नाइट्रोजन चक्र
250px|thumb|right|नाइट्रोजन चक्र
भूमि तथा पौधों में विभिन्न विधियों द्वारा वायुमंडल की स्वतंत्र नाइट्रोजन का नाइट्रोजनीय यौगिकों के रूप में स्थिरीकरण और उनके पुनः स्वतंत्र नाइट्रोजन में परिवर्तित होने का अनवरत प्रक्रम नाइट्रोजन चक्र (नाइट्रोजन साईकिल) कहलाता है।

Nitrogen gas makes up 70% of atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acid( DNA and RNA) and some vitamins. In nitrogen is found in other biologically important compounds such as alkaloids and urea nitrogen is an essential nutrient for all life forms and life would be simple if if all these life forms cloud use the atmospheric nitrogen directly.

However, other than a few forms of bacterial iPhones are not able to convert the compare MI note nitrogen molecule into forms like nitrates and nitrites which can be taken up and used to make the required molecules the nitrogen fixing bacteria may be free living or be associated with some species or of dicot plants most commonly, the nitrogen fixing bacteria are found in the roots of legumes (generally the plants which give us pulses) in special structures called root nodules. Other than this bacteria, the only other manner in which the nitrogen molecule is converted to nitrates and nitrites is by a physical process. During lightning the high temperature and pressure is created in the air convert nitrogen into oxides of Nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with rain this hour then utilised by various life forms.

What happens to the nitrogen once it is converted into forms that can be taken up and used to make nitrogen containing molecules ? Plants generally take up up nitrates and nitrites and convert them into amino acids which are used to make proteins. some other biochemical Pathways are used to make the other Complex compounds containing nitrogen these proteins and other Complex compounds are subsequently consumed by animals. Once the animal or plant dies, other bacteria in the soil convert the various compounds of Nitrogen into nitrates and nitrites. A different type of bacteria converts the nitrates and nitrites into elemental nitrogen. Thus, there is a nitrogen cycle in nature in which nitrogen and passes from its element from in the atmosphere into simple molecules in the soil and water which get converted to more Complex molecules in living beings and back again to the simple hydrogen molecule in the atmosphere. 
250px|thumb|right|नाइट्रोजन चक्र
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