Effects of land use and dissolved organic matter on pCO₂ are dependent on stream orders and hydrological seasonality in a low-order karst river

Human activities and stream accumulation influence carbon loadings, altering the distributions and characteristics of dissolved inorganic and organic carbon in rivers. It is widely recognized that such alterations affect dissolved organic matter (DOM) components, water environment and river carbon dioxide (CO₂) degassing, however, the control factors by which land use/land cover (LULC) and DOM components regulate the partial pressure of CO₂ (pCO₂) are unclear. Here, in the Daning karst river system, an extensive investigation was presented to investigate the role of LULC and DOM components in influencing the spatial and temporal variability of pCO₂, as well as to investigate the regulating effect of stream order and hydrological rhythm on this influence. DOM quality and pCO₂ levels exhibited significant spatial and temporal variations. In the 3rd - 4th order streams, pCO₂ was correlated with protein and lignin compounds in the wet period and with DOM molecular weight in the dry period. Relatively high protein-like components (54.83 % ~ 71.84 %, on average) and biological index (0.86-0.90, on average) indicated notable autochthonous processes. Significant relationships between pCO₂ and water quality parameters were observed in the 3rd - 4th order streams in the wet period, demonstrating the role of runoff and upstream accumulation. Farmland increased pCO₂ levels in the 3rd - 4th order streams, whereas forests could potentially mitigate river CO₂ saturation. River pCO₂ was well predicted by LULC under extended circular buffers (1000 and 2000 m in diameter). This study demonstrated that DOM and LULC directly or indirectly affect pCO₂ and that the influences are largely regulated by hydrological seasonality and stream orders, which is better for understanding aquatic CO₂ drivers.