2.3 Nitrogen Cycle

Overview

• The nitrogen cycle is a natural cycle that recycles nitrogen between plants and animals, and the atmosphere, soil and water.
• Approximately 78% of air is nitrogen.
• Plants cannot absorb nitrogen directly from the air; their roots absorb nitrogen as nitrate ions from the soil.
• Nitrogen-cycle processes either add nitrate ions to soil or extract nitrate ions from soil.

Flow of Nitrogen

$\text{Nitrogen in air}\stackrel{\text{nitrogen fixation}}{\to }\text{nitrate ions in soil}$

$\text{Plant protein}\stackrel{\text{animals eat plants}}{\to }\text{animal protein}$

$\text{Dead plants and animals}\stackrel{\text{decomposition}}{\to }\text{ammonium compounds}$

$\text{Ammonium compounds}\stackrel{\text{nitrification}}{\to }\text{nitrite ions}\stackrel{\text{nitrification}}{\to }\text{nitrate ions}$

$\text{Nitrate ions}\stackrel{\text{denitrification}}{\to }\text{nitrogen in air}$

Processes That Add Nitrate Ions to Soil

• Lightning and volcanic eruptions:
  1. High-temperature heat energy oxidises nitrogen in air to form nitrogen dioxide.
  2. Nitrogen dioxide dissolves in rainwater to form nitric acid.
  3. Nitric acid seeps into soil and reacts with minerals to form nitrate ions.
• Nitrogen fixation: Nitrogen-fixing bacteria in soil or water, or living symbiotically in root nodules of legume plants, convert nitrogen in air into nitrate ions.
• Nitrification:
  1. Decomposing bacteria convert animal and plant proteins into ammonium compounds.
  2. Nitrifying bacteria convert ammonium compounds into nitrite ions and then nitrate ions.
• Nitrogenous fertilisers also supply nitrate ions to soil.

Processes That Extract Nitrate Ions from Soil

• Absorption by plants: Roots absorb nitrate ions to form plant protein. Animals obtain nitrogen when they eat plants or other animals.
• Denitrification: Denitrifying bacteria convert nitrate ions in soil into nitrogen gas, which returns to the air.
• Nitrate leaching: Nitrate ions dissolve in water and flow into rivers or underground water.

Importance of the Nitrogen Cycle

• Maintains nitrogen in the atmosphere at about 78% by removing nitrogen to form nitrate ions and returning nitrogen to the air through denitrification.
• Maintains soil fertility and increases crop productivity:
  • Plants require nitrogen as a macronutrient.
  • Crop rotation with legume plants and use of fertilisers restore nitrate ions to soil.
  • Healthier crops increase yield and farmers’ income.
• Maintains a continuous supply of plant and animal proteins:
  • Nitrogen is needed to form protein.
  • Protein builds new cells and repairs damaged cells.
  • Plants form proteins from absorbed nitrate ions; animals obtain nitrogen by feeding.
• Reduces environmental pollution:
  • Decomposing bacteria and fungi convert dead remains into ammonium compounds and then nitrate ions.
  • Nitrification is applied in green technology to produce compost.