Human Alteration of the Global Nitrogen Cycle: Causes and Consequences
Human Alteration of the Global Nitrogen Cycle: Causes and Consequences Photo by Nadine Cavender I Published by the Ecological Society of America Number 1, Spring 1997 ssues in Eco logy
Issues in Ecology Number I Spring 1997 Human Alteration of the Global Nitrogen Cycle: Causes and Consequences SUMMARY Human activities are greatly increasing the amount of nitrogen cycling between the living world and the soil.water,and atmosphere.In fact,humans have already doubled the rate of nitrogen entering the land-based nitrogen cycle.and that rate is continuing to climb.This human-driven global change is having serious impacts on ecosystems around the world because nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioning of the world's ecosystems.In many eco osystems on land and sea,the supply of nitrogen is a key factor the unmanaged systems but in most croplands and forestry plantations as well.Excessive nitrogen additions can pollute ecosystems and alter both their ecological functioning and the living communities they support. Most of the human activities responsible for the increase in global nitrogen are local in scale,from the production and use of nitrogen fertilizers to the buming of fossil fuels in automobiles.power generation plants.and industries.However human activities have not eased the through air and w ause of this inc ty,ex nitrogen from human term environmental consequences for large regions of the Earth The impacts of human domination of the nitrogen cycle that we have identified with certainty include: Increased global concentrations of nitrous oxide (N).a potent greenhouse gas.in the atmosphere as vell as in nitrogen (including nitric oxide.NO)that drive the formation of photo ical smog; Losses of soil nutrients such as calcium and potassium that are essential for long-term soil fertility: Substantial acidification of soils and of the waters of streams and lakes in several regions: Greatly increased transport of nitrogen by rivers into estuaries and coastal waters where it is a major pollutant. We are also confident that human alterations of the nitrogen cycle have Accelerated losses of biological diversity,especially among plants adapted to low-nitrogen soils.and subsequently.the animals and microbes that depend on these plants: Caused changes in the plant and animal life and ecological processes of estuarine and nearshore ecosystems,and contributed to long-term declines in coastal marine fisheries. titetothe impt through the developmentand fam management practices that burgeoning demand for and release of nitrogenous fertilizers
SUMMARY Human activities are greatly increasing the amount of nitrogen cycling between the living world and the soil, water, and atmosphere. In fact, humans have already doubled the rate of nitrogen entering the land-based nitrogen cycle, and that rate is continuing to climb. This human-driven global change is having serious impacts on ecosystems around the world because nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioning of the worlds ecosystems. In many ecosystems on land and sea, the supply of nitrogen is a key factor controlling the nature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecological processes such as plant productivity and the cycling of carbon and soil minerals. This is true not only in wild or unmanaged systems but in most croplands and forestry plantations as well. Excessive nitrogen additions can pollute ecosystems and alter both their ecological functioning and the living communities they support. Most of the human activities responsible for the increase in global nitrogen are local in scale, from the production and use of nitrogen fertilizers to the burning of fossil fuels in automobiles, power generation plants, and industries. However, human activities have not only increased the supply but enhanced the global movement of various forms of nitrogen through air and water. Because of this increased mobility, excess nitrogen from human activities has serious and longterm environmental consequences for large regions of the Earth. The impacts of human domination of the nitrogen cycle that we have identified with certainty include: • Increased global concentrations of nitrous oxide (N2 O), a potent greenhouse gas, in the atmosphere as well as increased regional concentrations of other oxides of nitrogen (including nitric oxide, NO) that drive the formation of photochemical smog; • Losses of soil nutrients such as calcium and potassium that are essential for long-term soil fertility; • Substantial acidification of soils and of the waters of streams and lakes in several regions; • Greatly increased transport of nitrogen by rivers into estuaries and coastal waters where it is a major pollutant. We are also confident that human alterations of the nitrogen cycle have: • Accelerated losses of biological diversity, especially among plants adapted to low-nitrogen soils, and subsequently, the animals and microbes that depend on these plants; • Caused changes in the plant and animal life and ecological processes of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries. National and international policies should attempt to reduce these impacts through the development and widespread dissemination of more efficient fossil fuel combustion technologies and farm management practices that reduce the burgeoning demand for and release of nitrogenous fertilizers. Human Alteration of the Global Nitrogen Cycle: Causes and Consequences Issues in Ecology Number 1 Spring 1997 2
