Abstract

Isotopic signatures (δ15N and δ18O) of nitrate (NO3−) have been widely used to trace NO3− sources in rivers. These isotopic markers are often treated as “conservative tracers,” reflecting source mixing without significant isotope fractionation. This study investigated changes in the concentrations and isotopic signatures of NO3− and ammonium (NH4+) in the Fuji River, central Japan, using a novel longitudinal Lagrangian sampling approach to determine whether their isotopic signatures remained unchanged during downstream transport in a natural river. Dissolved inorganic nitrogen (DIN) concentrations and stable isotopic compositions of NH4+, nitrite (NO2−), and NO3− were analyzed across multiple sampling points along a 6 km reach. The results revealed significant δ15N enrichment in NH4+ as the concentrations decreased downstream, with apparent isotopic fractionation factors (15ε_NH4+consumption) ranging from 4.9 to 9.1‰, indicating active biogeochemical cycling. In contrast, δ15N and δ18O of NO3− exhibited minimal variation, confirming the stability of NO3− isotopic signatures and their utility as tracers for source identification. The apparent fractionation factors for NH4+ consumption were correlated with nutrient spiraling metrics, suggesting a potential proxy for 15ε_NH4+consumption for the in-stream processing of NH4+. These findings highlight the importance of isotopic approaches for uncovering in situ N dynamics, and propose isotopic fractionation as a novel indicator of nutrient cycling in fluvial ecosystems. This study advances our understanding of N biogeochemistry in river systems and its implications for nutrient management and ecosystem health.