The nature of nitrogen: an overview.

Most of the nitrogen available to the biosphere exists as N2 in the atmosphere, and is not useful to most organisms until it is "fixed" either biologically or abiotically (by lightning or aurorae, or industrially). Once it is fixed into NH3, usually it is either assimilated and transformed into organic N or nitrified into NO3-. Organic N can be transformed back into NH3 by ammonification. Nitrate can be converted into N2O by nitrification and denitrification, and to N2 by denitrification. Such N2O and N2 production results in nitrogen loss from ecosystems and a nitrogen gain to the atmospheric nitrogen reservoir. The different steps of the nitrogen cycle require different environmental conditions. These differences result in a spatial distribution pattern of the different nitrogen transformation reactions. Biological nitrogen fixation occurs universally in soils, sediments, fresh water, and marine systems that are both aerobic and anaerobic. Nitrification occurs primarily in aerobic habitats, whereas denitrification predominates in anaerobic habitats such as sediments and water-logged soils. In closed systems, such as plant growth chambers for CELSS, denitrification and nitrification result in a loss of fixed nitrogen available to plants.