Spatiotemporal variations and fluxes of dissolved nitrogen and carbon in a karst resurgence river

Karst Resurgence Rivers represent critical linkages between surface and subsurface hydrological systems, yet their coupled carbon–nitrogen dynamics remain insufficiently understood. This study investigates the spatiotemporal variations and transport fluxes of dissolved carbon (TDC, DIC, DOC) and nitrogen (TDN, NO₃⁻) in the Panyang River, a representative karst resurgence river in southwest China, based on monthly and rainfall-event sampling conducted from July 2022 to December 2023. Results showed that DIC (11.2 mg L-1) and TDC (18.7 mg L-1) were consistently lower during the rainy season due to hydrological dilution, while DOC and NO₃- increased in surface water but decreased in groundwater. Correspondingly, C/N ratios decreased from 12.13 before rainfall to 9.28 after rainfall, reflecting the higher dilution sensitivity of DIC relative to the cumulative enrichment of NO₃⁻. Spatially, surface water TDN, NO₃-, TDC, and DIC declined lower reaches due to hydrological dilution, while groundwater NO₃- increased from cumulative leaching. NO₃-/Cl- ratios identified sewage and fertilizer leaching as primary NO₃- sources in surface and groundwater, respectively. Annual surface water fluxes of TDN and NO₃- were 1063 and 879 kg km-² year-¹, respectively. These findings highlight distinct migration mechanisms of DIC and NO₃-, and clarify the effects of dilution and accumulation on carbon and nitrogen concentrations differ significantly between surface water and groundwater in Karst Resurgence River.