EarthTrends: Featured Topic Title. Nutrient Overload: Unbalancing the Global Nitrogen Cycle Author(s) taff of World Resources Program Source World Resonrces 1998-99 Date written: 1998 As a basic building block of plant contribute more to the glob vegan lakes have doubled in and animal proteins, nitrogen is a supply of fixed nitrogen each year less than a decade (vitousek et al nutrient essential to all forms of than natural processes do, with 1997: 10). Although many of the life. But it is possible to have too human-generated nitrogen totaling nitrogen trouble spots tend to be luch of a good thing recent about 210 million metric tons per in North America and Europe, the studies have shown that excess year, while natural processes threat of nitrogen overload is nitrogen from human activities contribute about 140 million global in scope, as both fertilizer such as agriculture, energy (Vitousek et al use and energy use are growing 1997: 5-6). See Figure 1: Global quickly in the developing world begun to overwhelm the natural Sources of Biologically Available In fact, global nitrogen deposition itrogen cycle with a range of ill (Fixed) Nitrogen may as much as double in the next effects-from diminished soil This influx of extra nitrogen 25 years as agriculture and energy fertility to toxic algal blooms has caused serious distortions of use continue to intensify(Asner et (Vitousek et al. 1997: 2, Jordan et the natural nutrient cycle, l.1997:228 al. 1996: 665: Asner et al. especially where intensive The effects of this surfeit of 1997:232) agriculture and high fossil fuel use nutrients reach Until recently, the supply of coincide. In some parts of environmental domain nitrogen available to plants-and northern Europe, for example, threatening air and water quality ltimately to animals-has been forests are receiving 10 times the and disrupting the health of quite limited. Although it is the natural levels of nitrogen from errestrial and aquatic ecosystem most abundant clement in the airborne deposition(Pearce Natural systems may be able to atmosphere, nitrogen from the air 1997: 10), while coastal rivers in absorb a limited amount of cannot be used by plants until it is the northeastern United States and additional nitrogen by producing chemically transformed, or fixed, northern Europe are receiving as more plant mass, just as garden nto ammonium or nitrate much as 20 times the natural vegetables do when fertilized ompounds that plants can amount from both agricultural Atmospheric deposition of metabolize In nature, only certain and airborne sources(Vitousek et nitrogen emissions on some bacteria and algae(and, to a lesser al. 1997: 10). Nitrate levels in many heavily cut forests in North extent, lightning) have this ability to fix atmospheric nitrogen, an the amount that they make A Global Glut of Nitrogen available to plants is comparatively gure 1: Global Sources of Biologically Available(Fixed)Nitrogen small. Other bacteria break down nitrogen compounds in dea ANNUAL RELEASE OF matter and release it to th ANTHROPOGENIC FIXED NITROGEN atmosphere again. As a SOURCES (TERAGRAMS) ence, nitrogen is a Fertilizer precious commodity-a limiting Legumes and other plants nutrient--in most undisturbed Fossil fuels 000 Biomass burning 40 All that has changed in the 10 past several decades. Driven by a Land clearing 20 massive increase in the use of fertilizer, the burning of fossil fuels, and a surge in land clearing Total from human sources 210 and deforestation the amount of nitrogen available for uptake at NATURAL SOURCES any given time has more than Soil bacteria, algae, lightning, etc. 140 doubled since the 1940s. In other words. human activities now Source: Vitousek et al. 1997: 4-6 CEarthTrends 2001 World Resources Institute. All nights reserved. Fair use is permitted on a limited scale and for educational purposes
©EarthTrends 2001 World Resources Institute. All rights reserved. Fair use is permitted on a limited scale and for educational purposes. EarthTrends: Featured Topic Title: Nutrient Overload: Unbalancing the Global Nitrogen Cycle Author(s): Staff of World Resources Program Source: World Resources 1998–99 Date written: 1998 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 (Vitousek et al. 1997:2; Jordan et al. 1996:665; Asner et al. 1997:232). 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 a surge 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 (Vitousek et al. 1997:5–6). (See Figure 1: Global Sources of Biologically Available (Fixed) Nitrogen.) 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 (Pearce 1997:10), 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 (Vitousek et al. 1997:10). Nitrate levels in many Norwegian lakes have doubled in less than a decade (Vitousek et al. 1997:10). 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 (Asner et al. 1997:228). The effects of this surfeit of nutrients reach 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 A Global Glut of Nitrogen Figure 1: Global Sources of Biologically Available (Fixed) Nitrogen ANNUAL RELEASE OF ANTHROPOGENIC FIXED NITROGEN SOURCES (TERAGRAMS) Fertilizer 80 Legumes and other plants 40 Fossil fuels 20 Biomass burning 40 Wetland draining 10 Land clearing 20 Total from human sources 210 NATURAL SOURCES Soil bacteria, algae, lightning, etc. 140 Source: Vitousek et al. 1997:4-6
