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Nitrogen Cycle

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Nitrogen Cycle Most nitrogen is found in the atmosphere. The nitrogen cycle is the process by which atmospheric nitrogen is converted to ammonia or nitrates. Nitrogen is essential to all living systems. To become a part of an organism, nitrogen must first be fixed or combined with oxygen or hydrogen. Nitrogen is removed from the atmosphere by lightening and nitrogen fixing bacteria. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid which is carried to the earth in rain producing nitrates. Nitrates are taken up by plants and are converted to proteins. Then the nitrogen passes through the food chain from plants to herbivores to carnivores. When plants and animals eventually die, the nitrogen compounds are broken down giving ammonia (ammonification). Some of the ammonia is taken up by the plants; some is dissolved in water or held in the soil where bacteria convert it to nitrates (nitrification). Nitrates may be stored in humus or leached from the soil and carried to lakes and streams. It may also be converted to free nitrogen (denitrification) and returned to the atmosphere. The nitrogen cycle is one of the most difficult of the cycles to learn, simply because there are so many important forms of nitrogen, and because organisms are responsible for each of the introversions. ...read more.


the nitrogen is returned to the soil. The usual form of nitrogen returned to the soil in animal wastes or in the output of the decomposers, is ammonia. Ammonia is rather toxic, but, fortunately there are nitrite bacteria in the soil and in the water which take up ammonia and convert it to nitrite, which is nitrogen with two oxygen's. Nitrite is also somewhat toxic, but another type of bacteria, nitrate bacteria, takes nitrite and converts it to nitrate, which can be taken up by plants to continue the cycle. We now have a cycle set up in the soil (or water), but what returns nitrogen to the air? It turns out that there are denitrifying bacteria which take the nitrate and combine the nitrogen back into nitrogen gas. Carbon Cycle The carbon cycle is relatively simple. From a biological perspective, the key events here are the complementary reactions of respiration and photosynthesis. Respiration takes carbohydrates and oxygen and combines them to produce carbon dioxide, water, and energy. Photosynthesis takes carbon dioxide and water and produces carbohydrates and oxygen. The outputs of respiration are the inputs of photosynthesis, and the outputs of photosynthesis are the inputs of respiration. ...read more.


This means that more carbon dioxide goes into the oceans, and more is present in the atmosphere. The latter condition causes global warming, because the carbon dioxide in the atmosphere allows more energy to reach the Earth from the sun than it allows escaping from the Earth into space. The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things extract carbon from their nonliving environment. For life to continue, this carbon must be recycled. That is our topic. Carbon exists in the nonliving environment as: * carbon dioxide (CO2) in the atmosphere and dissolved in water (forming HCO3-) * carbonate rocks (limestone and coral = CaCO3) * deposits of coal, petroleum, and natural gas derived from once-living things * dead organic matter, e.g., humus in the soil Carbon enters the biotic world through the action of autotrophs: * Primarily photoautotrophs, like plants and algae that use the energy of light to convert carbon dioxide to organic matter. * And to a small extent, chemoautotrophs - bacteria and archaeans that do the same but use the energy derived from an oxidation of molecules in their substrate. Carbon returns to the atmosphere and water by * respiration (as CO2) * burning * Decay (producing CO2 if oxygen is present, methane (CH4) if it is not. ...read more.

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