Global nitrogen cycle within the coupled CNP system

The system of C-N-P coupled biogeochemical cycles represents the main interactions between the biosphere and the inorganic world of the planetary surface environment. Within this system, nitrogen occupies a unique position owing to its overwhelmingly large atmospheric reservoir that exceeds by large factors its other major environmental reservoirs, such as the land phytomass, the oceans, and sediments. Our recent version of the global biogeochemical cycle of nitrogen emphasizes three major divisions of the environment: the land part, the coastal margins, and the open oceans. The reservoir masses and the values of some of the more important fluxes are summarized. Features of the nitrogen cycle which are particularly relevant to an understanding of the changing picture of the present-day cycle are discussed.