describe through a flow diagram the steps and process involved in nitrogen cycle
Answers
In general, the nitrogen cycle has five steps:
Nitrogen fixation (N2 to NH3/ NH4+ or NO3-)
Nitrification (NH3 to NO3-)
Assimilation (Incorporation of NH3 and NO3- into biological tissues)
Ammonification (organic nitrogen compounds to NH3)
Denitrification(NO3- to N2)
Nitrogen Fixation
Nitrogen fixation is the process by which gaseous nitrogen (N2) is converted to ammonia (NH3 or NH4+) via biological fixation or nitrate (NO3-) through high-energy physical processes. N2 is extremely stable and a great deal of energy is required to break the bonds that join the two N atoms. N2 can be converted directly into NO3- through processes that exert a tremendous amount of heat, pressure, and energy. Such processes include combustion, volcanic action, lightning discharges, and industrial means. However, a greater amount of biologically available nitrogen is naturally generated via the biological conversion of N2 to NH3/ NH4+. A small group of bacteria and cyanobacteria are capable using the enzyme nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen.
Nitrogenase only functions in the absence of oxygen. The exclusion of oxygen is accomplished by many means. Some bacteria live beneath layers of oxygen-excluding slime on the roots of certain plants. The most important soil dwelling bacteria, Rhizobium, live in oxygen-free zones in nodules on the roots of legumes and some other woody plants. Aquatic filamentous cyanobacteria utilize oxygen-excluding cells called heterocysts.
Nitrification
Nitrification is a two-step process in which NH3/ NH4+ is converted to NO3-. First, the soil bacteria Nitrosomonas and Nitrococcus convert NH3 to NO2-, and then another soil bacterium, Nitrobacter, oxidizes NO2- to NO3-. These bacteria gain energy through these conversions, both of which require oxygen to occur.
Assimilation
Assimilation is the process by which plants and animals incorporate the NO3- and ammonia formed through nitrogen fixation and nitrification. Plants take up these forms of nitrogen through their roots, and incorporate them into plant proteins and nucleic acids. Animals are then able to utilize nitrogen from the plant tissues.
Ammmonification
Assimilation produces large quantities of organic nitrogen, including proteins, amino acids, and nucleic acids. Ammonification is the conversion of organic nitrogen into ammonia. The ammonia produced by this process is excreted into the environment and is then available for either nitrification or assimilation.
Denitrification
Denitrification is the reduction of NO3- to gaseous N2 by anaerobic bacteria. This process only occurs where there is little to no oxygen, such as deep in the soil near the water table. Hence, areas such as wetlands provide a valuable place for reducing excess nitrogen levels via denitrification processes.
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