The Nitrogen Cycle

This chapter presents an overview of the major processes that comprise the nitrogen cycle and a discussion of how anthropogenic activities have significantly altered the various processes in the natural nitrogen cycle. Prior to the industrial fixation of dinitrogen gas by the Haber–Bosch process, bacteria and other microorganisms mediated or controlled essentially all the processes involving the transformation of nitrogen in soils, the unsaturated zone, surface water bodies, and in aquifers. The main processes that cycle the various forms of nitrogen between the atmosphere, hydrosphere, and biosphere, include mineralization/ammonification, nitrogen fixation, nitrification, denitrification, dissimilatory nitrate reduction to ammonia, and annamox reactions. Human management of soils and other terrestrial ecosystems has affected ecosystem nitrogen cycling. Major changes in the global distribution and commodities transport of reactive nitrogen need to be considered when assessing anthropogenic alteration of the nitrogen cycle from one part of the world to another.