Fixation by lightning: The energy of lightning causes the combination of nitrogen (N2) and water (H2O) to generate ammonia (NH3) and nitrates (NO3). Precipitation transports ammonia and nitrates to the soil, where plants can absorb them.
Biological fixation: Approximately 90 percent of nitrogen fixation is performed by bacteria. Nitrogen is converted by cyanobacteria into ammonia and ammonium: N2 + 3 H2 2 NH3. Then, plants can utilise ammonia directly. Ammonia and ammonium may undergo further reactions throughout the nitrification procedure.
Nitrification occurs by the following reactions:2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O2 NO2- + O2 → 2 NO3-
Aerobic bacteria convert ammonia and ammonium using oxygen. Nitrosomonas bacteria transform nitrogen into nitrite (NO2-), and subsequently Nitrobacter transforms nitrite into nitrate (NO3-). Some bacteria have a symbiotic connection with plants (legumes and certain root-nodule species), and plants use nitrate as a nutrition. Meanwhile, mammals receive nitrogen through consuming plants or animals that consume plants.
Bacteria turn nitrogen nutrients back into ammonium ions and ammonia when plants and animals die. This transformation is known as ammonification. Through the process of denitrification, anaerobic bacteria can convert ammonia into nitrogen gas.NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2ODenitrification returns nitrogen to the atmosphere, completing the cycle.