The Nitrogen Cycle

1. The Nitrogen Cycle Living things need nitrogen. It is an important element in proteins, DNA, and other chemical compounds. Nitrogen is found in fertilizers the we use in our garden and on farms. The earth’s atmosphere is 80 % nitrogen.

2. Nitrogen Fixation How do most plants get their supply of nitrogen? Most organisms cannot absorb nitrogen gas directly from the air. The nitrogen must be “fixed” or bonded to other elements to make new compounds. This process is called nitrogen fixing and is done by nitrogen fixing bacteria. Nitrogen fixing bacteria live in the roots of plants called legumes (beans, peas and clover).

3. These bacteria combine nitrogen with other elements such as hydrogen to make ammonia NH4+ and oxygen to form nitrate NO3-. Plants absorb these nitrogen compounds through their roots. Crops of legumes can add a lot of nitrogen to a farm field. If farmer does not plant legumes, they must fertilize.

4. Animals can obtain nitrogen only by eating plants or other animals. Nitrification Like other elements, nitrogen is returned to the ecosysytem through animal wastes and dead organisms. When producers (bacteria and fungi) break down these materials, ammonia is produced. Ammonia is converted back to nitrates by nitrifying bacteria. This process is called nitrification.

5. Denitrification The nitrates in the soil may be converted back to nitrogen gas by denitrifying bacteria. This process is call denitrification. The process starts all over again with the gaseous nitrogen.

6. Nitrogen fixation – the conversion of nitrogen gas to ammonia • Nitrifying bacteria – bacteria that convert ammonia to nitrates • Denitrifying bacteria – bacteria that converts nitrates to gaseous nitrogen.

7. In an aquatic ecosystem, nitrogen fixation is carried out by cyanobacteria (blue green algae).