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. Plain Language Summary Reactive nitrogen is an essential macronutrient and often limits primary production in marine ecosystems. The East China Sea (ECS) is highly productive biologically and is an important area for traditional fisheries. Nitrogen transfer processes and nitrate sources in the ECS were analyzed using dual isotopes of nitrate and nitrite, the concentration and isotopes of dissolved oxygen and nitrogen gases, nutrient concentrations, and the hydrological conditions. It was clear that nitrate in the Changjiang freshwater was 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 related to phytoplankton assimilation. In the lower water layers, organic matter remineralization, nitrification, and coupled sedimentary nitrification and denitrification resulted in low nitrate isotop