Nitrogen is now known to be unusual among the elements that have had their cycles significantly perturbed by human action.
As it moves along its biogeochemical pathway, the same atom of nitrogen can contribute to many different negative impacts in sequence. Once nitrogen is converted into reactive nitrogen, it can be transported to any part of the system, no matter where it was introduced into the environment. This sequence of effects has been termed the nitrogen cascade (Figure 1). The concept of the cascade, and the extensive research that underlies it, has allowed us not only to determine the linkages among the various aspects of the nitrogen cycle, but also to begin to assess how changes in one part of the cycling can delay or enhance the transfer of nitrogen to other parts of the cycle (Galloway and others 2003).

The Nitrogen Cascade

A single atom of nitrogen can have sequential effects in various parts of the environment after it has been converted from non-reactive N2 to a reactive form during energy or food production. There are three groups of reactive nitrogen compounds: inorganic reduced forms of N: ammonia [NH3] and others forms [NHX], such as ammonium [NH4+]; inorganic oxidized forms: nitrogen oxide [NOX], nitrous oxide [N2O], nitrate [NO3-] and others [NOY]; and organic compounds: urea, amines, proteins, nucleic acids and others [Norganic] Source: adapted from Galloway and others 2003 and redrawn by Robert Smith, Charlottesville, US

 

Nitrogen accumulates in the lower part of the atmosphere in the form of nitrous oxide that contributes to global warming and is also transported to the stratosphere where it contributes to ozone depletion. Excess reactive nitrogen in the air also results in higher concentrations of small particles (aerosols) and increased ozone levels (smog) in the lower part of the atmosphere, that cause respiratory ailments in people. It may fall back to the surface as acid rain, harming plants, corroding buildings, and acidifying soils, lakes, and streams. In addition to acidification effects, it can also fertilize grasslands and forests, causing changes in species composition and often an initial increase in plant growth, followed by, for some systems, a decrease in ecosystem health. Nitrates may seep into groundwater, making it unfit for human consumption. Excess reactive nitrogen is also transferred from the land into rivers, where it reduces biodiversity, and then to the seas and oceans where it can cause a variety of problems, including oxygen-starved coastal waters that harm many forms of marine life (Box 4). The cascade continues as long as the nitrogen remains active in the environment, and it ceases only when reactive nitrogen is stored for a very long time, or is converted back to non-reactive di-nitrogen (N2).

Box 4: Oxygen-starved coastal zones

Oxygen-starved areas in bays and coastal waters have been expanding since the 1960s. The number of known locations around the world has doubled since 1990. While many of these sites are small coastal bays and estuaries, seabed areas in marginal seas of up to 70 000 km2 are also affected. Increased flows of nitrogen from agricultural run-off and the deposition in coastal areas of air-borne nitrogen compounds from fossil-fuel burning stimulate blooms of algae in these waters. The algae sink to the bottom where they are decomposed by micro-organisms that use up most of the oxygen in the system, creating an inhospitable habitat for fish, shellfish, and most other living things. In recent decades, large areas of coastal waters with harmful algal blooms, severely depleted oxygen levels, and disappearing seagrass beds have been identified and clearly linked with increased inputs from the nitrogen cascade. The primary cause of these oxygen-starved areas varies. For example, the very large ‘dead-zone’ in the Gulf of Mexico is caused primarily by nitrogen from agricultural run-off, whereas the problems in the Baltic Sea, northern Adriatic Sea, Gulf of Thailand, Yellow Sea, and Chesapeake Bay result from a combination of agricultural run-off, nitrogen compounds from fossil-fuel burning being deposited from the air, and discharges of human wastes. Because of these different causal factors, different solutions are required. Severe oxygen depletion of coastal waters has significant negative consequences to economically important fisheries, ecosystem services, and biodiversity. These effects are difficult to quantify because the waters are often simultaneously affected by overfishing and habitat destruction related to extensive coastal development.

Global distribution of oxygen-depleted coastal zones. The 146 zones shown are associated with either major population concentrations or with watersheds that deliver large quantities of nutrients to coastal waters. (Annual – yearly events related to summer or autumnal stratification; Episodic – events occurring at irregular intervals greater than one year; Periodic – events occurring at regular intervals shorter than one year; Persistent – all-year-round hypoxia) Sources: Boesch 2002, Caddy 2000, Diaz and others (in press), Green and Short 2003, Rabalais 2002

 

The enormous significance of the nitrogen cascade comes into focus with the realization that it is linked with so many of the major global and regional environmental challenges that policymakers face at these levels today: ozone layer depletion, acidification of soils, global warming, surface and groundwater pollution, biodiversity loss, and human vulnerability. While the extent of its contribution to these problems is still unclear, addressing the issue of excess reactive nitrogen is one of the critical challenges for policymakers.