9.13 - The Nitrogen Cycle

The nitrogen cycle

• The air is about 78% nitrogen gas (N₂), but this gas is very unreactive, so plants and animals can't use it directly.
• Living things need nitrogen to make proteins for growth, so the nitrogen in the air has to be changed into a usable form.
• Before plants can take it up, nitrogen must be turned into mineral ions such as nitrates in the soil.
• Plants absorb nitrate ions through their roots and use the nitrogen to build amino acids and then proteins.
• Nitrogen then passes along food chains as protein - animals get their nitrogen by eating plants (and one another).

Microorganisms that recycle nitrogen

Nitrogen is returned to the soil and recycled by several groups of microorganisms (you don't need their specific names):

• Decomposers (bacteria and fungi) - Break down proteins in dead plants and animals, and the urea in animal waste, turning them into ammonia, which forms ammonium ions in the soil.
• Nitrifying bacteria - Convert ammonium compounds into nitrates, a process called nitrification, which plants can then absorb.
• Nitrogen-fixing bacteria - Carry out nitrogen fixation, turning atmospheric N₂ into nitrogen-containing compounds. Some live freely in the soil; others live in nodules on the roots of legumes such as peas and beans.
• Denitrifying bacteria - Work against the cycle by turning nitrates back into N₂ gas, which is lost to the air. This process is called denitrification.

Adding and losing soil nitrogen

• Nitrogen fixation also happens without bacteria: lightning has enough energy to make nitrogen react with oxygen to form nitrates.
• Nitrification and fixation need aerobic conditions.
• Denitrification happens mainly in waterlogged, oxygen-poor soils.

How farmers replace lost nitrogen

• Harvested crops are removed before they can decompose, so the nitrogen they contain isn't returned to the field - soil nitrate falls over time.
• Farmers replace it by:
  • Crop rotation - Growing a nitrogen-fixing crop such as peas or beans in the cycle to put nitrates back.
  • Fertilisers - Manure and compost, which decompose to release nutrients, or artificial fertilisers containing nitrates.