Nitrogen fixation assignment
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1. Nitrogen is an essential nutrient for plants that is required for proteins, chlorophyll, and genetic material. 2. Nitrogen exists in both organic and inorganic forms in soils, with plants taking up nitrogen mainly as nitrate and ammonium ions. 3. Biological nitrogen fixation is the conversion of atmospheric nitrogen to plant-usable forms by microorganisms, most notably rhizobia bacteria that form symbiotic nodules on legume roots, providing fixed nitrogen to the plant in exchange for energy.
Nitrogen fixation assignment
- 1. BIHAR AGRICULTURAL UNIVERSITY SABOUR, BHAGALPUR 813210 SUBMITTED BY SOUVIK SADHU ROLL NO- M-SSAC-00763-BAC-2020-21 DEPARTMENT OF SOIL SCIENCE AND AGRICULTURAL CHEMISTRY COURSE NAME- SOIL BIOLOGY AND BIOCHEMISTRY NITROGEN FIXATION
- 2. TABLE OF CONTENT • Role of Nitrogen in plants • Forms of Nitrogen • Nitrogen Fixation • Different Types of Nitrogen Fixation
- 3. ROLE OF NITROGEN IN PLANTS Nitrogen is an essential macronutrient for plant function and is a key component of amino acids, which form the building blocks of plant proteins and enzymes. N is considered as one of the significant macronutrient among all the mineral elements for all living tissues of the plant from metabolism to resource allocation, growth and development. Proteins make up the structural materials of all living matters and enzymes facilitate the vast array of biochemical reactions within a plant. Nitrogen is also a component of the chlorophyll molecule, which enables the plant to capture sunlight energy by photosynthesis, driving plant growth and grain yield. Role of Nitrogen can be highlighted through the following points:- 1. It is an essential component of amino acids, proteins, nucleic acids, porphyrins, flavins, purines and pyrimidine nucleotides, enzymes and alkaloids 2. Nitrogen containing chlorophyll in the presence of solar energy fixes atmospheric CO2 as carbohydrates. Figure 1 N sufficient (left) and N deficient(right) plants
- 4. 3. Being a constituent of nucleic acids such as RNA and DNA, nitrogen is responsible for transfer of genetic code to the off- springs 4. Nitrogen improves the quality of leafy vegetables and fodders. 5. Nitrogen improves the protein quality of food by enhancing glutamic acid, proline, phenylalanine, cysteine, methionine, valine and leucine in food grains.
- 5. Nitrogen in the soil system occurs mainly in two different forms that is organic and inorganic but the inorganic forms are most important from plant nutrition point of view because plant roots take up nitrogen from soil mostly as NO3- -N and to some extent NH4+ -N. Nitrogen from the organic sources become available to plants only after it is mineralized by microbes to inorganic form. FORMS OF NITROGEN Forms of Nitrogen Organic Hydrolyzable- N Amino sugar, Amino acids etc Non- hydrolyzable-N Fixed-NH4, Humin Inorganic NH4+ -N , NO3-- N and NO2--N
- 6. Nitrogen fixation, any natural or industrial process that causes free nitrogen (N2), which is a relatively inert gas plentiful in air, to combine chemically with other elements to form more-reactive nitrogen compounds such as ammonia, nitrates, or nitrites. Under ordinary conditions, nitrogen does not react with other elements. Yet nitrogenous compounds are found in all fertile soils, in all living things, in many foodstuffs, in coal, and in such naturally occurring chemicals as sodium nitrate (saltpetre) and ammonia. Nitrogen is also found in the nucleus of every living cell as one of the chemical components of DNA. BIOLOGICAL (SYMBIOTIC) NITROGEN FIXATION There are some organisms which have the ability to fix the gaseous nitrogen from atmosphere and this BNF ranges from 130 to 180 x 106 Mt. Among this the major portion comes from symbiosis between leguminous plants and microbes. In addition to this, N2 fixation by leguminous trees like mimosa, acacia etc. are also an important source of nitrogen in soil. NITROGEN FIXATION Figure 2 Types of Nitrogen Fixation
