Elucidating the isotopic dynamics of nitrate and ammonium in the Fuji River using a Lagrangian approach: stable nitrate and fractionating ammonium

Abstract Isotopic signatures ( δ 15 N and δ 18 O) of nitrate (NO 3 − ) have been widely used to trace NO 3 − 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 NO 3 − and ammonium (NH 4 + ) 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 NH 4 + , nitrite (NO 2 − ), and NO 3 − were analyzed across multiple sampling points along a 6 km reach. The results revealed significant δ 15 N enrichment in NH 4 + as the concentrations decreased downstream, with apparent isotopic fractionation factors ( 15 ε_NH 4 + consumption) ranging from 4.9 to 9.1‰, indicating active biogeochemical cycling. In contrast, δ 15 N and δ 18 O of NO 3 − exhibited minimal variation, confirming the stability of NO 3 − isotopic signatures and their utility as tracers for source identification. The apparent fractionation factors for NH 4 + consumption were correlated with nutrient spiraling metrics, suggesting a potential proxy for 15 ε_NH 4 + consumption for the in-stream processing of NH 4 + . 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.