Isotopes Reveal the Moderating Role of Ammonium on Global Riverine Water Nitrogen Cycling

Abstract

The relationship between δ 18 O and δ 15 N in aquatic nitrate (NO 3 − ) 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 δ 18 O/δ 15 N 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 NH 4 + /NO 3 − ratios and δ 18 O−H 2 O values. First, our analysis revealed that elevated δ 18 O/δ 15 N ratios (>0.6) were prevalent in rivers with high NH 4 + /NO 3 − ratios, suggesting reducing conditions that could potentially promote denitrification and/or ammonium accumulation. By contrast, lower δ 18 O/δ 15 N ratios (<0.5) predominated in rivers with low NH 4 + /NO 3 − conditions, suggesting oxidizing conditions favoring increased NH 4 + removal through nitrification. Second, when δ 18 O−H 2 O values were low, it resulted in reduced δ 18 O−NO 3− values during nitrification, which in turn lowered the δ 18 O/δ 15 N ratios. We discovered that the δ 18 O/δ 15 N ratios in nitrate were elevated in the fall, likely due to predominant processes, such as denitrification, and lower in the winter due to lower δ 18 O−H 2 O 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.