Diel variations of carbon and nitrogen cycling in a karst stream: Insights from high‑resolution monitoring of hydrochemistry, nitrate isotopes and ecosystem metabolism

High-resolution diel monitoring was conducted at a vegetated karst stream section in summer and winter to investigate diel nitrogen dynamics and its coupling with carbon cycling. Physicochemical parameters， major ions (Ca²⁺， HCO₃^-)， nitrate， ammonium， and stable isotopes (δ¹⁵N-NO₃^-， δ¹⁸O-NO₃^-， δ¹⁵N-NH₄⁺， δ¹³C-DIC) were measured over 24-h cycles. Net ecosystem production （NEP） was calculated to quantify metabolic activity. Pronounced diel variations were observed. During daytime， photosynthesis dominated （NEP > 0）， decreasing NO₃^- and NH₄⁺-N concentrations with concurrent enrichment of δ¹⁵N-NO₃^-， δ¹⁸O-NO₃^-， δ¹³C-DIC and δ¹⁵N-NH₄⁺; at night， respiration dominated （NEP<0）， reversing these patterns. The δ¹⁵N-NO₃^- vs. δ¹⁸O-NO₃^- covariation exhibited an enrichment ratio of ～0.79:1， with all data points falling outside the denitrification field， indicating denitrification was not a reach-scale dominant process. Assimilation and organic nitrogen mineralization were identified as the key processes controlling diel nitrogen cycling. Seasonal differences in isotopic enrichment and metabolic rates reflected variations in water temperature， flow velocity， and plant biomass. This study provides isotopic evidence that subaquatic vegetation metabolism drives coupled C-N diel cycles in karst streams， offering new insights into nitrogen biogeochemistry in high-DIC aquatic systems.