Carbon enters the atmosphere through either respiration/decay or the burning of fossil fuels. Plants take in the carbon in order to make food. From here, animals can eat the plant which transfers the carbon to them. For example, a cow eats grass. The carbon will be trapped inside the cow until it dies. When the cow has passed, decomposers will release the carbon through respiration. The carbon then restarts back in the atmosphere.
That is not the only path the carbon could take. If the plant dies before an animal consumes it, the carbon is released when the plant decays.
