Human Alteration of theGlobal Nitrogen Cycle:Causes and ConsequencesPhoto by Nadine CavenderPublished by the Ecological Society of America Number 1, Spring 1997Issues in EcologySUMMARYHuman activities are greatly increasing the amount of nitrogen cycling between the living world and the soil, water, andatmosphere. In fact, humans have already doubled the rate of nitrogen entering the land-based nitrogen cycle, and thatrate is continuing to climb. This human-driven global change is having serious impacts on ecosystems around the worldbecause nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioningof the worlds ecosystems. In many ecosystems on land and sea, the supply of nitrogen is a key factor controlling thenature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecologi-cal processes such as plant productivity and the cycling of carbon and soil minerals. This is true not only in wild orunmanaged systems but in most croplands and forestry plantations as well. Excessive nitrogen additions can polluteecosystems 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 useof 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 nitrogenthrough air and water. Because of this increased mobility, excess nitrogen from human activities has serious and long-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 (N2O), a potent greenhouse gas, in the atmosphere aswell as increased regional concentrations of other oxides of nitrogen (including nitric oxide, NO) thatdrive 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 majorpollutant.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, andsubsequently, the animals and microbes that depend on these plants;• Caused changes in the plant and animal life and ecological processes of estuarine and nearshoreecosystems, and contributed to long-term declines in coastal marine fisheries.National and international policies should attempt to reduce these impacts through the development and widespreaddissemination of more efficient fossil fuel combustion technologies and farm management practices that reduce theburgeoning demand for and release of nitrogenous fertilizers.Human Alteration of theGlobal Nitrogen Cycle:Causes and Consequences Issues in Ecology Number 1 Spring 19972INTRODUCTIONThis report presents an overview of the currentscientific understanding of human-driven changes to theglobal nitrogen cycle and their consequences. It alsoaddresses policy and management options that could helpmoderate these changes in the nitrogen cycle and theirimpacts.THE NITROGEN CYCLENitrogen is an essential component of proteins,genetic material, chlorophyll, and other key organic mol-ecules. All organisms require nitrogen in order to live. Itranks fourth behind oxygen, carbon, and hydrogen asthe most common chemical element in living tissues. Untilhuman activities began to alter the natural cycle (Figure1), however, nitrogen was only scantily available to muchof the biological world. As a result, nitrogen served as1Human Alteration of the Global Nitrogen Cycle:Causes and ConsequencesbyPeter M. Vitousek, Chair, John Aber, Robert W. Howarth,Gene E. Likens, Pamela A. Matson, David W. Schindler,William H. Schlesinger, and G. David Tilmanone of the major limiting factors that controlled the dy-namics, biodiversity, and functioning of many ecosystems.The Earths atmosphere is 78 percent nitrogengas, but most plants and animals cannot use nitrogengas directly from the air as they do carbon dioxide andoxygen. Instead, plants  and all organisms from thegrazing animals to the predators to the decomposers thatultimately secure their nourishment from the organicmaterials synthesized by plants  must wait for nitro-gen to be fixed, that is, pulled from the air and bondedto hydrogen or oxygen to form inorganic compounds,mainly ammonium (NH4) and nitrate (NO3), that theycan use.The amount of gaseous nitrogen being fixed atany given time by natural processes represents only asmall addition to the pool of previously fixed nitrogenthat cycles among the living and nonliving componentsof the Earths ecosystems. Most of that nitrogen, too,is unavailable, locked up in soil organic matter  par-Figure 1-Simplified diagram of the nitrogen cycle. Adapted from Environmental Science, Third Edition by JonathonTurk and Amos Turk, 81984 by Saunders College Publishing, reproduced by permission of the publisher.3 Issues in Ecology Number 1 Spring 1997a 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 majornatural suppliers of new biologically available nitrogen.Before the widespread planting of legume crops, terres-trial organisms probably fixed between 90 and 140 Tgof nitrogen per year. A reasonable upper bound for therate of natural nitrogen fixation on land is thus about140 Tg of N per year.HUMAN-DRIVEN NITROGEN FIXATIONDuring the past century, human activities clearlyhave accelerated the rate of nitrogen fixation on land,effectively doubling the annual transfer of nitrogen fromthe vast but unavailable atmospheric pool to the biologi-cally available forms. The major sources of this enhancedsupply include industrial processes that produce nitro-gen fertilizers, the combustion of fossil fuels, and thecultivation of soybeans, peas, and other crops that