The tissues of plants use the nitrogen atoms to produce amino acids, which are then built into the proteins. Animals, in turn, get the nitrogen that they need to make proteins by eating plants or other animals. When the plant or animal dies, decomposing bacteria and fungi cause the body of the plant to decay. Nitrite bacteria convert this poisonous ammonia into nitrate molecules, and other nitrate bacteria add a third atom to nitrites to form nitrates. Plants now have a usable form of nitrogen again.
Many plants can use ammonia directly.
These plants do not have to wait until the ammonia has been converted into a nitrate before they absorb it. Nitrogen can be removed from the nitrate in the soil by denitrifying bacteria. In this case, the released nitrogen is returned to the atmosphere and the nitrogen cycle is complete.