•   Biogeochemical cycle, any of the natural pathways by which essential elements of living matter are circulated. Biogeochemical cycles can be classed as:

•   Gaseous: In these cycles, reservoir is mainly air or the oceans. Examples are carbon, nitrogen, water etc.

•   Sedimentary: In these cycles, reservoir is mainly Earth’s crust. Examples are sulphur, phosphorous etc.

•   The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.

•   The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere.

•   Unlike other cycles, atmosphere does not play a major role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids.

•   The sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. Here both atmosphere and biosphere play a role in circulation of sulphur.

•   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.

•   Processes in Nitrogen Cycle:

o  Nitrogen fixation

o  Assimilation

o  Ammonification

o  Nitrification

o  Denitrification

o  Dissimilatory nitrate reduction to ammonium

o  Anaerobic ammonia oxidation

Nitrogen fixation

•   The conversion of nitrogen gas (N2) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation.

•   Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants.

•   Non-symbiotic bacteria or free-living bacteria such as Azobacter and Beijemickia (aerobic); Clostridium and Rhodosprillium (anaerobic), some symbiotic bacteria such as Rhizobium and some cyanobacteria, blue-green algae (Nostoc, Anabaena, Spirulina) are major sources of nitrogen fixation.

•   Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, beans, alfalfa, locust trees, red clover, peanut, lentil etc.).

•   Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils.

Assimilation

•   Plants can absorb nitrate or ammonium from the soil by their root hairs.

•   If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll.

Ammonification

•   Living organisms produce nitrogenous waste products like urea and uric acid.

•   These waste products and dead remains of the organisms are converted back into inorganic ammonia and ammonium ions by bacteria by ammonification.

•   When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+)

Nitrification

•   The conversion of ammonium to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2).

•   Ammonium ions are first oxidized to nitrite by Nitrosomonas/ Nitrococcus bacteria.

•   Other bacterial species such as Nitrobacter bacteria (chemoautotrophs), are responsible for the oxidation of the nitrites (NO2-) into nitrates (NO3).

•   It is important for the ammonia (NH3) to be converted to nitrates or nitrites because ammonia gas is toxic to plants.

•   Plants absorb these nitrates and convert them into amino acids.

Denitrification

•   The process of reducing nitrate in the soil to nitrogen is called denitrification.

•   Soil and oceans have denitrifying bacteria like Pseudomonas and Thiobacillus which convert nitrate/nitrites to elemental nitrogen, under anaerobic conditions.

•   This nitrogen is released into the atmosphere completing the cycle.

Dissimilatory nitrate reduction to ammonium

•   Dissimilatory nitrate reduction to ammonium (DNRA), or nitrate/nitrite ammonification, is an anaerobic respiration process.

Anaerobic ammonia oxidation

•   In this biological process, nitrite and ammonia are converted directly into molecular nitrogen (N2) gas.

•   This process makes up a major proportion of nitrogen conversion in the oceans.