Nitrogen is a very important element required for the formation of amino acids (basic protein units) and nucleic acids. Most parts of animal and plant bodies are made up of proteins. This explains why nitrogen is so important for proper growth to take place in plants and animals. Nitrogen, unfortunately, does not exist in a solid state. It occurs in the biosphere (especially atmosphere) in a gaseous state and is not available to plants and
animals in this state. Certain conversions/chemical processes have to take place, converting it to usable forms such as ammonium ions and nitrates before it becomes available for use by living things. Being a cycle, one is free to start describing it from any point in the biosphere. However, it is better to start from the atmosphere where nitrogen forms about 78% of available gases. Nitrogen cycle begins with a process of fixation. Nitrogen fixation occurs in the atmosphere during lightening and thunderstorms, causing the conversion of nitrogen to oxides of nitrogen. These oxides of nitrogen are further converted to nitrites, which subsequent oxidation during rains, converts them to nitrates which are washed into the soil. Nitrates are salts which contain nitrogen. It is in this form that most plants obtain nitrogen. Also, some nitrogen fixing microscopic organisms, especially monerans, blue-greens of clostridium, cyanobacteria and other free-living soil bacteria convert nitrogen gas present in soil air into ammonium ions (NH4+)
Some plants obtain and use nitrogen in this form, Symbiotic bacteria, living in root nodules of legumes also convert free nitrogen into ammonium ions, which their host use to build up amino acids. Certain plants are unable to use ammonium ions, directly. For such plants, ammonium ions, formed during fixation are converted into nitrates by nitrification. This is a process of converting ammonium ions into nitrates by nitrifying
bacteria in the soil. Nitrification takes place in two stages:
(a) conversion of ammonium ions into nitrites (NO2-) by nitrosomnas sp and
(b) further conversion of nitrites into nitrates (NO3-) by nitrobacter sp.
Plants then use the nitrates to synthesise amino acids and proteins. Animals derive nitrogen by eating plants or indirectly by eating herbivores.
When plants and animals die, or when animals defecate or urinate, decay occurs and ammonification takes place. During this process, amino acids from animal wastes and decaying dead bodies are converted into ammonium ion by bacteria. Nitrification again follows leading to the release of nitrates into the soil for use by living plants.
Not all the nitrates released into the soil by the various processes discussed above are used up by plants. In the soil, while the plants are absorbing the nitrates for use, denitrifying bacteria are busy reconverting some of the nitrates built up by nitrifying bacteria into atmospheric nitrogen or nitrogen gas, which is released again into the air, and the cycle starts again.