Isotopes Reveal the Moderating Role of Ammonium on Global Riverine Water Nitrogen Cycling
Ioannis Matiatos, Lucilena Rebêlo Monteiro, Mathieu Sébilo, David X. Soto, Daren C. Gooddy, Leonard I. Wassenaar
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.