What is ammonification,nitrification and denitrification in nitrogen cycle

The processes of the nitrogen cycle

Nitrogen fixation

Atmospheric nitrogen must be processed, or "fixed" (see page on nitrogen fixation), to be used by plants. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbioticbacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two component enzyme that has multiple metal-containing prosthetic groups.^[5] Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualisticrelationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form suchsymbioses. Today, about 30% of the total fixed nitrogen is manufactured in ammonia chemical plants.

Ammonification

When a plant or animal dies, or an animal expels waste, the initial form of nitrogen is organic. Bacteria, or fungi in some cases, convert the organic nitrogen within the remains back into ammonium(NH₄⁺), a process called ammonification or mineralization. Enzymes Involved:

• GS: Gln Synthetase (Cytosolic & PLastid)
• GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin & NADH dependent)
• GDH: Glu Dehydrogenase:
  • Minor Role in ammonium assimilation.
  • Important in amino acid catabolism.

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^-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO₃^-). It is important for the nitrites to be converted to nitrates because accumulated nitrites are toxic to plant life.

Due to their very high solubility, nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome.^[7] Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process that leads to high algal, especially blue-green algal populations and the death of aquatic life due to the algae's excessive demand for oxygen. While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. Since 2006, the application of nitrogen fertilizer has been increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.

Denitrification

Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N₂), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas andClostridium in anaerobic conditions.^[3] They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.