Corn and the Environment- Corn and Climate Change
According to that article, 17 times as much Co2 is removed from the air than is used in the production of corn. How fast that CO2 is released back into the air depends on how the corn and plant material is used. Take the corn, eat it, burning it to heat a house, or convert to ethanol it's released pretty quickly, if the remaining plant material is either fed to cows or left on the ground to rot/fertizise the ground it's released pretty quick too.
Burning Fossil fuels only releases.