Science, Tech, Math › Science Nitrogen Cycle Understand Nitrogen's Cycle Through Nature Share Flipboard Email Print Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 08, 2020 The nitrogen cycle describes the path of the element nitrogen through nature. Nitrogen is essential for life—it is found in amino acids, proteins, and genetic material. Nitrogen is also the most abundant element in the atmosphere (~78%). However, gaseous nitrogen must be "fixed" into another form so that it can be used by living organisms. 01 of 03 Nitrogen Fixation Xuanyu Han / Getty Images There are two main ways nitrogen can become "fixed:" Fixation by lightning: The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.Biological fixation: About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium: N2 + 3 H2 → 2 NH3. Ammonia can then be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process. 02 of 03 Nitrification Tony C French / Getty Images Nitrification occurs by the following reactions: 2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O2 NO2- + O2 → 2 NO3- Aerobic bacteria use oxygen to convert ammonia and ammonium. Nitrosomonas bacteria convert nitrogen into nitrite (NO2-), and then Nitrobacter converts nitrite to nitrate (NO3-). Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species), and plants utilize the nitrate as a nutrient. Meanwhile, animals obtain nitrogen by eating plants or plant-eating animals. 03 of 03 Ammonification Simon McGill / Getty Images When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification. Anaerobic bacteria can convert ammonia into nitrogen gas through the process of denitrification: NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2O Denitrification returns nitrogen to the atmosphere, completing the cycle.