Long-term spatial and temporal variation of CO<sub>2</sub> partial pressure in the Yellow River, China

Abstract

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.

Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems: Concepts, emerging trends, and research challenges

“…The annual DIC flux of the Mekong was 3.95 Tg, with DIC and alkalinity both negatively correlated with discharge (J. Richey, unpublished data). This trend, which has also been observed in the Pearl and Yellow River (Ran et al, 2015a), has been attributed to the dominance of a weathering-based source in the dry season which is diluted by less ion-rich source (rainwater) during the high-flow periods (Cai et al, 2008). The mean of reported values for the lower Mekong River is 1235 415 atm (Table 3).…”

“…3), the organic C in highly turbid waters of many Asian rivers can become an important source of CO2 during the fluvial transport and sediment storage. Although the rate of CO2 evasion from the water surface can increase in the impounded river reaches as longer residence times tend to create favourable environments for microbial biodegradation of organic C (Ittekkot et al, 270 1985;Ran et al, 2015a), the countering effect of enhanced planktonic uptake of CO2 in the euphotic reservoir surface has rarely been compared against biodegradation (refer to the wide range of pCO2 summarized in Table 3). Based on extensive CO2 evasion measurements in the river-reservoir-river continuum on the Loess Plateau, Ran et al (2017a) found that the Loess Plateau reservoirs acted as relatively small sources or even sinks of C, due largely to the significantly reduced turbulence and enhanced photosynthesis.…”

“…It is very difficult to evaluate overall pollution impacts on the mainstem Mekong River as very few systematic measurements of the nutrient and C cycles of the Mekong River have been made, except for the short reaches of the lower Mekong in Laos 405 and Cambodia (Alin et al, 2011;Ellis et al, 2012;Martin et al, 2013) Richey et al, 1990;Mayorga and Aufdenkampe, 2002) and high-alkalinity 410 rivers such as the Mississippi and Yellow River (1.5 -3 mM; Raymond and Cole, 2003;Ran et al, 2015a). The annual DIC flux of the Mekong was 3.95 Tg, with DIC and alkalinity both negatively correlated with discharge (J. Richey, unpublished data).…”

Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater

“…2). Very high levels of pCO 2 observed in the lower reach of the Han River (up to 4132 atm) and its three 290 urban tributaries (up to 11970 atm) fall in the high ranges found in some polluted rivers in Europe (Kempe, 1984;Frankignoulle et al, 1998;Borges et al, 2006) and China (Yao et al, 2007;Ran et al, 2015;Liu et al, 2016;. Consistent with these previous reports on pCO 2 and other studies reporting high levels of CH 4 and N 2 O in polluted urban waters (Garnier et al, 2013;Yu et al, 2013;Smith et al, 2017;Wang et al, 2017bWang et al, , 2018, the results observed in this study emphasize the dominant influence of urban tributaries carrying wastewater as a primary anthropogenic source of GHGs 295 in the highly urbanized river system.…”

Comparing soil carbon loss through respiration and leaching under extreme precipitation events in arid and semi-arid grasslands

“…It should be 20 mentioned that the DIC concentration may be underestimated due to CO 2 outgassing during leachate collection. However, the potential underestimation is lower than 7% owing to the low proportion of outgassed CO 2 in total DIC (Table S2) as calculated using formulas in Ran et al (2015). CO 2 concentration in the soil column headspace was determined by gas chromatograph (Agilent 7890A, USA) coupled with a flame ionization detector (FID).…”