Which of the following nutrient cycles is NOT considered a primary cycle?

The hydrogen cycle is not considered a primary nutrient cycle because it does not have the same importance and complexity as the other cycles listed. While hydrogen is essential for biological processes and is a component of water as well as organic compounds, it does not cycle in a distinct pathway comparable to the carbon, nitrogen, or phosphorus cycles.

The carbon cycle is fundamental to the flow of carbon among earth’s ecosystems, involving processes such as photosynthesis and respiration, which are vital for life. The nitrogen cycle is crucial for converting nitrogen into forms usable by living organisms, thus supporting growth and metabolism. The phosphorus cycle plays a key role in the formation of DNA, RNA, and ATP, making it essential for energy transfer within cells.

In contrast, the hydrogen cycle is often linked as a part of water cycling and does not have as significant a standalone pathway or major biological roles as the primary nutrient cycles. This distinction clarifies why hydrogen is not classified in the same category as the carbon, nitrogen, and phosphorus cycles.