Improved estimates on global carbon stock and carbon pools in tidal wetlands

The study addresses biases in global estimates of carbon stocks in tidal vegetated wetlands (mangroves and saltmarshes). It investigates whether conventional conversions from loss-on-ignition (LOI) to organic carbon (OC), commonly assumed constant across soils (e.g., OC/LOI = 0.58, LOI/OC = 1.724), are appropriate for mangrove and saltmarsh sediments, and quantifies how these assumptions affect global stock estimates. The work also evaluates overlooked carbon pools—sediment inorganic carbon (IC) and dead biomass—in ecosystem carbon accounting. The purpose is to derive ecosystem-specific OC–LOI relationships, correct sediment OC stock estimates, incorporate IC and dead biomass, and provide improved global totals. This is important because tidal wetlands are key blue carbon ecosystems and accurate accounting underpins climate mitigation assessments and emission estimates from mangrove loss.

Previous syntheses reported wide ranges for mangrove sediment OC stocks (e.g., 72–936 Mg ha^-1), often using mean values as representative central tendencies and applying generic SOM-to-SOC conversion factors derived from other soils or few mangrove sites. The widely used van Bemmelen factor (OC/LOI = 0.58; LOI/OC = 1.724) may be too low or inappropriate for many sediments, leading to biased OC contents and stock estimates. Some studies estimated conversion factors from limited regional datasets and did not apply them consistently. Additionally, global ecosystem carbon stocks usually included only living biomass and sediment OC, omitting sediment IC and dead biomass, despite evidence of significant IC in carbonate settings and substantial dead biomass in mangroves. These gaps likely led to overestimation of sediment OC stocks and underestimation of total ecosystem carbon.

Data compilation: The authors compiled 1534 paired observations of sediment OC and LOI from 30 studies spanning mangroves and saltmarshes (global biogeographic coverage) and assembled 1727 observations of mangrove sediment OC stock from 235 studies across 52 countries (covering ~91.9% of global mangrove area). Sediment IC data were compiled for 100 sites from recent reviews and additional sources. Dead biomass carbon data were compiled for 225 sites. Field sampling: Supplementary field data were collected in Hong Kong mangroves and saltmarshes (Ting Kok, Yung Shue O, Wu Shek Kok, Mai Po; April–October 2018). Sediment cores were collected to 1 m at Mai Po and 20–40 cm at other sites, sectioned by depth, stored cold, dried (60 °C) to constant weight for bulk density, and analyzed. Organic matter was measured by LOI at 550 °C for 4 h. Total C and N were measured by elemental analyzer; IC was derived from pre/post-LOI differences and additional LOI at 950 °C for 2 h, with carbonate mass estimated using a 1.36 factor. IC stock was computed as IC content times bulk density. Statistical analyses: Relationships between OC and LOI were assessed using regression (mangroves fitted with a polynomial/power model; saltmarshes linear). Normality was tested with Shapiro–Wilk; data were log-transformed where needed. Slopes and exponents were compared to standard factors via Wald tests. Multiple testing used Bonferroni correction (α adjusted to 0.017). Differences in OC stocks among latitude bands, salinity categories, forest conditions, mangrove types, and Holocene relative sea-level rise (RSLR) zones were tested using Kruskal–Wallis and Mann–Whitney tests. Global stock estimation: Individual study OC stocks originally derived from LOI using constant factors were recalculated using the new ecosystem-specific OC–LOI relationships. Due to non-normal distributions, medians were adopted as representative central tendencies for OC and IC. Global totals were obtained by multiplying per-hectare medians by global mangrove area (81,485 km^2 primary; also reported for 137,760 km^2). Ecosystem carbon stock summed sediment OC and IC with living biomass and dead biomass carbon. CO2 emissions from post-2000 mangrove deforestation were estimated assuming total carbon loss rates comparable to prior studies and an annual area loss rate of 0.17%, converting C to CO2.

