Nitrogen Fixing and Nutrient Solubilising Bacteria

After reading this article you will learn about the nitrogen fixing and nutrient solubilising bacteria.

Nitrogen-Fixing Bacteria:

Biological nitrogen-fixation is the biochemical process by which elemental nitrogen combined into organic (available) form by a number of organisms including sever species of bacteria, a few actinomycetes, and blue-green algae.

(1) Symbiotic Nitrogen-Fixation with Nodule Forming Legumes:

Rhizobium—A symbiotic bacterium, colonizes on the roots of specific legumes to form root nodules which fixes atmospheric nitrogen. Rhizobium lives in the soil independently when they come in contact with the roots of leguminous plants, they infect the roof hairs and penetrate through the root hairs into the root tissues. The number of infected root hairs increases until the nodule is formed.

The organisms entering into the roots derive carbohydrates from the root tissues of the host plants, fix nitrogen from atmosphere and synthesize amino acids and protein to make the cell material. In return, the host plants receive nitrogenous compounds synthesized by the bacteria for their own growth and development. Thus, the two organisms, the legume and bacteria, live in association and help each other in their growth. In this process, some amino acids are excreted into the soil and the nitrogen content of soil is enriched. Nitrogen is also contributed by the decaying root nodules.

There is considerable specificity between Rhizobium species and the host plants. A given Rhizobium species will inoculate some legumes but not others. On the basis of specificity of interaction, the Rhizobium and their host plants are classified into following seven groups.

The Rhizobium legume association could fix up to 100-300 kg N/ha in one crop season and in certain situation leave substantial N for the following crop.

Symbiotic N-Fixation with Nodule-Forming Non-Legumes:

A large number of non-legumes (several angiosperms) are known to develop nodules and perform symbiotic nitrogen-fixation. The examples of non-legumes angiosperms which are mostly found in forest areas form nodules in association with Actinomycetes: species of Gunnera, species of Casuarina.

The rate of nitrogen-fixation by the actinomycetes is comparable with legume—Rhizobium complexes in agricultural crops. Some blue-green algae (Nostac) are known to form nodules to develop nitrogen-fixing symbiotic relations with angiosperm (Gunnera). The rate of N-fixation by Nostac algae is typically 10-20 kg N/ha (Table 18.1).

(2) Non-Symbiotic Nitrogen-Fixers:

Certain free living microorganisms exist in soils and water that are able to fix molecular nitrogen from air without any symbiosis.

(a) Azotobacter and Clostridium (Heterotrophs):

Azotobacter is a heterotrophic aerobic bacterium. The free living bacteria fixes nitrogen in the rhizosphere and provides it to the plant. This inoculation is useful for cereals and non-leguminous crop plants. Azotobacter is sensitive to the acidic reaction (pH less than 6.0). Azotobacter, being aerobic, cannot tolerate water-logged conditions.

However, in sods growing rice, they act as facultative anaerobes. Probably they utilize oxygen liberated by algae which are found in these soils and thrive in the oxidized root zone of the rice plant.

Azotobacter is believed to be as good as Rhizobium under irrigated conditions but often relatively less effective in rainfed soils. This is because non-symbiotic organisms essentially depend on soil organic matter reserve for their energy requirement, and most of the rainfed soils are poor in organic matter. Therefore, only low to moderate contribution of N through these in dry land conditions could be expected, unless suitable application of an organic source is made to the soil.

Certain anaerobic bacteria of the genus Clostradium also are able to fix nitrogen. Because of pockets of low oxygen supply in most soils, even when they are in the best tilth, aerobic and anaerobic bacteria probably work side by side in many agricultural soils. Clostridium is a heterotrophic anaerobic bacterium, but is sometimes present in areable soils. This organism is tolerant to acidity (pH varies from 5.0 to 9.0 in the soil) and fixes less nitrogen than Azotobacter.

The amount of nitrogen fixed by these heterotrophs varies greatly with the pH, soil nitrogen level, and the organic matter available to the organism for energy. Generally, the rate of nitrogen-fixation by these organisms is much lower than those associated with legumes. The rate of N-fixation is in the range of 3-15 kg N/ha per year.

(b) Azospirillum:

It is an associative micro-aerophilic nitrogen-fixer. It colonizes the root mass and fixes N in loose association with plants. It fixes nitrogen in an environment of low oxygen tension. These bacteria induce the plant roots to secrete a mucilage which creates low oxygen environment and helps to fix atmospheric nitrogen.

