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

Surface rivers play an essential role in carbon cycle processes in karst regions. However, the CO2 diffusion flux from karst rivers under the influence of urbanization has been scarcely examined in the literature. Along these lines, in this work, the CO2 partial pressure (pCO2) and its degassing in a typical karst river (Nanming River and its tributaries) were thoroughly investigated, which are significantly affected by urbanization in Southwest China. From the acquired results, it was demonstrated that the average values of pCO2 in the main stream of the Nanming River 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. On the other hand, the tributary showed mean pCO2 values of 1770.46 ± 1120.79 μatm, 1638.13 ± 1121.82 μatm, and 1107.74 ± 824.03 μatm in the three different hydrographic periods. Overall, the pCO2 of the Nanming River basin decreased in the following order: wet season > dry season > flat season, while the mainstream of the Nanming River was slightly higher than that of the tributaries in the wet season. However, it was lower than that of the tributaries in the dry and flat seasons. Additionally, more than 90% of the samples displayed a supersaturated state of CO2, which acted as an important source of CO2 in the atmosphere. From a spatial point of view, pCO2 tended to be higher in the western region than in the eastern region, higher in the middle than in the immediate vicinity, and higher in the south during the three seasons. The higher urban areas showed also relatively higher pCO2 than the lower urban areas. The urban land along with the Nanming River’s mainstream exhibited a weaker correlation with pCO2 than the urban land along with the main tributaries due to the mainstream’s regular management in recent years. Moreover, the pCO2 was mainly influenced by the carbonate rocks dissolution, aquatic organism metabolic processes, and human activities. In the Nanming River basin, the average CO2 diffusion fluxes in the wet season, dry season, and flat season were 147.02 ± 100.3 mmol·m−2·d−1, 76.02 ± 67.45 mmol·m−2·d−1, and 119.28 ± 168.22 mmol·m−2·d−1, respectively, which indicates high potential CO2 emissions. In addition, it was found that urban construction could increase the pCO2 of karst rivers and cause an increase in the CO2 release flux during the regional urban expansion. In view of the increasing intensive and extensive urbanization in karst regions, our findings are helpful to elucidate the characteristics of carbon dioxide emissions from karst rivers under the disturbance of human activities and further deepen the understanding of the carbon balance in karst river basins.