Coastal wetland ecosystems deliver large carbon stocks in tropical Mexico

•The belowground C pool dominated total C stocks.•Swamp forests and marshes had substantial peat deposits.•The lowest carbon stock was in grazed flooded grasslands.•Wetland peat deposits should be a target for conservation. Tropical wetlands are important in the global carbon (C) cycle and climate system. To elaborate government policies that protect wetland ecosystem services, spatially explicit data on the existing C stocks are needed. This study quantified above and below ground C stocks in mangrove, swamp forests, marshes, and grazed wetlands in three different regions in Mexico (Veracruz, Tabasco/Campeche, and Chiapas states), using a combination of remote sensing techniques and field data collection. We report a 0.688 Gt C stock in the areas investigated showing that these wetlands hold important C stocks. The total C stock of Mexico’s wetlands is expected to be considerably larger as our study is not a complete survey of Mexico’s total wetland area. Across the three regions, the currently unprotected swamp forests and marshes had substantial peat deposits storing ≈ 0.232 and 0.375 Pg C, respectively. The high C stocks found in swamp forests (1801 ± 283 Mg C ha−1) and marshes (1100 ± 376 Mg C ha−1) are likely to be vulnerable to human interventions such as drainage. Flooded grasslands, used for grazing, had substantially lower C storage (271 ± 122 Mg C ha−1), showing that grazed areas are less effective as C stores. Across sites the belowground C pool that was two orders of magnitude greater than the aboveground biomass C stocks. Carbon storage in wetland vegetation types differed among the three study regions, indicating the need for developing government policies considering a specific regional approach to protect the C stocks of coastal wetlands. Given that freshwater swamps had the greatest C stocks and freshwater marshes stored most C, these wetland peat deposits should be prioritized for conservation, via new legislation or inclusion in payment for ecosystem services schemes.