Symbiotic Nitrogen Fixation

Nitrogen is one of the supreme significant elements for plants and is the most restrictive nutrient in terrestrial environments. Much of the worlds nitrogen is in the atmosphere, where it is the most ample gas. Symbiotic nitrogen fixation is the procedure that makes atmospheric nitrogen accessible to some plants. Plants from the legume family (Fabaceae ) usually involve in symbiotic nitrogen fixation. However, some nonlegume classes (mostly trees and shrubs) and free living algae are also gifted to behavior biological fixation. This conjointly advantageous organization happens among legumes and bacteria mutually known as Rhizobia. The legume plant supplies nutrients and energy to the bacteria that exist in root nodules. A bacterial enzyme, nitrogenase, alters nitrogen from the soil into ammonia (NH3), which is reduced to ammonium (NH4 ) that is used by the legume plant to custom amino acids and protein. This procedure is overpriced for the plant, and if nitrogen levels in the soil are great, plant may reduce N2 fixation levels. The whole response that encompasses the nitrogenase bacterial enzyme is

N2 + 8H+ + 16 ATP (energy) à 2NH3 + H2 + 16 ATP

Rhizobia that are existing in the soil, or provided in inocula to the seed, contaminate plant root hairs and inspire improvement of tumor-like nodules on the roots. A particular rhizobial species is required for a given legume species. For example, bacteria infecting and nodulating white clover will not successfully nodulate soybean. The quantity of fixed nitrogen fluctuates liable on the symbiosis (Table 1). Some of the fixed nitrogen can be relocated to nonfixing plants grown in mixture or may be used by consequent crops in crop rotations.

Table 1: Quantities of nitrogen fixed by various legumes. (Sheaffer et al., 2003)


Nitrogen fixed per year



Alfalfa 70-200 78-224
White clover 115-180 129-202
Sweet clover 120 135
Sub clover 52-163 58-183
Soybean 20-200 22-224
Red clover 60-200 67-224
Lentil 149-168 167-188
Cicer milkvetch 140 157
Nodule formation

The following is the process of nodule formation, as shown in figure 1.

  1. Root hairs develop. They discharge root exudates (flavonoids, sugars, amino acids and so on), which fascinates particular rhizobia to the root.
  2. The rhizobia confer to the root hair surface.
  3. The root hair whorls, trapping the rhizobia.
  4. The rhizobia digest the cell wall and form an infection filament into the epicenter of the root. Within the thread, the bacteria split and intensification in number.
  5. The rhizobia encourage division of the root cells.
  6. A nodule is formed from the protuberance of the root cells to the surface of the root.
  7. Within the distinct root cells, the bacteria become widowed of a cell wall and become bacteroids, which develop the nitrogenase enzyme and fix atmospheric nitrogen.


Nodules shapes contrast and can be the elongated lobes originate in roots of alfalfa and the clovers, or they can be round like those found in birdsfoot trefoil and soybean. Lobed nodules are perpetual. They overwinter and fixed nitrogen for more than one growing season, whereas round nodules die and reforms on roots each year. Upon segmentation, dynamic counties of nodules will be perceived to comprehend a pink pigment, leghemoglobin, that is answerable for oxygen regulation in the nodule. Nitrogen that is fixed by bacteroids move in the plant’s vascular structure and can be conveyed all through the plant.


  1. Sheaffer, C. C., Ehlke, N. J., Albrecth, K. A. and Peterson, P. R. (2003). Forage legumes: Clovers, birdsfoots trifold, cicer milkvetch, crownvetch and alfalfa. (2nd ed.). Station bulletin 608-2003. St. Paul MN: Minnesota Agricultural Experiment Station, University of Minnesota.
  2. Sheaffer, C. C. and Moncada, K. M. (2012). Introduction to Agronomy, Food, Crops and environment. (2nd ed.). Cengage learning India private limited, New Delhi.

About Author / Additional Info:
I have completed Ph.D. Agriculture in the discipline of Agronomy from Anand Agricultural University, Anand, Gujarat. I have also qualified national eligibility test conducted by ASRB, ICAR, New Delhi.