Rhizobium Bacteria in Soybeans Research Paper

Excerpt from Research Paper :

microorganism, Bradyrhizobium japonicum, displays a symbiotic relationship with soybean plants. There are different factors that may affect the relationship of this microorganism with plant biomass. These factors may be pH, temperature, the nutrition status and density of soil. The aim of this study was to determine the effects of different soil treatments, in different soil types, on nodule formation and the dry weight of the plant. For this purpose, soil samples were collected from three locations, namely, at a forest, near a stream and potted soil. Each sample was then analyzed with a hydrometer and classified to a soil type. The forest soil was of clay loam type, the pot soil was loam soil and the stream sample was loamy sand. The samples were subjected to different treatments, such as sterilization and inoculation. The dry weight and number of nodules on each soil type was measured. Results showed that the greatest number of nodules were in plants that were grown in soil samples grown from the stream and least for samples collected from the forest. However, no direct relationship was observed between the number of nodules and dry weight of plants. Moreover, according to observations, the most important factor in determining nodule formation and dry weight of plants was inoculation. These results provide a significant insight to certain factors that may enhance nodule formation and crop yield.


Soybeans are legumes that have a symbiotic relationship with the bacteria, rhizobium. Rhizobium are a class of nitrogen fixing bacteria that have many species, each of which is specific to different types of legumes, such as, peas, beans and lentils. These bacteria grow in nodules on plant roots. (Lindemann and Glover, 2003)

The reason why nitrogen fixing bacteria are so important is because they play a vital role in the maintenance of the ecosystem. Nitrogen forms the basic building block of amino acids which is essential for the existence of life. It is also the biological limiting nutrient for marine life. (Lindemann and Glover, 2003)

The availability of nitrogen is through many forms, namely, gaseous nitrogen, ammonium, nitrate and nitrite. About 79% of air is composed of gaseous nitrogen. However, this is unavailable for use to many living creatures on earth. Plants, animals and microorganisms could die, surrounded by an abundance of gaseous nitrogen, without the basic mechanism of nitrogen fixation. (Lindemann and Glover, 2003)

The microorganism, rhizobium, are gram negative, motile, non-sporulating rods that form a symbiotic relationship with legume species and are responsible for nitrogen fixation. The organism invades plant roots and divides within the cells of the cortex. It derives its energy and nutrition from the plant and within a week, visible nodules develop on the surface of the root. The rate of division of the bacterium is dependent on germinating conditions and legume specie. (Lindemann and Glover, 2003)

Nitrogen fixation is the process by which microorganisms convert unusable nitrogen (N2) to ammonium (NH4+). The nitrogen fixed by the organism rhizobium can be used by plants for the synthesis of amino acids. The abundance of nitrogen available to these plants are also beneficial for the plants around them. (Lindemann and Glover, 2003)

The process by which atmospheric nitrogen is fixed by nitrogen fixing microorganisms and is circulated through the ecosystem is known as the nitrogen cycle. Bacteria, such as Rhizobium and Frankia utilize N2 and convert it to an inorganic form, which is usually ammonium. The plants use the fixed nitrogen to produce amino acids. The amino acids form the building blocks for various proteins which form vital cellular elements. The plants are then consumed by animals, which utilize the amino acids in plants for protein synthesis. This is how fixed nitrogen and nitrogen products move up the food chain. When plants and animals die, decomposers act to return nitrogen back to the soil. Human-produced fertilizers are another source of nitrogen in the soil along with pollution and volcanic emissions, which release nitrogen into the air in the form of ammonium and nitrate gases. The gases react with the water in the atmosphere and are absorbed by the soil with rain water. Nitrogen is returned back to the atmosphere by denitrifying bacteria, which convert nitrates back to atmospheric nitrogen. (Lindemann and Glover, 2003)

The symbiosis between Rhizobia and legumes are precisely matched, which means Rhizobia are specific for each legume. For example, the soybean rhizobia for the soybean class of legumes are called Bradyrhizobiumjaponicum. Even among the rhizobia that can nodulate the same plant, some strains work better and more efficiently, resulting in faster nodule formation. (Lindemann and Glover, 2003)

