Zhu X.D., Sun C.Y. and Qin Z.C.. 2021. Journal of Geophysical Research-Biogeosciences, 126(8):e2021JG006416.
The importance of tidal mangroves in mitigating greenhouse gas (GHG) via sequestering atmospheric carbon dioxide (CO2) has been increasingly recognized, but this climate benefit comes at a biogeochemical cost of methane (CH4) 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 (CO2 and CH4) 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 CO2 sink of −1,075.8 g C m−2 and a CH4 source of 3.1 g C m−2, and the CH4-induced warming effect can offset 4.6% (9.8%) of the CO2-induced cooling effect at a 100-year (20-year) time horizon using the metric of sustained-flux global warming potentials; (b) net CO2 and CH4 fluxes showed different diurnal and seasonal variation patterns, with stronger CO2 sink and CH4 source in colder and warmer seasons, respectively; (c) drought-induced salinity enhancement due to reduced rainfall and river discharge weakened GHG cycling, lowering both CO2 sink and CH4 source in the drier year. This study confirms that ecosystem-level CH4 emissions from estuarine mangroves are not negligible and could substantially offset the CO2-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.
Figure 1. Temporal variations in daily (gray bars) and monthly (red dots) net ecosystem CO2 (NEE) and CH4 (NME) exchange from August 2019 to December 2020 (a), (b), where lighter gray bars denote the gap-filled daily values from the ANN model simulations. Monthly variations in SGWP and CH4 offset potential are also shown (c) with the conversion values of 45 and 96 for 100-year and 20-year time horizons, respectively.