Nitrogen-fixing bacteria are microorganisms that convert atmospheric nitrogen into ammonia, which is then used by plants to create proteins and other nitrogen-containing compounds. These bacteria are found in the root nodules of legumes, such as soybeans, peas, and beans, and in the soil around the roots of other plants.

Nitrifying bacteria convert ammonia into nitrite and nitrate, which are also used by plants. Nitrifying bacteria are found in the soil and in the water.

Denitrifying bacteria convert nitrates into nitrogen gas, which is released into the atmosphere. Denitrifying bacteria are found in the soil and in the water.

These three groups of microorganisms play a vital role in the nitrogen cycle, which is the process by which nitrogen is converted from one form to another in the environment. The nitrogen cycle is essential for plant growth and for the survival of all living organisms.