Absolutely every single gram of carbon in food waste came from the atmosphere. Trees grow up out of the ground and it appears that they get all of their mass from under the ground but it they actually only get water and some other elements like nitrogen sulfur phosphorus calcium and silicon. But the carbon that makes up most of the tree comes only from air. When plants decay it just goes back to the air. Of course it is in the form of methane lots of the time and methane actually floats up out of the atmosphere and gets taken away by the solar wind. The net effect is less carbon on earth. That is why carbon which is the fourth most abundant element in the universe is only the 14th most abundant element in the earth’s crust.