At this point, according to the BioScience review, humans were producing about 15 Tg of reactive nitrogen per year. Around this time, however, German scientists Fritz Haber and Carl Bosch developed a way to convert nonreactive atmospheric nitrogen to ammonia, the reactive compound that forms the base of nitrogen fertilizer. Currently, the Haber-Bosch process is used to produce about Tg of reactive nitrogen per year worldwide, most of which is used to produce nitrogen fertilizer.
Food grown with this fertilizer feeds some 2 billion people, estimates Vaclav Smil, a professor of geography at the University of Manitoba, writing in the July issue of Scientific American.
The past 15 years have seen a huge explosion in the amount of reactive nitrogen that humans have produced and injected into the environment, according to a report on relationships between the global nitrogen cycle and human health in volume 1, issue 5 of Frontiers in Ecology and the Environment by Alan Townsend, an assistant professor of ecology and evolutionary biology at the University of Colorado, and colleagues.
Human production of reactive nitrogen is currently estimated to be about Tg per year, write Galloway and colleagues in the BioScience review, and the global use of nitrogen fertilizers is increasing by about 15 Tg per year. The ratio of anthropogenic to natural reactive nitrogen creation is likely to increase with population increases, Galloway says. More mouths to feed will require both more reactive nitrogen fertilizers in the ground and the clearing of unspoiled, nitrogen-fixing lands to make farmland.
Where does all this human-generated reactive nitrogen come from? The largest contributor is nitrogen fertilizer.
As of , about Tg of reactive nitrogen were released each year from nitrogen fertilizer spread on farmlands around the world, according to the BioScience review. As modern farming methods have been increasingly adopted, so has the rate at which nitrogen is being fixed, with much of the increase coming in developing countries, according to Townsend and colleagues in Frontiers in Ecology and Environment. In their BioScience review, Galloway and colleagues write that widespread cultivation of nitrogen-fixing crops such as legumes has added another approximately 40 Tg of reactive nitrogen.
Burning of biomass—the use of wood for fuel and the clearing of forests and grasslands for agriculture—converts another 40 Tg or so. Draining wetlands allows organic material in the soil to oxidize, and clearing land of vegetation for crops can free reactive nitrogen from soils. These sources contribute about 10 and 20 Tg, respectively, according to an article in the Spring Issues in Ecology by a team led by Peter Vitousek, a professor of population and resource studies at Stanford University.
Fossil fuel combustion also contributes to the reactive nitrogen load. Cities are full of cars. Very few parts of the Earth now lack their own regional sources of reactive nitrogen pollution, says David Tilman, a professor of ecology at the University of Minnesota. Not just among the seven or eight most industrialized nations, but even among nations that are not industrial giants, the agricultural side has really pursued nitrogen fertilization.
As a result, Galloway says, there are significant sources of polluting reactive nitrogen in just about any corner of the Earth, with the unfortunate exception of much of Africa, which although spared much direct nitrogen pollution, is also deprived of the sorely needed fertilizer. There are areas there, for example, that are seeing deposition from the atmosphere that is ten times or more what it was prior to human activity. Some of this reactive nitrogen is, of course, put to good use, Townsend says.
Nitrogen fertilizers can take credit for reductions in starvation and malnutrition in many parts of the world, especially in Asia in the last decade. But as nitrogen levels continue to rise, Townsend says, the net health effects become increasingly negative.
Furthermore, says Galloway, reactive nitrogen can not only impact many different ecosystems, but a single atom also can make mischief repeatedly, unlike most better recognized pollutants. The effects of reactive nitrogen on ozone are profound, wreaking havoc at every elevation.
NO x , which can form from the application of nitrogen fertilizers, burning of biomass, and combustion of fossil fuels, is an important contributor to the formation of smog and ground-level ozone.
High concentrations of NO x , which are common in urban areas with their high car populations, can produce low-lying ozone, which in turn can cause or worsen asthma, cough, reactive airways disease, respiratory tract inflammation, and chronic respiratory disease.
High levels of NO x can also worsen viral infections such as the common cold. In addition to ground-level sources, where denitrification the conversion of reactive nitrogen to N 2 in soil also produces some N 2 O, aircraft inject NO x directly into the atmosphere. At mid-altitudes, N 2 O acts as a green-house gas, with each molecule absorbing about times as much outgoing radiation as carbon dioxide. And although at low altitudes reactive nitrogen increases ozone, at very high altitudes it actually destroys ozone.
In the stratosphere, ultraviolet light breaks N 2 O apart, producing NO, which in turn acts as a catalyst to break down ozone. The effects of N 2 O can persist for decades, with a residence time of years in the atmosphere, says Robert Howarth, a professor of ecology and environmental biology at Cornell University. If you did not have the nitrogen pollution, you would not have the ozone pollution. Other experts point to a lack of recognition—in U. Not all NO x stays aloft, says Aber.
In industrialized areas of the United States, nitric acid has become an increasingly significant component of acid raid, says Gene Likens, director of the Institute of Ecosystem Studies in Mill-brook, New York. Reactive nitrogen—whether from animal-raising facilities, manufactured fertilizer, septic systems, or other sources—has raised nitrate concentrations in the waterways of most industrialized nations. Without nitrogen to fertilize crops, the world couldn't feed itself.
But if humanity doesn't cut back on the nitrogen it pumps into the environment, we could choke the oceans and ourselves. The first, a review of earlier nitrogen pollution studies, charts the incredible growth of nitrogen in the environment. The second quantifies nitrogen added by human activity to the oceans. The problem isn't strictly nitrogen, which comprises more than three-quarters of the air we breathe, but so-called reactive nitrogen.
These are analogous to better-known free oxygen radicals: an altered electron configuration makes them especially unstable, and more likely to wreak environmental havoc. In , humanity produced 15 metric tons of reactive nitrogen. By , that number stood at tons, and swelled to tons by Those numbers are small in comparison to global CO2 emissions -- 27 billion tons annually -- but the impacts are magnified by what James Galloway, a University of Virginia biogeochemist and co-author of the review, calls the nitrogen cascade.
Over time, that moves through the atmosphere, into the soil, into the water, into the coastal systems and back into the atmosphere," he said. We already know how to reduce our nitrogen footprint: use less nitrogen fertilizer, eat fewer learn more What can I do to influence my N footprint? While some elements like electricity production are out of your control, there are many things learn more Copyright N-Print.
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