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.