Box 3: Our incremental understanding of nitrogen

The 7th element of the periodic table was named ‘nitrogene’ in 1790 by Jean Antoine Claude Chaptal (1756–1832). By 1850, the fundamental role of nitrogen in living systems and its beneficial role in crop production became clear: it was a common element in plant and animal tissues, indispensable for plant growth, underwent constant cycling between organic and inorganic compounds, and was an effective fertilizer. Knowledge on the negative impacts of nitrogen compounds on people and ecosystems also grew with time. The contribution of nitrogen compounds to air pollution was certainly known by the late 19th century, as was the impact of excess nutrients (including nitrogen) on coastal marine ecosystems, although for the latter, the focus was on the positive impacts of nutrients on marine productivity. It was not until the mid- 20th century that negative impacts of excess nutrients were recognized as a serious international problem.

Source: Nixon and Buckley 2002

Although the environmental significance of nitrogen has been studied for at least 150 years (Box 3) scientists have only recently documented how reactive nitrogen ‘cascades’ through ecosystems, creating environmental and human health problems all along the way (Galloway and others 2003). We have also recently come to a better understanding of the magnitude of the problem: human production of reactive nitrogen is now greater than the amount created by natural processes on land, leading to a large increase in the amount of nitrogen moving within the environment. Globally, humans create about 160 million tonnes of reactive nitrogen per year, compared to natural rates of terrestrial biological nitrogen fixation, which are between 90–120 million tonnes annually. However, we have not similarly increased de-nitrification processes which convert nitrogen primarily back to non-reactive N2. Reactive nitrogen is, therefore, accumulating in the environment.