Dynamic of riverine pCO_2, biogeochemical characteristics, and carbon sources inferred from δ¹³C in a subtropical...

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 pCO₂ by employing the pH-alkalinity-temperature method, and dissolved inorganic (DIC), dissolved organic (DOC), particulate organic (POC) carbon, and their isotopes (δ¹³C) 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 pCO₂ varies from (120 ppm) to (3400 ppm) with an average (1085 ppm) across the Yangtze River and pCO₂ is almost three times oversaturated than the ambient air (380 ppm). The downstream sites pronounced elevated pCO₂ than the upstream sites. The relationship of δ¹³C_DIC and pCO₂ indicated that pCO₂ control is seasonally independent. The significant correlations between DOC, POC, and pCO₂ revealed that organic carbon influenced pCO₂ in the river. The seasonal fluctuations of pCO₂ were observed with an average of (762.23 ppm) and (1407.35 ppm) in winter and summer, respectively. δ¹³C_DIC showed that the metabolic process has a negligible influence on DIC, δ¹³C_DIC, and pCO₂. δ¹³C_DIC values increased from -8.95‰ to -4.91‰ during summer, whereas winter increased from -19.76‰ to -1.97‰ suggesting that DIC derived from carbonate weathering, dissolution of atmospheric and soil CO₂. The δ¹³C_DOC (-30.43‰ to -24.05‰) and δ¹³C_POC (-29.87‰ to -23.37‰) values confirmed that organic carbon mainly derived from the degradation of organic materials in soil. δ¹³C_DIC revealed that anthropogenic sewage discharge slightly modified DIC composition. Overall, this study provides new insight into recent seasonal fluctuations of the pCO₂, DOC, POC, DIC, δ¹³C, 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.