How do nitrogen-fixing bacteria contribute to the nitrogen cycle?

Nitrogen-fixing bacteria play a crucial role in the nitrogen cycle primarily by converting atmospheric nitrogen (N₂) into ammonia (NH₃) or related compounds that are usable by plants. This process occurs through biological nitrogen fixation, where specialized bacteria, often found in the root nodules of legumes, utilize an enzyme called nitrogenase to break the strong triple bond of atmospheric nitrogen, making it accessible in a form that plants can absorb and utilize for growth and development.

This conversion is essential because atmospheric nitrogen is abundant but largely inert and cannot be directly taken up by most plants. By transforming it into ammonia, which can then be further processed into nitrates or nitrites, nitrogen-fixing bacteria ensure that plants have the necessary nutrients to synthesize proteins, nucleic acids, and other vital compounds.

In contrast, other choices involve processes that don't specifically contribute to the fixation of nitrogen from the atmosphere. For example, while decomposing organic material is an essential process, it primarily recycles nutrients rather than converting atmospheric nitrogen. Similarly, releasing nitrogen gas back into the atmosphere refers to denitrification, which occurs later in the nitrogen cycle, while absorbing nitrogen from the soil does not pertain to the role of nitrogen fixation directly.