Dynamic of riverine pCO2, biogeochemical characteristics, and carbon sources inferred from δ13C in a subtropical river system

Rivers significantly contribute to the global carbon budget, but data limitations and uncertainty are hampered by CO2 quantification in the global rivers. Thus, this study estimated riverine pCO(2) by employing the pH-alkalinity-temperature method, and dissolved inorganic (DIC), dissolved organic (DOC), particulate organic (POC) carbon, and their isotopes (delta C-13) with Chlorophyll-a (Chl a) were measured in river water samples from 26 sampling sites for characterization and source identification in the Yangtze River system. The estimated pCO(2) varies from (120 ppm) to (3400 ppm) with an average (1085 ppm) across the Yangtze River and pCO(2) is almost three times oversaturated than the ambient air (380 ppm). The downstream sites pronounced elevated pCO(2) than the upstream sites. The relationship of delta C-13(DIC) and pCO(2) indicated that pCO(2) control is seasonally independent. The significant correlations between DOC, POC, and pCO(2) revealed that organic carbon influenced pCO(2) in the river. The seasonal fluctuations of pCO(2) were observed with an average of (762.23 ppm) and (1407.35 ppm) in winter and summer, respectively. delta C-13(DIC) showed that the metabolic process has a negligible influence on DIC, delta C-13(DIC), and pCO(2). delta C-13(DIC) values increased from -8.95 parts per thousand to -4.91 parts per thousand during summer, whereas winter increased from -19.76 parts per thousand to -1.97 parts per thousand suggesting that DIC derived from carbonate weathering, dissolution of atmospheric and soil CO2. The delta C-13(DOC) (-30.43 parts per thousand to -24.05 parts per thousand) and delta C-13(POC) (-29.87 parts per thousand to -23.37 parts per thousand) values confirmed that organic carbon mainly derived from the degradation of organic materials in soil. delta C-13(DIC) revealed that anthropogenic sewage discharge slightly modified DIC composition. Overall, this study provides new insight into recent seasonal fluctuations of the pCO(2), DOC, POC, DIC, delta C-13, and their inputs. Thus, these variations and inputs of carbon transported by the Yangtze River could have a significant influence not only on the biogeochemical cycle and ecosystem process but also on the global carbon budget.

Automated summary

This study estimated riverine pCO(2) in the Yangtze River system and measured various carbon variables and isotopes. The results showed seasonal and spatial fluctuations in pCO(2), which were influenced by organic carbon and DIC. This study provides important insights into the carbon cycling in river systems and the global carbon budget.