In the nitrogen cycle, which process converts atmospheric nitrogen into a form usable by plants?

The process that converts atmospheric nitrogen into a form usable by plants in the nitrogen cycle is called nitrogen fixation. Here are the steps:

1. Nitrogen Fixation: Nitrogen gas (N2) in the atmosphere is converted into ammonia (NH3) or related nitrogenous compounds by nitrogen-fixing bacteria, such as Rhizobium, present in the root nodules of legumes, or by high-energy natural phenomena such as lightning.

2. Nitrification: The ammonia is then converted into nitrite ions (NO2-) by bacteria such as Nitrosomonas species. Other bacteria, such as Nitrobacter, further convert the nitrites into nitrates (NO3-).

3. Assimilation: Plants absorb the nitrates through their roots and use these compounds to create proteins, DNA, and other necessary materials.

4. Ammonification/Mineralization: When the plant or animal dies or an animal excretes waste, the nitrogen in the organic matter re-enters the soil where it is converted into ammonium (NH4+) by bacteria and fungi.

5. Denitrification: Finally, other bacteria in the soil convert nitrates back into nitrogen gas (N2), which returns to the atmosphere, completing the nitrogen cycle.