The article provides a comprehensive overview of the microbial nitrogen-cycling network, highlighting the complex interactions and metabolic processes involved in nitrogen transformation. Nitrogen, essential for all life, is primarily found in the atmosphere as dinitrogen gas, which is inaccessible to most organisms. Microorganisms, particularly nitrogen-fixing bacteria and archaea, can convert this inert gas into bioavailable forms like ammonia and nitrate. The availability of these forms depends on various microbial reactions that alter nitrogen's oxidation state. The review discusses the six classical processes of the nitrogen cycle—fixation, assimilation, nitrification, ammonification, denitrification, and anammox—and emphasizes the metabolic versatility of nitrogen-transforming microorganisms, which often engage in multiple processes simultaneously. It also covers the biochemical pathways, enzymes, and microorganisms involved, including novel processes such as complete ammonia oxidation and anaerobic ammonium oxidation. The environmental and industrial significance of these processes is explored, along with the impact of human activities, such as the use of nitrogen-based fertilizers, on global nitrogen cycling. The article concludes by discussing the robustness and modularity of microbial nitrogen networks in natural and anthropogenic ecosystems, emphasizing the need for further research to understand the factors shaping these networks.The article provides a comprehensive overview of the microbial nitrogen-cycling network, highlighting the complex interactions and metabolic processes involved in nitrogen transformation. Nitrogen, essential for all life, is primarily found in the atmosphere as dinitrogen gas, which is inaccessible to most organisms. Microorganisms, particularly nitrogen-fixing bacteria and archaea, can convert this inert gas into bioavailable forms like ammonia and nitrate. The availability of these forms depends on various microbial reactions that alter nitrogen's oxidation state. The review discusses the six classical processes of the nitrogen cycle—fixation, assimilation, nitrification, ammonification, denitrification, and anammox—and emphasizes the metabolic versatility of nitrogen-transforming microorganisms, which often engage in multiple processes simultaneously. It also covers the biochemical pathways, enzymes, and microorganisms involved, including novel processes such as complete ammonia oxidation and anaerobic ammonium oxidation. The environmental and industrial significance of these processes is explored, along with the impact of human activities, such as the use of nitrogen-based fertilizers, on global nitrogen cycling. The article concludes by discussing the robustness and modularity of microbial nitrogen networks in natural and anthropogenic ecosystems, emphasizing the need for further research to understand the factors shaping these networks.