生物固氮速率测定中15N2同位素示踪剂的引入

生物固氮作用是一个重要的海洋新氮来源，在海洋生物地球化学循环中扮演着不可替代的角色。基于稳定同位素（15N₂）示踪吸收法，是目前直接测定海洋生物固氮速率最有效的手段。其中，高效、洁净地将15N₂引入海水培养体系，并准确定量培养体系底物的同位素示踪水平，是同位素示踪吸收法准确获取固氮速率的关键。本研究针对15N₂同位素示踪剂引入这一关键环节进行了探讨，确认改进气泡法是将15N₂引入海水培养体系的首选操作。在对培养体系造成的较小扰动的情况下，可将培养体系氮气底物的15N原子丰度提升至10%以上，相对于另一种导入同位素示踪剂的手段——预溶解海水法，改进气泡法将培养瓶中氮气底物的15N原子丰度提升了近200%。此外，改进气泡法还具有最小化痕量金属沾污、操作简便等优点。将改进气泡法结合与稳定同位素比值质谱测定结合，是准确测定水体生物固氮速率的推荐方法。

On the introduction of 15N2 tracer in marine N2 fixation rate assay

N₂ fixation plays an essential role in marine biogeochemical cycles by supplying new N. Stable isotope tracer (¹⁵N₂) assay has been considered as the best approach for measuring in situ N₂ fixation rate. Introducing ¹⁵N₂ in an efficient and clean way, and precisely determining the isotopic enrichment level in the incubation system, is thus fundamental for obtaining true rates. In this study, we show that modified bubble treatment is a recommended practice in introducing ¹⁵N₂. It can create ¹⁵N atomic abundance of substrate N₂ in the incubation bottle higher than 10%. Such enrichment is twice higher when compared with gas pre-dissolution treatment. Moreover, modified bubble treatment is featured by much lowered risk of trace metal contamination and less time/labor consuming. We propose that the modified bubble method coupled to stable isotope mass spectrometry should be widely used in measuring N₂ fixation rate in aquatic environments.