Dissolved Organic Nitrogen (DON) is an essential component of Dissolved Organic Matter (DOM) and plays a critical role in determining the biogeochemical cycling of DOM in the marine environment. However, current researches on DON are confronted with several major challenges: 1) Lack of unified or officially recommended methods for concentration measurement; 2) The absence of automated analytical methods hinder the continuous generation of high-quality DON data; 3) There are limited data from nearshore or high DIN (particularly nitrate) areas, such as deep seas and estuaries; 4) Difficulties in separating DON molecules from DOM and analyzing; 5) Limited research methodologies, i.e. few DON isotope data and ultra-high resolution molecular data available.

        In response to the above research gaps, we conducted several trial studies. The first study of stepwise oxidation found that the oxidized-DON varied under different oxidant/sample ratios, exhibiting distinct isotopic signatures. Furthermore, we validated the oxidation process using FT-ICR MS. The next trial study is to enable automated analysis of Total Dissolved Nitrogen (TDN). We optimized the TDN determination method through the Shimadzu TOC-TNM module, ensuring it meets the analytical demands of seawater samples. The developed automated TDN measurement shows high consistency with the results produced by traditional potassium persulfate methods. Subsequently, we analyzed over 600 nearshore samples revealing differences in bioactivity of DON, with the order being Bohai Sea > East China Sea ≈ South China Sea. A third trial study concerns the DON compositions differences in Deep Ocean. The DON from different stations in the deep South China Sea exhibited very similar isotopic signatures（3.00 ± 0.08‰, n=5）and comparable molecular compositions by FT-ICR MS. We hypothesize that deep-sea DON possesses relatively stable ¹⁵N isotopes with minor molecular composition differences, which are so subtle that current isotopic analytical precision cannot adequately reflect them. ​Based on these findings, we suggest that understanding the transformation process of DON in the marine environment requires an integration of accurate concentration measurement, molecular compositions, isotope signature, and other chemical characteristics.​