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 (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

Isotopes reveal the moderating role of ammonium on global riverine water nitrogen cycling

The relationship between δ18O and δ15N in aquatic nitrate (NO3–) is used to assess nitrogen (N) cycling, primarily relying on controlled laboratory tests of isotope fractionation from nitrification and denitrification. Nevertheless, laboratory findings frequently contradict the evolution of the nitrate δ18O/δ15N ratios observed in natural river systems. We investigated this disparity by using moderated regression modeling, analyzing a global data set (n = 1303) of nitrate isotopes encompassing rivers with varying NH4+/NO3– ratios and δ18O–H2O values. First, our analysis revealed that elevated δ18O/δ15N ratios (>0.6) were prevalent in rivers with high NH4+/NO3– ratios, suggesting reducing conditions that could potentially promote denitrification and/or ammonium accumulation. By contrast, lower δ18O/δ15N ratios (<0.5) predominated in rivers with low NH4+/NO3– conditions, suggesting oxidizing conditions favoring increased NH4+ removal through nitrification. Second, when δ18O–H2O values were low, it resulted in reduced δ18O–NO3– values during nitrification, which in turn lowered the δ18O/δ15N ratios. We discovered that the δ18O/δ15N ratios in nitrate were elevated in the fall, likely due to predominant processes, such as denitrification, and lower in the winter due to lower δ18O–H2O values. This global river assessment suggests a more significant influence of ammonium and the role of water oxygen in riverine N-nutrient isotope cycling than was previously considered