There are also certain environmental factors that can affect the growth of different species of Rhizobia. The type of soil is one such factor. These microorganisms grow better in moist soils with a soil temperature of 250 -- 350 C. And a pH of 6.0 -- 6.8. The temperature and pH can vary slightly for different strains of rhizobia. (Jenny, 2005)

This effect of temperature and soil pH was analyzed in a study conducted on the legume specie PisumSavitum that was inoculated with Rhizobium leguminosarumbv. Viciae. The research concluded that soil pH should be maintained above 4.9 for optimum growth of the bacteria. No microorganisms were detected in high temperatures or in soils with pH lesser than 4.6. (Evans et al., 2002)

Inoculation of legumes with certain strains of Rhizobia improves biomass production through direct effects on root and shoots growth. They also prevent inoculations of pathogenic organisms which may have damaging effects on the crops. ("Introduction to Rhizobia")

There has been much research interest in the effects of Rhizobia on the growth of different legume species. Now, an increasing number of plant growth promoting Rhizobacteria are being commercialized for various crops. Several researches have also studied an association between growth promotion of legumes and Rhizobia species. (Saharan and Nehra, 2011)

The aim of this study is to detect the effect of BradyrhizobiumJaponicum on the growth of soybean plants in different soil types. Accurately identifying the optimum conditions required by the organism, BradyrhizobiumJaponicum, can have multiple effects on plant growth through increased nodulation in soybean roots, such as plant vigor, height, shoot weight, nutrient content of shoot tissues, early bloom, chlorophyll content, and increased yield.


"The effects of BradyrhizobiumJaponicum on the growth of soybean plants have a relationship to different soil types."


The content of organic matter was analyzed for each soil type. Potting soil was found to have the greatest percentage of organic matter as compared to sand and forest soil (Table 1). Next, hydrometer readings were taken at 40 second and 1 hour interval for each soil type. Results proved sand to be the densest, followed by potting and forest (Table 2).

Through results of the hydrometer, each soil sample was classified to a soil type closest to its composition. The sample collected from the forest resembled closest to the Clay Loam soil, whereas those of the potting and stream belonged to Loam and Loamy Sand soil respectively.

The number of nodules and dry weight of each plant growing in each soil type was recorded. The standard deviation and mean dry weight of each soil treatment was also calculated. The variables were described as Y1 for soil that was treated with inoculation and sterilization, Y2 for soil that was sterilized only, Y3 for soil that was inoculated only, and Y4 for soil that was neither treated with inoculation nor with sterilization. Results for the pot soil type showed the greatest dry mass with inoculation only, whereas the dry weight for forest and stream soil was greatest when neither treatment was conducted.(Table 3)





0.68 +/- 0.20

0.20 +/- 0.12

0.56 +/- 0.26


0.57 +/- 0.09

0.27 +/- 0.09

0.56 +/- 0.15


0.73 +/- 0.13

0.33 +/- 0.04

0.78 +/- 0.37


0.68 +/- 0.14

0.57 +/- 0.12

0.96 +/- 0.06

(Table 3)

Results of regression analysis showed no relationship between dry weight and nodule formation with a p value lesser than 0.05. (Figure 1) The number of nodules was greatest for samples collected from the stream and zero for those collected from the forest. In the pot type soil, nodules were only formed when the sample was inoculated and sterilized. Nodules were also not observed in the stream sample that was not inoculated.

(Figure 1)

Results from ANOVA revealed that the most significant factor affecting nodule formation resulted from the effects of inoculation on soil type and sterilization. The p value for inoculation and soil type was lesser than 0.0005 proving to be highly significant. The relationship between soil type and sterilization was also a significant factor.

To test the variances in means and standard deviation, the t- test analysis was used. The values were calculated to be greater than the value of alpha at 0.05. The overall variance was 0.0624.


To assess the effect of different treatment on different soil types and its effect on dry weight and nodule formation, different tests were implied. Inoculation was…

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