Drought-Induced Salinity Enhancement Weakens Mangrove Greennouse Gas Cycling

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. Plain Language Summary Tidal mangroves have a climate benefit via sequestering atmospheric carbon dioxide, but this benefit could be offset by their methane emissions. Due to the lack of long-term continuous and simultaneous measurements of these two greenhouse gas fluxes, we're short of the knowledge on the extent of this climate offset. Drought-induced salinity enhancement could suppress carbon dioxide and methane exchanges given the recognized role of salinity stress on mangrove biogeochemistry. Based on one-and-a-half-year flux tower measurements capturing a severe spring/summer drought event, we examine the responses of greenhouse gas fluxes to environmental factors in particular for drought in a subtropical estuarine mangrove in the Southeast China. As a carbon dioxide sink and a methane source, this mangrove sequesters more carbon dioxide in colder seasons but emits more methane in warmer seasons. Drought-induced salinity enhancement due to reduced rainfall and river discharge weakens greenhouse gas cycling, simultaneously lowering carbon dioxide sink and methane source. This study confirms that the warming effect from methane emissions could substantially offset the cooling effect from carbon dioxide uptake. Future climate change including higher temperature and salinity will likely weaken the climate benefits of mangroves.

Automated summary

This study confirms that estuarine mangroves play a significant role in both absorbing carbon dioxide and emitting methane, with the warming effect of methane emissions potentially offsetting the cooling effect of carbon dioxide uptake. The lack of long-term continuous measurements has led to uncertainties in understanding the climate offset caused by these greenhouse gas fluxes. Drought-induced salinity enhancement weakened the greenhouse gas cycling of mangroves, reducing both the carbon dioxide sink and methane source.