Spatial and temporal distribution characteristics of pCO2 and CO2 evasion in karst rivers under the influence of urbanization

Abstract Surface rivers play an essential role in the carbon cycle processes in karst regions; however, less attention has been given to CO2 emissions from karst rivers under the influence of urbanization. In this study, we investigated CO2 partial pressures (pCO2) and degassing in a typical karst river (Nanming River and its tributaries) affected by urbanization in southwest China. Field sampling and measurements were conducted in the area from July 2021 to March 2022 in an attempt to determine the effect of urbanization on riverine pCO2 and CO2 degassing. The results showed that the average values pCO2 of the Nanming River mainstream in the wet season, dry season and flat season were 1975.77 ± 714.45 µatm, 1116.08 ± 454.24 µatm, and 976.89 ± 746.37 µatm, respectively. The tributary had three phase mean pCO2 values of 1770.46 ± 1120.79 µatm, 1638.13 ± 1121.82 µatm, and 1107.74 ± 824.03 µatm. On the whole, the pCO2 of the mainstream of the Nanming River is only slightly higher than that of the tributaries in the wet season but lower than that of the tributaries in the other two hydrological periods. The pCO2 showed obvious seasonal changes in the whole basin; it was the highest in the wet season, followed by the dry season, and the lowest in the flat season. Additionally, more than 90% of the sample points show a supersaturated state of CO2, demonstrating the carbon source characteristics. Spatially, pCO2 tends to be higher in the west region than in the east, higher in the middle than in the immediate vicinity, and higher in the south during the three seasons. And, the pCO2 responds to the urbanization gradient across different rivers and in the upper, middle and lower reaches of the river; that is, the areas with high urbanization have higher pCO2. Urban land along the Nanming River's mainstream has a weaker correlation with pCO2 than urban land along the tributaries due to the mainstream's regular management in recent years. In addition, pCO2 is also positively correlated with TP and TN, which shows that nutrient input from human activities may result in an increase in river CO2 emissions. In the Nanming River Basin, the average CO2 diffusion fluxes at the water-air interface in the wet season, dry season and flat season were 147.02 ± 100.3 mmol/(m2d), 76.02 ± 67.45 mmol/(m2d), and 119.28 ± 168.22 mmol/(m2d), respectively, and the spatiotemporal distribution characteristics were basically the same as those of pCO2. The findings of this study are on the high side when compared to the CO2 fluxes of other rivers in the globe and the worldwide average CO2 fluxes of rivers. Therefore, the impact of urbanization on the carbon emissions of karst rivers cannot be ignored, but strengthening urban river governance can reduce its carbon emissions intensity.