The answer is yes. Fallen leaves give off CO2 when they decompose in the soil. This is because they are eaten by microbes that use oxygen and release carbon dioxide as part of their respiration. According to a study by MIT, the natural decay of organic carbon contributes more than 90 percent of the yearly carbon dioxide released into Earth’s atmosphere and oceans The rate of decomposition depends on various factors, such as temperature, moisture, and leaf composition. Decomposing leaves are also a source of nitrous oxide, another greenhouse gas that's more potent than carbon dioxide. However, not all trees lose their leaves in fall. Some trees, such as pines, spruces, and firs, are evergreen and keep their needles throughout the year. These trees can still take in carbon dioxide and produce oxygen even when their deciduous counterparts are dormant.