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

Abstract 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.