Explanation
Higher plants cannot directly use atmospheric nitrogen (N₂).
They absorb nitrogen mainly from the soil in the form of nitrates (NO₃⁻) and ammonium ions (NH₄⁺).
Importance of Nitrogen for Plants
• Essential for synthesis of proteins and amino acids
• Required for formation of nucleic acids (DNA and RNA)
• Important component of chlorophyll
• Necessary for plant growth and development
Nitrogen Cycle
Nitrogen cycle is the natural process through which nitrogen is converted into different chemical forms and circulated in nature.
Major Steps of Nitrogen Cycle
• Nitrogen Fixation → Conversion of atmospheric nitrogen into ammonia by bacteria (Rhizobium, Azotobacter)
• Nitrification → Conversion of ammonia into nitrites and then nitrates by nitrifying bacteria
• Assimilation → Plants absorb nitrates and ammonium from soil
• Denitrification → Conversion of nitrates back into nitrogen gas by bacteria
Important Microorganisms in Nitrogen Cycle
• Rhizobium → Nitrogen fixation in legume root nodules
• Nitrosomonas → Converts ammonia into nitrites
• Nitrobacter → Converts nitrites into nitrates
• Pseudomonas → Denitrification process
Important Exam Concepts
Leguminous plants → Pea, gram, beans (contain Rhizobium bacteria)
Biofertilizers → Microorganisms used to increase soil fertility
Ammonification → Conversion of organic nitrogen into ammonia