Abstract

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.