CO₂ outgassing from the Yellow River network and its implications for riverine carbon cycle

Abstract

CO 2 outgassing across water-air interface is an important, but poorly quantified, component of riverine carbon cycle, largely because the data needed for flux calculations are spatially and temporally sparse. Based on compiled data sets measured throughout the Yellow River watershed and chamber measurements on the main stem, this study investigates CO 2 evasion and assesses its implications for riverine carbon cycle. Fluxes of CO 2 evasion present significant spatial and seasonal variations. High effluxes are estimated in regions with intense rock weathering or severe soil erosion that mobilizes organic carbon into the river network. By integrating seasonal changes of water surface area and gas transfer velocity (k), the CO 2 efflux is estimated at 7.9 ± 1.2 Tg C yr À1 with a mean k of 42.1 ± 16.9 cm h À1. Unlike in lake and estuarine environments where wind is the main generator of turbulence, k is more correlated with flow velocity changes. CO 2 evasion in the Yellow River network constitutes an important pathway in its riverine carbon cycling. Analyzing the watershed-scale carbon budget indicates that 35% of the carbon exported into the Yellow River network from land is degassed during fluvial transport. The CO 2 efflux is comparable to the carbon burial rate, while both larger than the fluvial export to the ocean. Comparing CO 2 evasion with ecosystem productivity in the Yellow River watershed shows that its ecosystem carbon sink has previously been overestimated by >50%. Present efflux estimates are associated with uncertainty, and future work is needed to mechanistically understand CO 2 evasion from the highly turbid waters.

Riverine CO2 emissions in the Wuding River catchment on the Loess Plateau: Environmental controls and dam impoundment impact

“…This is probably due to the nearly constant depth measurements with limited variations. Contrast to large rivers with depth typically exceeding 10 m, the shallower depths (0.03–1.32 m; Table ) imply that the turbulence generated by bed traction can be efficiently advected to surface [ Ran et al , ]. However, the deeply incised streams are characterized by gentle slopes (0.0001–0.013) and uniform bed composition composed of loess soils.…”

Carbon dioxide and methane dynamics in a human-dominated lowland coastal river network (Shanghai, China)

“…Therefore, despite the potential underestimations on f CO 2 , the ratio of CO 2 evasion to DIC export for the mainland river network was approximately 4.1:1, higher than those for the Mississippi River basin (0.9:1 [ Dubois et al , ]) and the Yukon River basin (1:1 [ Striegl et al , ]). If the upstream watershed that contributed to the DIC export was included in the evasion estimate, the C evasion/export ratio of the Shanghai river network could be potentially comparable to those for the Yellow River basin (4.7:1 [ Ran et al , ]) and the Amazon basin (6.6:1 [ Richey et al , ]). The revealed high evasion/export ratio is partially due to the large river surface area in this lowland region (~9%) but also highlights the potential of lowland coastal rivers as open reactors mineralizing terrestrial and anthropogenic OC and wide conduits for CO 2 evasion near the end of the stream‐river continuum.…”

Regional-scale lateral carbon transport and CO<sub>2</sub> evasion in temperate stream catchments

“…C burial in sediments was neglected in this study but can make a significant contribution to catchment-scale C balances. Estimates vary between 22 % at a global scale (Aufdenkampe et al, 2011), 14 % for the conterminous US and 39 % for the Yellow River network (Ran et al, 2015). However, C storage in aquatic systems occurs mainly in lakes and reservoirs, which are virtually absent in the study area.…”