Nitrogen (N) limits ocean production in general. Growing inputs of anthropogenic N from land via riverine discharge seriously influence biogeochemical cycles of multiple elements at the land-ocean boundary, where the information of sources and transformations of particulate and dissolved organic N (PN/DON) is crucial, yet, remaining insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. We investigated the spatial distributions of concentrations and isotopic compositions of PN and DON in the Changjiang River (CJ) plume, the sixth largest river in the world, during summer flood period. We found that the highest concentrations of PN and DON appeared near the river mouth with an offshore decreasing pattern. DON and PN accounted for 25.6±12.1% and 8.1±9.1% (n=55), respectively, of the total N (TN) pool, with the remaining in the form of nitrate (NO₃^–) mainly. For entire CJ plume, the mean d¹⁵N value was the lowest for DON (–0.1±2.7‰, n=58) and slightly higher for PN (2.0±1.6‰, n=101), with the highest for NO₃^– (6.5±2.2‰, n=67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among three N pools. Combing the observed concentrations and isotopic compositions of DON with a conservative mixing model, we found obvious negative offsets in both DON (–3.5±3.7 μmol L^-1, n=43) and d¹⁵N_DON (–3.6±2.2‰) for entire CJ plume, revealing a nonconservative behavior of DON. In the offshore surface plume where Chl a was high, only phytoplankton uptake of DON with a mean isotope effect of –20‰ can explain the deficit of DON (–3.0±2.3 μmol L^-1, n=9) and negative shift in d¹⁵N_DON (–3.0±2.2‰). Such DON assimilation contributed ~16±12% to PN production, with the remaining supported by NO₃^– assimilation, producing an overall isotope effect of 8-13‰ during phytoplankton uptake. However, near the river mouth where turbidity was the highest and the bottom-hugging in the offshore plume where TSM was higher than 5 mg L^-1, the DON deficit was most likely induced by the selective absorption of ¹⁵N enriched moieties of DON onto particulate surfaces. All information was assembled into a conceptual diagram offering an overview of multiple transformations among three N forms in the CJ plume.

particulate nitrogen, dissolved organic nitrogen, Chnagjiang River Plume