During their lifetimes, plants generally give off about half of the carbon dioxide (CO₂), that they absorb, although this varies a great deal between different kinds of plants. Once they die, almost all of the carbon that they stored up in their bodies is released again into the atmosphere.

As you may know, plants use the energy in sunlight to convert CO₂ (from the air) and water (from the soil) into sugars. This is called photosynthesis. Plants use some of these sugars as food to stay alive, and some of them to build new stems and leaves so they can grow. When plants burn their sugars for food, CO₂ is produced as a waste product, just like the CO₂ that we exhale is a waste product from the food we burn for energy. This happens day and night, but since photosynthesis is powered by sunlight, plants absorb much more CO₂ than they give off during the daytime.

At night, when photosynthesis is not happening, they give off much more CO₂ than they absorb. While they're alive, overall, about half of the CO₂ that plants absorb is given off as waste.

When you look at a tree, almost all of the body of the tree is made of sugars, which are made from carbon (from CO₂) and hydrogen and oxygen (from water). When the tree dies, it rots as decomposers, like bacteria, fungi,and insects eat away at it. Those decomposers gradually release almost all of the tree's stored carbon back into the atmosphere as CO₂. Only a very small portion of the carbon in the tree ends up staying in the soil or washing out to sea without changing back into CO₂.

Note from ScienceLine Moderator:
Actually, the oxygen from water is split off to form O₂ gas, and only the hydrogen ends up in the sugar. That seems important to notice.