A cross-referenced isotope-hydrochemistry-statistics framework supports a legacy-N-constrained interpretation of...

In managed basins, reductions in contemporary nitrogen inputs do not necessarily translate into proportional declines in riverine nitrate export, making source-sink diagnosis increasingly challenging under strong hydrological and landscape heterogeneity. Here, we investigated nitrate dynamics in the monsoon-influenced Erhai Lake Basin following a basin-wide management program initiated in 2015, treating this intervention as a temporal reference rather than a formal causal counterfactual. We combined basin-wide river-mouth monitoring (51 sites; hydrochemistry from 2021 to 2023), two dry- and wet-season nitrate-isotope campaigns in 2023, and a cross-referenced, uncertainty-aware analytical framework integrating dual nitrate isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), hydrochemical diagnostics, Bayesian isotope mixing, and redundancy analysis. The basin exhibited pronounced spatial and seasonal heterogeneity. The western sub-basin was nitrate-dominated and functioned as a rapid export corridor, the southern sub-basin retained a stronger wastewater-related signal, and the northern sub-basin reflected a more buffered terrestrial-processing regime. Bayesian mixing indicated a minor fertilizer-derived contribution (posterior mean about 2%) and dominant soil- and manure-associated source space across most sub-basins, with a residual sewage-related component mainly in the south. Sensitivity analyses showed that this dominant source-ranking pattern remained stable despite overlap among soil, manure, and sewage endmembers. Isotope and hydrochemical evidence further indicated that exported nitrate was not a simple source signal, but was modified by seasonal remobilization, source mixing, and in-stream process overprinting. Under generally oxic conditions, the observed isotope relationships were more consistent with mixed controls involving nitrification-associated nitrate production, recycling, biological uptake, and localized fractionation than with pervasive basin-wide denitrification. Overall, nitrate export in the Erhai Basin is better interpreted as legacy-N-constrained, seasonally remobilized, and process-overprinted rather than dominated by present-day fertilizer inputs alone. These results support differentiated sub-basin management and demonstrate the value of combining isotopes, hydrochemistry, Bayesian mixing, and multivariate structure to diagnose nitrate source-sink reorganization in heterogeneous managed watersheds.