- All organisms need nitrogen to synthesize protein, nucleic acids, and other nitrogen containing compounds.
- Molecular nitrogen (N2) makes up almost 80% of the Earth’s atmosphere.
- For plants to assimilate and use nitrogen, it must be fixed, that is, taken up and combined into organic compounds.
- The activities of specific microorganisms are important to the conversion of nitrogen to usable forms.
- All the nitrogen in the soil exists in organic molecules.
- When an organism dies, the process of microbial decomposition results in the hydrolytic breakdown of proteins into amino acids.
- In a process called deamination, the amino groups of amino acids are removed and converted into ammonia (NH3).
- The release of ammonia is called ammonification.
Ammonification, brought about by numerous bacteria and fungi, can be represented as follows:
- Microbial growth releases extracellular proteolytic enzymes that decompose proteins.
- The amino acids are transported into the microbial cells, where ammonification occurs.
- The ammonia produced by ammonification depends on soil conditions because ammonia is a gas, it rapidly disappears from
dry soil, but in moist soil it becomes solubil ized in water, and ammonium ions (NH4 +) are formed:
- Ammonium ions from this sequence of reactions are used by bacteria and plants for amino acid synthesis.
- The next sequence of reactions in the nitrogen cycle involves the oxidation of the nitrogen in the ammonium ion to produce nitrate, a process called nitrification.
- Nitrifying bacteria, such as those of the genera Nitrosomonas and Nitrobacter.
- These microbes obtain energy by oxidizing ammonia or nitrite.
In the first stage, Nitrosomonas oxidizes ammonium to nitrites:
In the second stage, such organisms as Nitrobacter oxidize nitrites to nitrates:
- Plants use nitrate as their source of nitrogen for protein synthesis because nitrate is highly mobile in soil and is more likely to encounter a plant root than ammonium.
- Ammonium ions would actually make a more efficient source of nitrogen because they require less energy to incorporate into protein.
- The form of nitrogen resulting from nitrification is fully oxidized and no longer contains any biologically usable energy.
- However, it can be used as an electron acceptor by microbes metabolizing other organic energy sources in the absence of atmospheric oxygen.
- This process is called denitrification, can lead to a loss of nitrogen to the atmosphere, especially as nitrogen gas.
Denitrification can be represented as follows:
- Denitrification occurs in waterlogged soils, where little oxygen is available.
- In the absence of oxygen as an electron acceptor, denitrifying bacteria substitute the nitrates of agricultural fertilizer.
- This converts much of the valuable nitrate into gaseous nitrogen that enters the atmosphere and represents a considerable economic loss.
- The air we breathe is about 75% nitrogen.
- Some creatures on earth that can use it directly as a nitrogen source are a few species of bacteria, including cyanobacteria.
- The process by which they convert nitrogen gas to ammonia is known as nitrogen fixation.
- Bacteria that arc responsible for nitrogen fixation all depend on the same enzyme, nitrogenase.
- Nitrogen fixation is brought about by two types of microorganisms: Free-living and Symbiotic.
- (Agricultural fertilizers arc made up of nitrogen that has been fixed by industrial physical chemical processes.)
Free-living Nitrogen-Fixing Bacteria:
- Free-living nitrogen-fixing bacteria arc found in particularly high concentrations in the rhizosphere, a region of 2 millimeters from the plant root.
- Among the free-living bacteria that can fix nitrogen are aerobic species such as Azotobacter.
- Some anaerobic bacteria, such as certain species of Clostridium, also fix nitrogen .
- Most of the free-living nitrogen -fixing bacteria are capable of fixing large amounts of nitrogen under laboratory conditions.
Symbiotic Nitrogen-Fixing Bacteria:
- Symbiotic nitrogen-fixing bacteria play an important role in plant growth for crop production .
- Members of the genera Rilizobium, Bradyrhizobium, and others infect the roots of leguminous plants, such as soybeans, beans, peas, peanuts, alfalfa, and clover.
- Rhizobia, as these bacteria, specially adapted to particular leguminous plant species, on which they form root nodules.
- Nitrogen is then fixed by a symbiotic process of the plant and the bacteria.
- The plant provides anaerobic conditions and growth nutrients for the bacteria, and the bacteria fix nitrogen that can be incorporated into plant protein.
- Another important contribution to the nitrogen economy of forests is made by lichens, which are a combination of fungus and an alga or a cyanobacterium in a mutualistic relationship.
- When one symbiont is a nitrogen-fixing cyanobacterium, the product is fixed nitrogen that eventually enriches the forest soil.