Microbial-mediated CH4 and CO2 emissions and environmental controls in the fluvial sediment across hydrological seasons.

Fluvial sediments are recognized as critical sources of methane (CH4) and carbon dioxide (CO2) emissions. However, the interactions between microbial communities and carbon emissions in these sediments remain poorly understood. This study investigated carbon fluxes at the water-air interface in the mainstream of the Liao River, integrating environmental parameters and microbial communities within sediments to elucidate seasonal differences in carbon cycling mechanisms. The results highlighted the wet season as a hot period for CH4 and CO2 emissions, with fluxes in the lower reach (CH4: 78.6 ± 55.79 mg m-2·d-1; CO2: 4.82 ± 1.59 g m-2·d-1) notably exceeding those in other sections. During the wet season, Pseudomonadota (recently reclassified from Proteobacteria) predominated, whereas Acidobacteriota and Cyanobacteriota showed substantial increases in relative abundance by 41.87 % and 83.60 %, respectively, in the dry season. Methanoregula and Methanospirillum were significantly enriched in the lower reach (p < 0.05), with their abundance positively correlating with CH4 fluxes (r = 0.75). Methylocaldum reached peak abundance in the midstream (34.00 ± 35.61 %) and exhibited a significant correlation with CO2 fluxes (p < 0.03). Structural equation modeling revealed that water parameters (standardized path coefficient = 1.45) were the primary factors influencing carbon fluxes during the wet season, whereas microbial-environmental interactions diminished in the dry season. This study offers microbiological insights into the river carbon cycle's response to hydrological changes and elucidates the microbial-driven mechanisms that make urbanized downstream areas hotspots for carbon emissions.