6246lect07_S10 - Advanced Environmental Geochemistry, GLY...

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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 1 LECTURE 7 - NITROGEN CYCLE AND ACID RAIN Nitrogen Cycle Nitrogen is an essential nutrient. It is supplied in most commercial fertilizers. It is equally necessary for animals. Unlike sulfur, which is common in the oceans but often scarce over land, nitrogen is common on the continents and scarce in the oceans. Nitrogen compounds are also major pollutants, both in the atmosphere and in the ocean. In the atmosphere the bulk of the nitrogen occurs as N 2 , the largest component of the atmosphere and an innocuous compound. It also occurs as N 2 O (nitrous oxide), NO 2 (nitrogen dioxide), NO (nitrogen oxide), and NH 3 (ammonia). Nitrogen oxide and nitrogen dioxide are often called NO x . The intra conversions of all these forms of nitrogen in the atmosphere, and the inputs from and outputs to the land and oceans, make up the atmospheric nitrogen cycle. Chemically nitrogen is element number seven. It has five electrons in the n = 2 shell, and needs three more to complete the shell. When one nitrogen atom combines with another to form a nitrogen molecule, the bonding between the atoms involves the covalent sharing of three electrons. In other words the nitrogen-nitrogen bond is a triple bond. This makes it extremely strong and difficult to break. Thus the 79 + % of the atmosphere that is nitrogen is largely unreactive and not involved greatly in the nitrogen cycle. Some processes can break the strong nitrogen bonds and convert N 2 to a form more readily used by plants and animals. These processes are called nitrogen fixation processes. Nitrogen fixation is essential to all forms of life. Nitrogen fixation occurs both naturally and anthropogenically. The major natural sources of nitrogen fixation are biological in origin. Biological fixation involves the combination of nitrogen with hydrogen, carbon, and oxygen. In the oceans this is accomplished by cyanobacteria and on land by legumes and lichens. Legumes,
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Advanced Environmental Geochemistry, GLY 6246, ©David Warburton, 2010 2 7-1 7-2 sometimes described as any vegetable used for food, include many different plants. Peas and beans are grown for human food consumption. Alfalfa and clover are often grown as cover crops used for forage or as "green manure.” Green manure is a crop plowed under while green to enrich the soil. Nodule-forming Rhizobium bacteria inhabit the roots of leguminous plants and through a symbiotic relationship convert atmospheric nitrogen to a form the plant can use. Substantial amounts of atmospheric nitrogen can be transformed by legumes into nitrogen usable by plants, with a potential for several hundred lbN/acre/year to be fixed in an alfalfa crop. All parts of a legume crop, including roots and nodules, that is left after harvest, supplies N to the soil system. When the plant material is decomposed, N is released (O’Leary et al ., 1997). Many of these plants grown naturally and are thus considered a natural source of nitrogen fixation. To the extent that the
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This note was uploaded on 05/04/2011 for the course GLY 6246 taught by Professor Warburton during the Spring '11 term at FAU.

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6246lect07_S10 - Advanced Environmental Geochemistry, GLY...

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