Final answer:
Nitrogen fixation is the process where nitrogen-fixing bacteria convert atmospheric nitrogen into a form that is usable by plants, specifically ammonia. This step is identified as option 4) Nitrogen fixation among the given choices in the question about the steps of the nitrogen cycle.
Step-by-step explanation:
Understanding the Nitrogen Cycle
The nitrogen cycle consists of several processes that circulate nitrogen through the environment and living organisms. Nitrogen fixation is one of these processes and refers to the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) by nitrogen-fixing bacteria, which is then available for use by plants. This crucial step is performed by bacteria in the soil or those associated with the roots of legume plants. In the options provided to describe the steps of the nitrogen cycle, the correct choice for nitrogen fixation is 4) Nitrogen fixation.
Following nitrogen fixation, other steps in the cycle include ammonification, where nitrogenous waste is converted to ammonium (NH₄+); nitrification, the conversion of ammonium to nitrites (NO₂⁻) and nitrates (NO₃⁻); and denitrification, which involves converting nitrates back into gaseous nitrogen (N₂), thus completing the cycle and releasing it back into the atmosphere.