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 (δ¹⁵N, δ¹⁸O) help unravel spatiotemporal nitrogen(N)-cycling of aquatic nitrate (NO₃⁻). We synthesized nitrate isotope data (n = ~5200) for global rivers and shallow aquifers for common patterns and processes. Rivers had lower median NO₃⁻ (0.3 ± 0.2 mg L⁻¹, n = 2902) compared to aquifers (5.5 ± 5.1 mg L⁻¹, n = 2291) and slightly lower δ¹⁵N values (+7.1 ± 3.8‰, n = 2902 vs +7.7 ± 4.5‰, n = 2291), but were indistinguishable in δ¹⁸O (+2.3 ± 6.2‰, n = 2790 vs +2.3 ± 5.4‰, n = 2235). The isotope composition of NO₃⁻ was correlated with water temperature revealing enhanced N-cascading in warmer climates. Seasonal analyses revealed higher δ¹⁵N and δ¹⁸O 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.

Nitrogen-cascading increases in warmer climates and median nitrate concentrations are lower in rivers compared to aquifers globally, according to a global synthesis of nitrogen isotope data in rivers and aquifers.