Issues in Ecology Number I Spring 1997 Human Alteration of the Global Nitrogen Cycle: Causes and Consequences Gene E.Likens.PamelaAMatson,David W.Schindler. William H.Schlesinger,and G.David Tilman INTRODUCTION factors that the dy namics, f many ecosystems This report presents an overview of the current The Earth's atmosphere is 78 percent nitrogen scientific understanding of human-driven changes to the gas,but most plants and animals cannot use nitrogen global nitrogen cycle and their consequences.It also gas directly from the air as they do carbon dioxide and addresses policy and management options that could help oxygen.Instead,plants-and all organisms from the moderate these changes in the nitrogen cycle and their impacts. aaa山 ely secur thei from the synth zed by pla must wait THE NITROGEN CYCLE to hydrogen or oxygen to form inorganic compounds Nitrogen is an essential component of proteins. mainly ammonium (NH,)and nitrate (NO,).that they genetic material,chlorophyll.and other key organic mol can use r to live.It The amount of gaseous nitrogen being fixed at nd hyd t inliving tiss bd any e bv ts only human activities began to alter the natural cycle(Figure that cycles among the living and nonliving components 1).however,nitrogen was only scantily available to much of the Earth's ecosystems.Most of that nitrogen,too. of the biological world.As a result,nitrogen served as is unavailable,locked up in soil organic matter-par- Atmospheric nitrogen Amino acid NO and NH soll wat by Saders Collse ns d Ad apted from Environmental Science Third Edition by Jonathon 2
INTRODUCTION This report presents an overview of the current scientific understanding of human-driven changes to the global nitrogen cycle and their consequences. It also addresses policy and management options that could help moderate these changes in the nitrogen cycle and their impacts. THE NITROGEN CYCLE Nitrogen is an essential component of proteins, genetic material, chlorophyll, and other key organic molecules. All organisms require nitrogen in order to live. It ranks fourth behind oxygen, carbon, and hydrogen as the most common chemical element in living tissues. Until human activities began to alter the natural cycle (Figure 1), however, nitrogen was only scantily available to much of the biological world. As a result, nitrogen served as 1 Human Alteration of the Global Nitrogen Cycle: Causes and Consequences by Peter M. Vitousek, Chair, John Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, David W. Schindler, William H. Schlesinger, and G. David Tilman one of the major limiting factors that controlled the dynamics, biodiversity, and functioning of many ecosystems. The Earths atmosphere is 78 percent nitrogen gas, but most plants and animals cannot use nitrogen gas directly from the air as they do carbon dioxide and oxygen. Instead, plants and all organisms from the grazing animals to the predators to the decomposers that ultimately secure their nourishment from the organic materials synthesized by plants must wait for nitrogen to be fixed, that is, pulled from the air and bonded to hydrogen or oxygen to form inorganic compounds, mainly ammonium (NH4 ) and nitrate (NO3 ), that they can use. The amount of gaseous nitrogen being fixed at any given time by natural processes represents only a small addition to the pool of previously fixed nitrogen that cycles among the living and nonliving components of the Earths ecosystems. Most of that nitrogen, too, is unavailable, locked up in soil organic matter parFigure 1-Simplified diagram of the nitrogen cycle. Adapted from Environmental Science, Third Edition by Jonathon Turk and Amos Turk, 81984 by Saunders College Publishing, reproduced by permission of the publisher. 3 Issues in Ecology Number 1 Spring 1997
Issues in Ecology Number Spring 1997 tially rotted plant and animal remains-that must be a million metric tons of nitrogen.Worldwide.lightning decomposed by soil microbes. release for nst ce cycled through the food web.The two major natural natural suppliers of new biologically available nitrogen. sources of new nitrogen entering this cycle are nitrogen- Before the widespread planting of legume crops.terres fixing organisms and lightning. trial organisms probably fixed between 90 and 140 Tg Nitrogen-fixing organisms include a relatively of nitrogen per vear.a reasonable upper bound for the small number of algae and bacteria.Many of them live rate of natural nitrogen fixation on land is thus about 140 Tg of N per year Symbiotic nitrogen-fixing bacteria such as the Rhizobia, HUMAN-DRIVEN NITROGEN FIXATION for instance.live and work in nodules on the roots of peas,beans,alfalfa and other legumes.These bacteria During the past century,human activities clearly have accelerated the rate of nitrogen fixation on land effectively doubling the annual transfer of nitrogen from Lightning ma tly transform atmo the vast av lable atr spher ic pool to the biolog spheric nitrogen into itrates.which rain onto soil. cally available forms.The major sources of this enhance Quantifying the rate of natural nitrogen fixation supply include industrial processes that produce nitro prior to human alterations of the cycle is difficult but gen fertilizers.the combustion of fossil fuels,and the necessary for evaluating the impacts of human-driven cultivation of soybeans,peas,and other crops that host changes to the global cycling of nitrogen.The standard symbiotic nitrogen-fixing bacteria.Furthermore,human ctivity is also speeding up the release of nitrogen from g).which is equal to long-term storage in soi organic matter Figure 2-Nitrogen is the major factor limiting many terrestrial ecosystems.including most of those in the temperate zone,such as this oak savannah.The number and identities of the plant and animal species that live in such terrestrial ecosystems,and the functioning of the ecosystem,depends on the rate of nitrogen supply to the ecosystem
