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Particulate nitrogen (PN) dynamics in the oligotrophic northern South China Sea (around the SouthEast Asian Time‐series Study (SEATS) station) was explored by examining the isotopic compositions of suspended PN in the top 200 m over 3 years and sinking PN collected by sediment traps. The PN inventory (IPN) in the upper 100 m is larger than in the lower 100 m, exhibiting stronger seasonality. Both layers reveal significant seasonality in meanδ¹⁵N_PN, yet, the mean in the upper 100 m (2.0 to 5.3‰) is consistently smaller than that in the lower 100 m, implying the occurrence of vertical biological fractionation and/or an addition of ¹⁵N‐depleted N from the atmosphere. Theδ¹⁵N_PN surges in winter, when the mixed layer is deeper, indicate an intensified nitrate supply from thermocline, during which relatively stronger downward transfer efficiency was inferred by a small IPN gradient. The largest vertical gradient …