Nitrate isotopes reveal N-cycled waters in a spring-fed agricultural catchment.

Nitrate stable isotopes provide information about nitrate contamination and cycling by microbial processes. The Fischa-Dagnitz (Austria) spring and river system in the agricultural catchment of the Vienna basin shows minor annual variance in nitrate concentrations. We measured nitrate isotopes (δ¹⁵N, δ¹⁸O) in the source spring and river up to the confluence with the Danube River (2019-2020) with chemical and water isotopes to assess mixing and nitrate transformation processes. The Fischa-Dagnitz spring showed almost stable nitrate concentration (3.3 ± 1.0 mg/l as NO₃^--N) year-round but surprisingly variable δ¹⁵N, δ¹⁸O-NO₃^- values ranging from +5.5 to +11.1‰ and from +0.5 to +8.1‰, respectively. The higher nitrate isotope values in summer were attributed to release of older denitrified water from the spring whose isotope signal was dampened downstream by mixing. A mixing model suggested denitrified groundwater contributed > 50 % of spring discharge at baseflow conditions. The isotopic composition of NO₃^- in the gaining streams was partly controlled by nitrification during autumn and winter months and assimilation during the growing season resulting in low and high δ¹⁵N-NO₃^- values, respectively. NO₃^- isotope variation helped disentangle denitrified groundwater inputs and biochemical cycling processes despite minor variation of NO₃^- concentration.