Unit 5: Environment, Its Resources and Their Conservation PDF

# Unit 5: Environment, Its Resources and Their Conservation ## 5. (a) Nitrogen Cycle: Life cannot exist without nitrogen. The atmospheric air contains 77-17% nitrogen. It is an essential component of all proteins. It is required immensely for the synthesis of amino acids, enzymes, chlorophylls, nu...

# Unit 5: Environment, Its Resources and Their Conservation ## 5. (a) Nitrogen Cycle: Life cannot exist without nitrogen. The atmospheric air contains 77-17% nitrogen. It is an essential component of all proteins. It is required immensely for the synthesis of amino acids, enzymes, chlorophylls, nucleic acids, etc. Nitrogen gas forms approximately 4/5th of the atmosphere and neither plants nor animals can fix N₂. Plants can take nitrogen in the form of nitrates which they absorb from the soil, and animals obtain nitrogen by eating plants. The mechanisms which replace nitrates in the soil may be divided into 2 types - (i) those which transform atmospheric nitrogen into soil nitrates (N₂ fixer), and (ii) those which transform plant and animal protein into nitrates (decomposers). ### (a) Definition of Nitrogen cycle: The complete series of cyclical events which occur partly in the micro-organisms of the soil and partly in the tissues of higher plants and animals are collectively known as the Nitrogen cycle. | | | | ----------- | ----------------------------------------------------------------------- | | Denitrifying bacteria in soil (Thiobacillus denitrificans) | Atmospheric Nitrogen (N) | | Nitrate, Nitrite in soil | Symbiotic Biological N₂ fixation (Rhizobium leguminosarium) | | | Thunder etc. Non biological Nitrogen fixation | | | Nitrates in soil | | | NHii in plants | | | Amino acids in plants | | | Plant proteins Food | | | Animals | | | Death Excreta | | | Commercial fertiliser in soil Urea [CO(NH2)2] | | Nonsymbiotic Nitrifying bacteria in soil, (Nitrosomonas, Nitrobacter) | | | NH, in soil | | | Ammonifying bacteria in the soil | | | Nitrogenous Organic matter in soil | | *Fig. 5.1: Nitrogen cycle-a diagrammatic sketch.* ## (b) Stages of Nitrogen cycle: Nitrogen cycle consists of the various steps such as, (1) Nitrogen fixation, (2) Nitrogen assimilation, (3) Ammonification, (4) Nitrification, (5) Denitrification and (6) Sedimentation. ### 1 Nitrogen fixation: The process of conversion of nitrogen of atmosphere into the biologically acceptable form or nitrogenous compounds is referred to as nitrogen fixation. This process is of two types such as physico-chemical or non-biological nitrogen fixation and biological nitrogen fixation. #### (i) Physico-chemical process of nitrogen fixation: In this process the atmospheric nitrogen combines with oxygen during lightning of electrical discharges in the clouds and produces different nitrogen oxides. $N_2 + 2(0) →2NO$ (Nitric oxide) $2NO + 2(0) 2NO_2$ (Nitrogen dioxide) $2NO_2 + (0) →N_2O_5$ (Nitrogen pentoxide) These nitrogen oxides (NOX) get dissolved in rain water and on reaching earth surface react with mineral compounds to form nitrates and other nitrogenous compounds. $N_2O_5 + H_2O → 2HNO_3$ $2HNO_3 + CaCO_3 → Ca(NO_3)_2 + CO_2 + H₂O$ #### (ii) Biological nitrogen fixation: The process of conversion of molecular nitrogen of the atmosphere into nitrogenous compounds through the agency of some living organisms is called biological nitrogen fixation. Certain bacteria, blue-green algae, leguminuous plants etc. can fix atmospheric nitrogen and are grouped as follows- - **Autotrophic** - (i) Aerobic e.g, blue-green algae (Nostoc), (ii) Anaerobic e.g., Rhodospirillum, Chromatium etc. - **Heterotrophic** - (i) Aerobic e.g., Azotobacter, (ii) Anaerobic e.g., Clostridium. - **Symbiotic bacteria** - Bacteria of this type use carbohydrate and atmospheric nitrogen to make compounds which are eventually released into the soil as nitrates. They live inside the root cells of leguminous plants e.g., peas, beans, clover, etc. where they cause tiny swellings called root nodules. They obtain carbohydrate from the plant cells and in return release nitrates into the plant tissues and the soil. An association of this kind in which two different organisms benefit from living together is called symbiosis. e.g., Rhizobium leguminosarum. Root hair cells of leguminous plant produces leghaemoglobin. Many non-leguminous plants are able to fix atmospheric nitrogen. They are - Casuarino, Alnus, Podocarpus, Pinus