Nitrogen cycle

Lesson 10: Bio-geo-chemical cycles

Nitrogen cycle

Carnivores feed on herbivores that in turn, feed on plants. When animals defecate, this waste material is broken down by worms and insects, mostly beetles and ants. These small soil animals break the waste material into smaller bits on which microscopic bacteria and fungi can act. This material is thus broken down further into nutrients that plants can absorb and use for their growth. In this manner, nutrients are recycled back from animals to plants. Similarly, the bodies of dead animals are also broken down into nutrients that are used by the plants for their growth. Thus the nitrogen cycle, on which life is dependent, is completed.

The nitrogen-fixing bacteria and fungi in the soil give this important element to plants, which absorb it as nitrates. These nitrates are a part of the plant’s metabolism, which help in forming new plant proteins. This is used by the animals that feed on the plants. The nitrogen is then transferred to the carnivores when they feed on the herbivores. After the death and decay of carnivores, decomposers act upon them and release nitrates to the soil, making it as a vicious cycle.

Nitrogen makes up 79% of Earth's atmosphere, but most organisms cannot use nitrogen gas (N2). N2 enters the trophic system through a process called nitrogen fixation. Bacteria found on the roots of some plants can fix N2 to organic molecules, making proteins. Again, animals get their nitrogen by eating plants. But after this point, the nitrogen cycle gets far more complicated than the carbon cycle.

Animals releases nitrogen in their urine. Fish releases NH3, but NH3 when concentrated, is poisonous to living organisms. So organisms must dilute NH3 with a lot of water. Living in water, fish have no problem with these requirements, but terrestrial animals have problems. They convert NH3 into urine, or another chemical that is not as poisonous as NH3. The process of releases NH3 is called

Because NH3 is poisonous, most of the NH3 which is released is untouchable. But soil bacteria have the ability to assimilate NH3 into proteins. These bacteria effectively eat the NH3, and make proteins from it. This process is called

Some soil bacteria do not convert NH3 into proteins, but they make nitrate NO3- instead. This process is called nitrification. Some plants can use NO3-, consuming nitrate and making proteins. Some soil bacteria, however, takes NO3-, and converts it into N2, returning nitrogen gas back into the atmosphere. This last process is called denitrification, because it breaks nitrate apart.

Last modified: Thursday, 29 December 2011, 9:41 AM