NITROGEN FIXATION-DEFINITION, PROCESS& TYPES

NITROGEN FIXATION-DEFINITION, PROCESS& TYPES

DEFINITION

Nitrogen fixation is the process by which atmospheric nitrogen is converted by a natural or an industrial means into Ammonia (NH3), Nitrate or Nitrites. In nature, most nitrogen is fetched from the air by microorganisms. In fact, N-fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds like amino acids and proteins, nucleoside triphosphates and nucleic acids. It is essential for agriculture and the manufacturing of fertilizers.

PROCESS OF FIXATION

1. Non-Biological- Nitrogen is fixed by lightning that converts Nitrogen and Oxygen into N-oxides. Later N-oxides react with water in the soil and makes nitrate that is used by plants.
2. Biological- It is the natural process of fixation of atmospheric nitrogen through the activity of soil microorganisms. Biological nitrogen fixation was discovered by German agronomist Hermann Hellriegel and Dutch microbiologist Martinus Beijerinck. Biological nitrogen fixation occurs when atmospheric nitrogen is converted to ammonia by a nitrogenase enzyme. Biological N-fixation is done by activity of some saprophytic bacteria or symbiotic bacteria or by activity of blue green algae.

• Symbiotic N-fixation– Symbiotic nitrogen fixation occurs in plants that harbor nitrogen-fixing bacteria within their tissues. The best example is the association of legumes and Rhizobium bacteria. Symbiotic N-fixation can be through nodule forming or without nodule.
• Nodule forming- With legumes plant like soya bean, alfalfa, clover etc. contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. Leghaemoglobin acts as oxygen carrier in nodules and also protect N-fixing enzyme (Nitrogenase) from oxygen. Rhizobium bacteria are aerobic and heterotrophs, hence excess moisture is harmful. Lucerne can fixes more than 250 kg N/ha, while alfalfa and ladino clover can fix up to about 195 kg/ha.

With non-legumes – important group is angiosperm. The roots of non-legumes plants are inoculated by soil Actinomycetes of the genus frankia and casuarina alder lives in association with casuarina tree. Legumes fail to develop if pH is being less than 5.

• Without Nodule- the Anabaena azollae (Blue green algae) is having a symbiotic association with Azolla pinnata (water fern). It can fixes 30-40 kg N/ha. Azolla is applied in the main field as a green manure and dual crop.
• Non-symbiotic Fixation– fixation is carried out by free living microorganisms.
  Aerobic- Azotobacter (Temperate zone, pH- 6.5-8.0), & Beijerinckia (Tropical soils, pH- 5.0-9.0)
  Anaerobic- clostridium (tolerant to wider pH)
  Photosynthetic Bacteria- Chlorobium, Rhodospirillum and BGA

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