This illustration shows the role of bacteria in the nitrogen cycle. Nitrogen-fixing bacteria in root nodules of legumes convert nitrogen gas, or N2, into organic nitrogen found in plants. Nitrogen-fixing soil bacteria produce ammonium ion, or NH4+. Decomposers, including bacteria and fungi, decompose organic matter, also releasing NH4+. Nitrification is the process by which nitrifying bacteria produce nitrites (NO2-) and nitrates (NO3-). Nitrates are assimilated by plants, then animals, then decomposers. Denitrifying bacteria convert nitrates to nitrogen gas, completing the cycle.
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