Isotope constraints on nitrogen dynamics in the upper water column of the South China Sea

The supply of nitrogen (N) from various external and internal sources into the euphotic zone, e.g., atmospheric N deposition (AND), upwelling, lateral intrusion, and remineralization, modulates the biogeochemical and climatic roles of oligotrophic oceans and complicates N dynamics in the upper water column (≤200 m). However, our ability to resolve the mechanisms controlling upper-ocean N cycling is limited by the lack of high-resolution vertical observations. Here, we analyzed concentrations and dual isotopes of nitrate (NO 3 – ) in the upper 200 m of the oligotrophic South China Sea. By examining dual isotopic signatures of NO 3 – (δ 15 N NO3 and δ 18 O NO3 ) and multiple associated parameters vertically throughout the upper water column, we resolved the dominant N sources and processes, including AND/N 2 -fixation, assimilative fractionation, and nitrification, and quantitatively evaluated their contributions in the vertical distribution of NO 3 – , which can be separated into the Δδ 18 O NO3 -positive (δ 18 O NO3−obs −δ 18 O NO3−200m >0) and Δδ 18 O NO3 -negative layers (δ 18 O NO3−obs −δ 18 O NO3−200m