The climate benefit of seagrass blue carbon is reduced by methane fluxes and enhanced by nitrous oxide fluxes

Blue carbon is carbon stored long-term in vegetated coastal ecosystems, which constitutes an important sink for atmospheric carbon dioxide (CO2). However, because methane (CH4) and nitrous oxide (N2O) have higher global warming potentials (GWP) than CO2, their production and release during organic matter diagenesis can affect the climate benefit of blue carbon. Here, we present a meta-analysis synthesizing seagrass CH4 and N2O fluxes and long-term organic carbon burial rates, and use these data to estimate the reduced climate benefit (offsets) of seagrass blue carbon using three upscaling approaches. Mean offsets for individual seagrass species (34.7% GWP20;1.0% GWP100) and globally (33.4% GWP20;7.0% GWP100) were similar, but GWP20 offsets were higher, and GWP100 offsets were lower than globally, for the Australian region (41.3% GWP20;1.1% GWP100). This study highlights the importance of using long-term organic carbon burial rates and accounting for both CH4 and N2O fluxes in future seagrass blue carbon assessments. Methane and nitrous oxide release from seagrass ecosystems offset around 7% to 33% of the global climate benefits of atmospheric carbon dioxide drawdown by seagrasses, suggest in situ measurements and a global synthesis of seagrass greenhouse gas fluxes and long-term organic carbon burial rates.

Automated summary

Blue carbon refers to carbon stored in vegetated coastal ecosystems for a long time, which is an important sink for atmospheric CO2. However, the production and release of methane and nitrous oxide during organic matter decomposition can affect the climate benefit of blue carbon. A meta-analysis of seagrass methane and nitrous oxide fluxes and organic carbon burial rates was conducted, and the reduced climate benefit of seagrass blue carbon was estimated using three upscaling approaches. The study found that methane and nitrous oxide release from seagrass ecosystems offset around 7% to 33% of the global climate benefits of CO2 drawdown by seagrasses. This highlights the importance of considering methane and nitrous oxide fluxes and using long-term organic carbon burial rates in future assessments of seagrass blue carbon.