Nitrogen Cycle

The nitrogen cycle proceeds through five linked stages. In fixation, the nitrogenase enzyme complex progressively reduces atmospheric di-nitrogen (N-N, triple bonded) by adding pairs of hydrogen atoms, breaking the triple bond and producing ammonia (Figure 12.7); this reaction needs the cofactor minerals molybdenum, iron and sulphur, anaerobic conditions, ATP, electrons, and glucose-6-phosphate as the hydrogen donor. In nitrification, the bacterium Nitrosomonas oxidises ammonia (NH3) to nitrite (NO2-), and the bacterium Nitrobacter then oxidises that nitrite to nitrate (NO3-); plants are better adapted to absorb nitrate from soil than ammonium. In nitrate assimilation, absorbed nitrate is reduced back down to ammonia inside the plant in two enzymatic steps - nitrate reductase (which needs molybdenum) converts nitrate to nitrite, and nitrite reductase (which needs copper and iron) converts nitrite to ammonia - so that the nitrogen can be built into amino acids. In ammonification, bacteria including Bacillus ramosus and Bacillus vulgaris decompose the organic nitrogen locked up in the proteins and amino acids of dead plants and animals, releasing it back into the soil as ammonia. Finally, in denitrification, bacteria including Pseudomonas, Thiobacillus and Bacillus subtilis convert soil nitrate back into atmospheric molecular nitrogen (N2), completing the cycle and returning nitrogen to the atmosphere it was originally fixed from. Figure 12.8 draws all five stages together into a single circular diagram of the overall nitr …