FALL SEMESTER, 1997

Although humans and other land animals live in an ocean of air that is 79 percent nitrogen, their supply of food is limited more by the availability of fixed nitrogen than by that of any other plant nutrient (see companion reading assignment: nitrogenfixation.htm). By "fixed" is meant nitrogen incorporated in a chemical compound that can be utilized by plants and animals, i.e. converted into a metabolically "active" form. As it exists in the atmosphere nitrogen is an inert gas except to the comparatively few organisms that have the ability to convert the element to a metabolically active form. A smaller but still significant amount of atmospheric nitrogen is fixed by ionizing phenomena such as cosmic radiation, meteor trails and lightning, which momentarily provide the high energy needed for nitrogen to react with oxygen or the hydrogen of water. Nitrogen is also fixed by marine organisms, but the largest single natural source of fixed nitrogen is terrestrial microorganisms and symbiotic associations between such microorganisms and plants.

Of all man's recent interventions in the cycles of nature the industrial fixation of nitrogen far exceeds all the others in magnitude. Since 1950 the amount of nitrogen annually fixed for the production of fertilizer has increased approximately fivefold, until it now equals the amount that was fixed by all terrestrial ecosystems before the advent of modern agriculture. In 1968 the world's annual output of industrially fixed nitrogen amounted to about 30 million tons of nitrogen; by the year 2000 the industrial fixation of nitrogen may well exceed 100 million tons.

To appreciate the intricate web of nitrogen flow in the biosphere let us trace the course of nitrogen atoms from the atmosphere into the cells of microorganisms, and then into the soil as fixed nitrogen, where it is available to higher plants and ultimately to animals. Plants and animals die and return the fixed nitrogen to the soil, at which point the nitrogen may simply be recycled through a new generation of plants and animals or it may be broken down into elemental nitrogen and returned to the atmosphere.

Because much of the terminology used to describe steps in the nitrogen cycle evolved in previous centuries it has an archaic quality. Antoine Laurent Lavoisier, who clarified the composition of air, gave nitrogen the name azote, meaning without life. The term is still found in the family name of an important nitrogen-fixing bacterium: the Azotobacteraceae. One might think that fixation would merely be termed nitrification, to indicate the addition of nitrogen to some other substance, but nitrification is reserved for a specialized series of reactions in which a few species of microorganisms oxidize the ammonium ion (NH₄⁺) to nitrite (NO₂^-) or nitrite to nitrate (NO₃^-). When nitrites or nitrates are reduced to gaseous compounds such as molecular nitrogen (N₂) or nitrous oxide (N₂O), the process is termed denitrification. "Ammonification" describes the process by which the nitrogen of organic compounds (chiefly amino acids) is converted to ammonium ion. The process operates when microorganisms decompose the remains of dead plants and animals. Finally, a word should be said about the terms oxidation and reduction, which have come to mean more than just the addition of oxygen or its removal. Oxidation is any process that removes electrons from a substance. Reduction is the reverse process: the addition of electrons. Since electrons can neither be created nor destroyed in a chemical reaction, the oxidation of one substance always implies the reduction of another.

One may wonder how it is that some organisms find it profitable to oxidize nitrogen compounds whereas other organisms—even organisms in the same environment—owe their survival to their ability to reduce nitrogen compounds. Apart from photosynthetic organisms, which obtain their energy from radiation, all living forms depend for their energy on chemical transformations.

These transformations normally involve the oxidation of one compound and the reduction of another, although in some eases the compound being oxidized and the compound being reduced are different molecules of the same substance, and in other cases the reactants are fragments of a single molecular species. Nitrogen can be cycled because the reduced inorganic compounds of nitrogen can be oxidized by atmospheric oxygen with a yield of useful energy. Underage aerobic conditions the oxidized compounds of nitrogen can act as oxidizing agents for the burning of organic compounds (and a few inorganic compounds), again with a yield of useful energy.

Nitrogen is able to play its complicated role in life processes because it has an unusual number of oxidation levels, or valences. All oxidation level indicates the number of electrons that an atom in a particular compound has "accepted" or "donated." In plants and animals most nitrogen exists either in the form of the ammonium ion or of amino (—NH₂) compounds. In either case it is highly reduced; it has acquired three electrons by its association with three other atoms and thus is said to have a valence of minus 3. At the other extreme, when ni-trogen is in the highly oxidized form of the nitrate ion (the principal form it takes in the soil), it shares five of its electrons with oxygen atoms and so has a valence of plus 5. To convert nitrogen as it is found in the ammonium ion or amino acids to nitrogen as it exists in soil nitrates involves a total valence change of eight, or the removal of eight electrons. Conversely, to convert nitrate nitrogen into amino nitrogen requires the addition of eight electrons.

