The Nitrogen Cycle All proteins and nucleic acids contain nitrogen. An amino acid and a DNA nucleotide are shown here. The cycling of nitrogen is critical to life. Bacteria play a crucial role in recycling nitrogen between the atmosphere, the soil, and living things. Some bacteria live symbiotically in the roots of certain species of plants, supplying their hosts with a direct source of usable nitrogen. The largest group of plants with this mutualistic relationship is the legumes, a family that includes peanuts, soybeans, and alfalfa. Free-living nitrogen-fixing bacteria in soil or water convert N2 to ammonia (NH3), which then picks up another H+ ion to become ammonium (NH4+) ion. After nitrogen is "fixed", some the NH4+ is taken up and used by plants. Nitrifying bacteria in the soil also convert some of the NH4+ to nitrate (NO3-), which is more readily absorbed by plants. Plants use the nitrogen they assimilate to synthesize molecules such as amino acids, which are then incorporated into proteins. When a herbivore eats a plant, it digests the proteins it needs. Nitrogen-containing waste products are formed during protein metabolism; consumers excrete some nitrogen as wells as incorporate some in their body tissues. Organisms that are not consumed eventually die and become detritus, which is decomposed by prokaryotes and fungi. Decomposition releases NH4+ from organic compounds back into the soil, replenishing the soil reservoirs of NH4+ and, with the help of nitrifying bacteria, NO3-. Under low-oxygen conditions, however, soil bacteria known as denitrifiers strip the oxygen atoms from the NO3-, releasing N2 back into the atmosphere and depleting the soil reservoir of usable nitrogen.