As a basic building block of plant and animal proteins, nitrogen is a nutrient essential to all forms of life. But it is possible to have too much of a good thing. Recent studies have shown that excess nitrogen from human activities such as agriculture, energy production, and transport has begun to overwhelm the natural nitrogen cycle with a range of ill effects from diminished soil fertility to toxic algal blooms   .
Until recently, the supply of nitrogen available to plants – and ultimately to animals – has been quite limited. Although it is the most abundant element in the atmosphere, nitrogen from the air cannot be used by plants until it is chemically transformed, or fixed, into ammonium or nitrate compounds that plants can metabolize. In nature, only certain bacteria and algae (and, to a lesser extent, lightning) have this ability to fix atmospheric nitrogen, and the amount that they make available to plants is comparatively small. Other bacteria break down nitrogen compounds in dead matter and release it to the atmosphere again. As a consequence, nitrogen is a precious commodity – a limiting nutrient – in most undisturbed natural systems.
All that has changed in the past several decades. Driven by a massive increase in the use of fertilizer, the burning of fossil fuels, and an upsurge in land clearing and deforestation, the amount of nitrogen available for uptake at any given time has more than doubled since the 1940s. In other words, human activities now contribute more to the global supply of fixed nitrogen each year than natural processes do, with human-generated nitrogen totaling about 210 million metric tons per year, while natural processes contribute about 140 million metric tons . (See A Global Glut of Nitrogen.)
|A global glut of nitrogen|
|Global sources of biologically available (Fixed) nitrogen)|
|ANNUAL RELEASE OF|
|Legumes and other plants||40|
|Total from human sources||210|
|Soil bacteria, algae, lightning, etc.||140|
|Source: Peter M. Vitousek et al., “Human Alteration of the Global Nitrogen Cycle: Causes and Consequences,” Issues in Ecology, No. 1 (1997), pp. 4-6.
This influx of extra nitrogen has caused serious distortions of the natural nutrient cycle, especially where intensive agriculture and high fossil fuel use coincide. In some parts of northern Europe, for example, forests are receiving 10 times the natural levels of nitrogen from airborne deposition , while coastal rivers in the northeastern United States and northern Europe are receiving as much as 20 times the natural amount from both agricultural and airborne sources . Nitrate levels in many Norwegian lakes have doubled in less than a decade . Although many of the nitrogen trouble spots tend to be in North America and Europe, the threat of nitrogen overload is global in scope, as both fertilizer use and energy use are growing quickly in the developing world. In fact, global nitrogen deposition may as much as double in the next 25 years as agriculture and energy use continue to intensify .
The effects of this surfeit of nutrients reach into every environmental domain, threatening air and water quality and disrupting the health of terrestrial and aquatic ecosystems. Natural systems may be able to absorb a limited amount of additional nitrogen by producing more plant mass, just as garden vegetables do when fertilized. Atmospheric deposition of nitrogen emissions on some heavily cut forests in North America and Europe seems to have spurred additional growth in this manner. But there is a limit to the amount of nitrogen that natural systems can take up; beyond this level, serious harm can ensue. In terrestrial ecosystems, nitrogen saturation can disrupt soil chemistry, leading to loss of other soil nutrients such as calcium, magnesium, and potassium and ultimately to a decline in fertility .
Excess nitrogen can also wreak havoc with the structure of ecosystems, affecting the number and kind of species found. Researchers in the United Kingdom and the United States have found that applying nitrogen fertilizer to grasslands enables a few nitrogen-responsive grass species to dominate, while others disappear. In one British experiment, this effect led to a fivefold reduction in the number of species in the most heavily fertilized plots  . In the Netherlands, where nitrogen deposition rates are among the highest in the world, whole ecosystems have been altered because of this shift in dominant plants, with species-rich heathlands being converted to species-poor forests and grasslands that better accommodate the nitrogen load .
References and notes
1. Peter M. Vitousek et al., “Human Alteration of the Global Nitrogen Cycle: Causes and Consequences,” Issues In Ecology, No. 1 (February 1997), p. 2.
2. Thomas E. Jordan and Donald E. Weller, “Human Contributions to Terrestrial Nitrogen Flux: Assessing the Sources and Fates of Anthropogenic Fixed Nitrogen,” BioScience, Vol. 46, No. 9 (1996), p. 665.
3. Gregory P. Asner, Timothy R. Seastedt, and Alan R. Townsend, “The Decoupling of Terrestrial Carbon and Nitrogen Cycles: HumanInfluences on Land Cover and Nitrogen Supply Are Altering Natural Biogeochemical Links in the Biosphere,” BioScience, Vol.47, No. 4 (1997), p. 232.
4. Op. cit. 1, pp. 5-6.
5. Fred Pearce, “Planet Earth Is Drowning in Nitrogen,” New Scientist (April 12, 1997), p. 10.
6. Op. cit. 1, p. 10.
7. Op. cit. 1, p. 10.
8. Op. cit. 3, p. 228.
9. Op. cit. 1, pp. 7-9.
10. Op. cit. 1, pp. 9-10.
11. David Wedin and David Tilman, “Influence of Nitrogen Loading and Species Composition on Carbon Balance of Grasslands,” Science, Vol. 274 (1996), pp. 1720-1721.
12. Op. cit. 1, pp. 9-10.