Colonies of Rhizobia fix nitrates into ammonia. true or false
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Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.
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THE NITROGEN FIXATION PROCESS
The element nitrogen, or “azote,” meaning “without life,” as Antonie Lavoisier called it about 200 years ago, has proved to be anything but lifeless, since it is a component of food, poisons, fertilizers, and explosives (277). The atmosphere contains about 1015 tonnes of N2 gas, and the nitrogen cycle involves the transformation of some 3 × 109 tonnes of N2 per year on a global basis (244). However, transformations (e.g., N2 fixation) are not exclusively biological. Lightning probably accounts for about 10% of the world’s supply of fixed nitrogen (301). The fertilizer industry also provides very important quantities of chemically fixed nitrogen. World production of fixed nitrogen from dinitrogen for chemical fertilizer accounts for about 25% of the Earth’s newly fixed N2, and biological processes account for about 60%. Globally the consumption of fertilizer-N increased from 8 to 17 kg ha−1 of agricultural land in the 15-year period from 1973 to 1988 (107). Significant growth in fertilizer-N usage has occurred in both developed and developing countries (238). The requirements for fertilizer-N are predicted to increase further in the future (306); however, with the current technology for fertilizer production and the inefficient methods employed for fertilizer application, both the economic and ecological costs of fertilizer usage will eventually become prohibitive.
For more than 100 years, biological nitrogen fixation (BNF) has commanded the attention of scientists concerned with plant mineral nutrition, and it has been exploited extensively in agricultural practice (50, 91). However, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N have been used for the production of food and cash crops (238). However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture (91, 238). The expanded interest in ecology has drawn attention to the fact that BNF is ecologically benign and that its greater exploitation can reduce the use of fossil fuels and can be helpful in reforestation and in restoration of misused lands to productivity (50, 301).
Currently, the subject of BNF is of great practical importance because the use of nitrogenous fertilizers has resulted in unacceptable levels of water pollution (increasing concentrations of toxic nitrates in drinking water supplies) and the eutrophication of lakes and rivers (19, 91, 301). Further, while BNF may be tailored to the needs of the organism, fertilizer is usually applied in a few large doses, up to 50% of which may be leached (301). This not only wastes energy and money but also leads to serious pollution problems, particularly in water supplies.
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NITROGEN-FIXING ORGANISMS
Organisms that can fix nitrogen, i.e., convert the stable nitrogen gas in the atmosphere into a biologically useful form, all belong to a biological group known as prokaryotes. All organisms which reduce dinitrogen to ammonia do so with the aid of an enzyme complex, nitrogenase. The nitrogenase enzymes are irreversibly inactivated by oxygen, and the process of nitrogen fixation uses a large amount of energy (91, 244). Nitrogenase activity is usually measured by the acetylene reduction assay, which is cheap and sensitive (91, 141, 301). The 15N isotopic method, which is also used to measure N2 fixation, is accurate but expensive.
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