrove soils reveals specific patterns that offer insights into biogeochemical interactions influencing carbon dynamics. The moderate positive correlation between SOC and Copper (Cu) (r ≈ 0.43) suggests that Cu may play a contributory role in SOC stabilization. Copper is known to participate in microbial enzymatic processes and has a strong affinity for humic substances, often forming stable organo-metallic complexes (Fu et al., 2023). Nickel (Ni), exhibiting a positive correlation with SOC (r ≈ 0.26), may indicate partial association with organic matter. Though typically considered a lithogenic or mineral-associated element, Ni can interact with organic ligands under certain pH and redox conditions.
On the other hand, the weak or negligible correlations observed between SOC and Manganese (Mn), Zinc (Zn), and Lead (Pb) suggest limited geochemical or biological coupling in this context. These elements may be present in forms not readily complexed with organic matter or may be governed by separate geochemical pathways. For instance, Mn is redox-sensitive and may be more influenced by oxidation-reduction dynamics than organic matter content (Badawy et al., 2024; Islam et al., 2023). Similarly, Zn and Pb, despite known complexation potentials, may preferentially associate with inorganic mineral phases or precipitate under prevailing soil conditions, thereby weakening their correlation with SOC (Chakraborty, 2024).
Overall, the modest positive correlations with Cu and Ni imply selective trace element roles in SOC dynamics, while the lack of correlation with Mn, Zn, and Pb highlights the complexity of trace metal behaviour in estuarine mangrove systems.
4.4. Variation of Clay Mineral Composition
The presence of glauconite in shallow soils (1 m depth) across all three estuarine systems explains the influence of saline water intrusion and tidal dynamics. Glauconite formation in such environments is facilitated by sodium ion exchange, as previously reported in saline coastal soils (Choudhury et al., 2022). This uniformity across sites indicates a common geochemical process. At greater depths (5 m), the detection of argentopyrite and pyrite indicates the possibility of sulphate reduction under anoxic conditions. These minerals are indicative of active sulphur cycling and organic matter decomposition, consistent with reducing environments found in water-logged soils.
In Dayapur (Bidyadhari estuarine), the detection of zinnwaldite at 2.5 m depth suggests lithogenic contributions or weathering of mica-rich parent materials, emphasizing the influence of local geology. The presence of sepiolite at 5 m depth further reflects prolonged water stagnation and alkaline soil conditions, likely driven by magnesium leaching under constrained hydrology (El Rasafi et al., 2024; Hamid et al., 2021). In Sagar Island (Hooghly estuarine), tamarugite (sodium and aluminium-rich sulphate hydrate) at 5 m signifies extreme salinity and acidic conditions driven by seawater and brine deposits. In Maipith (Matla estuarine), the detection of chlorite-vermicullite at 1m depth indicates partial weathering of chlorite under alternating wet and dry conditions (Wang et al., 2021), while the presence of argentopyrite and montmorillonite at 5m depth highlights reducing conditions and sedimentation processes that favour fine-grained clay accumulation (Ferreira et al., 2022). These mineral transformations are indicative of dynamic redox conditions and sediment deposition patterns in mangrove ecosystems.
4.5. Mangrove Species Distribution and Density Patterns across Estuarine Systems
The observed variation in mangrove species composition and density across the three estuarine sites highlights the ecological heterogeneity of the Indian Sundarbans. The dominance of Avicennia alba across all sites, particularly at Dayapur, suggests its broad ecological tolerance and adaptive capacity to varying salinity and tidal regimes. The exclusive presence of Ceriops decandra at Dayapur and Avicennia marina at Maipith further indicates the influence of site-specific microenvironmental conditions such as soil salinity, hydrology, and sediment characteristics on species distribution.
Sagar Island, with the highest total vegetation density, is dominated by Sonneratia apetala, a fast-growing species known for its tolerance to dynamic estuarine environments and potential contribution to biomass accumulation. This dense vegetation cover may enhance organic matter input to the soil, indirectly influencing soil organic carbon (SOC) accumulation and stabilization through increased litter deposition and root biomass (Nayak et al., 2024).
While this study primarily focuses on geochemical controls of carbon sequestration, incorporating biotic data provides a more integrated understanding of SOC dynamics. Variations in species-specific traits—such as litter quality and biomass productivity—can significantly modulate the input, composition, and turnover of organic matter in mangrove soils. These insights highlight the importance of considering vegetation structure alongside geochemical parameters to better evaluate carbon sequestration potential in mangrove ecosystems.
5. Conclusion
Our study showed that there was not much geological influence on the soil organic carbon storage and stability of SOC. Surface soils serve as active carbon accumulation zones due to mangrove litter and root biomass, while deeper layers act as long-term carbon reservoirs, safeguarded by anoxic conditions that inhibit decomposition. This signifies the importance of both surface and subsurface soils in mangrove carbon dynamics. SiO2 concentrations was higher at greater depths across all three estuarine systems. Hooghly estuary showed higher surface Fe2O3 concentration and moderate SiO2 concentration. Bidyadhari estuary had the lowest SiO2 and Fe2O3 concentrations but the highest surface Al2O3, which increased with depth. Sedimentation effects were found to be negligible except in the case of copper (Cu) concentration, where localized sedimentary processes may have influenced trace element deposition. The concentration of chromium (Cr) and lead (Pb) was clearly influenced by anthropogenic activities in the study area, indicating the trends observed in these two trace elements. Despite these influences, all toxic trace elements, including cadmium (Cd) and lead (Pb), were found to be within permissible limits as per environmental safety standards, suggesting minimal immediate ecological risks while highlighting the resilience of the Sundarban mangrove soils to contamination. Overall, this study emphasizes the important role of Sundarban degraded mangrove ecosystem in global carbon sequestration and the need for site-specific conservation and management strategies. Future research should integrate geochemical modelling and long-term monitoring to fully leverage the carbon sequestration potential of these unique degraded mangrove systems, which have become an integral part of the wetland ecosystem, contributing to climate change mitigation and sustainable ecosystem management.