Mechanistic Characterization of Nitrate Transport and Transformation in Typical Semiarid River-Lake Systems, Northwest China: Implications by Dual Nitrogen–Oxygen Stable Isotopes

Nitrogen pollution from agricultural activities is widespread in semiarid river-lake systems, yet mechanisms governing nitrate migration and transformation across the lake water-pore water-sediment continuum remain unclear. This study investigates nitrate sources, transport, and transformation in Hongjiannao Lake and its tributaries using dual nitrogen and oxygen isotopes coupled with Bayesian mixing modeling and partial least-squares path modeling (PLS–PM). The two tributaries exhibited contrasting nitrogen speciation: Zhasake River (ZR) exported nitrate-dominated dissolved inorganic nitrogen, whereas the Manggaitu River was ammonium-dominated, likely reflecting differences in hydrological regulation and nitrification potential within the respective watersheds. Lake water nitrate was significantly higher in the wet season than the dry season, and source apportionment indicated that pore water release contributed 30.2 ± 5.6% of the wet season nitrate load, followed by ZR (25.7 ± 2.9%). Isotope enrichment during nitrate reduction (ε = −28.4‰) indicates sedimentary denitrification, and a significant positive correlation between pore water and sedimentary δ15N-NO3– suggests active nitrate exchange. PLS–PM further indicated that pore water replenishment and lake water diffusion were key pathways regulating sedimentary denitrification during the wet season. This study is based on two seasonal surveys, and longer-term monitoring is warranted to resolve interannual dynamics.