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Global Population - Global Population and the Nitrogen...

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D uring the 20th century, hu- manity has almost quadru- pled its numbers. Although many factors have fostered this unprece- dented expansion, its continuation dur- ing the past generation would not have been at all possible without a wide- spread yet generally unappreciated activity: the synthesis of ammonia. The ready availability of ammonia, and oth- er nitrogen-rich fertilizers derived from it, has effectively done away with what for ages had been a fundamental restric- tion on food production. The world’s population now has enough to eat (on the average) because of numerous ad- vances in modern agricultural practices. But human society has one key chemical industry to thank for that abundance the producers of nitrogen fertilizer. Why is nitrogen so important? Com- pared with carbon, hydrogen and oxy- gen, nitrogen is only a minor constituent of living matter. But whereas the three major elements can move readily from their huge natural reservoirs through the food and water people consume to be- come a part of their tissues, nitrogen re- mains largely locked in the atmosphere. Only a puny fraction of this resource exists in a form that can be absorbed by growing plants, animals and, ultimate- ly, human beings. Yet nitrogen is of decisive importance. This element is needed for DNA and RNA, the molecules that store and trans- fer genetic information. It is also required to make proteins, those indispensable messengers, receptors, catalysts and structural components of all plant and animal cells. Humans, like other higher animals, cannot synthesize these mole- cules using the nitrogen found in the air and have to acquire nitrogen compounds from food. There is no substitute for this intake, because a minimum quantity (consumed as animal or plant protein) is needed for proper nutrition. Yet getting nitrogen from the atmosphere to crops is not an easy matter. The relative scarcity of usable nitro- gen can be blamed on that element’s pe- culiar chemistry. Paired nitrogen atoms make up 78 percent of the atmosphere, Global Population and the Nitrogen Cycle Feeding humankind now demands so much nitrogen-based fertilizer that the distribution of nitrogen on the earth has been changed in dramatic, and sometimes dangerous, ways by Vaclav Smil 76 S cientific A merican July 1997 Copyright 1997 Scientific American, Inc.
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but they are too stable to transform easily into a reactive form that plants can take up. Lightning can cleave these strongly bonded molecules; however, most natural nitrogen “fixation” (the splitting of paired nitrogen molecules and subsequent incorporation of the el- ement into the chemically reactive com- pound ammonia) is done by certain bacteria. The most important nitrogen- fixing bacteria are of the genus Rhizo- bium, symbionts that create nodules on the roots of leguminous plants, such as beans or acacia trees. To a lesser extent, cyanobacteria (living either freely or in association with certain plants) also fix nitrogen.
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