Research progress of urban nitrogen cycle and metabolism

•A comprehensive and systematic review of urban nitrogen cycle.•Covering budget, evaluation, simulation and regulation of urban nitrogen cycle.•Data accuracy of nitrogen flow accounting need to be improved.•Extending to city-cluster scale from a metabolic perspective is important. The anthropogenic impact on nitrogen cycle is exacerbated by urbanization. Previous reviews of urban nitrogen have mainly focused on the summary of research methods and development stages. They only focus on one aspect of urban nitrogen research but lack a holistic view. Thus, there is a need to sort out urban nitrogen research from a systematic perspective. With the idea of problem identification, analysis and solution, this article systematically reviewed the contents and methods of urban nitrogen research from four aspects: budget, evaluation, simulation and regulation. That is, applying material flow analysis to discover the sources of urban nitrogen problems, using relevant indicators to evaluate the influences of nitrogen problems, constructing models to simulate the urban nitrogen metabolism processes and explore the distribution pattern of nitrogen pollution as well as its synergistic relationship with other elements, then applying factor decomposition analysis to identify the influencing factors and carry out scenario simulation to guide the optimal regulation of nitrogen problems. This article also proposed future directions for urban-scale nitrogen research in terms of data accuracy improvement, construction of comprehensive evaluation index system, refinement of research contents, and expansion of research scales. [Display omitted]