Nitrogen-Fixing Plants: Which of the Six Actually Qualify

Why plants cannot use air nitrogen

• Air is about 78% nitrogen gas (N₂).
• Its two atoms share a very strong bond.
• Plants cannot break it with their own enzymes.
• They absorb nitrogen mainly as ammonium or nitrate.
• Microbes use an enzyme called nitrogenase to turn N₂ into ammonia.
• This uses a lot of energy.

Which microbes fix nitrogen?

• Some bacteria fix nitrogen while living free in soil or water. Certain cyanobacteria do.
• Others live with plants in symbiosis. The plant supplies sugars. The microbe supplies nitrogen.
• Legumes are in the pea and bean family. Many form root nodules.
• Bacteria called rhizobia live in the nodules.
• Alder trees partner with Frankia bacteria.
• The water fern Azolla hosts a nitrogen-fixing cyanobacterium.
• Sources: NMSU, legume nitrogen fixation and NCERT mineral nutrition.

Nodule formation and oxygen

• Rhizobia respond to chemicals from a legume root.
• Root hairs curl around them. An infection thread carries them in.
• Root cells divide and form a nodule.
• Oxygen can damage nitrogenase, yet the bacteria need oxygen for energy.
• The protein leghaemoglobin keeps free oxygen low.
• Active nodules often look pink inside because of it.

The six plants in the UPSC question

• A legume uses much of its fixed nitrogen for its own growth.
• The next crop gains only what remains in roots, leaves and residues.
• Removed grain and fodder carry nitrogen away.
• Green manure releases nutrients as it decays.
• When soil already has plenty of nitrate, a legume may form fewer or less active nodules.
• An inoculant adds bacteria to seeds or soil. The strain must suit the crop, and heat or drying can kill it.
• Acid soil, waterlogging, drought and nutrient shortages reduce fixation.
• Source: NMSU, inoculating legumes.

Exam reminders

• “Nitrogen-fixing plant” means a plant-microbe partnership.
• Legumes are common hosts, not the only ones.
• Fixation does not give the next crop an instant supply of fertiliser.