Nitrogen Circulation on Earth and Bacteria

Abstract

About 78% of atmospheric gas of Earth is nitrogen (N₂). Many readers of this book may think that nitrogen gas in the atmosphere is unchangeable forever as it is a stable substance. However, when they know that proteins of animals, plants, and bacteria contain nitrogen atoms, they will understand that nitrogen atoms are present also in the organisms in other forms than gas. Ammonia (NH₃) is formed when dead bodies of animals (carcasses), their excreta, and dead plants are decomposed by bacteria. Ammonia thus formed and released from nitrogen fertilizer (ammonium sulfate, urea, etc.) is partially absorbed by plants, and its remaining part which has not been utilized by plants is oxidized to nitrite (NO₂⁻) by ammonia-oxidizing bacteria. Resulting nitrite is not utilized by plants because it is poisonous. Nitrite is quickly oxidized to nitrate (NO₃⁻) by nitrite-oxidizing bacteria. Through these processes, ammonia is actually oxidized to nitric acid, and nitric acid formed reacts with calcium carbonate in soil to form calcium nitrate. Nitrate is a good nitrogen source for all plants. Nitrate remained unabsorbed by plants is reduced to nitrogen gas by denitrifying bacteria. In some cases, the denitrifying bacteria compete with plants in utilizing nitrate.