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Lateral input of dissolved organics may play a significant role to support productivity in oligotrophic ocean although associated biogeochemical evidences are lacking in the field. Ammonia oxidation (AO), the first step of nitrification that bridges organic remineralization and nitrate, is potentially an immediate responder. By using N-15-NH4+, the spatial distribution of AO was investigated in the northern South China Sea, where Kuroshio Current intrudes frequently. AO ranged widely (0.001 to 134 nmol.L-1.day(-1)) in space and the depth-integrated (200 m) AO peaked where the Kuroshio influence is moderate suggesting that enhanced AO had occurred due to lateral mixing. Since oligotrophic Kuroshio is characterized by high dissolved organic nitrogen (DON), such lateral mixing not only introduces external DON into the northern South China Sea but also enhances NH4+ regeneration and subsequent oxidation to complicate the conventional new production in the boundary zone with DON gradient.

Plain Language Summary The horizontal mixing is widespread in oligotrophic ocean; however, the mixing-induced biogeochemical response is hard to detect and remains less explored. We measured ammonia oxidation (AO), a critical process immediately responds to remineralization of dissolved organic matter (DOM), along the pathway of Kuroshio intrusion into the South China Sea. We found AO peaked at stations where the influence of Kuroshio Current intrusion is moderate. We hypothesized that intrusion of oligotrophic water brings on-site recalcitrant DOM into marginal seas and enriches ammonium level via bacteria decomposition of the foreign DOM. Accompanied is the enhancement of AO, which transfers DOM into nitrate, further complicating the conventional concept of N-based new production. Such lateral intrusion or transport appears also along a meridional direction. For example, the western boundary current (warm and saline) carrying DOM-rich water flows toward higher latitudes where nutrients are high. According to our findings, such hemispheric scale transport may exert considerable influence on high-latitude nitrogen biogeochemistry. Knowledge of the impact of Kuroshio intrusion on AO in the northern South China Sea was a step forward in documenting this type of response in frontal zone, but more studies will be needed to validate the underlying mechanisms.