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

Abstract

Abstract River ecosystems contribute significantly to CO 2 emissions. However, estimates of global riverine CO 2 emissions remain greatly uncertain owing to the absence of a comprehensive and spatially resolved CO 2 emission measurement. Based on intensive field measurements using floating chambers, riverine CO 2 evasion in the Wuding River catchment on the Loess Plateau was investigated. Lateral carbon derived from soil respiration and chemical weathering played a central role in controlling the variability of riverine CO 2 partial pressure ( p CO 2 ). In addition, in‐stream processing of allochthonous organic carbon was an also important source of CO 2 excess, modulating the influence of lateral carbon inputs. All the surveyed streams were net CO 2 sources, exhibiting pronounced spatial and seasonal variabilities. The mean CO 2 efflux was 172, 116, and 218 mmol m −2 d −1 in spring, summer, and autumn, respectively. Unlike the commonly observed strongest CO 2 emissions in headwater streams, the increasing CO 2 efflux with stream order in the Wuding River catchment reflects its unique geomorphologic landscape in controlling CO 2 emissions. While in reservoirs, the p CO 2 was more controlled by primary production with aquatic photosynthetic assimilation constraining it to a lower level. Both the magnitude and direction of CO 2 evasion from reservoirs have been greatly altered. Contrast to streams with large CO 2 effluxes, reservoirs were small carbon sources and even carbon sinks, due primarily to greatly reduced turbulence and enhanced photosynthesis. In view of the large number of reservoirs on the Loess Plateau, assessing the resulting changes to CO 2 emissions and their implications for regional carbon budgets warrants further research.