suffered the most so far. They are troubling aspects of this nutrient the ultimate receptacles of much assault on aquatic systems has f the nutrient overload, which been a steady rise in toxic alg ends to accumulate in runoff blooms which can take a heay be delivered directly in the oll on fish, seabirds, and marine form of t treated mammals(Anderson 1994: 62-68) ewage is very high in nitrogen The nitrogen glut also from protein in the human diet. impinges on the health of the South Florida Water Management District In these aquaticsystems, excess atmosphere when the nitrogen- have spurred additional growth in When this extra plant matter dies the air, either from fossil fuey 9 nitrogen can often stimulate the containing gases-nitric oxide and America and Europe seems to growth of algae and other plants. nitrous oxide-are released this manner. But there is a limit to and decays, it can rob the water of burning, land clearing, or the amount of nitrogen that its dissolved oxygen, suffocating agriculture-related activities. Nitric natural systems can take up; many aquatic organisms oxide, for example, is a potent beyond this level, serious harm This overfertilization proces precursor of smog and acid rain can ensue. In terrestrial called eutrophication, is one of the and nitrous oxide is a long-lived ecosystems, nitrogen saturation most serious threats to aquatic greenhouse gas that traps some can disrupt soil chemistry, leading environments today, particularly 200 times more heat than carbon to loss of other soil nutrients such as calcium, magnesium, and waters where most commercial play a role in depleting the potassium and ultimately to a fish and shellfish species breed stratospheric ozone layer decline in fertility (Vitousek et al. (Vitousek et al. 1997: 11; Diaz et concentrations in the atmosphere 1997:7-9 al. 1995: 245). Partially enclosed are rising rapidly-about 0.2 to Excess nitrogen can also seas such as the Baltic Sea, the 0.3 percent per year Socci 1997, reak havoc with the structure of Black Sea, and even the Vitousek et al. 1997: 6-7) ecosystems, affecting the number Mediterranean have also been Curbing the world,'s nitroger nd kind of species found. hard hit by nitrogen-ca overload will mean acting on Researchers in the united eutrophication, and an extensive several fronts. Making fertilizer Kingdom and the United States ' dead zone of diminished applications more efficient is one have found that applying nitrogen productivity has developed at the of the most promising options fertilizer to grasslands enables a mouth of the Mississippi River in Agriculture accounts for by far th ew nitrogen-responsive grass the Gulf of Mexico because of the largest amount of human species to dominate, while others large influx of nitrogen from generated nitrogen-some 86 disappear. In one British agricultural runoff (Warrick percent (ordan et al. 1996: 655) experiment, this effect led to a 1997: A1). One of the more Fertilizer use was scant until the fivefold reduction in the number of species in the most heavily fertilized plots(Vitousek et al. More fertilizer: more food, But more pollution too 1997:9-10: Wedin et al Figure 2: Trends in Fertilizer Consumption, 1961-1997 1996:1720-1721). In the Netherlands, where nitroger 120 deposition rates are among the ighest in the world, whole 100 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 (Vitousek et al. 1997: 9-10) Although terrestrial 196119651970197519801985199019951997 ecosystems are vulnerable to the global nitrogen glut, aquatic ecosystems in lakes, rivers, and OUTH AMERIC -O OCEANIA coastal estuaries have probably ood and Agriculture Organization of the United Nations(FAO) CEarthTrends 2001 World Resources Institute. All nights reserved. Fair use is permitted on a limited scale and for educational purposes
