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Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen. But most organisms cannot use nitrogen in this form. Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as: nitrate ions (NO3") ammonia (NH3) urea (NH2)2CO Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants). N& P3 Pl P&3l  MFour processes participate in the cycling of nitrogen through the biosphere: NN  N 8nitrogen fixation decay nitrification denitrification9 9(b          0 0 0' 0)8 Nitrogen Fixation    ! Atmospheric Fixation   fThe enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5 8% of the total nitrogen fixed. 4 4 4 " Industrial Fixation   BUnder great pressure, at a temperature of 600C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH4NO3). LC t$  !  0"# Biological Fixation   !The ability to fix nitrogen is found only in certain bacteria and archaea. Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa). Some establish symbiotic relationships with plants other than legumes (e.g., alders). Some establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-eating bivalves). Some nitrogen-fixing bacteria live free in the soil. Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies." Z"5        J   \  05=  0BI  0AI  0go  0$Decay   oThe proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia. (p f2_      0_l% Nitrification    Ammonia can be taken up directly by plants  usually through their roots. However, most of the ammonia produced by decay is converted into nitrates. This is accomplished in two steps: Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2"). Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3"). These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants.  P<      8X   8   ^    0|&Denitrification   The three processes above remove nitrogen from the atmosphere and pass it through ecosystems. Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere. Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.  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