Although ocean observations indicated a negative relationship between δ15NPOM and nitrate concentration based on Rayleigh fractionation (Sigman et al., 2009) has been supported in the ocean observations (Kodama et al., 2021; Ho et al., 2021),  the negative relationship in the SOJ remained equivocal. Even in an open system, kinetic isotope effects (δ15N difference between reactant and product) and the degree of consumption of reactant theoretically determine the δ15N value of the product, and when the remaining reactant is zero (i.e., completely consumed), the δ15N of a product is equal to the original δ15N of reactant (Sigman et al., 2009). In this case, the kinetic isotope effect of nitrate on POM is ~3‰ (Sigman et al., 2009). Thus, δ15NPOM is theoretically 0–3‰ lower than δ15N of nitrate (δ15NNO3), and with nitrate consumption, it increases and approaches the original δ15NNO3 value. The monthly variations in the surface layer along the SI-line indicated that the δ15NPOM increases from winter to summer, and the GLM method confirms this trend. This also indicates that nitrate depletion partly contributes to the increase in δ15NPOM. Because δ15NPOM was not normally distributed, the association was insignificant (p = 0.4467) when we removed class I POM, indicating that the relationships between δ15NPOM and the environment, especially the nitrate concentration in the SOJ, were unique.