©EarthTrends 2001 World Resources Institute. All rights reserved. Fair use is permitted on a limited scale and for educational purposes. 2 South Florida Water Management District 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 (Vitousek et al. 1997:7–9). 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 (Vitousek et al. 1997:9–10; Wedin et al. 1996:1720–1721). 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 (Vitousek et al. 1997:9–10). Although terrestrial ecosystems are vulnerable to the global nitrogen glut, aquatic ecosystems in lakes, rivers, and coastal estuaries have probably suffered the most so far. They are the ultimate receptacles of much of the nutrient overload, which tends to accumulate in runoff or to be delivered directly in the form of raw or treated sewage. (Sewage is very high in nitrogen from protein in the human diet.) In these aquaticsystems, excess nitrogen can often stimulate the growth of algae and other plants. When this extra plant matter dies and decays, it can rob the water of its dissolved oxygen, suffocating many aquatic organisms. This overfertilization process, called eutrophication, is one of the most serious threats to aquatic environments today, particularly in coastal estuaries and inshore waters where most commercial fish and shellfish species breed (Vitousek et al. 1997:11; Diaz et al. 1995:245). Partially enclosed seas such as the Baltic Sea, the Black Sea, and even the Mediterranean have also been hard hit by nitrogen-caused eutrophication, and an extensive “dead zone” of diminished productivity has developed at the mouth of the Mississippi River in the Gulf of Mexico because of the large influx of nitrogen from agricultural runoff (Warrick 1997:A1). One of the more troubling aspects of this nutrient assault on aquatic systems has been a steady rise in toxic algal blooms, which can take a heavy toll on fish, seabirds, and marine mammals (Anderson 1994:62–68). The nitrogen glut also impinges on the health of the atmosphere when the nitrogencontaining gases—nitric oxide and nitrous oxide—are released into the air, either from fossil fuel burning, land clearing, or agriculture-related activities. Nitric oxide, for example, is a potent precursor of smog and acid rain, and nitrous oxide is a long-lived greenhouse gas that traps some 200 times more heat than carbon dioxide. Nitrous oxide can also play a role in depleting the stratospheric ozone layer; concentrations in the atmosphere are rising rapidly—about 0.2 to 0.3 percent per year (Socci 1997; Vitousek et al. 1997:6-7). Curbing the world's nitrogen overload will mean acting on several fronts. Making fertilizer applications more efficient is one of the most promising options. Agriculture accounts for by far the largest amount of humangenerated nitrogen—some 86 percent (Jordan et al. 1996:655). Fertilizer use was scant until the Figure 2: Trends in Fertilizer Consumption, 1961-1997 More Fertilizer: More Food, But More Pollution, Too 0 20 40 60 80 100 120 140 1961 1965 1970 1975 1980 1985 1990 1995 1997 million metric tons ASIA (EXCL. MIDDLE EAST) EUROPE MIDDLE EAST & N. AFRICA SUB-SAHARAN AFRICA NORTH AMERICA C. AMERICA & CARIBBEAN SOUTH AMERICA OCEANIA WORLD Source: Food and Agriculture Organization of the United Nations (FAO)
1950s but since then has increased and more accurate delivery could up excess nitrogen before it can exponentially. (See Figure 2: cut this waste substantially. damage aquatic systems. Trends in Fertilizer Consumption, Cutting airborne nitr t none of these steps is easy 1961-1997) emissions from fossil fuels will or obvious. and there seems little In fact, one half of all the also be important and will benefit likelihood of concerted action commercial fertilizer ever from many of the same strategies until the nitrogen threat is produced has been applied since used to reduce carbon dioxide elevated to a higher global profile. 1984 Socci 1997). The problem is emissions, including a greater While the risks of global warming hat about one half of every mphasis on energy efficiency, a from a buildup of greenhouse metric ton of fertilizer applied gradual shift toward alternative gases in the atmosphere are fairly fields never even makes it into ergy sources, and the use of common knowledge today, the plant tissue but ends up ow-nitrogen technology in power dangers of the worlds heavy evaporating or being washed into plants and cars. Other strategies habit have gone largely local watercourses (Vitousek et al. make as well, such as unheralded so far, although this 1997: 13). A combination of better restoration of wetlands, which are habit may be as pervasive and as timing of fertilizer applications, natural nutrient traps that sponge hard to address as cutting more exact calculation of doses greenhouse gas emission REFERENCES Anderson, D M. 1994. Red Tides, "Scientific American(August): 62-68 Asner, G.,T. Seastedt, and A. Townsend. 