Nitrogen Fixation and the Nitrogen Cycle
In a symbiotic relationship with the soil bacteria known as 'rhizobia', legumes form nodules on
their roots to 'fix' nitrogen into a form usable by plants (and animals). The process of biological
nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck. Rhizobia
(e.g., Rhizobium, Mesorhizobium, Sinorhizobium) fix atmospheric nitrogen or dinitrogen, N2,
into inorganic nitrogen compounds, such as ammonium, NH4+, which is then incorporated into
amino acids, which can be utilized by the plant. Plants cannot fix nitrogen on their own, but need
it in one form or another to make amino acids and proteins. Because legumes form nodules with
rhizobia, they have high levels of nitrogen available to them. Their abundance of nitrogen is
beneficial not only to the legumes themselves, but also to the plants around them. There are other
sources of nitrogen in the soil, but are not always provided at the levels required by plants,
making the symbiotic relationship between legumes and rhizobia highly beneficial. In return for
the fixed nitrogen that they provide, the rhizobia are provided shelter inside of the plant's nodules
and some of the carbon substrates and micronutrients that they need to generate energy and key
metabolites for the cellular processes that sustain life (Sprent, 2001). Nodulation and nitrogen
fixation by rhizobia is not exclusive to legumes; rhizobia form root nodules on Parasponis Miq.,
a genus of five species in the Ulmaceae (see 'Rosales').
The nitrogen cycle describes the series of processes by which the element nitrogen, which
makes up about 78% of the Earth’s atmosphere, cycles between the atmosphere and the
biosphere. Plants, bacteria, animals, and manmade and natural phenomena all play a role in the
nitrogen cycle. The fixation of nitrogen, in which the gaseous form dinitrogen, N2) is converted
into forms usable by living organisms, occurs as a consequence of atmospheric processes such as
lightning, but most fixation is carried out by free-living and symbiotic bacteria. Plants and
bacteria participate in symbiosis such as the one between legumes and rhizobia or contribute
through decomposition and other soil reactions. Bacteria like Rhizobium, or the actinomycete
Frankia which nodulates members of the plant families Rosaceae and Betulaceae, utilize
atmospheric nitrogen and convert it to an inorganic form (usually ammonium, NH4+) that plants
can use. The plants then use the fixed nitrogen to produce vital cellular products such as proteins.
The plants are then eaten by animals, which also need nitrogen to make amino acids and
proteins. Decomposers acting on plant and animal materials and waste return nitrogen back to
the soil. Human-produced fertilizers are another source of nitrogen in the soil along with
pollution and volcanic emissions, which release nitrogen into the air in the form of ammonium
and nitrate gases. The gases react with the water in the atmosphere and are absorbed by the soil
with rain water. Other bacteria in the soil are key components in this cycle converting nitrogen
containing compounds to ammonia, NH3, nitrates, NO3-, and nitrites, NO2-. Nitrogen is returned
back to the atmosphere by denitrifying bacteria, which convert nitrates to dinitrogen gas.