The synergy of environmental and microbial variations caused by hydrologic management affects the carbon emission in the Three Gorges Reservoir

The synergy of environmental and microbiological changes caused by hydrologic management on carbon emissions of river reservoirs remains unknown. Here, we investigated physiochemistry parameters, compositions of dissolved organic matter (DOM), carbon fluxes (CH4 and CO2), and microbial communities in the surface waters of the Three Gorges Reservoir (TGR) within one whole hydrological year. The results showed that hydrologic management significantly changed physiochemistry and DOM composition of the TGR water, and further influenced microbial community composition and functions. DOM content during the drainage period was much lower than during the impoundment period. During the impoundment period, humification extent of DOM became decreasing, while biotransformation extent became increasing compared with the drainage period. DOM composition and water pH exhibited significant correlation with the fluxes of CH4 and CO2, respectively. Microbial community composition and function significantly differed between the drainage and impoundment periods. Most of the differential microbial taxa were affiliated with functional groups involved in carbon cycle such as methanotrophy and phototrophy, which showed significant correlation with carbon fluxes. CH4 and CO2 fluxes can be mostly explained by synergy of microbial function with DOM composition and water pH, respectively. Such synergistic effect may account for the observed temporal variations of CH4 fluxes and spatial variations of CO2, and for the relatively low annual carbon emissions in the TGR. In summary, the synergy of environmental and microbial variations caused by hydrologic management affects carbon emissions from river reservoirs. [Display omitted] •Hydrologic management changed the physiochemistry, microbial communities, and C emission.•In the impoundment period, humification index of DOM decreased, but biological index increased.•Microbes showed more spatial difference in the drainage period than in the impoundment period.•The differential microbial taxa were affiliated with functional groups involved in carbon cycle.•The synergy of physiochemical and microbial changes controls the C emissions from the TGR.