Which of the following is not a major flux in the global carbon cycle?

• A. Photosynthesis.
• B. Nitrogen fixation.
• C. Respiration.
• D. Exchange with the oceans.

Answer: (B)

Global carbon cycling revolves around transfers of carbon in CO2 and organic carbon among the atmosphere, living organisms, oceans, and sediments. Photosynthesis pulls CO2 from the atmosphere and stores carbon in plants as organic matter, while respiration releases CO2 back to the air as organisms break down that carbon. The oceans also act as a major carbon reservoir, exchanging CO2 with the atmosphere through gas transfer and carbonate chemistry. Nitrogen fixation, by contrast, is part of the nitrogen cycle: it converts inert N2 into biologically usable forms of nitrogen. It does not move carbon through ecosystems, so it does not function as a carbon flux. While nitrogen availability can influence plant growth and thus carbon storage indirectly, the process itself does not transfer carbon, making it not a major flux in the global carbon cycle.