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Nitrogen is essential for the synthesis of proteins in plants and animals. Atmospheric air contains about 79 per cent nitrogen, but it is not of much use for plants and animals. Animals need nitrogen inthe form of amino acids to form proteins, and plants in the form of soluble salts, nitrates for the synthesis of amino acids and proteins.  Nitrogen compounds from dead and organisms, animal excreta are broken down into ammonia by bacteria. Theses bacteria are chemosynthetic bacteria. Ammonia is converted into soluble ammonium compounds, which break down into NH4 and NH2 ions. Nitrifying bacteria (e.g., Nitrosomonas) convert ammonia into soluble nitrite in soil. Nitrobacteria convert nitrites into nitrates. These soluble nitrates dissolve in soil water and are absorbed by the roots of plants.  Nitrogen fixation in soil and plants provides nitrogen to the plants. All photosynthetic, chemosynthetic and heterotrophic micro-organisms help in the process of nitrogen fixation. Some bacteria live freely in the soil, some like Rhizobium, etc., are symbiotic live in the plants roots. These bacteria fix nitrogen into nitrates for the use of plants and bacteria on the other hand get food (carbohydrate) and O2. 
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