The microbial nitrogen-cycling network is essential for life on Earth, as nitrogen is a critical component of all living organisms. Atmospheric dinitrogen is the largest reservoir of freely available nitrogen, but most organisms rely on more bioavailable forms such as ammonium and nitrate. The availability of these nitrogen forms is controlled by microbial reactions that alter nitrogen's oxidation state. Human activities, particularly the use of industrial nitrogen-based fertilizers, have significantly increased bioavailable nitrogen, affecting food production and ecosystems. Microorganisms play a key role in nitrogen cycling through various processes, including nitrogen fixation, ammonia oxidation, nitrite oxidation, nitrate reduction, nitrite reduction, nitric oxide reduction, and hydrazine synthesis. These processes are carried out by diverse microorganisms, some of which can perform multiple reactions. The network of these microorganisms is complex and involves interactions that influence global nitrogen biogeochemistry. Nitrogen fixation is performed by certain bacteria and archaea, which convert atmospheric dinitrogen into ammonia. Ammonia oxidation to hydroxylamine and nitrite is carried out by bacteria and archaea, while nitrite oxidation to nitrate is a major pathway for nitrate production. Nitrate reduction to nitrite and ammonium is important for nitrogen assimilation and dissimilatory processes. Nitric oxide reduction to nitrous oxide or dinitrogen gas is a significant source of nitrous oxide, a potent greenhouse gas. Hydrazine synthesis and oxidation to dinitrogen gas are unique processes that contribute to nitrogen cycling. The microbial network of nitrogen-transforming microorganisms is highly diverse and complex, with interactions that influence nitrogen availability and cycling in various ecosystems. Understanding these processes is crucial for managing nitrogen cycles and mitigating environmental impacts.The microbial nitrogen-cycling network is essential for life on Earth, as nitrogen is a critical component of all living organisms. Atmospheric dinitrogen is the largest reservoir of freely available nitrogen, but most organisms rely on more bioavailable forms such as ammonium and nitrate. The availability of these nitrogen forms is controlled by microbial reactions that alter nitrogen's oxidation state. Human activities, particularly the use of industrial nitrogen-based fertilizers, have significantly increased bioavailable nitrogen, affecting food production and ecosystems. Microorganisms play a key role in nitrogen cycling through various processes, including nitrogen fixation, ammonia oxidation, nitrite oxidation, nitrate reduction, nitrite reduction, nitric oxide reduction, and hydrazine synthesis. These processes are carried out by diverse microorganisms, some of which can perform multiple reactions. The network of these microorganisms is complex and involves interactions that influence global nitrogen biogeochemistry. Nitrogen fixation is performed by certain bacteria and archaea, which convert atmospheric dinitrogen into ammonia. Ammonia oxidation to hydroxylamine and nitrite is carried out by bacteria and archaea, while nitrite oxidation to nitrate is a major pathway for nitrate production. Nitrate reduction to nitrite and ammonium is important for nitrogen assimilation and dissimilatory processes. Nitric oxide reduction to nitrous oxide or dinitrogen gas is a significant source of nitrous oxide, a potent greenhouse gas. Hydrazine synthesis and oxidation to dinitrogen gas are unique processes that contribute to nitrogen cycling. The microbial network of nitrogen-transforming microorganisms is highly diverse and complex, with interactions that influence nitrogen availability and cycling in various ecosystems. Understanding these processes is crucial for managing nitrogen cycles and mitigating environmental impacts.