Once ammonia or the ammonium ion has appeared in the soil, it can be absorbed by the roots of plants and the nitrogen can be incorporated into amino acids and then into proteins. If the plant is subsequently eaten by an animal, the nitrogen may be incorporated into a new protein. In either case the protein ultimately returns to the soil, where it is decomposed (usually with bacterial help) into its component amino acids. Assuming that conditions are aerobic, meaning that an adequate supply of oxygen is present, the soil will contain many microorganisms capable of oxidizing amino acids to carbon dioxide, water and ammonia. If the amino acid happens to be glycine, the reaction will yield 176 kilocalories per mole.

A few microorganisms represented by the genus Nitrosomonas employ nitrification of the ammonium ion as their sole source of energy. In the presence of oxygen, ammonia is converted to nitrite ion (NO₂^-) plus water, with an energy yield of about 65 kilocalories per mole, which is quite adequate for a comfortable existence. Nitrosomonas belongs to the group of microorganisms termed autotrophs, which get along without an organic source of energy. Photoautotrophs obtain their energy from light; chemoautotrophs (such as Nitrosomonas) obtain energy from inorganic compounds.

There is another specialized group of microorganisms, represented by Nitrobacter, that are capable of extracting additional energy from the nitrite generated by Nitrosomonas. The result is the oxidation of a nitrite ion to a nitrate ion with the release of about 17 kilocalories per mole, which is just enough to support the existence of Nitrobacter.

In the soil there are numerous kinds of denitrifying bacteria (for example Pseudomonas denitrificans) that, if obliged to exist in the absence of oxygen, are able to use the nitrate or nitrite ion as electron acceptors for the oxidation of organic compounds. In these reactions the energy yield is nearly as large as it would be if pure oxygen were the oxidizing agent. When glucose reacts with oxygen, the energy yield is 686 kilocalories per mole of glucose. In microorganisms living under anaerobic conditions the reaction of glucose with nitrate ion yields about 545 kilocalories per mole of glucose if the nitrogen is reduced to nitrous oxide, and 570 kilocalories if the nitrogen is reduced all the way to its elemental gaseous state.

The comparative value of ammonium and nitrate ions as a source of nitrogen for plants has been the subject of a number of investigations. One might think that the question would be readily resolved in favor of the ammonium ion: its valence is minus 3, the same as the valence of nitrogen in amino acids, where-as the valence of the nitrate ion is plus 5.

On this basis, plants must expend energy to reduce nitrogen from a valence of plus 5 to one of minus 3. The fact is, however, there are complicating factors; the preferred form of nitrogen depends on other variables. Because the ammonium ion has a positive charge it tends to be trapped on clay particles near the point where it is formed (or where it is introduced artificially) until it has been oxidized. The nitrate ion, being negatively charged, moves freely through the soil and thus is more readily carried downward into the root zone. Although the demand for fertilizer in solid form (such as ammonium nitrate and urea) remains high, anhydrous ammonia and liquid ammoniacal fertilizers are now widely applied. The quantity of nitrogen per unit weight of ammonia is much greater than it is per unit of nitrate; moreover, liquids are easier to handle than solids.

Until the end of the 19th century little was known about the soil organisms that fix nitrogen. In fact, at that time there was some concern among scientists that the denitrifying bacteria, which had just been discovered, would eventually deplete the reserve of fixed nitrogen in the soil and cripple farm productivity. In an address before the Royal Society of London, Sir William Crookes painted a bleak picture for world food production unless artificial means of fixing nitrogen were soon developed. This was a period when Chilean nitrate reserves were the main source of fixed nitrogen or both fertilizer and explosives. As it urned out, the demand for explosives provided the chief incentive for the invention of the catalytic fixation process by Fritz Haber and Karl Bosch of Germany in 1914. In this process atmospheric nitrogen and hydrogen are passed over a catalyst (usually nickel) at temperature of about 500 degrees Celcius and a pressure of several hundred atmospheres.

As the biological fixation of nitrogen and the entire nitrogen cycle became better understood, the role of the denitrifing bacteria fell into place. Without such bacteria to return nitrogen to the atmosphere most of the atmospheric nitrogen would now be in the oceans locked up in sediments. Actually, of course, there is not enough oxygen in the atmosphere today to convert all the free nitrogen into nitrates. One can imagine, however, that if a one-way process were to develop m the absence of denitrifying bacteria, the addition of nitrates to the ocean would make seawater slightly more acidic and start the release of carbon dioxide from carbonate rocks. Eventually the carbon dioxide would be taken up by plants, and if the carbon were then deposited as coal or other hydrocarbons, the remaining oxygen would be available in the atmosphere to be combined with nitrogen. Because of the large number of variables involved it is difficult to predict how the world would look without the denitrification reaction, but it would certainly not be the world we know.