The Nitrogen Cycle  All organisms need nitrogen to make proteins  Nitrogen gas makes up 78% of the atmosphere, but it cannot be used directly by most organisms

Nitrogen Cycle  They get their nitrogen from substances such as ammonia that contain nitrogen.  Nitrogen fixation is the converting nitrogen gas into ammonia

Nitrogen-fixing  Bacteria perform this critical step  Without these bacteria, movement of nitrogen would stop almost completely.  Lightning is the only other natural nitrogen fixing process. It accounts for only about 1% of the world’s nitrogen fixation.

Nitrogen Fixation Bacteria  Plants called legumes (peas, bean and alfalfa) have nodules on their roots that contain nitrogen-fixing bacteria  NF bacteria supply usable nitrogen directly to the plants.  They also live freely in the soil and water  Most plants cannot use the ammonia these bacteria produce.

Nitrifying bacteria  Convert ammonia into nitrites and then nitrates, which plants absorb through their roots.  Animals get the nitrogen they need by eating plants or animals.  When animals digest protein, a by-product is ammonia.  Ammonia is toxic to animals and they get rid of it in their wastes.

Nitrogen Cycle  Bacteria and fungi in the soil break down the ammonia in wastes and dead organisms into nitrates and release them into the soil where they can be adsorbed by plants

Returning Nitrates to Nitrogen Gas  Denitrifying bacteria- in the soil convert nitrates back into nitrogen gas, which returns to the atmosphere

 s07_int_nitrogen/ s07_int_nitrogen/