Short notes: (i) Symbiotic nitrogen fixation (ii) Ammonification

(i) Symbiotic Biological Nitrogen Fixation:

This refers to mutualistic associations between nitrogen-fixing microbes and host plants. The most prominent example is the legume-Rhizobium relationship.

Mechanism:

• Rhizobium bacteria colonize legume roots and induce formation of root nodules
• Nodules provide anaerobic conditions (via leghaemoglobin) and energy (ATP) from host respiration
• Bacteria provide fixed nitrogen (ammonia) to the host
• Both partners benefit: bacteria receive carbohydrates; plant receives nitrogen

Other symbiotic associations:

• Frankia with non-legumes (e.g., Casuarina)
• Cyanobacteria with Azolla (aquatic fern)

(ii) Ammonification:

Ammonification is the decomposition of organic nitrogen (from dead plants, animals, excreta) into ammonia (NH₃) by decay microorganisms.

Process:

1. Proteins → Amino acids (proteolysis)
2. Amino acids → Organic acids + NH₃ (deamination)
3. Organic acids are utilized by microbes for energy

Microorganisms:Bacillus ramosus, B. vulgaris, Actinomyces

Equation:Proteins → Amino acids → Organic acids + NH₃

Ammonification replenishes soil nitrogen pools, making nitrogen available for nitrification and plant uptake.