1997. The Decoupling of Terrestrial Carbon and Nitrogen Cycles luman Influences on Land Cover and Nitrogen Supply Are Altering Natural Biogeochemical Links in the Biosphere, "BioScience 47(4): 226-234 Diaz, R.J. and R. Rosenberg. 1995. " Marine Benthic Hypoxia: A Review of Its Ecological Effects and the Behavioral Responses of Benthic Macrofauna, "Oceanography and Marine Biology: An Annual Review 33: 245- 02. Food and Agriculture Organization of the United Nations. 1999. FAOSTAT on-line statistical service(FAO, Rome).Availableonlineathttp://www.fao.org ordan, T. and D. Weller. 1996. Human Contributions to Terrestrial Nitrogen Flux: Assessing the Sources and Fates of Anthropogenic Fixed Nitrogen, BioScience 46(9): 655-664 Pearce, F. 1997. "Planet Earth Is Drowning in Nitrogen, New Scientist(April 12: 10 Socci, T. 1997. "Ecological Consequences of Human-Induced Changes in the Global Nitrogen Cycle, "U.S Global Change Research Program briefing paper(February 26 Vitousek, P. et al. 1997. "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences, " Issues In Ecology 1: 2-16 Warrick, ]. 1997. Dead Zone'Plagues Gulf Fishermen, The Washington Post(August 24): 1, sec. A Wedin, D and D. Tilman. 1996. Influence of Nitrogen Loading and Species Composition on Carbon Balance of Grasslands. " Science 274: 1720-1723. CEarthTrends 2001 World Resources Institute. All nights reserved. Fair use is permitted on a limited scale and for educational purposes
©EarthTrends 2001 World Resources Institute. All rights reserved. Fair use is permitted on a limited scale and for educational purposes. 3 1950s but since then has increased exponentially. (See Figure 2: Trends in Fertilizer Consumption, 1961–1997) In fact, one half of all the commercial fertilizer ever produced has been applied since 1984 (Socci 1997). The problem is that about one half of every metric ton of fertilizer applied to fields never even makes it into plant tissue but ends up evaporating or being washed into local watercourses (Vitousek et al. 1997:13). A combination of better timing of fertilizer applications, more exact calculation of doses, and more accurate delivery could cut this waste substantially. Cutting airborne nitrogen emissions from fossil fuels will also be important and will benefit from many of the same strategies used to reduce carbon dioxide emissions, including a greater emphasis on energy efficiency, a gradual shift toward alternative energy sources, and the use of low-nitrogen technology in power plants and cars. Other strategies make sense as well, such as restoration of wetlands, which are natural nutrient traps that sponge up excess nitrogen before it can damage aquatic systems. But none of these steps is easy or obvious, and there seems little likelihood of concerted action until the nitrogen threat is elevated to a higher global profile. While the risks of global warming from a buildup of greenhouse gases in the atmosphere are fairly common knowledge today, the dangers of the world's heavy nitrogen habit have gone largely unheralded so far, although this habit may be as pervasive and as hard to address as cutting greenhouse gas emissions. REFERENCES Anderson, D. M. 1994. "Red Tides," Scientific American (August):62–68. Asner, G., T. Seastedt, and A. Townsend. 1997. "The Decoupling of Terrestrial Carbon and Nitrogen Cycles: Human Influences on Land Cover and Nitrogen Supply Are Altering Natural Biogeochemical Links in the Biosphere," BioScience 47(4):226–234. Diaz, R. J. and R. Rosenberg. 1995. "Marine Benthic Hypoxia: A Review of Its Ecological Effects and the Behavioral Responses of Benthic Macrofauna," Oceanography and Marine Biology: An Annual Review 33:245– 302. Food and Agriculture Organization of the United Nations. 1999. FAOSTAT on-line statistical service (FAO, Rome). Available online at: http://www.fao.org. Jordan, T. and D. Weller. 1996. "Human Contributions to Terrestrial Nitrogen Flux: Assessing the Sources and Fates of Anthropogenic Fixed Nitrogen," BioScience 46(9):655–664. Pearce, F. 1997. "Planet Earth Is Drowning in Nitrogen," New Scientist (April 12):10. Socci, T. 1997. "Ecological Consequences of Human-Induced Changes in the Global Nitrogen Cycle," U.S. Global Change Research Program briefing paper (February 26). Vitousek, P. et al. 1997. "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences," Issues In Ecology 1:2–16. Warrick, J. 1997. "Dead Zone' Plagues Gulf Fishermen," The Washington Post (August 24):1, sec. A. Wedin, D. and D. Tilman. 1996. "Influence of Nitrogen Loading and Species Composition on Carbon Balance of Grasslands," Science 274:1720–1723
