Table of Contents
Introduction
All plants, including fodder plants, need a large amount of nitrogen (N) to grow well and develop. Biological nitrogen fixation (BNF) is the term used for the process by which nitrogen gas (N2) from the atmosphere is injected into a particular plant organ. Only a selected group of plants can detect N in this way, with the help of microorganisms in the soil. Among fodder plants, a group of plants known as legumes (plants in the plant family Fabaceae) is best known for their ability to detect N in the air N2.
In fodder production, this process can be very important because it means that the much-needed N can be obtained from three sources: atmosphere through BNF, soil, and fertilizer. Fodder producers who find ways to increase the amount of N found in the atmosphere through BNF will be able to reduce their fertilizer costs while maintaining soil fertility, high levels of fodder protein, and higher yields.
The process by which other fodder plants can attach the N2 from the air to their tissues involves a host plant (also known as microsymbiont). For example, alfalfa and microorganism (also known as microsymbiont) are associated with the activity of a plant called symbiotic or symbiosis.
A symbiotic relationship is one in which two organisms form a mutually beneficial relationship. In most fodder plants the second animal is the bacteria that occur naturally in the soil. The bacterium that is most often involved in cabbage plants is best known as rhizobia because it is classified as part of a type of bacterium known as Rhizobium.
These germs attack plant roots and form structures are known as nodules. Chemical reactions, a process known as BNF, occur in nodules.
Although this process involves a complex biochemical reaction, it can be summed up in a simple way by the following statistics:
N2 + 8H2 + 16ATP ——> 2 NH3 + 2H2 + 16ADP + 16 Pi
The above statistics show that one nitrogen gas molecule (N2) combines with eight hydrogen ions (also known as protons) (8H +) to form two ammonia molecules (2NH3) and two gas molecules of hydrogen (2H2). This reaction is caused by an enzyme known as nitrogenase. The 16 ATP molecules (ATP = Adenosine Triphosphate, a combination of energy storage) represent the energy needed for a BNF reaction.
In biochemical terms, 16 ATP represents a large amount of plant energy. Thus, the BNF process is ‘expensive’ in the industry in terms of energy consumption. What source of energy is needed in the BNF? The sun, through the process of photosynthesis. As ammonia (NH3) is synthesized it is converted into an amino acid similar to glutamine. Nitrogen in amino acids can be used by a plant to synthesize proteins for growth and development.
Nitrogen Preparation by Free Living Heterotrophs
Many heterotrophic bacteria live in the soil and repair important nitrogen levels without direct contact with other organisms. Examples of these types of nitrogen-fixing bacteria include Azotobacter, Bacillus, Clostridium, and Klebsiella.
As noted earlier, these creatures have to find their source of energy, usually by incorporating organic molecules released from other organisms or by decomposing. There are other free organisms with chemolithotrophic potential and thus can use inorganic compounds as a source of energy.
Because nitrogenase can be inhibited by oxygen, free-living organisms act as anaerobes or microaerophiles while fixing nitrogen. Due to the lack of suitable carbon sources and energy sources for these organisms, their contribution to global nitrogen fixation levels is generally considered to be minimal.
However, a recent study in Australia of the wheat rotation system showed that free-living organisms contribute 20 kg per hectare per year to the long-term nitrogen requirements of this planting program (30-50% of total demand; Vadakattu & Paterson 2006). The retention of wheat varieties and reduced farming in this program provided the required environment with high carbon, low nitrogen content to improve the activity of free organisms.
Associative Nitrogen Fixation
Azospirillum species are able to form close relationships with several members of the Poaceae (grass), which includes agronomically cereal crops, such as rice, wheat, corn, oats, and barley. These bacteria fix the ideal amount of nitrogen within the rhizosphere of host plants. Efficiency of 52 mg N2 g-1 malate has been reported (Stephan et al. 1979).
