Asymmetric isotopic constraints on nitrogen and phosphorus source apportionment in a plain river network

Nitrogen and phosphorus source apportionment in plain river-network systems is complicated by urban-agricultural land-use mosaics, seasonal hydrological disturbance, and process-driven overprinting during transport. A key uncertainty is whether nitrate and phosphate isotopes preserve comparable source information under such conditions. Taking the Fuyang River plain river-network area as a case study, this study integrated nitrate dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), phosphate oxygen isotopes (δ¹⁸O_p), overlap analysis, Bayesian mixing modelling, bootstrap resampling, and sensitivity analysis to compare N and P source constraints and uncertainty structures. δ¹⁸O_p generally deviated from theoretical equilibrium values, with larger deviations in the dry season, indicating non-equilibrium P cycling and process overprinting. In contrast, nitrate dual isotopes did not show a coherent denitrification trajectory and mainly reflected source mixing. Overlap analysis showed that nitrate source separability increased in the wet season, whereas δ¹⁸O_p end-members remained substantially overlapped. MixSIAR indicated persistent WWTP-related control of nitrate, with WWTP posterior means of 0.58 and 0.491 in the dry and wet seasons, respectively. Phosphate was dominated by a diffuse background, with Diffuse posterior means of 0.872 and 0.710, respectively, while non-dominant sources increased during the wet season. Robustness tests supported these ranking patterns but also highlighted nutrient-specific uncertainty. These results reveal an asymmetric source-constraint pattern: nitrate isotopes better preserve point-source-driven source competition, whereas δ¹⁸O_p mainly records diffuse-background inputs and process-driven overprinting. Coupled N-P tracing in river networks should therefore shift from parallel source identification toward nutrient-specific signal interpretation.