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.

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.

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.

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).

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.

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.

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.

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

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.

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.

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).

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.

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

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) ).

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.

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 ).

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.

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.

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.

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.