OVERVIEW

Although 80% of the Earth's atmosphere is made up of molecular nitrogen (N2), only a small but very important fraction of this nitrogen is converted to a form that can be used by plants and animals, a form known as "fixed" nitrogen. Until recently, this "fixed" atmospheric nitrogen has been thought of as beneficial to all living things. However, industrial and other human-derived sources of fixed nitrogen have now doubled the rate that is now available.

This global overload of fixed nitrogen, despite being one of nature's essential life-giving and life-limiting nutrients, now poses a suite of very serious environmental concerns. For example, too much nitrogen can result in: 1) Loss of commercially important fish stocks and ecosystems by promoting algal blooms, which result in oxygen deprivation and reduced sunlight in coastal and aquatic ecosystems; 2) local extinction of terrestrial plant, animal, and microbial species, thereby reducing biodiversity and ecosystem health; 3) an increase in the greenhous gas nitrous oxide (N2O), which is contributing to global warming; and 4) an increase in the concentration of nitric oxide (NO), which contributes to acid rain and smog.

The Global Nitrogen Cycle: Natural and Humanly Altered Conditions

Molecular nitrogen is the most abundant gas in the Earth's atmosphere. However, in order for nitrogen to be useful to life it must first be transformed, naturally, into forms that are useful to living organisms, a process known as "nitrogen-fixation." Life depends on "fixed" nitrogen that can be absorbed by plants and subsequently used by animals linked together in the "food chain." The amount of nitrogen available at any one time and place has a direct effect on the growth of plants (on land and in the sea) because fixed nitrogen is an essential life-giving and life-limiting nutrient. Thus, the health of the biosphere is strongly dependent upon the availability of nitrogen in a chemical form that is useful to life.

Under natural conditions, a small amount of nitrogen is fixed through chemical processes such as lightning. A much larger amount is fixed by biological processes involving nitrogen-fixing bacteria in the soil and on the roots of certain plants. Once plants die, however, this fixed nitrogen is subsequently returned to the atmosphere by the decomposition of dead tissue by bacteria and is then recycled for later use.

To enhance the availability of nitrogen for living things such as food and fiber products, humans produce nitrogen in the form of fertilizer. With the growth of agriculture, half of all industrial nitrogen fertilizer used in human history has been applied since 1984. In addition to their intentional creation of fixed nitrogen, humans also inadvertently produce fixed nitrogen through the burning of fossil fuels. On a global scale, the fixation of nitrogen by humans now roughly equals the amount of nitrogen that was formerly made available naturally to life by the combined activity of all bacteria on land. In other words, our society has now doubled the amount of fixed nitrogen available to all living things.

Growing amounts of fixed nitrogen are showing up in remote locations, leading to significant impacts. The concentration of N2O in the Earth's atmosphere is rising at about 0.3%/yr. Perhaps even more ominously, N2O has roughly 200 times the global warming potential of carbon dioxide, and remains in the Earth's atmosphere for approximately 150 years, thus making it a long-lived and potent problem. Nitrous oxide is also implicated in the loss of stratospheric ozone.

Increases in the emissions of NO due to the combustion of coal and oil also contribute directly to higher levels of acid rain and ozone (smog) in the lower atmosphere. Atmospheric deposition of nitrogen is the largest single source of human-derived nitrogen in the eastern U.S. coastal waters. There is now 10-20 times more nitrogen entering coastal rivers in the northeastern U.S. and northern Europe than in pre-industrial times.

Excess nitrogen flushed from fertilized farmlands, sewage treatment plants, and fossil-fuel combustion ultimately ends up in streams, rivers, and coastal waters, where it provokes and enhances the growth of microscopic plants that form the base of the food chain upon which more complex and larger plants and animals later feed. However, during the prolific, nitrogen-driven growth and life cycle of these marine and aquatic microscopic plants, they tend to cloud the waters, thus shutting out essential sunlight for other plants. Furthermore, upon the death of these microscopic and larger plants, their once living tissue is consumed by bacteria which proliferate due to the excess nitrogen, and deplete the surrounding water of oxygen necessary for the metabolic processes of marine and aquatic animals, including important commercial fish and shellfish stocks.

Impacts of Nitrogen Deposition on Terrestrial Ecosystems

Since 1982, Dr. Tilman and his colleagues have been engaged in an experiment in which fixed nitrogen was systematically added to 207 plots of grassland and savanna throughout Minnesota. In each instance, nitrogen was added at rates that have been observed in a variety of locations around the world, so as to mimic and replicate real levels of nitrogen deposition in a variety of places. The results of this 12-year experiment reveal that high levels of nitrogen can have a number of serious impacts on these and other ecosystems, including:

1. The addition of nitrogen to these grasses caused major changes in plant species composition, insect species composition, and soil fungal composition. Higher nitrogen levels led to decreased abundances of native plants and to increased abundances of non-native plants, especially certain non-indigenous grasses.
2. The addition of nitrogen caused a significant decrease in plant species diversity. The lowered diversity in these experimental grassland sites corresponded with lowered stability of primary productivity in the face of a major disturbance such as a flood or drought.
3. When nitrogen was added at low rates to plots dominated by native prairie grasses, most of the nitrogen was retained within the ecosystem. However, at higher rates of nitrogen addition, the native species were replaced by non-native species, while most of the added nitrogen was leached into the groundwater. Ultimately, higher levels on nitrogen in the groundwater (in the form of nitrates) can provoke toxic algal blooms in waterways and seriously impair the quality of drinking water.
4. The highest rates of carbon storage occurred at the lowest rates of nitrogen addition, in direct contrast to anticipation. In instances where high levels of nitrogen were available, the ability of the plant community to effectively store carbon was lowered. A shift in plant species composition was also observed to occur in instances involving higher rates of nitrogen addition. This resulted in low rates of carbon storage, because these plant species decayed too rapidly to effectively store carbon.
5. High rates of nitrogen deposition are likely to greatly impact the composition, functioning, and stability of many terrestrial, aquatic, and marine ecosystems, with profound implications for food supplies and other ecosystems services.