Tidal-driven N2O emission is a stronger resister than CH4 to offset annual carbon sequestration in mangrove ecosystems.

The mangrove ecosystems store a significant amount of "blue carbon" to mitigate global climate change, but also serve as hotspots for greenhouse gases (GHGs: CO2, CH4 and N2O) production. The CH4 and N2O emissions offset mangrove carbon benefits, however, the extent of this effect remains inadequately quantified. By applying the 36 h time-series observations and mapping cruises, here we investigated the spatial and temporal distribution of GHGs and their fluxes in Dongzhaigang (DZG) bay, the largest mangrove ecosystem in China, at tidal and monthly scales. The spatiotemporal variation of GHGs were mainly controlled by tidal-forced water mixing, outgassing and multiple biogeochemical processes. Tidal-driven porewater outwelling and sediment resuspension largely explained the excess addition of dissolved GHGs in tidal creeks. These lateral export combined with river input contribute significantly to surrounding water emission. Salinity controls CH4 emission in river-tidal creek-bay continuum, with concentration decreased by ∼100-fold from freshwater to seawater sites. N2O concentration was controlled by substrate supply for both nitrification and denitrification. Overall, the GHGs emissions in tidal creeks equal to 669-39,000 (7521 ± 6401) g CO2-eq/m2/yr (CO2 contributed ∼88-91 %) to atmosphere. In particular, CH4 and N2O contribute 8-366 (124 ± 78) g CO2-eq/m2/yr and 59-2260 (712 ± 525) g CO2-eq/m2/yr, together offsetting 8.7 ± 2.1 % of annual mangrove carbon sequestration, with a larger contribution from N2O (7.4 %). Our findings highlight the importance of simultaneous quantification of non-CO2 GHGs to accurately assess blue carbon capacity.