The Nitrogen Cycle: From Air to Living Things

0:00Living things need nitrogen. How does it get from the air into plants and animals?

0:06Air is about 78 percent nitrogen gas, made of pairs of tightly bonded nitrogen atoms.

0:13But plants and animals cannot use nitrogen gas directly: they cannot break that strong bond.

0:20Nitrogen fixation turns nitrogen gas into compounds that living things can use.

0:26Nitrogen-fixing bacteria do most of it, in the soil and in root nodules: swellings on the roots of peas, beans and clover.

0:36Lightning fixes some too, making nitrates that rain carries into the soil.

0:41Bacteria fix it as ammonium, and other soil bacteria change much of that into nitrates.

0:48Plants absorb ammonium and nitrates through their roots and use them to build proteins and DNA.

0:55Animals get their nitrogen from food, by eating plants or other animals.

1:00When living things die or leave waste, decomposers such as bacteria and fungi break it down, returning ammonium to the soil.

1:10Denitrifying bacteria turn nitrates back into nitrogen gas, which returns to the air.

1:16So nitrogen cycles between the air, the soil and living things. This is the nitrogen cycle.

1:24People also add nitrogen to fields as fertilizer.

1:28Rain can wash extra fertilizer into lakes, where it feeds algal blooms: sudden, thick growths of algae.

1:36Nitrogen gas must be fixed first, then it is used by living things and returned to the air.