Abstract: Organic nitrogen is one of the primary forms of nitrogen in coastal sediments, playing a crucial role in biogeochemical cycles. The molecular composition and mineralization processes of organic nitrogen in coastal sediments exert significant influences on coastal eutrophication and the emission of major greenhouse gases (such as N₂O). We systematically review the molecular composition, sources, and mineralization processes of organic nitrogen in coastal sediments, summarize their responses to environmental changes, including human activities and climate changes. The molecular composition of coastal sediment organic nitrogen is complex and challenging to characterize. Conventional methods offer only preliminary insights, as they fail to provide detailed molecular composition information or track transformation processes. Our understanding is primarily confined to the metabolic processes of proteins and nucleic acids. Advanced techniques, particularly FTICR-MS, provide critical technical support for unraveling the molecular composition of sediment organic nitrogen. Factors such as eutrophication, warming, and acidification affect the mineralization processes of coastal sediment organic nitrogen. They influence the microbial communities involved in mineralization, which alters the characteristics of coastal sediment organic nitrogen (e.g., molecular composition and content). Considering the knowledge gaps, we propose three directions for future research of coastal sediment organic nitrogen: (1) employing new technologies and methods to further decipher the molecular composition and sources of sediment organic nitrogen, (2) using genomics, transcriptomics, and other omics techniques to delve deeper into the mineralization processes and pathways of sediment organic nitrogen, (3) investigating how sediment organic nitrogen mineralization responds to human activities and climate change, with a particular focus on its impact on the emission of N-related greenhouse gases.

Key words: marine sediment, organic nitrogen, molecular composition, mechanism of mineralization, environmental change