Abstract
Maximizing carbon sequestration in mangroves is part of the global effort to combat the climate crisis. However, methane (CH4) emissions can partially offset carbon sequestration in mangroves. Previous estimates have suggested that CH4 emissions offset organic carbon burial by 20% in mangroves with substantial freshwater inputs and/or in highly impacted mangroves. Here we resolve the magnitude and drivers of the mangrove CH4 offset using multiple isotopic tracers across a latitudinal gradient. CH4 emission offsets were smaller in high-salinity (~7%) than in freshwater-influenced (~27%) mangroves. Carbon sequestration was disproportionally high compared with CH4 emissions in understudied tropical areas. Low CH4 emissions were explained by minor freshwater inputs minimizing CH4 production in saline, high-sulfate conditions and intense CH4 oxidation in porewaters and surface waters. CH4 oxidation in mangrove surface waters reduced potential aquatic CH4 emissions by 10–33%. Overall, carbon sequestration through mangrove preservation and restoration is less affected by CH4 emissions than previously thought.
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The raw datasets of all new observations and literature compilation are available on figshare (https://doi.org/10.6084/m9.figshare.24204351)75. Source data are provided with this paper.
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Acknowledgements
Funding for field investigations and analytical instrumentation was provided by the Australian Research Council to I.R.S. (LE120100156 and DE140101733). Some of the analysis and travel were funded by the Swedish Research Council to I.R.S. (2020-00457). G.A. was supported by the French-Brazilian International research project VELITROP (CNRS, INEE). L.C.C. thanks the Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP, INT-00159-00009.01.00/19) and the Post Graduate Program in Tropical Marine Sciences of the Federal University of Ceará (UFC-PRPPG) for a visiting researcher grant at the Marine Sciences Institute (LABOMAR). L.C.C. also thanks the German Federal Ministry of Education and Research through the Project Carbostore, grant no. 03F0875B, for the Postdoctoral Fellowship Grant.
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L.C.C. performed most of the data analysis, made tables and figures, and wrote the first draft with support from I.R.S., G.A. and others. I.R.S. designed, managed and obtained funding for the project. G.A. supported the interpretation of methane isotopic composition and calculation of oxidation rates. C.J.S. was responsible for sediment analysis and carbon burial estimates. D.R.T., J.Z.S., C.H. and D.T.M. helped design the field campaign and performed field and laboratory work. D.T.M. and J.Z.S. calculated methane emissions and drafted some of the methods section. Y.Y.Y.Y. drafted the introduction and performed some of the literature review. All authors edited the paper and approved its submission.
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Extended data
Extended Data Fig. 1 Global map of study areas included in the compilation of CO2 and CH4 emissions.
Global map of study areas including CO2 and CH4 emission sites (blue dots, compiled data) and six field study sites in Australia (red numbered dots) showed in Fig. 1 of the main manuscript. The six panels represent digital elevation models of the field sites and their catchments. Image of global mangrove area modified from Giri et al.74. The reference list containing all compiled data can be found at: https://doi.org/10.6084/m9.figshare.24204351.
Extended Data Fig. 2 Time series observations of CH4 concentrations.
Time series observations of CH4 concentrations (Y left axis) and tidal height (Y right axis) in the six marine-dominated mangroves. Note the different Y-axes (left and right) scales. Grey shaded area represents tidal height.
Extended Data Fig. 3 Time series observations of δ13C-CH4 signatures.
Time series observations of δ13C-CH4 signatures (Y left axis) and tidal height (Y right axis) in the six marine-dominated mangroves. Note the different Y-axes (left and right) scales. Grey shaded area represents tidal height.
Extended Data Fig. 4 Time series observations of water CO2 partial pressure (pCO2) values.
Time series observations of water CO2 partial pressure (pCO2) values (Y left axis) and tidal height (Y right axis) in the six marine-dominated mangroves. Note the different Y-axes (left and right) scales. Grey shaded area represents tidal height.
Extended Data Fig. 5 Time series observations of δ13C-CO2 signatures.
Time series observations of δ13C-CO2 signatures (Y left axis) and tidal height (Y right axis) in the six marine-dominated mangroves. Note the different Y-axes (left and right) scales. Grey shaded area represents tidal height.
Extended Data Fig. 6 Relationship between pCO2 and CH4 in the six marine-dominated mangroves.
The color gradient represents 222Rn concentrations.
Extended Data Fig. 7 Global upscaling of CH4 emissions and organic carbon burial in sediments.
A) Median (interquartile range) rates of CH4 emissions and organic carbon burial in global seawater-dominated mangroves. Mangrove ecosystems were considered being inundated 65% of the time (water-atmosphere flux) and exposed 35% of the time (sediment-atmosphere flux) in seawater-dominated mangroves. B) Median (interquartile range) rates of CH4 emissions and organic carbon burial in global freshwater-influenced mangroves. Mangrove ecosystems were considered being inundated 50% of the time (water-atmosphere flux) and exposed 50% of the time (sediment-atmosphere flux) in freshwater-influenced mangroves. Details on the upscaling calculation appear in the methods section. Credits: trees, Diana Kleine, Marine Botany UQ, under Creative Commons license CC BY SA 4.0; intertidal mud, Dieter Tracey, Coastal CRC, under Creative Commons license CC BY SA 4.0.
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Supplementary Information
Supplementary Discussion, Figs. 1–7, Tables 1–8 and References.
Source data
Source Data Fig. 1
Raw data used to construct Fig. 1.
Source Data Fig. 2, 3 and Source Data Extended Data Fig. 1-6
Raw data including all observations of CH4 concentrations, emissions and ancillary parameters in surface waters and porewaters used to calculate the CH4 emissions and the offsets.
Source Data Fig. 4 and Source Data Extended Data Fig. 7
Global data compilation used to calculate global-scale CH4 offsets to organic carbon burial in mangroves.
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Cotovicz, L.C., Abril, G., Sanders, C.J. et al. Methane oxidation minimizes emissions and offsets to carbon burial in mangroves. Nat. Clim. Chang. 14, 275–281 (2024). https://doi.org/10.1038/s41558-024-01927-1
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DOI: https://doi.org/10.1038/s41558-024-01927-1