Dissolved organic nitrogen (DON) is one of the major chemical forms in marine nitrogen inventory. The DON cycling is closely coupled with DOC cycling, so that it significantly impact the DOC cycling and stocks in the ocean. Even though substantial progresses have been achieved in the nitrogen cycling, such as nitrate, nitrite, and particulate nitrogen cycling, we barely have a clear picture of the cycling processes of DON in the marine environment as a result of very limited DON concentration data, isotopic data and other parameters that might assist the interpretation of DON cycling like optical spectrum, amino acid enantiomers etc.. Thus, we should take several measures, separately or in a combined manner, to accelerate our understanding of the sources, sinks and transformations of DON from coastal zones to the pelagic seas. Firstly, high quality data of DON should be produced with satisfactory spatial and temporal sampling density, under one consensus standard procedure, so that the DON data of different research group are comparable and a compiled DON dataset on coastal zones and deep sea could be available for modelling study. Secondly, more DON isotope of coastal samples and deep sea samples should be measured with fair quality either by traditional isotope mass balance method or other innovative method. Thirdly, optical properties, molecular analysis should be combined in analyses to unveil the DON black box and to benefit the interpretation of DON isotope and cycling. Finally, extraction of large quantity of DON sample, if operational, should greatly advance the analysis and examination of DON bioavailability, functional group characterization, and novel N containing molecules screening. Hereby, a few study cases on DON cycling in Yangtze River, East China Sea, Northwestern Pacific and Mariana-Yap Trench are presented to emphasize the unique role of DON cycling in different types of marine environment.