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The nitrogen cycle is a process in which nitrogen converted into different forms, from the environment to soil, and then becomes part of living beings. N2 is a component of the food chain and found in organic and inorganic compounds and it changes to various forms to retain stability in the ecosystem. Nitrogen cycle comprised of following steps nitrogen fixation, nitrification, assimilation, denitrification, ammonification, putrefaction, and decomposition.
Image Source: Britannica.
Nitrogen is an unreactive molecule and for becoming a part of the biosphere high amount of energy is utilized. The reduction of one mole of molecular nitrogen needs 16 ATP molecules and it is said to be an energetically expensive process. Nitrogen is called fixed when it converts to its various compounds. Molecular N2 transformed into ammonia biologically by nitrogenase enzyme action.
Nitrogen fixation processes
Nitrogen is incorporated in soil by three processes.
- Atmospheric incorporation
- Industrial incorporation
- Biological incorporation
The extreme lightning energy split N2 molecule which reacts with O2 to form oxides of N2. These nitrogen oxides mix up with rain and taken towards the earth. Out of total fixed N2, 5%-8% is fixed by atmospheric incorporation.
The Haber-Bosch process is used for the formation of ammonia at high pressure and temperature from nitrogen and added in crops to enhance growth. This is a synthetic fertilizer utilized highly by farmers for crop(s) yield.
These chemical fertilizers are advantageous for increasing soil efficiency, nitrate absorption in soil, increased plantation. But they have some harmful effects also such as rising soil pH to acidic condition, ammonia liberation, and nitrate elution with high amount.
Biological Fixation is an essentially significant phenomenon, proceeded by diverse range actinomycetes, cyanobacteria, heterotrophic and autotrophic bacteria, aerobic or non-aerobic bacteria. These could be free-living organisms or live in an association. They produce nitrogenase enzymes for fixation in the biosphere. The incorporated nitrogenous compounds into soil consumed as a nutritional substance.
These organisms are contributing nitrogenous compounds without an alliance with other organisms. They have nitrogenase enzyme which catalyzes the reaction but this enzyme is sensitive to O2. These microbes act as microaerophilic or anaerobes or they may develop a unique system against O2 such as Azotobacter respiration rate is elevated which eliminates O2 from the environment and no harm occurs to the activity of nitrogenase enzyme.
There are different species of free-living bacteria found in the soil for example Bacillus, Clostridium, Azotobacter, Klebsiella, Desulfovibrio, cyanobacteria, etc; these all fix N2 but the there overall ratio of fixing N2 is considerably less due to insufficiency of adequate nutrients required for energy production.
They form a relationship with the host plant. The plant is responsible for providing carbon source to bacteria as energy and microorganisms return fixed N2 to plants. This mutual coordination helps both in growth and survival.
In this group species of Rhizobia, Azospirillum, Frankia, etc are included. They associate completely or loosely with host plants. Their contribution to N2 fixation is higher which provides high-quality crops.
Leguminous Plants and Rhizobium species
Leguminous plants have a distinctive relation with Rhizobia species known as symbiosis. Plant permits the interaction of roots with bacteria which results in nodule formation of roots. The nutrient ratio gets increased in legume plants as they interact with Rhizobium species and they obtained fixed nitrogen for growing rapidly. This interactive relationship (have) has beneficial results for plants but the overall influence on soil biome is not known.
Leguminous plants secrete chemicals “flavonoids” as an attractant for Rhizobium species, in order to interact with them. Bacteria sense these attractants and move towards plants. Flavonoids also stimulate Nod genes to produce Nod factors. Nod genes help in the formation of symbiosis and branching root hair growth.
Rhizobium as bio-fertilizer
Biofertilizers are free from the bad impacts on soil and the environment and they enhance the productivity of crops. Rhizobium is a soil habitant and predominant biofertilizer among other organisms. They were grown at an industrial scale to inoculate in soil for increasing growth rate. Their important role in incorporating fixed nitrogen, phosphorous solubilization, and promote plant growth.
Nitrification is a process of oxidation of ammonia by soil bacteria especially Nitrosomonas species which form nitrites from ammonia and subsequently nitrites converted to nitrates by Nitrobacter species in soil. This transformation is beneficial and crucial as ammonia is poisonous to plants and cause harsh damages to them. The process reaction occurs as follow:
2NH4+ + 3O2 → 2NO2– + 4H+ + 2H2O
2NO2– + O2 → 2NO3–
It is a process that proceeds by a few selected species of bacteria (Nitrosomonas, Nitrococcus, and Nitrospira). A new discovery about archaea for ammonia oxidization proves nitrification is not restricted to bacteria only. But there is limited knowledge about archaeal activities for nitrification because only Nitrospomuillus maritimus cultured purely in the laboratory.
It is the uptake of nitrite, nitrate, and ammonium ions by plants for synthesizing proteins for plants and animals. Thus, nitrogen became part of the food chain when primary consumers utilize plants as food.
It is a process of returning biologically accessible N2 to the environment. It is similar to nitrogen fixation carried by a number of prokaryotic species. It is an anaerobic phenomenon that occurred in soil, sediment, and O2 fewer zones of lakes and seas. Genus of denitrifying bacteria includes Bacillus, Pseudomonas and Paracoccus. These are chemoorganotrophs in nature and utilize organic carbon sources for energy production and utilization.
Ammonification is a process in which organic nitrogen source (dead plant or excreted waste material) is converted to ammonia by decomposing tissues of them. This process involves different fungi and bacteria. The converted ammonia released in the inorganic form to the ecosystem.
It is a check system for returning of N2 in the ecosystem, nutrient amount, tendency level of eutrophication and its anticipation, soil productivity, reduction of harmful effects of ammonia, nitrites, and nitrates.
Nitrogen is monitored via different techniques few of them are plant tissue examination, spectroradiometers, chlorophyll and polyphenol fluorescence, leaf chlorophyll meters, nitrate sap content, and electrical variables.
Nitrogen fertilizers have a useful impact on environment and economics but these fertilizers also have bad effects due to which N2 management is necessary. Techniques used for N2 determination are advantageous as whole field analysis allowed, plant tissue properties, non-interfering due to high correlation of N2 status, and chlorophyll content. But on other hand, these techniques are time-consuming, use of radioactive elements, electrode polarization effects, etc.
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