Global Nitrogen Cycle: Critical Enzymes, Organisms, and Processes for Nitrogen Budgets and Dynamics.

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.