Which element is primarily involved in the nitrogen cycle?

The nitrogen cycle is a crucial biogeochemical cycle that describes the movement of nitrogen in various forms through nature, involving processes such as nitrogen fixation, nitrification, denitrification, and ammonification. Nitrogen, which is abundant in the atmosphere as N₂ gas, is essential for the creation of amino acids, proteins, and nucleic acids in living organisms.

In this cycle, nitrogen is transformed by certain bacteria into forms that plants can uptake, such as ammonia (NH₃) or nitrate (NO₃⁻). These forms can then be incorporated into organic compounds by plants, thereby entering the food chain. Animals consume plants, utilizing the nitrogen to build their own proteins and nucleic acids. Eventually, through processes like decomposition and denitrification, nitrogen is returned to the atmosphere, completing the cycle.

In contrast, other elements listed, such as oxygen, phosphorus, and carbon, while also important in their respective cycles—like the oxygen cycle, phosphorus cycle, and carbon cycle—do not play the central role that nitrogen does in the nitrogen cycle. Thus, nitrogen's key participation in these transformations and its critical role in sustaining life clearly positions it as the primary element involved in the nitrogen cycle.