mycorrhiza, Pavetta, Chomelia, etc. The whole process of biological nitrogen fixation is controlled by the action of different enzymes like nitrogenase, nitrate synthetase, nitrate reductase, hydroxylamine reductase and so on. These enzymes are present in some bacteria, few blue green algae etc. but not in other plants and animals (including man). So these microbes can fix nitrogen whereas animals and plants can not. Moreover, a special Nif gene is present in N₂-fixing organisms but not in other plants and animals. The overall process of nitrogen fixation may be represented as follows : $N_2 + H_2 + Rhizobium Clostridium, Azotobacter etc. (Reduction) → NH_3 + O_2 → NO_2 + O_2 → NO_3 $ ### (iii) Industrial nitrogen fixation: Nitrogen fixation is essential for agriculture and manufacture of fertilizer. Ammonia is a required precursor to fertilizers. The most common industrial method is the Haber process (Haber-Bosch process). $N_2 + 3H_2 →2NH_3 $ ### 2. Ammonification: The process of release of ammonia and its formation to ammonium ions is known as ammonification. The proteins of the dead plants and animals pass into the soil, that are acted upon by soil micro-organism, which decompose protein with the liberation of NH3 (ammonia). Soil water contains a large number of hydrogen ions (H+) which chemically unite with the free ammonia to form ammonium ions (NH₄⁺). The micro-organisms influencing the process are mainly, Bacillus mycoides, Bacillus ramosus and Bacillus vulgaris. ### 3. Nitrification: The process of converting ammonia to nitrate via nitrite is known as nitrification. The ammonium ions are oxidised by a group of bacteria in the soil known as the nitrifying bacteria. Nitrosomonas converts ammonium (NH*) ions into nitrite ions (NO₂), and Nitrobacter converts nitrite ions to nitrate ions (NO₂), $2NH4 + 3O2 → 2NO2 + 2H2O + 4H*$ $2NO2 + O2 → 2NO3$ Nitrates are also available through weathering of nitrate containing rocks. ### 4. Nitrogen assimilation: The process by which inorganic nitrogen in the form of nitrates, nitrites and ammonia are absorbed by green plants and are converted into nitrogenous organic compounds is called nitrogen assimilation. Nitrates are converted into ammonia which combines with organic acids to form amino acids. Amino acids are used for the synthesis of protein, enzymes, nucleic acids, chlorophylls, etc. During digestion in animals, plant proteins are broken into amino acids which are transformed into animal proteins, nucleic acids, etc. ### 5. Denitrification: The process which involves conversion of nitrates and nitrites into ammonia, nitrous oxide and nitrogen is called denitrification. The process is accomplished by the denitrifying bacterias like-Bacillus denitrificans, species of Pseudomonas, Micrococcus, etc. Several autotrophs like Thiobacillus denitrificans, Thiobacillus thioparus, etc., also take part in this process. $2NO3 → 2NO2 → 2NO N2O N2$ ## Human activities and nitrogen cycle : Human activities (such as use of fertilizers) greatly increase the amount of nitrogen in the environment. This excess quantity of nitrogen cycles between the living world and biosphere (soil, water and air). This may result into some disastrous effects as follows : - (a) Increased global concentration of nitrous oxide (N₂O) may cause serious greenhouse effect (since N₂O is a potent greenhouse gas). - (b) Increased regional concentration of other nitrogen oxides like NO may result into bad air pollution that may cause bronchitis, pneumonia and other lung diseases. - (c) Due to deposition of acids of nitrogen, substantial acidification of soil and water may cause soil pollution and water pollution respectively. - (d) Nitrogen oxides (NOX) damages leaves of plant, growth of saplings, reduction in the rate of photosynthesis. - (e) Excessive NOX is responsible for acid rain which is very harmful to flora and fauna. ## (c) Significance of Nitrogen cycle : - To maintain nitrogen balance in nature. - Nitrogen is an essential component of all protein. The cell which is the unit of living organism is made up of protein. Hence, life cannot exist without nitrogen. - Nitrogen is the most important component of protoplasm. - Nitrogen is the essential component of deoxyribonucleotide of DNA and ribonucleotide of RNA. So, nitrogen cycle represents an excellent example-how nitrogen circulates around and through the physical and biological world, restoring the balance of nature.