Nitrogen cycle in ocean is mainly driven by microbial processes, which plays important roles in the marine production and influences the global climate change. Different  microbial nitrogen transformation processes, including nitrogen assimilation, nitrogen fixing, nitrification and denitrification, are all associated with different isotope effect of both the nitrogen and oxygen atoms, impacting the composition of the nitrogen and oxygen isotope of nitrate in the sea or ocean. Therefore, N-isotopic tracers can provide qualitative and quantitative information on extant processes on local scales. Bacterial denitrification method provides  an important and powerful approach for this kind of study, which can transform the signal of N/O isotope in nitrate into N₂O. Through investigating the composition of coupled N/O isotope in N₂O, each of microbial process for nitrogen transformation can be determined exactly.