Which set of processes is included in the atmosphere reservoir of the carbon cycle?

• A. Decomposition.
• B. Transpiration.
• C. Weathering.
• D. Photosynthesis, respiration, combustion, and burial.

Answer: (D)

This question tests how carbon moves into and out of the atmosphere. The atmosphere acts as a reservoir for carbon primarily as CO2, so the processes that directly alter the amount of carbon in the air are those that transfer carbon to and from this reservoir.

Photosynthesis pulls CO2 out of the atmosphere as plants convert carbon dioxide into organic matter. In contrast, respiration releases CO2 back into the air when organisms break down organic matter for energy. Decomposition similarly returns carbon to the atmosphere as microbes break down dead material. Combustion, whether from burning fossil fuels or standing vegetation, releases stored carbon as CO2 into the atmosphere. Burial removes carbon from the atmospheric pool by sequestering it in sediments and long-term geological storage, reducing atmospheric CO2 over time.

These processes together—photosynthesis, respiration, decomposition, combustion, and burial—cover the major ways carbon enters or leaves the atmospheric reservoir. Transpiration and weathering involve carbon in other parts of the cycle (water transport in plants or chemical weathering of rocks) and are either indirect or less central to the atmosphere’s immediate carbon balance, so they don’t form the best description of what directly governs the atmosphere’s carbon content.