The carbon source is CO2 in the air. Digesting the food creates CO2, which completes the cycle of carbon neutrality.
The amount of carbon in the atmosphere, that used to be underground in coal, oil, and gas.
More than 170 gigatons. (approximately, this is just the 40% higher concentration than historical high point, based on 412 ppm co2 in atmosphere vs 300 ppm historical high point, back of the envelope).
This seems like a lot of land, a lot of trees, and would have to include harvesting and storing the trees after 100 years.
Reforesting an area would be carbon negative, and eventually the first will saturate. It's much much more carbon negative to produce biochar, biotar, or mass timber buildings.