What is the Difference Between Symbiotic and Nonsymbiotic Nitrogen Fixation

The key difference between symbiotic and nonsymbiotic nitrogen fixation is that symbiotic nitrogen fixation is carried out by nitrogen-fixing bacteria that live in symbiotic relationships with host plants, while free-living nitrogen-fixing bacteria in the soil carry out nonsymbiotic nitrogen fixation.

Nitrogen fixation is a process that results in the transformation of relatively non-reactive atmospheric N₂ into its more reactive compounds, such as nitrates, nitrites, or ammonia. These reactive forms are extremely important for plant growth. More than 90% of N₂ fixation is carried out through N₂-fixing bacteria. Symbiotic and nonsymbiotic nitrogen fixation are two different types of nitrogen fixation processes that are regulated by different bacteria.

CONTENTS

1. Overview and Key Difference
2. What is Symbiotic Nitrogen Fixation
3. What is Nonsymbiotic Nitrogen Fixation
4. Similarities – Symbiotic and Nonsymbiotic Nitrogen Fixation
5. Symbiotic vs. Nonsymbiotic Nitrogen Fixation in Tabular Form
6. Summary – Symbiotic vs. Nonsymbiotic Nitrogen Fixation

What is Symbiotic Nitrogen Fixation?

Symbiotic nitrogen fixation is a type of biotic N₂ fixation process regulated by mutualistic (symbiotic) bacteria such as Rhizobium (associated with leguminous plants like members of the pea family), Frankia (associated with certain dicotyledonous species like actinorhizal plants), and certain Azospirillum species (associated with cereal grasses).

Symbiotic nitrogen-fixing bacteria usually invade the root hairs of the host plant. Then they multiply and stimulate the formation of root nodules. In root nodules, bacteria and enlarged plant cells are in intimation association. Therefore, within this root nodule, bacteria convert free nitrogen to ammonia. The host plant uses this reactive compound (ammonia) for development and growth. Commercially, this N₂ fixing technique is used in modern agriculture to ensure sufficient nodule formation and optimum growth of legumes.  The seeds are usually inoculated with commercial cultures of Rhizobium species by farmers. This N₂ fixing technique is especially useful in soils lacking the required bacterium.

What is Nonsymbiotic Nitrogen Fixation?

Approximately 90% of the natural N2 fixation in the planet is biotic and occurs due to soil microorganisms. The fixation of atmospheric nitrogen by free-living soil bacteria is known as nonsymbiotic nitrogen fixation. One of the main free-living anaerobic soil bacteria that fix atmospheric nitrogen is Clostridium pasteurianum. The other examples include Azotobacter chroococcum (aerobic-free living) and Beijerinckia. Both Azotobacter and Beijerinckia are bacterial species that fix atmospheric nitrogen in tropical climate regions. Nonsymbiotic nitrogen fixation also supplies nutrients for the growth of numerous crops.

Furthermore, the main steps in the atmospheric N₂ fixation by soil bacteria may include ammonia production (by the reduction of N_2, the ammonia is created) and nitrification (ammonia is converted to nitrite or nitrate.

What are the Similarities Between Symbiotic and Nonsymbiotic Nitrogen Fixation?

• Symbiotic and nonsymbiotic nitrogen fixation are two different types of nitrogen fixation processes that are regulated by different bacteria.
• They are biological, atmospheric N₂ fixing methods.
• Both processes produce ammonia from atmospheric N₂.
• Both processes supply nutrients for the growth of numerous crops.

What is the Difference Between Symbiotic and Nonsymbiotic Nitrogen Fixation?

Symbiotic nitrogen fixation is carried out by the nitrogen-fixing bacteria that live in symbiotic relationships with the host plants, while nonsymbiotic nitrogen fixation is carried out by the free-living nitrogen-fixing bacteria in the soil. Thus, this is the key difference between symbiotic and nonsymbiotic nitrogen fixation. Furthermore, symbiotic nitrogen fixation fixes nitrogen for its host, while nonsymbiotic nitrogen fixation makes nitrogen available in the soil.

The infographic below presents the differences between symbiotic and nonsymbiotic nitrogen fixation in tabular form for side by side comparison.

Summary – Symbiotic vs. Nonsymbiotic Nitrogen Fixation

Atmospheric nitrogen fixation is of mainly two types: physical and biological (natural) nitrogen fixation. Physical nitrogen fixation is mainly carried out through lightning and UV radiation, while bacteria mainly regulate biological nitrogen fixation. Symbiotic and nonsymbiotic nitrogen fixation are two different types of biological nitrogen fixation processes regulated by different bacteria. Symbiotic nitrogen fixation is a process that involves atmospheric nitrogen fixation by mutualistic (symbiotic) bacteria associated with plants, whereas nonsymbiotic nitrogen fixation is a process that involves atmospheric nitrogen fixation by free-living soil bacteria. So, this summarizes the difference between symbiotic and nonsymbiotic nitrogen fixation.

Reference:

1. “Symbiotic Nitrogen Fixation – An Overview.” ScienceDirect.
2. “Nonsymbiotic Nitrogen Fixation in Soil.” JSTOR.

Image Courtesy:

1. “Nitrogen fixation Fabaceae en” By Nefronus – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Azotobacter cells” By DAN H. JONES – JONES D. H. FURTHER STUDIES ON THE GROWTH CYCLE OF AZOTOBACTER // JOURNAL OF BACTERIOLOGY, 1920, VOL. 5, NO. 4 Р. 325-341 [1] (Public Domain) via Commons Wikimedia