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

Nitrogen transfer processes and NO 3 − sources in the East China Sea (ECS) were analyzed using dual isotopes of NO 3 − and NO 2 − , the concentration and isotopes of dissolved O 2 and N 2 gases, nutrient concentrations, and the hydrological conditions. It was clear that the δ 15 N and δ 18 O values of NO 3 − 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 NO 3 − concentration was lower or exhausted in the upper water layers, where both available δ 15 N and δ 18 O values for NO 3 − were high related to phytoplankton assimilation. In the lower water layers, organic matter remineralization, nitrification, and coupled sedimentary nitrification and denitrification resulted in low NO 3 − isotope values. Moreover, in the upper water layers of the ECS continental slope, NO 3 − showed high δ 15 N and δ 18 O values and low Δ(15, 18) values affected by assimilation, nitrification, and N 2 fixation. NO 2 − in the ECS was dominated by NH 4 + oxidation, and NO 2 − oxidation plays an important role in depleting NO 2 − in δ 15 N values. An overall NO 3 − budget is built for the ECS shelf, indicating that open boundary exchanges of NO 3 − 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 NO 3 − assimilation and nitrification) play an important role in nitrogen cycle, and NO 3 − is modified on the ECS shelf.