Animals get nitrogen through the consumption of plants. Plants get there nitrogen through a process in which certain organisms turn nitrogen into the 2 usable forms for a plant, ammonium and nitrate. Legumes have nitrogen-fixing bacteria in their root nodules. These nitrogen-fixing bacteria take nitrogen and convert it to a usable form for the legume. For other plants, nitrogen-fixing bacteria in the soil take nitrogen (N2) and convert it to ammonium (NH4+) through a process called ammonification. Then, the nitrifying bacteria convert the ammonium into nitrites (NO2-). The nitrifying bacteria then convert to nitrites to nitrates(NO3-). The nitrogen is now in a usable form for the plant (Denitrifying bacteria can also take the nitrates and convert back into N2 and put it back into the atmosphere). Animals then may eat the plants and this is how they get their nitrogen. When plants and animals die, decomposers (aerobic and anaerobic bacteria and fungi) take the nitrogen in the dead plant or animal and turns it into ammonium where the nitrifying bacteria then turn the ammonium into nitrates again. The organisms that fix the nitrogen are only fulfilling their own metabolic requirements, but they also leave excess ammonia that becomes available for other organisms. In terrestrial ecosystems, the nitrogen is fixed by free-living (nonsymbiotic) soil bacteria as well as Rhizobium in the root nodules of legumes and other plants. In aquatic ecosystems some cyanobacteria fix nitrogen. Agriculture may now be responsible for one-half of the nitrogen fixation on Earth through the use of fertilizers produced by industrial fixation. .
