Reactive nitrogen
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 NOx;
- 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.
