Metabolic processes drive spatio-temporal variations of carbon sink/source in a karst river

Riverine carbon dioxide (CO₂) exchange is a crucial component of the global carbon cycle. However, the changes in the CO₂ sink/source in karst rivers caused by differences in lithological features and climate, hindered the resolution of the spatio-temporal heterogeneity of global inland water carbon emissions. Here, we use hydrochemical data and CO₂ gas isotopic data to reveal the spatio-temporal variations of CO₂ sink/source in karst rivers and their controlling mechanisms. Fifty-two monitoring transects were set up in the subtropical Lijiang River in southwest China in June and December 2019. Our results indicated that the CO₂ flux across the water-air interface (FCO₂) in the Lijiang River basin ranged from -43.77 to 519.67 mmol/(m²·d). In June, the Lijiang River acted as an atmospheric carbon source due to higher water temperatures (T_water). However, driven by hydrodynamic conditions and the metabolism of aquatic photosynthesis, the river shifts from being an atmospheric carbon source in June to an atmospheric carbon sink in December. The stable isotopes of CO₂ (δ¹³C-CO₂) show significant differences in the spatio-temporal variations of CO₂ sink/source. In December, the transects of the Lijiang River basin with a negative CO₂ flux are significantly negatively correlated with dissolved oxygen (DO) and chlorophyll-a (Chl-a) concentration (p < 0.05). This confirms that the enhancement of aquatic photosynthesis efficiency increased water DO concentrations, which resulted in the positive movement of water δ¹³C-CO₂ and a decrease in the partial pressure of CO₂ (pCO₂) and FCO₂. Comparative analysis with global river FCO₂ indicates that under the combined driving forces of metabolic processes of aquatic photosynthetic organisms and hydrodynamic conditions, rivers tend to act more frequently as CO₂ sinks, particularly in subtropical and temperate rivers. In conclusion, this study represents a new example focusing on CO₂ dynamics to address the spatio-temporal heterogeneity of carbon emissions in inland waters on a global scale.