🐛 Nitrogen-Fixing Biofertilizers

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Nitrogen-Fixing Biofertilizers

Nitrogen is the most limiting nutrient in crop production. Atmospheric nitrogen (N2) constitutes 78% of the air, yet plants cannot use it directly. Biological nitrogen fixation (BNF) by diazotrophic microorganisms converts atmospheric N2 into ammonia (NH3) using the enzyme nitrogenase, contributing an estimated 175 million tonnes of nitrogen globally each year.

Rhizobium — Symbiotic Nitrogen Fixer

Rhizobium and related genera (Bradyrhizobium, Mesorhizobium, Sinorhizobium) form root nodules on leguminous plants through a highly specific symbiotic relationship.

Host Specificity (Cross-Inoculation Groups)

Rhizobium Species	Host Legume
R. leguminosarum bv. viciae	Pea, lentil, faba bean
R. leguminosarum bv. trifolii	Clover
R. leguminosarum bv. phaseoli	Common bean (Rajma)
Bradyrhizobium japonicum	Soybean
Mesorhizobium ciceri	Chickpea
Rhizobium sp. (cowpea group)	Groundnut, cowpea, moong, urd

Nodulation Process

1. Root exudates (flavonoids) attract Rhizobium and activate nod genes
2. Bacteria produce Nod factors (lipochitooligosaccharides) that trigger root hair curling
3. Infection thread carries bacteria into root cortical cells
4. Bacteria differentiate into bacteroids within the nodule
5. Nitrogenase enzyme fixes N2 into NH3; leghemoglobin maintains microaerobic conditions

Rhizobium inoculation can fix 50-300 kg N/ha/year depending on the legume species, saving significant fertilizer costs.

Azotobacter — Free-Living Aerobic Fixer

Azotobacter chroococcum is the most commonly used free-living nitrogen fixer in Indian agriculture. It is a gram-negative, obligately aerobic bacterium found in neutral to alkaline soils (pH 6.5-7.5).

• Fixes 15-25 kg N/ha/year in non-leguminous crops
• Produces phytohormones (IAA, gibberellins), vitamins (B group), and siderophores
• Secretes antifungal compounds that suppress soil-borne pathogens
• Recommended for wheat, rice, maize, sorghum, cotton, mustard, and vegetables

Azospirillum — Associative Nitrogen Fixer

Azospirillum lipoferum and A. brasilense colonize the rhizosphere and root interior (endorhizosphere) of grasses and cereals. They fix nitrogen associatively — not through true symbiosis but through close root association.

• Fixes 15-40 kg N/ha/year in cereals and millets
• Major benefit is through phytohormone production (IAA), which promotes root elongation and branching
• Particularly effective in rice, sugarcane, sorghum, millets, and maize
• Thrives in moist, low-oxygen microsites near roots

Blue-Green Algae (Cyanobacteria)

Nitrogen-fixing cyanobacteria such as Anabaena, Nostoc, Aulosira, and Tolypothrix fix atmospheric nitrogen using heterocysts — specialized thick-walled cells that provide an anaerobic environment for nitrogenase. They contribute 20-30 kg N/ha/year in lowland rice paddies. BGA also adds organic matter to submerged soils, improving soil structure upon decomposition.

Azolla — The Biofertilizer Fern

Azolla is a small floating aquatic fern that harbours the symbiotic cyanobacterium Anabaena azollae in its leaf cavities. The Azolla-Anabaena complex can fix 40-60 kg N/ha in a single rice season. Azolla is used as a dual crop (grown alongside rice) or incorporated as green manure before transplanting. It doubles its biomass in 3-5 days under favourable conditions (25-30 degrees C, pH 5.5-7.0), making it a highly efficient biofertilizer for wetland rice systems.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Nitrogen-Fixing Biofertilizers — Rhizobium, Azotobacter, Azospirillum, Blue-Green Algae, and Azolla in agricultural systems.
Section context	Revise this lesson with the rest of Biopesticides and Biofertilizers for stronger conceptual continuity.