固氮作用是海洋氮循环的关键过程之一，它直接影响到海洋氮收支平衡及海洋净吸收大气CO2的能力。传统观点认为海洋固氮作用仅局限于真光层，但最新研究发现，该过程亦可发生于热带/亚热带海区的弱光层，其贡献的新氮通量可与真光层相媲美，是一个不可忽视的新氮来源，弱光层固氮作用的研究对于重新评估海洋氮循环具有重要意义。目前相关研究在国际上刚刚起步，认识十分肤浅。南海是研究弱光层固氮作用的理想区域，本研究将聚焦于南海弱光层，结合固氮速率测定（15N2吸收法）和天然氮同位素组成（颗粒氮、硝酸盐的δ15N），确定南海弱光层的固氮作用存在与否，定量其速率特征，深入探讨弱光层固氮作用的调控因素，评估弱光层固氮作用对海区新氮收支的贡献，并探明弱光层固氮作用是否主要由粒径<10 μm的固氮生物所贡献。此项研究的开展，可望在海洋弱光层固氮作用这一前沿领域取得富有南海特色的创新性成果。

Nitrogen fixation plays a key role in marine nitrogen cycle, influencing marine nitrogen balance and the capacity of net sequestration of atmospheric CO2 by the ocean. Marine nitrogen fixation has been traditionally thought to occur only in the euphotic zone of the tropical/subtropical areas. However, new evidence indicates that it can also actively occur in the twilight zone of the ocean and contributes a new nitrogen flux comparable to the euphotic zone. Although it has significant implications for the re-evaluation of marine nitrogen cycle, study of nitrogen fixation in the twilight zone is still at its starting stage. The South China Sea is an ideal environment to study twilight zone nitrogen fixation. In this study, we will test whether nitrogen fixation occurs in the twilight zone of the South China Sea via 15N tracer technique. The quantitative method (15N2 tracer assay) and stable nitrogen isotopic composition (δ15N of PON and NO3-) will both be employed. We will analyze the controlling factors on nitrogen fixation in the twilight zone of the South China Sea. By comparing the nitrogen fixation flux with the euphotic zone, the relative importance of twilight zone nitrogen fixation to the new nitrogen budget will be assessed. We will also test whether nitrogen fixation in the twilight zone is contributed mainly by the diazotrophs <10 μm. The study in this cutting-edge field will lead to innovative results with a South China Sea feature.