a million metric tons of nitrogen. Worldwide, lightning, for instance, fixes less than 10 Tg of nitrogen per year maybe even less than 5 Tg. Microbes are the major natural suppliers of new biologically available nitrogen. Before the widespread planting of legume crops, terrestrial organisms probably fixed between 90 and 140 Tg of nitrogen per year. A reasonable upper bound for the rate of natural nitrogen fixation on land is thus about 140 Tg of N per year. HUMAN-DRIVEN NITROGEN FIXATION During the past century, human activities clearly have accelerated the rate of nitrogen fixation on land, effectively doubling the annual transfer of nitrogen from the vast but unavailable atmospheric pool to the biologically available forms. The major sources of this enhanced supply include industrial processes that produce nitrogen fertilizers, the combustion of fossil fuels, and the cultivation of soybeans, peas, and other crops that host symbiotic nitrogen-fixing bacteria. Furthermore, human activity is also speeding up the release of nitrogen from long-term storage in soils and organic matter. tially rotted plant and animal remains that must be decomposed by soil microbes. These microbes release nitrogen as ammonium or nitrate, allowing it to be recycled through the food web. The two major natural sources of new nitrogen entering this cycle are nitrogenfixing organisms and lightning. Nitrogen-fixing organisms include a relatively small number of algae and bacteria. Many of them live free in the soil, but the most important ones are bacteria that form close symbiotic relationships with higher plants. Symbiotic nitrogen-fixing bacteria such as the Rhizobia, for instance, live and work in nodules on the roots of peas, beans, alfalfa and other legumes. These bacteria manufacture an enzyme that enables them to convert gaseous nitrogen directly into plant-usable forms. Lightning may also indirectly transform atmospheric nitrogen into nitrates, which rain onto soil. Quantifying the rate of natural nitrogen fixation prior to human alterations of the cycle is difficult but necessary for evaluating the impacts of human-driven changes to the global cycling of nitrogen. The standard unit of measurement for analyzing the global nitrogen cycle is the teragram (abbreviated Tg), which is equal to 4 Figure 2-Nitrogen is the major factor limiting many terrestrial ecosystems, including most of those in the temperate zone, such as this oak savannah. The number and identities of the plant and animal species that live in such terrestrial ecosystems, and the functioning of the ecosystem, depends on the rate of nitrogen supply to the ecosystem. Photo by D. Tilman Issues in Ecology Number 1 Spring 1997
Issues in Ecology Number Spring 1997 Sources of Human-Caused Alteration to the Global Nitrogen Cycle Figure 3-The pace of many hu man-caused global changes has increased starkly in modern his- 75 tory,but none so rapidly as in dustria pr .Deforestation f nitrogen fertiliz .CO.release ponentially since the 1940 509% Human population The chart shows the vear which changes in human population growth carbon dioxide release deforestation.and fertilizer pro 25% duction had r reached25% and 759 the exten 1700 1800 1900 107 50 seen in the late 190s Revised from Kates et al.(1990). Nitrogen Fertilizer nitrogen directly from the atmosphere and greatly in ndustrial fixation of nitrogenfor use as fertilizer crease the rate of nitrogen f ation previously occurring currently totals approximately 80 Tg per year and repre- on those lands. Substantial levels of nitrogen fixation sents by far the largest human contribution of new nitro also occur during cultivation of some non-legumes,nota gen to the global cycle (Figure 3).That figure does not bly rice.All of this represents new,human-generated etatansterofaheayfiag stocks of biologically available nitrogen.The quantity of which r rogen fron nitrog crop difficult to ana e tha one o anothe tha new ind duction Estim e from 32 The process of manufacturing fertilizer by indus rial nitrogen pro to 53 Tg per year.As an average.40 Tg will be used trial nitrogen fixation was first developed in Germany here. during world War l.and fertilizer production has grown exponentially since the 1940s.In recent years,the in- Fossil Fuel Burning creasing pace of production and use has been truly phe The burning of fossil fuels such as coal and oil nomenal.The unt of industrially fixed nit releases fixed nitrog lon -te stor the decade f m1980019960 age in geol al for ck to the osphere more than equaled all i industrial fertilizer applied previ the form of nitrogen-base ed trace