Deciphering and quantifying nitrate sources and processes in the central Yellow Sea using dual isotopes of nitrate

Anthropogenic activities pose significant challenges to the accumulation of coastal nitrogen (N). Accurate identification of nitrate (NO₃^-) sources is thus essential for mitigating excessive N in many marginal seas. We investigated the dual isotopes of NO₃^- in the central Yellow Sea to elucidate the sources and cycling processes of NO₃^-. The results revealed significant spatial variability in NO₃^- concentrations among the Yellow Sea Surface Water (YSSW), Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM), and Taiwan Warm Current Water (TWCW). Stratification played a crucial role in restricting vertical nutrient transport, leading to distinct nutrient sources and concentrations in different water masses. The dual NO₃^- isotopic signature indicated that atmospheric deposition was the primary source of surface NO₃^-, contributing approximately 30 % to the NO₃^- in the YSSW. In the NO₃^--rich CDW, the heavier δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- suggested incomplete NO₃^- assimilation. Organic matter mineralization and water stratification played crucial roles in the accumulation of nutrients within the YSCWM and TWCW. Notably, regenerated NO₃^- accounted for approximately half of the NO₃^- stored in the YSCWM. A synthesis of NO₃^- dual isotope data across the coastal China seas revealed significant spatial and seasonal variations in the N source. The study emphasized the dynamics of coastal NO₃^- supply, which are shaped by the complex interconnections among marine, terrestrial, and atmospheric processes. Our approach is a feasible method for exploring the origins of N amidst the escalating pressures of anthropogenic nutrient pollution in coastal waters.