Drought‐Induced Salinity Enhancement Weakens Mangrove Greenhouse Gas Cycling

Abstract The importance of tidal mangroves in mitigating greenhouse gas (GHG) via sequestering atmospheric carbon dioxide (CO 2 ) has been increasingly recognized, but this climate benefit comes at a biogeochemical cost of methane (CH 4 ) emissions. Previous studies have assessed the net radiative effect of mangrove GHG fluxes, however, large uncertainty still exists due to the very limited availability of long‐term continuous measurements. In this study, we analyzed the temporal variations of GHG (CO 2 and CH 4 ) fluxes and their environmental controls based on eddy covariance measurements in a subtropical estuarine mangrove in the Southeast China during 2019 and 2020, when a severe drought occurred. The results showed (a) annually this mangrove acted as a CO 2 sink of −1,075.8 g C m −2 and a CH 4 source of 3.1 g C m −2 , and the CH 4 ‐induced warming effect can offset 4.6% (9.8%) of the CO 2 ‐induced cooling effect at a 100‐year (20‐year) time horizon using the metric of sustained‐flux global warming potentials; (b) net CO 2 and CH 4 fluxes showed different diurnal and seasonal variation patterns, with stronger CO 2 sink and CH 4 source in colder and warmer seasons, respectively; (c) drought‐induced salinity enhancement due to reduced rainfall and river discharge weakened GHG cycling, lowering both CO 2 sink and CH 4 source in the drier year. This study confirms that ecosystem‐level CH 4 emissions from estuarine mangroves are not negligible and could substantially offset the CO 2 ‐induced cooling effect. Future increases in temperature and salinity with expected global warming and sea level rise will likely weaken the climate benefits of mangroves.