Which organisms do participate in the nitrogen cycle?

Just about all the major classes of living things
participate in the nitrogen cycle. Nitrogen is a major component of the Earth's
atmosphere, where is exists as dinitrogen, or N2, gas. Nitrogen gas can be converted to
nitrates such as nitrogen monoxide (NO) or nitrogen dioxide (NO2) by
Rhizobium species of bacteria, which live in nodules on the roots
of plants of the Legume family. Lightning can also cause the same conversion from N2 to
nitrate to occur.

Plants take up nitrates from the soil and
incorporate them into their structure.  Animals consume the plants and absorb the
nitrates along with other nutrients. Some of the nitrates are incorporated into the
animal's body, and some are eliminated, mostly in the form of ammonia (NH3) in the
urine. The stored nitrogen eventually returns to the earth when the animal dies. The
nitrogen in urine and in carcasses is processed by bacteria of the
Nitrosomonas and Nitrobacter genera, which
turn the ammonia compounds back into nitrate compounds. These nitrates may either be
taken up by plants and used again, or be further altered by other denitrifying bacteria,
which can turn the nitrates into N2 gas and release it back to the atmosphere,
completing the cycle.