The level of nitrogen fixation is determined by a number of factors, including soil temperature (species of Azospirillum grows well in areas with higher temperatures and/or temperatures), the ability of the host plant to provide a lower rhizosphere for oxygen uptake, photosythate exposure, bacterial competition, as well as nitrogenase efficiency (Vlassak & Reynders, 1979).
Symbiotic Nitrogen Fixation
Many microorganisms fix nitrogen in combination with a host plant. This plant provides the sugar from photosynthesis used by the nitrogen-fixing microorganism to get the energy we need to fix nitrogen. By interacting with these carbon sources, the microbe delivers unchanging nitrogen to the host plant to grow.
One example of this type of nitrogen fixation is water fern Azolla’s symbiosis with cyanobacterium Anabaena azollae. Anabaena colonized the holes dug under the leaves of Azolla. When cyanobacteria secrete large amounts of nitrogen into special cells called heterocysts. This symbiosis has been used for at least 1000 years as a biofertilizer in wetlands in Southeast Asia.
Rice pastures are often covered with Azolla “flowers” of up to 600 Kg N ha-1 yr-1 during the growing season (Postgate 1982, Fattah 2005).
Another example is a symbiosis between actinorhizal trees and shrubs, such as Alder (Alnus sp.), And actinomycete Frankia. These plants are native to North America and often thrive in nitrogen-free areas.
In many areas, they are the most common non-leguminous nitrogen fixer and are usually the successive plant species. Actinorhizal plants are found in many species of alpine, xeric, chaparral, forest, glacial till riparian, coast dune, and arctic tundra (Benson & Silvester, 1993) .
Although the symbiotic partners described above play an important role in the global ecology of nitrogen fixation, the most important associations for nitrogen fixation are the relationship between legumes and Rhizobium and Bradyrhizobium bacteria.
Essential vegetables used in agricultural systems include alfalfa, beans, clover, cowpeas, lupines, peanuts, soybeans, and vetches. In legumes in agricultural production, soybeans are grown at 50% of the global legume-focused area and represent 68% of the total global production (Vance 2001).
Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted into ammonia by an enzyme called nitrogenase. Many nitrogen-fixing organisms exist only in anaerobic conditions, which breath in lowering oxygen levels or bind oxygen to proteins. All plants, including fodder plants, need a large amount of nitrogen (N) to grow well and develop.
Biological nitrogen fixation (BNF) is the term used for the process by which nitrogen gas (N2) from the atmosphere is injected into a particular plant organ. Only a selected group of plants can detect N in this way, with the help of microorganisms in the soil. Among fodder plants, a group of plants known as legumes (plants in the plant family Fabaceae) is best known for their ability to detect N in the air N2.
FAQs:
1. What is biological nitrogen fixation (BNF)?
Ans: It is the process by which nitrogen (N2) present in the atmosphere is converted into plants that can be used by plants. The reaction is caused by an enzyme called nitrogenase, which is found in all nitrogen-fixing bacteria. In the case of agriculture, the symbiosis between nitrogen-fixing bacteria (called rhizobia) and vegetables (a plant family that includes soybeans, beans, peas, etc.) is the most important.
2. Are all plants fixing nitrogen biologically in symbiosis and rhizobia?
Ans: Unfortunately, not everyone is doing it. Such symbiosis is limited to legume plants and is characterized by the formation of special root structures known as nodules, in which the BNF process takes place. After the formation of root nodules, bacteria begin to convert atmospheric nitrogen into organic compounds used by plants, eliminating or reducing the need for nitrogen fertilizer.
3. Does that mean that legumes are the only plants that benefit from BNF?
Ans: No, other strains of bacteria that are able to stabilize the N2 atmosphere have been found to interact with grasses such as corn, wheat, and sugarcane. In these plants, root nodules are not formed, and constant N values are very low. Therefore, it is not possible to work with such plants without the use of nitrogen fertilizers. Nitrogen fixing microorganisms have also been found in plants such as coffee, African palm oil, cassava, papaya, and bananas, and their contribution to these plants has been the subject of numerous studies.
