Tidal variation and litter decomposition co-affect carbon emissions in estuarine wetlands

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Estuarine wetlands play important roles in the regional and global carbon cycle as well as greenhouse gas emissions; however, the driving factors and potential carbon emissions mechanisms are unclear. Here, the carbon emission fluxes were investigated in situ from different vegetated areas in the Chongming wetlands. The results showed that the highest methane (CH4) and carbon dioxide (CO2) emissions of 178.1 and 21,482.5 mg∙m−2∙min−1 were in Scirpus mariqueter and Phragmites australis dominated areas, respectively. A series of microcosms was strategically designed to simulate the influence of tidal variation on carbon emissions and the litter decomposition on daily- and monthly-timescales in estuarine wetlands. All added litter promoted CH4 and CO2 emissions from the wetland soils. The CH4 and CO2 emission fluxes of the S. mariqueter treatment were higher (367.7 vs. 108.4; 1607.9 vs. 1324.3 mg∙m−2∙min−1) than those of the P. australis treatment without tidal variation on a monthly timescale, due to the higher total organic carbon (TOC) content of S. mariqueter. The decomposition of litter also released a large amount of nutrients, which enhanced the abundance of methane-producing archaea (MPA) and methane-oxidizing bacteria (MOB). However, the tidal water level was negatively correlated with CH4 and CO2 emission fluxes. The CH4 and CO2 emission fluxes in the S. mariqueter treatment at the lowest tide were 556.02 and 604.99 mg∙m−2∙min−1, respectively. However, the CH4 and CO2 emission fluxes did not change significantly on the daily timescale in the S. mariqueter treatment without tidal variations. Therefore, the prolonged timescales revealed increases in litter decomposition but a decrease in the contribution of tidal variations to carbon emissions in estuarine wetlands. These findings provide a theoretical basis for evaluating the carbon cycle in estuarine wetlands.