Do a research on nitrogen fixing organisms, their mode of living
and ecological and economic significance. Write a report in about
250-300 words. Include facts, figures, diagrams and table of
information in the report.
Answers
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.
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.
SIGNIFICANCE OF BIOLOGICAL N2 FIXATION TO SOIL FERTILITY
BNF is an efficient source of nitrogen (238). The total annual terrestrial inputs of N from BNF as given by Burns and Hardy (49) and Paul (235) range from 139 million to 175 million tonnes of N, with symbiotic associations growing in arable land accounting for 25 to 30% (35 million to 44 million tons of N) and permanent pasture accounting for another 30% (45 million tons of N). While the accuracy of these figures may be open to question (301), they do help illustrate the relative importance of BNF in cropping and pasture systems and the magnitude of the task necessary if BNF is to be improved to replace a proportion of the 80 to 90 million tonnes of fertilizer-N expected to be applied annually to agricultural land by the end of the decade (238, 239). Much land has been degraded worldwide, and it is time to stop the destructive uses of land and to institute a serious reversal of land degradation (50). BNF can play a key role in land remediation.
Environmental Conditions
Several environmental conditions are limiting factors to the growth and activity of the N2-fixing plants. A principle of limiting factors states that “the level of crop production can be no higher than that allowed by the maximum limiting factor” (46). In the Rhizobium-legume symbiosis, which is a N2-fixing system, the process of N2 fixation is strongly related to the physiological state of the host plant. Therefore, a competitive and persistent rhizobial strain is not expected to express its full capacity for nitrogen fixation if limiting factors (e.g., salinity, unfavorable soil pH, nutrient deficiency, mineral toxicity, temperature extremes, insufficient or excessive soil moisture, inadequate photosynthesis, plant diseases, and grazing) impose limitations on the vigor of the host legume (46, 239, 315).
Typical environmental stresses faced by the legume nodules and their symbiotic partner (Rhizobium) may include photosynthate deprivation, water stress, salinity, soil nitrate, temperature, heavy metals, and biocides (337). A given stress may also have more than one effect: e.g., salinity may act as a water stress, which affects the photosynthetic rate, or may affect nodule metabolism directly. The most problematic environments for rhizobia are marginal lands with low rainfall, extremes of temperature, acidic soils of low nutrient status, and poor water-holding capacity (44). Populations of Rhizobium and Bradyrhizobium species vary in their tolerance to major environmental factors; consequently, screening for tolerant strains has been pursued (176). Biological processes (e.g., N2 fixation) capable of improving agricultural productivity while minimizing soil loss and ameliorating adverse edaphic conditions are essential.