Abstract The multiple and complex N sources in coastal upwelling areas make it difficult to understand the behaviour of the N cycle. Stable N and O isotopes (δ15N and δ18O) of nitrate (NO3−) from coastal upwelling off eastern Hainan Island in the South China Sea (SCS) identified the main NO3− sources and the processes that control NO3− distribution in the area. Our results revealed that upwelling occurred in the nearshore area of transects Q2 and Q3, and that the sources of NO3− in the surface water were mainly upwelling from the bottom water (50%), followed by nitrification (34%) and atmospheric deposition (AD) (16%). In the nearshore surface water of Transect Q1, the sources of NO3− were mostly manure and sewage (31%), followed by AD (24%), soil N (24%), and N in fertiliser (21%). Complete assimilation was an important process in the surface water. For the offshore area, NO3− assimilation and AD were the primary processes that affected N concentrations, and the proportion of AD was 30% in the upper water of the eastern coast of Hainan Island in the northern SCS. In the bottom water, coupled nitrification-denitrification could occur in sediment porewater and/or sediment-water interface, the isotope signal of which could then be diffused into the bottom water. The degree of denitrification for NO3− in the bottom water of the eastern Hainan coast ranged from 7.2% to 29.8% with an average of 22.5%. Our results suggested that upwelling plays an important role in the N cycle on the eastern coast of Hainan Island.

中文翻译：

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海南岛东岸氮氧同位素识别硝酸盐来源及生物地球化学过程

摘要 沿海上升流区氮源的多样性和复杂性使得理解氮循环的行为变得困难。来自南海东部沿海上升流的硝酸盐 (NO3−) 的稳定 N 和 O 同位素 (δ15N 和 δ18O) 确定了主要的 NO3− 来源和控制该地区 NO3− 分布的过程。我们的研究结果表明，上升流发生在 Q2 和 Q3 样线的近岸区，地表水中 NO3− 的来源主要来自底水（50%），其次是硝化作用（34%）和大气沉降（广告）（16%）。Transect Q1近岸地表水中NO3−的来源主要是粪便和污水（31%），其次是AD（24%）、土壤N（24%）和肥料中的N（21%）。完全同化是地表水中的一个重要过程。对于近海区域，NO3−同化和AD是影响N浓度的主要过程，在南海北部海南岛东海岸上层水域，AD的比例为30%。在底水中，沉积物孔隙水和/或沉积物-水界面可能发生硝化-反硝化耦合，其同位素信号可以扩散到底水中。海南东部沿海底水中NO3−反硝化程度为7.2%～29.8%，平均为22.5%。我们的研究结果表明，上升流在海南岛东海岸的 N 循环中起着重要作用。南海北部海南岛东海岸上层水域AD的比例为30%。在底水中，沉积物孔隙水和/或沉积物-水界面可能发生硝化-反硝化耦合，其同位素信号可以扩散到底水中。海南东部沿海底水中NO3−反硝化程度为7.2%～29.8%，平均为22.5%。我们的研究结果表明，上升流在海南岛东海岸的 N 循环中起着重要作用。南海北部海南岛东海岸上层水域AD的比例为30%。在底水中，沉积物孔隙水和/或沉积物-水界面可能发生硝化-反硝化耦合，其同位素信号可以扩散到底水中。海南东部沿海底水中NO3−反硝化程度为7.2%～29.8%，平均为22.5%。我们的研究结果表明，上升流在海南岛东海岸的 N 循环中起着重要作用。然后可以将其同位素信号扩散到底部水中。海南东部沿海底水中NO3−反硝化程度为7.2%～29.8%，平均为22.5%。我们的研究结果表明，上升流在海南岛东海岸的 N 循环中起着重要作用。然后可以将其同位素信号扩散到底部水中。海南东部沿海底水中NO3−反硝化程度为7.2%～29.8%，平均为22.5%。我们的研究结果表明，上升流在海南岛东海岸的 N 循环中起重要作用。