The nitrogen cycle is one of the 'magical' cycles of nature in which essential nutrients for life, such as nitrogen, are cycled between atmosphere, soil and water without the elements vanishing. It enables components such as nitrogen to be used for one generation and recycled for future generations. All living organisms need nitrogen to live and grow.
Nitrogen makes up 78% of the earth's atmosphere and is thus by far the major component in terms of amount. Although there is more nitrogen in the atmosphere than any other component, the supply of food for the world population is more limited by nitrogen than by any other nutrient. This is because in the atmosphere, nitrogen is in the stable N2 form. It cannot be used by plant or animal life in this form and has to be broken down into other forms before it can be used. Fortunately, nature has ways of breaking down a small part of the atmosphere's nitrogen into forms that can be used by plants, animals and humans, through a process called nitrogen fixing. Even so, this form of biologically available nitrogen is often in short supply due to the inert nature of the N2 gas in the atmosphere from which it is derived.
This is the critical stage of converting small amounts of the highly stable atmospheric nitrogen (N2) into forms that can enter the biological system. There are two main ways in which this is done. The main one is by certain types of bacteria in the soil which convert the N2 from the atmosphere into NH4, the ammonium ions form of nitrogen. This process is undertaken by bacteria of the genus Rhizobium that have a symbiotic relationship with the roots of certain host plants, particularly legumes such as alfalfa, clover, peas, beans. The bacteria enter the plant mainly through the root hairs and pass into the inner root tissues of the plant where they form nodules, which serve as the site of nitrogen fixation. The bacteria supply nitrogen that they have fixed to plants, and the plants supply nutrients and organic compounds needed by the bacteria in return. In addition to the Rhizobia, there are some bacteria that live freely in the soil that are capable of fixing nitrogen, species such as Clostridium and Azotobacter. These use energy from decaying organic matter to enable them to fix nitrogen. The other main way in which the inert atmosphere nitrogen gas is changed into useable nitrogen forms is by natural phenomena such as lightning and forest fires. These can lead to the fixation of smaller amounts of nitrogen. It should be stressed that the the total fixation of nitrogen from the atmosphere amounts to only a small fraction of that held in the atmosphere and is very small in terms of the needs of plants, animals and humans.