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

Tidal vegetated wetlands, including mangroves and saltmarshes, are vital blue carbon ecosystems efficiently sequestering and storing carbon, mitigating climate change. Their anaerobic sediments, high productivity, and ability to trap sediment contribute to long-term carbon storage. Mangrove conservation and reforestation are promoted internationally for climate change mitigation. While numerous studies have estimated carbon stocks in these ecosystems, current estimates for mangrove sediment carbon stocks lack precision due to large variations in individual observations and the use of conversion factors derived from other habitats to estimate organic carbon (OC) from loss-on-ignition (LOI). The widely used conversion factors (e.g., LOI/OC = 1.724) are often unsuitable for mangrove sediments, leading to biased global estimates. Furthermore, sediment inorganic carbon (IC) and dead biomass, significant contributors to overall carbon stock, have been largely neglected in previous assessments. This study aims to refine global estimates of mangrove and saltmarsh carbon stocks by improving the LOI-to-OC conversion and incorporating IC and dead biomass. By combining existing data with new field measurements, we aim to establish more accurate relationships between OC and LOI for both mangrove and saltmarsh sediments and consequently, more reliable estimates of global carbon stocks in these vital ecosystems.

Numerous studies have investigated carbon stocks in blue carbon ecosystems, including mangroves and saltmarshes. However, inconsistencies and inaccuracies exist, particularly in estimating mangrove sediment carbon stocks. A significant challenge lies in the conversion of loss-on-ignition (LOI) measurements to organic carbon (OC). Studies have often relied on generalized conversion factors, such as 0.58 or 1.724, which are not universally applicable across diverse sediment types and ecosystems. The use of these factors, often derived from other habitats, introduces substantial bias. Furthermore, previous assessments have often overlooked the contributions of sediment inorganic carbon (IC) and dead biomass to the total carbon stock. The variability in reported sediment OC stocks (ranging from 72–936 Mg ha⁻¹), highlights the need for improved methodologies and more comprehensive data sets to accurately quantify global carbon stocks in mangroves. This research builds upon previous studies but addresses the limitations of using generalized conversion factors and the omission of significant carbon pools by providing a more robust and comprehensive assessment.

This study combined literature data with new field measurements to improve estimates of carbon stocks in mangrove and saltmarsh ecosystems. The researchers compiled a database of published sediment OC stock data from mangroves, encompassing 1727 observations from 52 countries, including new data from four additional countries. A separate database containing 1534 coupled measurements of OC and LOI was also created (Supplementary Table 1). Furthermore, data on sediment IC stock from 100 sites and dead biomass carbon from 225 sites were compiled. Field data on LOI and IC were collected from mangroves and saltmarshes in Hong Kong. Sediment samples were collected using a corer, analyzed for organic matter content using loss-on-ignition (LOI) method, and analyzed for carbon and nitrogen content using an elemental analyzer. The relationships between OC and LOI were analyzed separately for mangroves and saltmarshes using linear regression for saltmarshes and polynomial regression for mangroves. Statistical tests, including Shapiro-Wilk normality test, Kruskal Wallis rank sum test, Mann-Whitney U-tests, and Wald test, were conducted to assess the significance of relationships and differences in carbon stocks across various factors. Global mangrove OC and IC stock were estimated using a bottom-up approach, utilizing either median or mean values depending on the normality assumption of the data. Finally, ecosystem carbon stock was estimated by summing sediment, living, and dead biomass carbon stocks, considering five scenarios based on different studies on living biomass carbon stocks. The Bonferroni correction was applied to adjust for multiple comparisons.

The study revealed distinct relationships between OC and LOI for mangroves and saltmarshes. A strong significant polynomial relationship (R² = 0.86, *P* < 0.001) was observed for mangroves, while a linear relationship (R² ≥ 0.99, *P* < 0.001) was found for saltmarshes. The slope of the regression line for saltmarshes (0.52 ± 0.003) was significantly lower than the conventional conversion factor of 0.58. The exponent of the polynomial relationship for mangroves (1.12) was significantly different from 1, indicating the inadequacy of a linear approximation. Analysis of mangrove sediment OC stocks revealed significant variations across latitude, salinity, forest conditions, and mangrove types. The median sediment OC stock was 237.4 Mg ha⁻¹, resulting in a revised global mangrove sediment OC stock of 1.93 Pg, considerably lower than previous estimates. The median sediment IC stock was 34.7 Mg ha⁻¹, contributing 0.28 Pg to the total sediment carbon stock. The total sediment carbon stock (OC + IC) was estimated at 2.21 Pg. Ecosystem carbon stock, incorporating living and dead biomass, ranged from 3.7 Pg to 6.2 Pg, depending on the estimate of living biomass used. Mangrove deforestation after 2000 is estimated to cause CO₂ emissions of 23.5–38.7 Tg yr⁻¹.

This study's revised estimates of global mangrove and saltmarsh carbon stocks significantly improve upon previous assessments. The use of more appropriate conversion factors for LOI-to-OC, accounting for the distinct relationships in mangroves and saltmarshes, and the inclusion of IC and dead biomass resulted in a more accurate representation of carbon storage in these vital ecosystems. The finding that mangrove sediment OC stock has been overestimated in the past, and ecosystem carbon stock underestimated, highlights the importance of using robust methodologies. The substantial variations in sediment OC stocks observed across various factors underscore the need for geographically and ecologically specific estimations. The considerable contribution of sediments to the total ecosystem carbon stock further emphasizes the importance of protecting and restoring these habitats to mitigate climate change. The estimated CO₂ emissions from mangrove deforestation highlight the urgent need for conservation efforts.

This research provides significantly improved estimates of global carbon stocks in mangrove and saltmarsh ecosystems by employing more precise methodologies and including previously neglected carbon pools. The revised estimates highlight the substantial role of these ecosystems in carbon sequestration and the significant carbon emissions resulting from deforestation. Future research should focus on further refining the relationships between OC and LOI across diverse mangrove and saltmarsh environments, improving the accuracy of biomass estimation techniques, and investigating the long-term dynamics of carbon storage under various environmental and anthropogenic influences. This will enhance our understanding of these crucial ecosystems and their contributions to climate change mitigation.

The study's estimates are based on available data, which may not fully represent the global distribution of mangrove and saltmarsh ecosystems. The accuracy of the estimates relies on the quality and representativeness of the compiled datasets, and uncertainties remain in extrapolating data from specific locations to global scales. The use of a top-meter sediment depth for carbon stock estimation might underestimate the total carbon stored, as mangrove sediments can extend to much greater depths. Furthermore, the variability in living biomass estimations, stemming from diverse methodologies and sources, impacts the uncertainty associated with the estimated ecosystem carbon stock.