The path of nitrogen through its biogeochemical cycle involves the path it travels from the atmosphere through fixation in soil, uptake in animals and eventual release back into the atmosphere. The nitrogen biogeochemical cycle is extremely important for life on earth because nitrogen is needed for all of the major biological molecules, including amino acids, DNA and RNA.
The nitrogen cycle includes four major processes: nitrogen fixation, nitrification, denitrification and ammonification.
Nitrogen fixation is an essential process carried out by bacteria or archaea called diazotrophs that are capable of either living on their own in the soil, or in the root nodes of plants, primarily legumes. The diazotrophs have a specific enzyme called a nitrogenase that is capable of facilitating a reaction between atmospheric nitrogen and hydrogen molecules to create ammonia. Ammonia can be absorbed into plants and used to create the biological molecules necessary for life.
Nitrification is carried out by different species of bacteria that are able to convert the ammonia into nitrites and nitrates. The conversion of nitrites into nitrates is important because too much nitrite in the soil damages plant life. Similar to ammonia, nitrates are absorbed by plants and incorporated into biological molecules.
Denitrification is carried out by some bacteria species. This results in the reduction of the nitrogen in nitrates back into nitrogen gas which is released into the atmosphere.
Ammonification occurs when decaying plants and animals are broken down. The nitrogen is converted back into ammonium and can then re-enter the nitrogen cycle as ammonia.