What is the nitrogen cycle; what are its components; and how does it operate? How does the operation of the nitrogen cycle affect humans and other living organisms? What impact are humans having on this natural cycle and the movement of nitrogen in various forms, between living organisms, soil, water and the atmosphere? How fast are these changes taking place? What are the consequences and implications of these changes?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. James N. Galloway, Professor, Environmental Sciences Department, and Chair of the International Nitrogen Initiative, University of Virginia, Charlottesville, VA
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Dr. William H. Schlesinger, James B. Duke Professor of Biogeochemistry & Dean of the Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC
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Nitrogen: An Essential Ingredient for Life and Food
The chemical elements nitrogen (N), carbon (C), phosphorus (P), oxygen (O) and sulfur (S) are all necessary for life. With one exception, they are generally available in global reservoirs to sustain life forms ranging from singlecell organisms to vertebrates. It is ironic that while nitrogen has the greatest total abundance, it is also the element that is the least readily available to sustain life. The total amount of nitrogen in the atmosphere, soils, and waters of the Earth is more than the mass of all five of these other elements combined. However, more than 99% of this nitrogen is not available to > 99% of living organisms. The reason for this seeming contradiction is that while there is an abundance of nitrogen in nature, it is almost entirely in a chemical form (N2) that is not usable by most organisms.
At the very end of the 19 th century, it was realized that there was not enough useable nitrogen available from naturally occurring sources to provide food for a growing global population. The pressure to obtain additional nitrogen for food production led to the 1913 development of the HaberBosch process in Germany to produce NH3 from N2 and H2.
We are now at the beginning of the 21 st century. It is estimated that 40% of the worlds population owes its existence to the discovery of the HaberBosch process. This is a tremendous achievement, but in the process humans have become the primary source of the introduction of biologicallyactive nitrogen to continents. There are three general consequences. On the positive side, there is more food for more people. On the negative side, there are large regions of the world where there is too much nitrogen with resulting negative consequences to both ecosystem and human health. The multiple linkages among the ecological magnify the consequences and human health effects of nitrogen as it moves from one environmental system to another. This phenomenon is called the nitrogen cascade, which is defined as the sequential transfer of nitrogen through environmental systems, which results in environmental changes as nitrogen moves through or is temporarily stored within each system. In addition, there are also other regions, notably Africa, where there is still not enough nitrogen to sustain the human population.
Implications of Excess Nitrogen
Nitrogen is, of course, a major component of plant fertilizer, which we all put on our garden in the spring to ensure a bountiful crop. But, too often, some of this nitrogen escapes its intended purpose and moves into the atmosphere or to runoff waters, with unexpected consequences. Nitrogen in runoff waters, usually found as nitrate (NO3) causes blooms of algae growth in downstream regions, including the coastal estuaries that are so important to our fisheries. When the algae dies and sinks to the bottom, its decomposition consumes oxygen, depriving fish and shellfish in those deep waters of oxygen, a condition known as hypoxia. Large areas of the Gulf of Mexico, which receive runoff waters from the Mississippi, are depleted of oxygen nearly every summer, with catastrophic losses to the traditional coastal fishery in that region. Similar, hypoxic conditions are seen in waters affected by leakage from lagoons built to contain the wastes of hog and chicken farms in North Carolina. Excess nitrate in freshwater is a direct human health hazard and an indirect hazard in some areas where it leads to a release of arsenic from sediments. The problem of fertilizer runoff is exacerbated by the channelization of headwater streams, so that runoff from agricultural operations is often shunted directly into larger rivers.
Excess nitrogen fertilizer also results in the emission of various nitrogencontaining gases to the atmosphere, especially ammonium (NH3), nitric oxide (NO) and nitrous oxide (N2O) to the atmosphere. All three have deleterious effects on our environment. Ammonium, which is highly soluble in rainwater, is quickly removed from the atmosphere, joining nitrogen in runoff waters that contaminate estuaries. About 20% of the nitrogen delivered to the Chesapeake Bay is deposited from the atmosphere, with direct agricultural runoff contributing most of the remainder. Nitric oxide is a precursor to the formation of ozone, which is a potent air pollutant and health hazard for those that suffer from emphysema and asthma. Nitric oxide is also a component of acid rain, and excessive deposition of nitrogen from the atmosphere leads to losses of species diversity and increases in invading grasses that are a fire hazard in the arid Southwest. Nitrous oxide is a gas that contributes to the warming of Earths atmosphere, where it is roughly 200 times more powerful in global warming than carbon dioxide.
Excess nitrogen in our environment represents a human perturbation of the natural cycle of nitrogen in the environment. Industrial emissions of nitric oxide to the atmosphere must be reduced as soon as possible. The problem of excess nitrogen can be addressed by more judicious and efficient applications of nitrogen fertilizer in agriculture, and by better management of wetland ecosystems that return nitrogen to the atmosphere in its nearly inert or unreactive form, N2.
Dr. James N. Galloway is Professor of Environmental Sciences at the University of Virginia. Following a postdoctoral appointment with Gene Likens at Cornell University, he accepted a position as Assistant Professor of Environmental Sciences at the University of Virginia in 1976. He served as President of the Bermuda Biological Station for Research from 1988 to 1995, and as chair of Environmental Sciences, University of Virginia from 1996 to 2001. Dr. Galloway is currently chair of the International Nitrogen Initiative, a program sponsored by SCOPE and IGBP, and is a member of the USA EPA Science Advisory Board. In 2002, he was elected a Fellow of the American Association for the Advancement of Science. His research on biogeochemistry includes the natural and anthropogenic controls on chemical cycles at the watershed, regional and global scales. His current research focuses on beneficial and detrimental effects of reactive nitrogen as it cascades between the atmosphere, terrestrial ecosystems and freshwater and marine ecosystems. Dr. Galloway received the B.A. degree in Chemistry and Biology from Whittier College in 1966 and the Ph.D. degree in Chemistry from the University of California, San Diego in 1972. Dr. Galloway is the author of over 140 peerreviewed papers in the scientific literature.
Dr. William H. Schlesinger is James B. Duke Professor of Biogeochemistry and, Dean of the Nicholas School of the Environment and Earth Sciences at Duke University. Dr. Schlesinger was elected to The National Academy of Sciences in 2003. He was President of the Ecological Society of America for 20032004. His research work has taken him to diverse habitats ranging from the Okefenokee Swamp in southern Georgia to the Mojave Desert of California. Portions of his research have also been featured on NOVA, CNN, NPR, and on the pages of Discover, National Geographic, The New York Times, and Scientific American. Completing his A.B. at Dartmouth (1972), and Ph.D. at Cornell (1976), Dr. Schlesinger joined the faculty at Duke University in 1980. He is the author or coauthor of over 160 scientific papers and the widelyadopted textbook Biogeochemistry: An Analysis of Global Change (Academic Press, 2nd ed. 1997).
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