Insight into nitrous oxide production processes in the western North Pacific based on a marine ecosystem isotopomer model

Based on the observed inverse relationship between the dissolved oxygen and N₂O concentrations in the ocean, previous models have indirectly predicted marine N₂O emissions from the apparent oxygen utilization (AOU), In this study, a marine ecosystem model that incorporates nitrous oxide (N₂O) production processes (i.e., ammonium oxidation during nitrification and nitrite reduction during nitrifier denitrification) was newly developed to estimate the sea-air N₂O flux and to quantify N₂O production processes. Site preference of ¹⁵N (SP) in N₂O isotopomers (¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O) and the average nitrogen isotope ratio (δ¹⁵N) were added to the model because they are useful tracers to distinguish between ammonium oxidation and nitrite reduction. This model was applied to two contrasting time series sites, a subarctic station (K2) and a subtropical station (S1) in the western North Pacific. The model was validated with observed nitrogen concentration and nitrogen isotopomer datasets, and successfully simulated the higher N₂O concentrations, higher δ¹⁵N values, and higher site preference values for N₂O at K2 compared with S1. The annual mean N₂O emissions were estimated to be 34 mg N m⁻² yr⁻¹ at K2 and 2 mg N m⁻² yr⁻¹ at S1. Using this model, we conducted three case studies: 1) estimating the ratio of in-situ biological N₂O production to nitrate (NO₃⁻) production during nitrification, 2) estimating the ratio of N₂O production by ammonium oxidation to that by nitrite reduction, and 3) estimating the ratio of AOA ammonium oxidation to AOB ammonium oxidation. The results of case studies estimated the ratios of in situ biological N₂O production to nitrate production during nitrification to be ~0.22% at K2 and ~0.06% at S1. It is also suggested that N₂O was mainly produced via ammonium oxidation at K2 but was produced via both ammonium oxidation and nitrite reduction at S1. It is also revealed that ~80% of the ammonium oxidation at K2 was caused by archaea in the subsurface water. The results of isotope tracer incubation experiments using an archaeal activity inhibitor supported this hypothesis.