Diurnal variations in greenhouse gas diffusive emissions from a river covered with submerged macrophytes: environmental and biotic driving factors

Rivers are globally important sources of carbon emissions, but the diurnal pattern of greenhouse gas (GHG) emissions from rivers colonized with submerged macrophytes has rarely been explored. We determined the dissolved concentrations and diffusive fluxes of CO₂, CH₄, and N₂O from a river covered with submerged macrophytes under different meteorological conditions (i.e., nighttime, cloudy, and sunny daytime) over a continuous 36-h period. Overall, the river functioned as a CO₂ sink during the daytime, while it transitioned to a CO₂ source at nighttime, primarily because of diurnal variations in plant metabolisms. No similar diurnal fluctuations in CH₄ or N₂O emissions were detected, and the highest fluxes of CH₄ or N₂O were measured during sunny daytime. In total, the river emitted more carbon at nighttime, with CH₄ contributing most to the total emissions on the basis of a CO₂ equivalent. Dissolved organic matter (DOM) in the river, whose properties were potentially modulated by submerged macrophytes, considerably influenced GHG emissions. Distinct dissimilarities were observed in the microbial communities inhabiting the river sediment and biofilms on plant leaves. Microbial communities in the sediment played more important roles in biogeochemical cycles, while the regulatory effects of epiphytic microbiota on GHG emissions should not be ignored. Multiple regression and structural equation modeling analyses revealed that the dissolved oxygen concentration, temperature, humification index (HIX) of DOM, and NH₄⁺–N concentration significantly influenced CO₂ diffusive fluxes, while CH₄ dynamics were predominantly influenced by DOM properties. Salinity, HIX, NH₄⁺-N concentration, and microbial consortia were the predominant factors influencing N₂O emissions.