gases such as nitr ously in human history. oxide.High-temperature combustion also fixes a sma Until the late 1970s,most industrially produced amount of atmospheric nitrogen directly.Altogether,the fertilizer was applied in developed countries.Use in these operations of automobiles,factories,power plants,and regions has now stabilized while fertilizer applications in other combustion processes emit more than 20 Tg per developing countries have risen dramatically.The mo year of fixed nitrogen to the atmosphere. All of it is of human popu h a and inc ated her s wly fixed se it has bee zation ens es t locked up for ns of years and would remain locke continue at high and likely accelerating rates for decades up indefinitely if not released by human action. in order to meet the escalating demand for food. Mobilization of stored nitrogen Nitrogen-Fixing Crops Besides enhancing fixation and releasing nitro Nearly one third of the Earth's land surface is gen from geological reservoirs.hur man activities also lib ultural and astoral uses and human genfomlongermbiologtcalstorag have replaced large areas of diverse natural vegetatior such as soil organ c matter an uting with monocultures of soybeans,peas,alfalfa,and other further to the proliferation of biologically available nitro leguminous crops and forages.Because these plants sup gen.These activities include the burning of forests,wood port symbiotic nitrogen-fixers,they derive much of their fuels,and grasslands,which emits more than 40 Tg per
5 Nitrogen Fertilizer Industrial fixation of nitrogen for use as fertilizer currently totals approximately 80 Tg per year and represents by far the largest human contribution of new nitrogen to the global cycle (Figure 3). That figure does not include manures and other organic nitrogen fertilizers, which represent a transfer of already-fixed nitrogen from one place to another rather than new fixation. The process of manufacturing fertilizer by industrial nitrogen fixation was first developed in Germany during World War I, and fertilizer production has grown exponentially since the 1940s. In recent years, the increasing pace of production and use has been truly phenomenal. The amount of industrially fixed nitrogen applied to crops during the decade from 1980 to 1990 more than equaled all industrial fertilizer applied previously in human history. Until the late 1970s, most industrially produced fertilizer was applied in developed countries. Use in these regions has now stabilized while fertilizer applications in developing countries have risen dramatically. The momentum of human population growth and increasing urbanization ensures that industrial fertilizer production will continue at high and likely accelerating rates for decades in order to meet the escalating demand for food. Nitrogen-Fixing Crops Nearly one third of the Earths land surface is devoted to agricultural and pastoral uses, and humans have replaced large areas of diverse natural vegetation with monocultures of soybeans, peas, alfalfa, and other leguminous crops and forages. Because these plants support symbiotic nitrogen-fixers, they derive much of their nitrogen directly from the atmosphere and greatly increase the rate of nitrogen fixation previously occurring on those lands. Substantial levels of nitrogen fixation also occur during cultivation of some non-legumes, notably rice. All of this represents new, human-generated stocks of biologically available nitrogen. The quantity of nitrogen fixed by crops is more difficult to analyze than industrial nitrogen production. Estimates range from 32 to 53 Tg per year. As an average, 40 Tg will be used here. Fossil Fuel Burning The burning of fossil fuels such as coal and oil releases previously fixed nitrogen from long-term storage in geological formations back to the atmosphere in the form of nitrogen-based trace gases such as nitric oxide. High-temperature combustion also fixes a small amount of atmospheric nitrogen directly. Altogether, the operations of automobiles, factories, power plants, and other combustion processes emit more than 20 Tg per year of fixed nitrogen to the atmosphere. All of it is treated here as newly fixed nitrogen because it has been locked up for millions of years and would remain locked up indefinitely if not released by human action. Mobilization of Stored Nitrogen Besides enhancing fixation and releasing nitrogen from geological reservoirs, human activities also liberate nitrogen from long-term biological storage pools such as soil organic matter and tree trunks, contributing further to the proliferation of biologically available nitrogen. These activities include the burning of forests, wood fuels, and grasslands, which emits more than 40 Tg per Figure 3-The pace of many human-caused global changes has increased starkly in modern history, but none so rapidly as industrial production of nitrogen fertilizer, which has grown exponentially since the 1940s. The chart shows the year which changes in human population growth, carbon dioxide release, deforestation, and fertilizer production had reached 25%, 50%, and 75% of the extent seen in the late 1980s. Revised from Kates et al. (1990). 25% 50% 75% 1700 1800 1900 1975 Deforestation CO release 2 Human population Industrial N fertilizer Sources of Human-Caused Alteration to the Global Nitrogen Cycle Issues in Ecology Number 1 Spring 1997