• OC–LOI relationships:
  • Mangroves: Strong polynomial (power) relationship, OC = 0.21 × LOI^1.12 (R^2 = 0.86, P < 0.001); exponent (1.12) significantly differs from 1, so a linear model is inappropriate.
  • Saltmarshes: Strong linear relationship (R^2 ≥ 0.99, P < 0.001) with slope 0.52 ± 0.003 (OC:LOI), implying LOI/OC = 1.92 ± 0.01, significantly different from the commonly used 1.724 (P < 0.001). The conventional OC/LOI = 0.58 overestimates saltmarsh OC and biases mangrove OC when used universally.
• Geographic and environmental patterns of mangrove sediment OC stocks:
  • Significant differences across latitude bands 0–40° (K–W χ^2(3) = 116.9, P < 0.001); 0–10° > all others; 10–20° > 20–30° and 30–40° (both P < 0.001); no difference between 20–30° and 30–40° (P = 0.15).
  • Higher OC stocks in zones with high late-Holocene RSLR (zones I–II) vs others (M–W W = 80,070, P < 0.001).
  • Significant differences among salinity categories (K–W χ^2(2) = 23.5, P < 0.001), forest conditions (K–W χ^2(3) = 6.6, P < 0.05), and mangrove types (K–W χ^2(4) = 70.4, P < 0.001).
• Distribution and central tendencies:
  • Individual mangrove sediment OC stocks range: 6.1–1526 Mg ha^-1 (mean 293.9 ± 208.25 Mg ha^-1; CV 70.9%). Data are non-normal; median is more representative.
• Revised per-hectare and global stocks (top 1 m):
  • Sediment OC: median 237.4 Mg ha^-1; global total 1.93 Pg C (81,485 km^2), 26% lower than 2.6 Pg and 35% lower than 2.96 Pg from previous studies.
  • Sediment IC: median 34.7 Mg ha^-1 (14.6% of OC); mean 263.7 Mg ha^-1; global total 0.28 Pg C. High IC confined to carbonate settings (up to ~549 Mg ha^-1 in karst; ~93 Mg ha^-1 near reefs).
  • Total sediment carbon (OC + IC): 272.1 Mg ha^-1; global total 2.21 Pg C.
  • Biomass carbon: living biomass 102.5 ± 12.3 Mg ha^-1 (range 75.8–150.3); dead aboveground 16.7 Mg ha^-1; dead root 59.3 Mg ha^-1.
  • Ecosystem carbon stock (sediment + living + dead biomass): 454.5 Mg ha^-1; global total 3.7 Pg (area 81,485 km^2) to 6.2 Pg (area 137,760 km^2).
• Emissions from mangrove loss (post-2000):
  • Estimated global carbon loss 6.4–10.6 Tg C yr^-1 leading to CO2 emissions of 23.5–38.7 Tg CO2 yr^-1, slightly lower than a prior estimate for the same baseline area due to revised stock baselines.

By deriving ecosystem-specific OC–LOI relationships, the study corrects a key methodological bias in estimating sediment OC from LOI. For mangroves, a polynomial relationship reflects underlying differences in organic matter quality and composition across forest structures and settings, while saltmarshes conform to a linear model with a conversion (LOI/OC ≈ 1.92) higher than the commonly used 1.724. Applying these relationships and using medians to accommodate non-normality reduces overestimation of global mangrove sediment OC stocks and reveals that prior reliance on mean values and inappropriate conversion factors inflated global totals. Incorporating sediment IC and dead biomass rectifies previous underestimates of ecosystem carbon stocks, emphasizing that sediments constitute the majority (~60%) of ecosystem carbon. The documented spatial patterns (latitude, RSLR zones, salinity, forest condition, mangrove type) align with ecological drivers of productivity, accommodation space, and redox conditions, offering a framework to refine spatially explicit carbon models. The revised global stocks underpin more accurate assessments of emission factors from land-use change and strengthen the basis for blue carbon policy and restoration planning.

This study provides improved, ecosystem-specific OC–LOI conversions for mangroves (polynomial) and saltmarshes (linear; LOI/OC ≈ 1.92), recalculates mangrove sediment OC stocks using a robust central tendency (median), and explicitly includes sediment inorganic carbon and dead biomass to estimate total ecosystem carbon. The corrected top-1 m mangrove sediment OC stock is 1.93 Pg, with total sediment carbon 2.21 Pg and ecosystem carbon 3.7–6.2 Pg, and updated emissions from deforestation of 23.5–38.7 Tg CO2 yr^-1. These results indicate previous overestimation of sediment OC and underestimation of ecosystem stocks. Future work should: (1) extend measurements beyond the top 1 m to capture full soil profiles where feasible; (2) refine spatial models with drivers such as rainfall, canopy height, species composition, salinity, and geomorphic setting; (3) improve global coverage in underrepresented regions; (4) better distinguish autochthonous vs allochthonous IC sources and their climatic implications; and (5) integrate updated conversions into standardized blue carbon accounting and remote-sensing products.

• Depth constraint: Global sediment carbon estimates are limited to the top 1 m, while many mangrove soils exceed 3 m, leading to underestimation of total sediment carbon.
• Data representativeness: Although covering ~91.9% of mangrove area, the OC stock dataset spans ~50% of mangrove-supporting countries; some regions and settings (e.g., carbonate systems) may be underrepresented.
• Allochthonous IC handling: High IC values from carbonate-rich settings and allochthonous inputs were treated cautiously or excluded in medians; this choice affects representativeness of global IC estimates and associated climatic interpretation.
• Statistical assumptions: Strong non-normality necessitated use of medians; while robust, this reduces sensitivity to genuine high-end stocks and may underrepresent extremes.
• Emission estimates rely on assumed annual loss rate (0.17%) and similarity of pool-specific losses to prior studies, introducing uncertainty.
• OC–LOI relationships, especially for mangroves, may vary with local sediment composition, vegetation structure, and mineral inputs; applying a single global relationship may not capture all site-level variability.