Photosynthesis actually consumes carbon dioxide, converting it into organic matter using sunlight, so it doesn't add CO2 to the atmosphere. Respiration by plants and animals converts organic matter back into carbon dioxide, releasing it into the atmosphere. The decay of organic matter by decomposers also releases carbon dioxide back into the atmosphere as part of the decomposition process. Volcanic action emits carbon dioxide directly into the atmosphere from the Earth's interior. Therefore, the processes that add carbon dioxide to the carbon cycle are 2, 3, and 4.