Strong CH4 emissions modulated by hydrology and bed sediment properties in Qinghai-Tibetan Plateau rivers

Fluvial systems evade massive amounts of greenhouse gases to the atmosphere where stream hydrology and riverbed biogeochemistry imposes important in situ controls. Current paradigm however lacks an effective framework to account for this effect. Here, we investigated CH4 and CO2 emissions from 28 rivers on the QinghaiTibetan Plateau, an area of strong fluvial emissions and high climate sensitivity. We found two orders of magnitude higher CH4 fluxes in silt-dominated (the finest riverbed sediment type) than in sand- or graveldominated rivers (1,200 +/- 1,290 vs 88 +/- 83 and 93 +/- 77 mu mol m- 2 d-1; p < 0.05). Importantly, sediment silt% scaled closely with CH4 concentration and flux across the rivers (R2 = 0.16-0.30). Bed sediment properties (i.e., organic substrate contents, redox conditions and methanogen abundance) together explained 76 % of CH4 emission from the rivers and a higher percentage of porewater (87 %) than of surface CH4 concentration (69 %), suggesting a direct control of fluvial CH4 emission by hydrology and sediment properties in these rivers. We propose particle size served as an integral proxy for favorable conditions of fluvial CH4 production, due to strong dependence of benthic substrates, anaerobic conditions and microbial colonization on fine particles. The dependence of fluvial CO2 emission on riverbed sediment properties was however weak due to multiple production pathways not controlled by sediment anaerobic metabolisms. These findings extended our understandings on the important in situ controls of fluvial CH4 and CO2 emissions and suggest the possibility of improving the estimates by referring to riverbed sediment properties.

Automated summary

Fluvial systems on the Qinghai-Tibetan Plateau emit large amounts of CH4 and CO2, with silt-dominated rivers showing significantly higher CH4 fluxes compared to sand- or gravel-dominated rivers. Sediment silt percentage is closely related to CH4 concentration and flux, suggesting a direct control of hydrology and sediment properties on CH4 emission in these rivers.