The nitrogen cycle is one of Earth's most important biogeochemical cycles. Although nitrogen gas makes up 78% of our atmosphere, most living organisms cannot use this atmospheric nitrogen directly. The strong triple bond between nitrogen atoms makes it very stable and unreactive. For nitrogen to be useful to life, it must first be converted or 'fixed' into more reactive forms like ammonia and nitrates through various biological and chemical processes.
Nitrogen fixation is the crucial first step in the nitrogen cycle, converting inert atmospheric nitrogen gas into biologically available ammonia. This process occurs through three main pathways. Biological nitrogen fixation is performed by specialized bacteria like Rhizobium, which live in symbiotic relationships within root nodules of legume plants. These bacteria use the enzyme nitrogenase to break the strong triple bond in nitrogen gas. Industrial fixation uses the Haber-Bosch process under high temperature and pressure. Lightning provides enough energy to break nitrogen bonds naturally. Biological fixation accounts for most of the nitrogen fixed in natural ecosystems.
After nitrogen fixation, the ammonia produced undergoes nitrification, a crucial two-step oxidation process carried out by specialized soil bacteria. In the first step, Nitrosomonas bacteria oxidize ammonia to nitrites. In the second step, Nitrobacter bacteria further oxidize nitrites to nitrates. This process is important because nitrates are the preferred nitrogen source for most plants. During assimilation, plant roots actively absorb these nitrogen compounds from the soil and incorporate them into amino acids, proteins, and nucleic acids, which are essential for plant growth and development.
Decomposition and mineralization are essential processes that recycle nitrogen from dead organic matter back into the soil. When plants and animals die, or when waste products are produced, decomposer organisms including bacteria and fungi break down the complex organic nitrogen compounds like proteins and nucleic acids. Through the process of mineralization, also called ammonification, these decomposers convert the organic nitrogen back into ammonia, which can then re-enter the nitrogen cycle. This recycling process ensures that nitrogen is continuously available for new plant growth and maintains the balance of nitrogen in ecosystems.
Denitrification is the final step in the nitrogen cycle that returns nitrogen gas to the atmosphere. This process occurs in anaerobic environments such as waterlogged soils, sediments, and groundwater where oxygen levels are very low. Specialized denitrifying bacteria use nitrates as an alternative electron acceptor for respiration. They systematically reduce nitrates through a series of intermediate compounds: nitrate to nitrite, nitrite to nitric oxide, nitric oxide to nitrous oxide, and finally nitrous oxide to nitrogen gas. The nitrogen gas then escapes back to the atmosphere, completing the nitrogen cycle and maintaining the global nitrogen balance.