In the nitrogen cycle, different processes represent different chemical shifts. Atmospheric nitrogen exists as N2 (dinitrogen) with a triple bond. Breaking this bond to form NH3 (ammonia) is specifically defined as Nitrogen Fixation.
Nitrogen is the most abundant gas in our atmosphere (78%), yet it is often the limiting nutrient for plant growth. The reason lies in its chemical structure: the two nitrogen atoms are held together by a very strong triple covalent bond (N≡N). Converting this inert gas into reactive forms like NH3 is essential for the synthesis of amino acids, proteins, and DNA.
Nitrogen fixation (N2 → NH3) occurs through three main pathways:
The enzyme nitrogenase is a Mo-Fe (Molybdenum-Iron) protein. It is highly sensitive to oxygen. In symbiotic relationships like Rhizobium in legumes, the plant produces Leghaemoglobin to act as an oxygen scavenger, ensuring the enzyme stays active.
$$N_2 + 8e^- + 8H^+ + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i$$
Ammonia in the soil is oxidized into more mobile nitrate forms. This is a two-step process by chemoautotrophs:
It is vital to distinguish these. While fixation brings new nitrogen into the ecosystem from the air (N2 → NH3), ammonification recycles existing nitrogen. When plants or animals die, decomposers convert their protein-based organic nitrogen back into ammonia (Organic-N → NH3).
To complete the cycle, nitrates in the soil are reduced back to gaseous N2 by bacteria like Pseudomonas and Thiobacillus, usually in anaerobic (waterlogged) conditions.