What is the Nitrogen Cycle ?
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The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature.\n\nNitrogen (Latin nitrogenium, where nitrum (from Greek nitron) means "native soda" (see niter), and genes means "forming") is formally considered to have been discovered by Daniel Rutherford in 1772, who called it noxious air or fixed air. That there was a fraction of air that did not support combustion was well known to the late 18th century chemist. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air.\n\nEarth's atmosphere is about 79% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; it is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA. In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth.
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The process in which people take in nutrients and use them to nourish their body, grow, and produce energy.
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The Krebs cycle, also called the Citric Acid Cycle, is a method by which cells undergo aerobic cellular respiration.
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The nitrogen gas in air gets converted to 1) ammonia 2) nitrates (from nitrites)
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The Amino group (NH2) breaks off the amino acid molecule by a process called deamination and forms ammonia. Toxic ammonia is rapidly converted to urea by a process called the ornithine cycle.\nUrea is only slightly toxic.
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Ammonia is a chemical compound, and Excretion is to actively transport a substance outside of an area. So Ammonia Excretion is to move Ammonia outside of the cell/organism.
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nitrogen-80%, oxygen-18%, 2%trace elements
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http://en.wikipedia.org/wiki/Kreb%27s_cycle
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Do you mean nutrient cycles?\n\nhttp://www.ppi-ppic.org/ppiweb/ppibase.nsf/$webindex/article=5203CB87852569B50057E346C0116272
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http://www.lloydminsterheavyoil.com/nitrogen.jpg\nhttp://www.tcm.phy.cam.ac.uk/castep/MD/img43.png
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http://www.soils.agri.umn.edu/academics/classes/soil3612/Symbiotic_Nitrogen_Fixation/Specificity.htm\nhttp://edis.ifas.ufl.edu/ag152\nhttp://jan.ucc.nau.edu/~gaud/bio326/class/ecosyst/Ncycling2.html
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Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide).\n\nNitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, actinobacteria and heterocysts, certain types of anaerobic bacteria. Many higher plants, and some animals (termites), have formed associations with these microorganisms.
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Nitrogen is the major gas present in the earth's atmosphere(78%), followed by Oxygen(21%) and the rest 1% of other gases like Argon, hydrogen etc.
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Calvin cycle'''' (or Calvin-Benson cycle or carbon fixation) is a series of biochemical reactions that takes place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin and Andrew Benson at the University of California, Berkeley. James Bassham also made important contributions to elucidating this pathway. It is one of the light-independent reactions or dark reactions.
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Its possible to artificially 'cycle ' a tank by the carefull addition of ammonia, building up to the ammount of ammonia slowly up to the amount that will be produced by a fully occupied tank.  Then when the fish are added there is enough bacteria to break the ammonia down into nitrite/ nitrate, the tank is therefore already cycled and you then get no 'new tank syndrome'.  I pressume they would do it this way.
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Nitrates are actually the substance in the nodules on a plant's roots that humans eat and take it the nitrogen because we need nitrogen to grow and change.
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The Urea (or the Krebs-Hensliet) cycle outlines the mechanism by which ammonia is finally produced in the final reaction (involving Arginine, if I am right)\n\nAmmonia excretion can also be regarded as excretion of Nitrogen from different intermediates in the body. All of these N containing intermediates converge into the TCA cycle from where they become amino acids through enzymes known as deaminases or transminases. These amino acids converge to a common amino acid (alanine in humans, if i recollect well) which then travels to Kidney and initiates the UREA cycle to finally result in the production of Ammonia