o:9027 Isotopic Evidence of Nitrogen Dynamics in Oxic and Anoxic Groundwater en Groundwater is a very important resource, providing nearly half of the world’s drinking water and supporting around 70% of agricultural needs. Sustainable management requires understanding groundwater origin, age, flow paths, and recharge dynamics. Environmental isotopes, particularly stable isotopes, offer powerful tools for tracing these processes, identifying pollution sources, and evaluating contaminant transport. Nitrate is one of the most widespread groundwater pollutants, originating from untreated sewage, fertilizers, manure application, and the oxidation of soil organic matter. Due to a wide range of sources and the prevalence of different transformation processes under specific conditions, accurate interpretation requires a comprehensive analysis of hydrogeological, physicochemical, microbiological, and isotopic data. During nitrification, ammonia is oxidized to nitrate via nitrite, preferentially incorporating lighter isotopes. Denitrification reduces nitrate to nitrogen gas and enriches the residual nitrate in heavier isotopes. Dissimilatory nitrate reduction to ammonium may compete with denitrification under high carbon-to-nitrate ratios, while anaerobic ammonium oxidation and ammonium oxidation coupled to Fe(III) reduction further influence nitrogen cycling in oxygen-depleted environments. This study highlights contrasting nitrogen isotopic signatures in the oxic Ključ spring and the anoxic Kovin–Dubovac drainage system, revealing differences in nitrogen origin and transformation processes in shallow, agriculture-influenced groundwater. At the oxic site, δ¹⁵N–NO₃⁻ values ranged from +6.70 to +12.90‰ and δ¹⁸O–NO₃⁻ from +0.28 to +9.70‰, revealing three distinct influence zones: sewage and manure impact, mixed sewage–fertilizer inputs, and riparian denitrification. At the anoxic site, δ¹⁵N–NH₄⁺ values were lower from −0.84 to +6.93‰, indicating contributions from soil organic matter mineralization, fertilizer application, and autotrophic denitrification. The research assesses the impact of agricultural practices and the aquifer's potential for nitrogen compound attenuation and self-purification. groundwater: nitrogen isotopes; nitrogen transformation processes 1552100 978-86-6022-762-3 2026-01-30T13:36:23.098Z 44 no 46 Marija Perović Jaroslav Černi Water Institute, Belgrade, Serbia 0000-0003-2045-001X Tatjana Mitrović Institute of Physics Belgrade, University of Belgrade, Belgrade, Serbia person 0000-0002-5991-1445 application/pdf 866593 https://unilib.phaidrabg.rs/o:9027 no yes 1 70 92000004 71A04 5th International Student Conference – DISC2025, hybrid event, Abstract book, 11-12th December 2025, Novi Sad 38 Faculty of Technical Sciences, Novi Sad, Republic of Serbia 2025