Abstract. Carbon transport in river systems is an important component of the global carbon cycle. Most rivers of the world act as atmospheric CO2 sources due to high riverine CO2 partial pressure (pCO2). By determining the pCO2 from alkalinity and pH, we investigated its spatial and temporal variation in the Yellow River watershed using historical water chemistry records (1950s–1984) and recent sampling along the mainstem (2011–2012). Except the headwater region where the pCO2 was lower than the atmospheric equilibrium (i.e. 380 μatm), river waters in the remaining watershed were supersaturated with CO2. The average pCO2 for the watershed was estimated at 2810 ± 1985 μatm, which is 7-fold the atmospheric equilibrium. As a result of severe soil erosion and dry climate, waters from the Loess Plateau in the middle reaches had higher pCO2 than that from the upper and lower reaches. From a seasonal perspective, the pCO2 varied from about 200 μatm to > 30 000 μatm with higher pCO2 usually occurring in the dry season and lower pCO2 in the wet season (at 73% of the sampling sites), suggesting the dilution effect of water. While the pCO2 responded exponentially to total suspended solids (TSS) export when the TSS concentration was less than 100 kg m−3, it decreased slightly and remained stable if the TSS concentration exceeded 100 kg m−3. This stable pCO2 is largely due to gully erosion that mobilizes subsoils characterized by low organic carbon for decomposition. In addition, human activities have changed the pCO2 dynamics. Particularly, flow regulation by dams can diversely affect the temporal changes of pCO2, depending on the physiochemical properties of the regulated waters and adopted operation scheme. Given the high pCO2 in the Yellow River waters, large potential for CO2 evasion is expected and warrants further investigation.

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Carbon dioxide (CO2) evasion from inland waters is an important component of the global carbon cycle. However, it remains unknown how global change affects CO2 emissions over longer time scales. Here, we present seasonal and annual fluxes of CO2 emissions from streams, rivers, lakes, and reservoirs throughout China and quantify their changes over the past three decades. We found that the CO2 emissions declined from 138 ± 31 Tg C yr−1 in the 1980s to 98 ± 19 Tg C yr−1 in the 2010s. Our results suggest that this unexpected decrease was driven by a combination of environmental alterations, including massive conversion of free-flowing rivers to reservoirs and widespread implementation of reforestation programs. Meanwhile, we found increasing CO2 emissions from the Tibetan Plateau inland waters, likely attributable to increased terrestrial deliveries of organic carbon and expanded surface area due to climate change. We suggest that the CO2 emissions from Chinese inland waters have greatly offset the terrestrial carbon sink and are therefore a key component of China’s carbon budget.

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River ecosystems contribute significantly to CO2 emissions. However, estimates of global riverine CO2 emissions remain greatly uncertain owing to the absence of a comprehensive and spatially resolved CO2 emission measurement. Based on intensive field measurements using floating chambers, riverine CO2 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 CO2 partial pressure (pCO2). In addition, in‐stream processing of allochthonous organic carbon was an also important source of CO2 excess, modulating the influence of lateral carbon inputs. All the surveyed streams were net CO2 sources, exhibiting pronounced spatial and seasonal variabilities. The mean CO2 efflux was 172, 116, and 218 mmol m−2 d−1 in spring, summer, and autumn, respectively. Unlike the commonly observed strongest CO2 emissions in headwater streams, the increasing CO2 efflux with stream order in the Wuding River catchment reflects its unique geomorphologic landscape in controlling CO2 emissions. While in reservoirs, the pCO2 was more controlled by primary production with aquatic photosynthetic assimilation constraining it to a lower level. Both the magnitude and direction of CO2 evasion from reservoirs have been greatly altered. Contrast to streams with large CO2 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 CO2 emissions and their implications for regional carbon budgets warrants further research.