-Nitrogen gas is too unreactive to be used by plants, and must be converted into a comound such as nitrate or ammonia in order to become available to them. This is called nitrogen fixation. some bacteria are important nitrogen fixers.
-Animals obtain thier nitrogen by feeding on plants.
-Remains of dead plants and animals, and thier waste products, contain protiens and other nitrogenous substances. These are broken down by saprobiotic decomposers to form ammonia.
-Nitrifying bacteria oxidise ammonia to nitrite and nitrate ions.
-Denitrifing bacteria reduce nitrate ions to nitrogen.
-Pollution of waterways by nitrogen-containing fertilisers leads to eutrophication, in which excessive growth of the population of oxygen-requiring bacteria depletes the water of disolved oxygen.
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