The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. 1
Source of Nitrogen
Nitrogen, which has stable valence states ranging from -3, as in ammonia (NH₃) to +5, as in nitrate (NO3-), occurs in numerous oxidation states. Nitrogen is a constituent of amino acids, nucleic acids, amino sugars, and their polymers. A large, slowly cycled reservoir for nitrogen (3.8 x 10^15 metric tons) is nitrogen gas of the atmosphere (79%). Large but essentially unavailable reservoirs of nitrogen are present in igneous (1.4 * 10^16 metric tons) and sedimentary (4.0 * 10^15 metric tons) rock as bound, non exchangeable ammonia. Physiochemical and biological weathering releases ammonia from the reservoirs so slowly that it has little influence in early cycling.
The inorganic nitrogen ions such as ammonium, nitrite, and nitrate occur as salts that are highly water soluble and consequently are distributed in dilute aqueous solution throughout the ecosphere. They form small, actively cycled reservoirs. Living and dead organic matter also provide relatively small, actively cycled reservoirs of nitrogen.
Plants, animals, and most microorganisms require combined forms of nitrogen for incorporation into cellular biomass, but the little ability to fix atmospheric nitrogen is restricted to a limited number of bacteria, archaea, and symbiotic associations. The biogeochemical cycling of the element nitrogen is highly dependent on the activities of microorganisms.
The release of ammonia from a simple nitrogenous organic compound, urea, can be described as follows.
- NH2CONH2 + H2O = 2NH3 + CO2.
- Urease enzyme helps.
In nitrification, ammonia or ammonium ions are oxidized to nitrite ions (first equation) and then to nitrate ions (next equation).
- NH4+ + 1.5 O2 = NO2- + 2H+ + H2O
- NO2- + 1.5 O2 = NO3-
Denitrifying bacteria such as Paracoccus denitrificans, ThiobacilLus denitrificans, various Pseudomonads, Bacillus etc. converting NO3- through NO2 to nitric oxide (NO), and nitrous oxide (N2O) to molecular nitrogen. The denitrification sequence is as follows:
- NO3- → NO2- → NO → N2O → N