Aquatic ecology and Biodiversity

Unit 6- Biogeochemical cycles

6.3. Nitrogen cycle


Nitrogen exists in a variety of forms in natural systems and its compounds are involved in numerous biological and abiotic processes. Nitrogen makes up almost 80 percent of the atmosphere. This constitutes the major storage pool of nitrogen through ecosystems. Some of these gases are converted in the soil and water to ammonia (NH₃), ammonium (NH₄⁺) or many other nitrogen compounds. and In the absence of chemical fertilizers, living organisms fix nitrogen and the process is known as nitrogen fixation. Biological nitrogen fixation is mediated by special nitrogen-fixing bacteria and algae. On the other land, these bacteria often live on nodules on the roots of legumes where they use energy from plants to do their work. In freshwater and possibly in marine systems, cyanobacteria fix nitrogen. Once nitrogen has been fixed in the soil or aquatic system, it can follow two different pathways. It can be oxidized for energy in a process called nitrification or assimilated by an organism into its biomass in a process called ammonia assimilation.
Plants incorporate the appropriate forms of fixed nitrogen into their tissues through their root systems. The plants then use it to manufacture amino acids and convert it into proteins. Nitrogen fixed as proteins in the bodies of living organisms, eventually returns via the nitrogen cycle to its original form of nitrogen gas in the air. The process of denitrification starts when plants containing the fixed nitrogen are either eaten or die. Fixed nitrogen products in dead plants, animal bodies and animal excreta encounter denitrifying bacteria that undo the work done by the nitrogen-fixing bacteria. Generally, nitrogen is the end-product of denitrification, but nitrous oxide (N₂O) is also produced in smaller quantities (upto ten percent) and the free nitrogen released into the atmosphere.