Global patterns of nitrate isotope composition in rivers and adjacent aquifers reveal reactive nitrogen cascading

Abstract Remediation of nitrate pollution of Earth’s rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes ( δ 15 N, δ 18 O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO 3 − ). We synthesized nitrate isotope data ( n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO 3 − (0.3 ± 0.2 mg L −1 , n = 2902) compared to aquifers (5.5 ± 5.1 mg L −1 , n = 2291) and slightly lower δ 15 N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ 18 O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO 3 − was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ 15 N and δ 18 O values in wintertime, suggesting waste-related N-source signals are better preserved in the cold seasons. Isotopic assays of nitrate biogeochemical transformations are key to understanding nitrate pollution and to inform beneficial agricultural and land management strategies.