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.