A global assessment of mangrove soil organic carbon sources and implications for blue carbon credit

Mangrove forests are highly productive blue carbon ecosystems that provide fisheries, coastal protection, sediment stabilization, and carbon sequestration. Mangrove sediments store about 70% of ecosystem carbon, with geographic variability primarily determined by coastal environmental settings (estuarine vs. marine). Mangrove sediments accumulate autochthonous OC from mangrove vegetation and allochthonous OC from marine and terrestrial sources via tidal exchange. Accurate partitioning of these sources is required by certification standards like the Verified Carbon Standard (VCS, VM0033), which deducts allochthonous carbon from carbon credit calculations to satisfy additionality. This study aims to quantify the provenance of SOC sources in mangroves globally, compare SOC stocks between estuarine and marine mangroves, and identify environmental and socioeconomic drivers of OC source contributions, thereby informing climate mitigation efforts and blue carbon credit allocation.

Prior work has highlighted mangroves as carbon-rich tropical forests and identified global controls on mangrove soil carbon storage. Studies in Australian coastal wetlands and other systems have examined autochthonous versus allochthonous contributions and processes by which multiple OC sources contribute to sequestration. Verified Carbon Standard methodologies for tidal wetland and seagrass restoration require accounting for OC provenance. Previous syntheses reported differing SOC densities between estuarine and marine mangroves, potentially due to limited sampling (e.g., 81 observations from 27 sites), and estuarine systems have been shown to be influenced by anthropogenic inputs affecting OC decomposition and emissions. Source-specific characteristics such as lignin-rich terrestrial OM and lower C/N marine OM have implications for decomposition and retention.

The study compiled global isotope (δ¹³C, δ¹⁵N) and N/C data for mangrove sediments from 100 peer-reviewed studies (441 observations), meeting criteria that required reporting of parameters for OC source calculation and field studies under natural conditions. Relative contributions of mangrove (autochthonous), marine (phytoplankton and macroalgae combined), and terrestrial OC to top-meter sediment SOC were estimated using Bayesian mixing models (MixSIAR) in R with two-tracer stable isotope analysis, preferentially using δ¹³C and N/C, or δ¹⁵N and N/C when δ¹³C was unavailable. Endmember isotope and N/C values were compiled from literature for mangrove tissues, riverine POM, phytoplankton, and microalgae; average endmember values were applied to the geographically nearest sites, assuming source SD of 0.5 (isotopes) or 0.005 (N/C). Sampling site classifications into estuarine or marine mangroves relied on original definitions, maps in the literature, or custom global estuarine/marine mangrove mapping. Additional datasets included soil properties (pH, salinity, particle size), vegetation canopy height (from GEE), geomorphic and climatic data (tidal range, mean sea level, coastal elevation, Z_MHHW*, relative sea level rise, MAT, MAP), and socioeconomic indicators (GDP, HDI, population density, urbanization). Random forest models (randomForest, rfPermute) assessed the importance and significance of drivers for marine, mangrove, and terrestrial OC contributions. Generalized additive models (GAM) explored nonlinear relationships for top-ranked factors. For SOC stocks, a comprehensive database of 2356 observations was built, combining Ouyang and Lee’s global dataset, additional post-2020 studies, unpublished field data across China, and Coastal Carbon ATLAS entries. One-way ANOVA compared SOC per unit area between estuarine and marine mangroves globally and by country. Total SOC stocks were estimated using Kriging interpolation in GEE, overlaying SOC data with mapped estuarine/marine mangrove distributions.

• Autochthonous mangrove-derived OC is the dominant contributor to top-meter SOC in both settings: 49% in estuarine mangroves and 62% in marine mangroves. • In estuarine mangroves, allochthonous terrestrial OC (30%) exceeds marine OC deposition (21%). • SOC stock per unit area is higher in estuarine mangroves (282 ± 8.1 Mg C ha⁻¹) compared to marine mangroves (250 ± 5.0 Mg C ha⁻¹; P < 0.05). • Total SOC stocks: estuarine mangroves store 1502 ± 154 Tg C, marine mangroves store 3025 ± 345 Tg C; marine mangroves hold 67% of global mangrove SOC. • δ¹³C values of mangrove sediments span −30.7‰ to −6.20‰ with a mean of −25.1‰; variation is driven by location (latitude/longitude), tidal range, MAT, salinity, total nitrogen, and particle size. • Marine OC contributions increase with particulate organic carbon (POC), while taller canopy height and higher mean annual precipitation are associated with lower marine OC proportions. • Autochthonous OC contributions are influenced by location, soil properties, and climate; they decrease with longitude and increase with higher MAT and MAP, and may be elevated in areas with higher population density/anthropogenic activity. • Terrestrial OC contributions are largely confined to estuarine mangroves and negatively correlated with GDP and HDI, consistent with anthropogenic impacts (e.g., water use, dams) reducing riverine carbon delivery. • Continental variability is pronounced, with autochthonous OC contributions ranging widely (e.g., 15% in South Africa estuarine sites to 57% in South America).

The study resolves the provenance of SOC in mangrove sediments across coastal environmental settings, directly addressing the need for accurate OC source partitioning central to blue carbon accounting and VCS methodologies. It shows that mangrove autochthonous OC predominates globally, but estuarine systems receive substantial terrestrial allochthonous inputs that elevate SOC per unit area relative to marine systems. The identified environmental drivers (POC, CAR, canopy height, MAT, MAP) and socioeconomic influences (GDP, HDI, population density) clarify how natural processes and human activities shape OC source contributions and retention. Differences from earlier syntheses are attributed to expanded sampling (2356 SOC observations, 441 isotope observations), improving global representativeness. The results emphasize that estuarine mangroves, despite holding a smaller fraction of global SOC, face heightened threats from terrestrial pollution and anthropogenic change, which may alter OC decomposition and emissions and thus affect carbon credit additionality considerations.

This global assessment quantifies the relative contributions of autochthonous and allochthonous OC to mangrove sediment SOC and reveals systematic differences between estuarine and marine settings. It provides source-resolved data and identifies key environmental and socioeconomic drivers, supporting robust blue carbon credit accounting under standards that require deduction of allochthonous carbon. Future research should deepen source identification using expanded isotopic and biomarker analyses, address sampling biases, and evaluate impacts of climate change, deforestation, land use change, and infrastructure (e.g., dams) on OC delivery and retention. Enhanced, high-resolution mapping and monitoring will further improve carbon credit calculations and management strategies to safeguard mangrove ecosystem services.

Autochthonous OC contributions may be underestimated because endmember δ¹³C values were primarily derived from mangrove leaves, whereas roots and woody materials can be more 13C-enriched. Sampling locations were predominantly fringe mangroves (129 of 164 sites with reported positions), potentially biasing OC source estimates; analyses were performed to test location effects, but residual bias may remain. Not all studies reported soil or vegetation properties; nearest-site data were used to fill gaps, introducing uncertainty. Estuarine/marine classification sometimes relied on mapping and manual boundary delineation when original site descriptions were lacking. Endmember averaging and assumptions about source variability (fixed SDs) may oversimplify natural heterogeneity. Prior literature discrepancies in SOC stocks reflect differences in sampling sizes; while this study’s larger dataset improves confidence, unbalanced geographic coverage could still affect generalizability.