Nitrogen Fixation

<h1>Text Preview</h1> <p>Nitrogen is the most abundant element in our atmosphere. 78% of the earth’s atmosphere is molecular nitrogen. Nitrogen is required by plants and all other organisms to synthesize amino acids, nucleic acids, and other essential organic molecules. Molecular nitrogen is extremely stable. A triple bond holds the nitrogen atoms together. The amount of energy required to break the triple bond is very high and only a few organisms have evolved the enzyme to do it. </p> <p>Plants don’t have the enzymatic machinery to break the nitrogen triple bond. So even though nitrogen is abundant in the air, its triple bond must first be broken or fixed by another organism before a plant can use it.</p> <p>One way nitrogen enters a plant’s life cycle is through lightning strikes. The tremendous energy of the lightening breaks the triple bond and reacts the nitrogen with atmospheric oxygen.</p> <p>Another way is through nitrogen fixation. Nitrogen fixation is the conversion of molecular nitrogen or nitrogen gas to ammonia. Nitrogen is fixed by a few strains of bacteria, called diazotrophs. Diazotrophs have evolved a specialized enzyme complex called nitrogenase. Nitrogenase uses the energy stored in ATP to reduce nitrogen gas to ammonia. It combines nitrogen gas with protons and electrons to make ammonia and hydrogen gas. </p> <p>Ammonia is converted into ammonium when it reacts with water. Ammonium is either directly used by plants or it is oxidized to nitrate, and then used by plants. Nitrifying bacteria combine ammonium with oxygen gas to make nitrate, protons, and water. Not all nitrate is taken up by plants. Some of the nitrate is returned to the atmosphere by denitrifying bacteria. About 10-20% of the nitrogen in the soil is lost in this way.</p> <p>Some nitrogen gas is fixed by free-living bacteria in soil. Examples of free-living bacteria are species of Clostridium and Azobacter. However, most nitrogen is fixed by bacteria that live in legumes. Legumes make up the pea family, and include peas, beans, soybeans, alfalfa, and clover. Most of the nitrogen-fixing bacteria that live in legumes are from the genus Rhizobium. These bacteria live in enlarged sections of the roots called nodules.</p> <p>Rhizobium live in a symbiotic relationship with legumes. Recall that symbiosis is the living together of two dissimilar organisms in close association. The legume and its nitrogen-fixing bacteria both benefit—the bacteria give the legumes fixed nitrogen, and the legumes provide the bacteria with carbohydrates and other organic compounds.</p> <p>How many of these crops are legumes? Click on each legume, and click Submit when you're done. Click Jump Ahead to skip this step.</p> <p>Yes, that’s right!</p> <p>Sorry, that’s not right. </p> <p>Corn is not a legume. Try again.</p> <p>Sorry, that’s not right. Wheat is not a legume. Try again.</p> <p>Sorry, that’s not quite right. There is at least one more legume here. Try again.</p> <p>A bog is an area characterized by soft, watery, acidic soil. Most legumes and nitrogen-fixing bacteria can’t live in nutrient-poor soils such as bogs. How do plants in bogs obtain their essential nutrients?</p> <p>Carnivorous plants, such as the Venus flytrap, get certain minerals and nitrogen by digesting insects and other organisms. The flytrap has modified leaves that snap together to capture flies, ants, and grasshoppers. Special glands then secrete enzymes that digest the nutrient rich prey, and nutrients are absorbed directly into the leaves.</p> <p>Plants that are not in a symbiotic relationship with nitrogen fixing bacteria must obtain nitrogen directly from the soil. After a while, these plants remove all of the useable nitrogen from the soil. Several methods are used to replenish the soil’s nitrogen. One way to restore nitrogen to the soil is by crop rotation. A farmer alternates planting two or more crops in the same field from year to year. For example, one year a farmer might plant a field with corn, which removes most of the useable nitrogen from the soil. The next year, the farmer plants the field with a legume, such as soybeans, which returns nitrogen compounds back to the soil. </p> <p>Farmers also use fertilizers to add nitrogen and other nutrients to soil. Natural fertilizers, such as animal droppings, have been used for centuries to return organic materials and nitrogen to farmlands. Plowing legumes, such as alfalfa and clover, back into the soil is another way to restore nitrogen. Most commercial fertilizers contain nitrogen compounds, phosphorous, and potassium, which are the most commonly deficient nutrients in soils. Unfortunately, excess fertilizer that gets washed off farms by rainwater can accumulate in lakes and streams. Commercial fertilizer is also very expensive for farmers, so they are looking for safe, new ways to return nitrogen to the soil and keep costs down for consumers.</p> <p>Some researchers are trying to introduce the nitrogen-fixing genes of bacteria directly into plants. Other scientists are focusing on the bacteria themselves, by engineering bacteria that have multiple copies of nitrogen-fixing genes, and by designing nitrogen-fixing bacteria that can live in the tissues of non-legumes. This ongoing research has the potential to increase the yield of crops that farmers can harvest while keeping their costs low.</p> <p>Perhaps you can think of other ways to improve farming practices that reduce pollution, save natural resources, and help feed the growing world population.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>