Nitrogen (N) is used in many of life's fundamental biomolecules, and it is also a participant in environmental redox chemistry. Biogeochemical processes control the amount and form of N available to organisms ("fixed"N). These interacting processes result in N acting as the proximate limiting nutrient in most surface environments. Here, we review the global biogeochemical cycle of N and its anthropogenic perturbation. We introduce important reservoirs and processes affecting N in the environment, focusing on the ocean, in which N cycling is more generalizable than in terrestrial systems, which are more heterogeneous. Particular attention is given to processes that create and destroy fixed N because these comprise the fixed N input/output budget, the most universal control on environmental N availability. We discuss preindustrial N budgets for terrestrial and marine systems and their modern-day alteration by N inputs from human activities. We summarize evidence indicating that the simultaneous roles of N as a required biomass constituent and an environmental redox intermediate lead to stabilizing feedbacks that tend to blunt the impact of N cycle perturbations at larger spatiotemporal scales, particularly in marine systems. As a result of these feedbacks, the anthropogenic "N problem"is distinct from the "carbon dioxide problem"in being more local and less global, more immediate and less persistent.
All Science Journal Classification (ASJC) codes