1 Answers
Option 3 : 2 and 3 only
The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo: rocks, air, water). Nutrient cycles are of two types: (a) gaseous and (b) sedimentary.
The reservoir for the gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in the atmosphere and for the sedimentary cycle (e.g., sulfur and phosphorus cycle), the reservoir is located in Earth’s crust.
Environmental factors, e.g., soil, moisture, pH, temperature, etc., regulate the rate of release of nutrients into the atmosphere. The function of the reservoir is to meet the deficit which occurs due to an imbalance in the rate of influx and efflux.
THE NITROGEN-CYCLE-
- Nitrogen gas makes up 78% of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acids (DNA and RNA), and some vitamins.
- Nitrogen is found in other biologically important compounds such as alkaloids and urea too. Nitrogen is thus an essential nutrient for all life forms and life would be simple if all these life forms could use the atmospheric nitrogen directly.
- There are few types of organisms like certain species of soil bacteria and blue-green algae are capable of utilizing it directly in its gaseous form.
- However, other than a few forms of bacteria, life-forms are not able to convert the comparatively inert nitrogen molecule into forms like nitrates and nitrites which can be taken up and used to make the required molecules. Hence, 1 statement is not correct.
- These ‘nitrogen-fixing’ bacteria may be free-living or be associated with some species of dicot plants. Most commonly, the nitrogen-fixing bacteria are found in the roots of legumes in special structures called root nodules.
- Ninety percent of fixed nitrogen is biological. The principal source of free nitrogen is the action of soil microorganisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil.
- Other than these bacteria, the only other manner in which the nitrogen molecule is converted to nitrates and nitrites is by a physical process. During lightning, the high temperatures and pressures created in the air convert nitrogen into oxides of nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with rain. These are then utilized by various lifeforms.
- After atmospheric nitrogen has been fixed into an available form, green plants can assimilate it. Herbivorous animals feeding on plants, in turn, consume some of it. Dead plants and animals, excretion of nitrogenous wastes are converted into nitrites by the action of bacteria present in the soil. Some bacteria can even convert nitrites into nitrates that can be used again by green plants. There are still other types of bacteria capable of converting nitrates into free nitrogen, a process known as denitrification. Hence, 2 and 3 statement is correct.
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