Plant respiration
Explanation
Nitrogen fixation is the essential process by which relatively inert atmospheric nitrogen ($N_2$) is converted into ammonia ($NH_3$) or other nitrogenous compounds (like nitrates and nitrites) that are accessible to plants. This process occurs naturally through biological and physical mechanisms.
- Symbiotic action of Rhizobium is a natural method: Rhizobium is a bacterium that establishes a symbiotic relationship with the root nodules of leguminous plants. It biologically converts atmospheric nitrogen into ammonia, which is then utilized by the host plant.
- Activity of blue-green algae is a natural method: Blue-green algae (Cyanobacteria), such as Nostoc and Anabaena, are free-living or symbiotic organisms capable of fixing atmospheric nitrogen, particularly in aquatic environments like paddy fields.
- Atmospheric lightning is a natural method: Atmospheric lightning provides the high energy required to break the strong triple bonds of nitrogen molecules. The nitrogen then reacts with oxygen to form nitrogen oxides, which dissolve in rainwater to reach the soil as nitrates and nitrites.
- Plant respiration is NOT a method of nitrogen fixation: Plant respiration is a catabolic process in which glucose is broken down to release energy, carbon dioxide, and water. It is unrelated to the conversion of atmospheric nitrogen. Plants themselves do not possess the nitrogenase enzyme required to fix nitrogen.
Key Takeaway: Natural nitrogen fixation is driven by specific microorganisms (biological fixation) or high-energy atmospheric events (physical fixation). Metabolic processes like plant respiration do not contribute to the fixation of nitrogen.