 The movement of nitrogen, in its many forms, between the biosphere, atmosphere, and animals, is described by the nitrogen cycle.

 Nitrogen gas makes up about 80 percent of the Earth’s atmosphere.

 Nitrifying bacteria can fix nitrogen and convert it into ammonia (NH 3 ) or nitrates (NO 3 ). These bacteria are found in the soil and in the roots of some plants.

 The family of plants that includes peas, beans, soybeans, lentils, and alfalfa is called the legumes. These plants have special swellings on their roots called nodules. These swellings house and feed bacteria, which fix nitrogen into the soil.

 Plants cannot use nitrogen gas directly. They can only absorb it when nitrogen is found in its converted forms, such as ammonia or nitrates. These chemical compounds are commonly found in products known as fertilizers.

 Animals can gain nitrogen by eating plants.

 Bacteria and fungi that break down plant and animal materials are called decomposers. These bacteria return nitrogen to the soil.  Some denitrifying bacteria can release nitrogen into the atmosphere.