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

Remediation of nitrate pollution of Earth's rivers and aquifers is hampered by cumulative biogeochemical processes and nitrogen sources. Isotopes (delta N-15, delta O-18) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO3-). We synthesized nitrate isotope data (n=similar to 5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO3- (0.30.2mgL(-1), n=2902) compared to aquifers (5.5 +/- 5.1mgL(-1), n=2291) and slightly lower delta N-15 values (+7.1 +/- 3.8 parts per thousand, n=2902 vs +7.7 +/- 4.5 parts per thousand, n=2291), but were indistinguishable in delta O-18 (+2.3 +/- 6.2 parts per thousand, n=2790 vs +2.3 +/- 5.4 parts per thousand, n=2235). The isotope composition of NO3- was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher delta N-15 and delta O-18 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.