Which step in the nitrogen cycle involves the removal of nitrogen from the atmosphere by bacteria?

Nitrogen fixation is the step in the nitrogen cycle that involves the conversion of atmospheric nitrogen (N₂) into a usable form, primarily ammonia (NH₃), by specific bacteria. These bacteria, such as those found in the root nodules of leguminous plants or free-living in the soil, possess the enzyme nitrogenase, which facilitates the conversion process. This biological process is crucial because atmospheric nitrogen is inert and not directly usable by most living organisms. By fixing nitrogen, these bacteria play an essential role in introducing nitrogen into the ecosystem, enabling plants to absorb it and incorporate it into essential biomolecules like amino acids and nucleotides.

In the context of the nitrogen cycle, this step sets the stage for subsequent processes such as nitrification, where ammonia is converted into nitrites and then nitrates, and denitrification, where nitrates are converted back into nitrogen gas, returning it to the atmosphere. Ammonification, on the other hand, involves the breakdown of organic nitrogen compounds back into ammonia, which is different from the direct removal of nitrogen from the atmosphere. Thus, nitrogen fixation is foundational to the nitrogen cycle, as it initiates the process of making this vital nutrient available to living organisms.