Humans have doubled the annual rate of nitrogen fixation in the past 150 years. This is caused by transformation of inert atmospheric molecular nitrogen (N₂) to reactive forms of nitrogen (Nr). Reactive N is now accumulating in the environment on all local and regional scales. Nr accumulation contributes to many contemporary environmental problems, including eutrophication, ozone formation and acidification.

Causes of increasing reactive nitrogen (Nr)

During the last few decades, the production of Nr by humans has been greater than production of Nr from all natural terrestrial systems. The global increase in Nr production has three main causes:

• widespread cultivation of legumes, rice, and other crops promoting conversion of N₂ to organic N through biological N₂-fixation;
• combustion of fossil fuels, which converts both atmospheric N₂ and fossil N to reactive NO_x;
• the Haber-Bosch process in fertilizer production, which converts non-reactive N₂ to reactive NH₃ to sustain food production and some industrial activities.

Effects of reactive nitrogen

The same atom of reactive nitrogen (Nr) can cause multiple effects in the atmosphere, terrestrial ecosystems and freshwater and marine systems; there are also effects on human health. We call this sequence of effects the nitrogen cascade.

Source: Galloway et al. (2003) The nitrogen cascade. Bioscience 53: 341-356

As the cascade progresses, the origin of Nr becomes unimportant. The only way to eliminate Nr accumulation and stop the cascade is to convert Nr back to non-reactive N₂. Finding solutions to problems associated with Nr is a difficult task because of this nitrogen cascade.

• back to dossier

For more on integral nitrogen see

• Reactive nitrogen
• Global nitrogen cycle
• Gas fluxes from N₂O, NO and NH₃
• Surface N balance
• Reactive N in the global hydrologic system
• Denitrification

For policy analyses see

• Policy analyses