- 7. TABLE- ECONOMICALLY IMPORTANT MICROBES INVOLVED IN BNF Organism General properties Agricultural importance Azotobacter Aerobic, free fixers, live in soil, water, rhizosphere and leaf surface Minor benefit to agriculture Azospirilium Microaerobic, free fixers or found in association with roots Inoculation benefits some non leguminous crop increase root development. Rhizobium Fix N2 in legume- rhizobium symbiosis Legume crops are benefitted by inoculation with proper strains. Actinomycetes Fix N2 in symbiosis with non legume tree Important in reforestation N2 FIXATION BY LEGUMES When the root hairs of legumes grows N2 fixing bacteria invades them and multiply within it. Plant roots response by forming tumor like growth called nodules on root surface. The specialized bacteria called rhizobia inside the nodule absorb N2 from soil air and converts it to NH4+ . Rhizobia use the enzyme nitrogenase and energy from the transformation of ATP to Figure 3 Nodule formation in roots after microbial invation
- 8. ADP to break the triple bond in N2. In addition to this enzyme a source of reducing equivalents (ferredoxin, ATP and protons are required. N2 + 16 ATP + 2H+ 2NH4+ + 16 ATP + H2 The symbiotic relationship between host plant and nodule bacteria are mutually beneficial where host plant provides energy for Rhizobia to fix N2 and Rhizobia in turn provides NH4+ to host plant for making protein. The presence of nodule on roots does not Nitrogenase Figure 4 Nitrogen Fixation Figure 5 Symbiotic relationship between host and microbes
- 9. necessarily indicate N2 fixation by rhizobia. Mature effective nodules tend to be elongated, clustered on the primary roots and have a pink to red centers due to presence of leghemoglobin . Numerous Rhizobium species exist in soil each requiring a specific host legume plant. Inoculation of the legume with the correct inoculum is very important for satisfactory N2 fixation. Few Rhizobium species and their specific host are mentioned below- Figure 6 Rhizobium species and their hosts FACTORS AFFECTING NITROGEN FIXATION There are certain factors which influence the growth and activity of these microbes and hence N2 fixation. These factors included pH, salinity, moisture, temperature, microorganisms, organic matter and soil texture. The overall conclusion is that symbiotic nitrogen fixation by rhizobium is a critically biological process. Environmental stresses are generally the limiting factors of the symbiotic N2 fixation. With the
- 10. selection of appropriate legume and rhizobial inoculant, N2 fixation can be increased and food production can be improved even under environmentally stressed condition. NON- SYMBIOTIC NITROGEN FIXATION Non-symbiotic (NS) N2 fixation includes N2 fixation by free-living soil bacteria (autotrophic and heterotrophic) that are not in a direct symbiosis with plants, and associative N2-fixation (e.g. associated with the rhizospheres of grasses and cereals). N2 fixation by free living organism was identified in 19th century by winogradsky (1894). He isolated free living anaerobic bacterium Clostridium pastorianum which is a N2 fixing organism. The Non- symbiotic N2 fixation can be broadly classified intro 3 categories i.e aerobic, anaerobic and blue green algae. Some N2 fixing organism are Free living aerobic Azotobacter, Beijerinckia, Free living anaerobic Clostridium, Desulfovibrio, Purple sulpher bacteria, Green sulpher bacteria Free living associative Azospirrilum Free living photosynthetic Chlorobium, Rhodopseudomonas
- 11. NON-BIOLOGICAL NITROGEN FIXATION Non biological N2 fixation are generally two types 1. N compounds in the atmosphere are deposited with rainwater and lightning as NH4+ , NO3- and NO2- . Total N deposition as NH4+ and NO3- in rainfall is usually <8 lb N/acre/year. 2. In fertilizer manufacturing industries N2 fixation is done by Haber-Bosch process. The NH3 produced can be directly used as fertilizer. Figure 7 Haber-Bosch process of industrial N2 fixation