Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution...

Karst streams are ideal natural systems for studying diel biogeochemical dynamics, yet the diel dynamics of nitrogen (N) cycling and its coupling with carbon (C) cycling remain poorly understood under high-resolution monitoring. In this study, two monitoring sections were established: one at the outlet of the Dawang Cave subterranean stream (with no subaquatic vegetation) and another downstream in Dawang Lake (with abundant subaquatic vegetation). A 24-h high-resolution field campaign (12:00 on 6 September to 12:00 on 7 September 2024) was conducted to simultaneously measure hydrochemical parameters (pH, DO, EC, temperature), major ions (Ca²⁺, HCO₃^-), nitrogen species (NO₃^-, NH₄⁺-N), and their stable isotopic compositions (δ¹⁵N‑NO₃^-, δ¹⁸O‑NO₃^-, and δ¹⁵N‑NH₄⁺). Net ecosystem production (NEP) was calculated. At the unvegetated site, all parameters exhibited weak diel variations, reflecting groundwater-dominated "chemostatic behavior." At the downstream vegetated site, pronounced diel cycles were observed: during daytime, photosynthesis dominated (NEP > 0), with decreasing NO₃^- and NH₄⁺-N concentrations and enrichment of δ¹⁵N‑NO₃^-, δ¹⁸O‑NO₃^-, and δ¹⁵N‑NH₄⁺; at night, respiration dominated (NEP < 0), reversing these patterns. A significant negative correlation was observed between nitrogen concentrations and their isotope ratios. The covariation between δ¹⁵N‑NO₃⁻ and δ¹⁸O‑NO₃⁻ exhibited an enrichment ratio of approximately 0.79:1, and all data points fell outside the typical denitrification field, indicating that denitrification was negligible. Assimilation and organic nitrogen mineralization were identified as the key processes controlling diel nitrogen cycling. This study provides high-resolution isotopic evidence that subaquatic vegetation metabolism drives the coupled C-N diel cycles in karst streams, offering new insights into nitrogen biogeochemistry in high-DIC aquatic systems.