Information on the sources and transformations of particulate organic N (PN) and dissolved organic N (DON) at the coastal interface remains insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. Here, we investigated the spatial distribution of concentrations and isotopic compositions of PN and DON in the Changjiang plume during the summer flood period. In average, DON and PN accounted for 25.6 ± 12.1% and 8.1 ± 9.1% (n = 55), respectively, of the total N pool, with the remaining N primarily in the form of nitrate (NO₃⁻). Mean δ¹⁵N values were the lowest for DON (−0.1 ± 2.7‰, n = 58) and slightly higher for PN (2.0 ± 1.6‰, n = 101), and the highest for NO₃⁻ (6.5 ± 2.2‰, n = 67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among the three N pools. By applying a conservative mixing model, we found DON deficits (−3.5 ± 3.7 μmol L⁻¹, n = 43) and negative shift in δ¹⁵N_DON (−3.6 ± 2.2‰, n = 43) in the Changjiang plume, revealing nonconservative DON behaviors. In the offshore surface plume where Chlorophyll a was high, the most likely cause is the DON uptake by phytoplankton with a strong inverse isotope effect (around −40‰). This DON assimilation by phytoplankton contributed to ~16 ± 12% of the PN production, with the remaining supported by NO₃⁻ assimilation, producing an overall isotope effect of 4–9‰. However, in waters near the river mouth and at the bottom of the offshore plume where total suspended matter concentrations were high (>5 mg L⁻¹), the DON deficit was most likely induced by the selective adsorption of ¹⁵N enriched moieties of DON onto particulate surfaces (with an isotope effect of −20‰ to −5‰). Unlike dissolved organic carbon to behave conservatively in most estuaries, our results show that active transformations had occurred between the DON and PN pools in the Changjiang plume.

由于技术难度大、物理混合强度大、生物活动快等复杂特点，海岸界面颗粒有机氮（PN）和溶解有机氮（DON）的来源及转化信息尚不充分。在这里，我们调查了夏汛期长江羽流中 PN 和 DON 浓度和同位素组成的空间分布。平均而言，DON 和 PN 分别占总 N 库的 25.6 ± 12.1% 和 8.1 ± 9.1% ( n  = 55)，其余 N 主要以硝酸盐 (NO ₃ ^- ) 的形式存在。DON 的平均 δ ¹⁵ N 值最低（-0.1 ± 2.7‰，n  = 58），PN 的平均值略高（2.0 ± 1.6‰，n = 101），NO ₃ ^-最高（6.5±2.2‰，n  = 67），表明发生了多次转化以区分三个N库之间的同位素特征。通过应用保守的混合模型，我们发现长江羽流中的 DON 缺陷（-3.5 ± 3.7 μmol L ^-1，n = 43）和 δ ¹⁵ N _DON 负移（-3.6 ± 2.2‰，n = 43），揭示非保守的 DON 行为。在叶绿素a的近海地表羽流中高，最可能的原因是浮游植物对 DON 的吸收具有强烈的反同位素效应（约 -40‰）。浮游植物对 DON 的同化贡献了约 16 ± 12% 的 PN 产量，其余由 NO ₃ ^-同化支持，产生 4-9‰ 的整体同位素效应。然而，在河口附近和近海羽流底部总悬浮物浓度高（> 5 mg L ^-1）的水域中，DON 缺乏很可能是由^{15的选择性吸附引起的。}DON 的 N 富集部分在颗粒表面上（具有 -20‰ 至 -5‰ 的同位素效应）。与溶解有机碳在大多数河口的行为保守不同，我们的研究结果表明，长江羽流中的 DON 和 PN 池之间发生了积极的转变。