Methane and Nitrous Oxide Emissions Complicate Coastal Blue Carbon Assessments

The role of coastal wetlands as natural ‘blue carbon’ sinks may be counter‐acted by emissions of methane (CH4) and nitrous oxide (N2O). Site‐specific fluxes of these two potent greenhouse gases from coastal wetlands show high spatial and temporal variability , making global estimates sensitive to statistical assumptions and uncertainties. Here, we review the magnitude of CH4 and N2O fluxes from mangroves, saltmarshes and seagrasses and identify direct and indirect drivers that can control production and consumption pathways. Significant research is required to better quantify fluxes and to understand factors causing flux variability from different transport pathways before restoration of coastal blue carbon ecosystems can be reliably used for climate mitigation. Plain Language Summary Mangroves, saltmarshes, and seagrass meadows are ‘blue carbon’ ecosystems that provide a wide range of societal benefits, including carbon storage in their sediments and biomass in the case of mangroves. Yet they can also be a source—and sometimes a sink—of two powerful greenhouse gases, methane and nitrous oxide. Many processes are involved in the release and uptake of these gases; as a result, the local variability of coastal wetland emissions is high and global budgets are uncertain. The restoration of coastal blue carbon ecosystems is worthwhile for many reasons, and their existing carbon stores need to be kept intact. Nevertheless, their potential as a natural climate solution needs further attention, since it may have been over‐stated. Key Points Methane and nitrous oxide fluxes from coastal vegetated ecosystems are highly variable The potential global offsets of blue carbon sinks by CH4 and N2O emissions remain uncertain An adaptive management framework is needed for site‐specific monitoring for blue carbon benefits