Source Versus Recycling Influences on the Isotopic Composition of Nitrate and Nitrite in the East China Sea

Nitrogen transfer processes and NO₃⁻ sources in the East China Sea (ECS) were analyzed using dual isotopes of NO₃⁻ and NO₂⁻, the concentration and isotopes of dissolved O₂ and N₂ gases, nutrient concentrations, and the hydrological conditions. It was clear that the δ¹⁵N and δ¹⁸O values of NO₃⁻ in the Changjiang freshwater were 5.6–6.6‰ and 0.6–1.0‰, respectively, affected by human activities (fertilizer, sewage, and manure) and nitrification. Off the Changjiang Estuary to the ECS continental slope, the NO₃⁻ concentration was lower or exhausted in the upper water layers, where both available δ¹⁵N and δ¹⁸O values for NO₃⁻ were high related to phytoplankton assimilation. In the lower water layers, organic matter remineralization, nitrification, and coupled sedimentary nitrification and denitrification resulted in low NO₃⁻ isotope values. Moreover, in the upper water layers of the ECS continental slope, NO₃⁻ showed high δ¹⁵N and δ¹⁸O values and low Δ(15, 18) values affected by assimilation, nitrification, and N₂ fixation. NO₂⁻ in the ECS was dominated by NH₄⁺ oxidation, and NO₂⁻ oxidation plays an important role in depleting NO₂⁻ in δ¹⁵N values. An overall NO₃⁻ budget is built for the ECS shelf, indicating that open boundary exchanges of NO₃⁻ flux and isotopes through Kuroshio invasion and Taiwan Warm Current Water are comparable to outflow off the ECS shelf, and nitrogen transformation processes (such as NO₃⁻ assimilation and nitrification) play an important role in nitrogen cycle, and NO₃⁻ is modified on the ECS shelf.