These are found in association with the roots of cereals, millets, grasses etc. These are found to be specially effective for N-fixation in lowland rice. High N- fixation capacity, low energy requirement and abundant establishment in the roots of cereals and tolerance to high soil temperature (30—40—C) makes these most suitable for tropical conditions. Use of Azospirillum inoculum under saline-alkali conditions is also possible because their strains are known to maintain high nitrogenase activity under such stress conditions.

(c) Azolla:

It fixes atmospheric nitrogen in symbiotic association with blue-green algae, Anabaena. Azolla is an aquatic fern. It is widely found both in temperate and tropical natural ecosystems as well as in lowland rice growing regions. The association of Azolla with blue-green algae (Anabaena) atmospheric N, amounting to 1.00-150 kg/ha annually from 40-60 tonnes of biomass.

Azolla could be used either as a green manure before transplanting or as a dual crop after transplanting of rice, the latter practice is more beneficial. However, the biggest constraint in the large scale use of Azolla is inadequate availability of its location specific strains. Also its multiplication is a cumbersome process as it has to be grown and multiplied in separate ponds.

(d) Blue -green Algae (BGA):

Among the autotrophs able to fix nitrogen, the most important is blue-green algae (Cyanophyta). In the presence of light, BGA is able to fix both carbon dioxide and nitrogen simultaneously. BGA is a free living N-fixer. Its potential in lowland rice is well-known and it could contribute 20-30 kg N/ha per year.

Nitrogen-fixation by BGA in upland soils also occurs, but the level is much lower than is found under wetland condition. The nitrogen-fixing capacity is increased by the application of phosphate and molybdate. They, in fact, help in the nutrition of rice plants in two ways—by fixing atmospheric nitrogen, and by giving oxygen near the root zone through photosynthetic activity.

The biological nitrogen-fixation by free living, blue-green algae discussed above is non-symbiotic. But, there are certain blue- green algae which possess the character of symbiotic nitrogen- fixation in association with other organisms, such as aquatic fern Azolla. One such association of economic importance is with a species of BGA Anabaena associated with fern, Azolla.

Some of the problems of BGA commercial production are:

(i) Freshly inoculated BGA are often overwhelmed by the indigenous algae population.

(ii) Use of large scale production of BGA is very costly.

(iii) Like Azolla, growth and development of BGA is sensitive to temperature fluctuations and water level in the nursery. However, in this regard it is relatively less sensitive than Azolla.

(iv) Technology is yet to be developed to have location specific thermo insensitive strains and formulation of inoculant in dry form for better feasibility in their handling and transportation to different locations.

Nutrient Solubilizing Bacteria:

(i) Phosphorus Solubilizing Bacteria (PSB):

Phosphate solubilizing microorganisms, particularly the soil bacteria belonging to the genera Pseudomonas and Bacillus, possess the ability to transform insoluble forms. To work on phosphate solubilizing bacteria began in India with Phosphobacterin culture obtained from USSR and an indigenous culture obtained from Cassia Occidentalis.

Phosphorus solubilizing bacteria possess the ability to bring sparingly insoluble inorganic or organic phosphates into soluble form by secreting organic acids. These organic acids lower soil’s pH, and in turn, bring about dissolution of immobile forms of soil phosphate.

Some of the hydroxy acid may Chelate, Ca, Al, Fe and Mg resulting in effective availability of soil P and hence, its higher utilization by plants. In general, response to phosphobacterin is found in soils high in organic matter and low in available P. Inoculation of seed or seedling of wheat, rice, potato, chickpea with phosphobaceria increases the grain yield by 7-50 per cent.

(ii) Vesicular Arbuscular Mycorrhiza (VAM):

Some mycorrhizal fungi that penetrate roots and form specialized structures such as vesicles and arbuscles within the cortex are called Vesicular Arbuscular Mycorrhizae (VAM). VAM-fungi are considered to be obligate symbionts. Almost 90 per cent of plants, including the most important agricultural crops, are known to be associated with VAM-fungi.

VAM-fungi have been reported to increase the uptake of phosphorus. It is believed to increase the uptake of Zn, Cu, Mn and Fe. Production of growth promoters, tolerance to pathogeneus and boosting synergistic interaction with beneficial soil microorganisms such as N-fixers and P-solubilizes, are the other advantages associated with the use of VAM.

Till date, availability of VAM-fungi culture is limited because it is an obligate symbiont and have to be maintained and multiplied on live plants. Therefore, it is difficult to produce its clear and pure inoculum on large scale.