Significant CH4 Emissions from the Yellow River: Importance of Suspended Sediment

Rivers play a non-negligible role in global methane (CH4) emissions. However, little research has focused on CH4 emissions from rivers with high suspended sediment (SPS) concentrations, and how SPS influences riverine CH4 emissions is still unclear. In this study, significant CH4 emissions were observed in the upper reaches of the Yellow River, despite the extremely low organic-C level (DOC = 2.2 mg/L). The average value of CH4 flux was 107.7 μmol m–2 d–1, which was even higher than that of urban rivers with high organic pollution. The CH4 emission pattern in the Yellow River differed from that of typical rivers; SPS rather than sediment plays an important role. In the upper reaches, anaerobic/aerobic microenvironments are more easily established on SPS under lower DO conditions, resulting in a high abundance of methanogens and functional genes. The genus Methanobacterium, typically found in sediments, was dominant in the water column and positively related to SPS concentrations. The strong winds and water flow further enhanced the mass transfer from the SPS surface to the atmosphere. Overall, this study demonstrates the significant potential of low-organic-country rivers to act as CH4 hotspots in the presence of SPS.